N/A V6?

[autoxgalant72]

I know another N/A post just what you guys need. However I've been thinking about another way of going about it.

Honda's tend to make up for lack of displacement through spinning rediculously high rpms, this can get them to make closer to 100 hp per liter, and they seem to do it fairly reliably.

My question now is seeing as even large pushrod V8's can spin up to 7-8000 rpms somewhat reliably what keeps the 6G72 from attaining these rpms with its advantages such as overhead cams.

I figure less restrictive intake and exhaust are most certainly required as well as stiffer valve springs, stronger retainers, and a more rpm optimised camshaft. Headwork would be preferable for improved flow.

[McLarenross]

The stock valve train above ~6500 RPMs starts to float. Full headwork is required to up the RPMs.

[autoxgalant72]

I figured the stock valve train wouldnt be up to the task, but what about the intake design and that sort of stuff, about how high can it spin until the bottom end starts to complain. Would Stage 2 cams, springs, and retainers be sufficient to spin 7500, valvetrain wise. Will it continue making power as it spins higher with the proper mods.

Can you list out everything you mean when you say full headwork.

[Tearstone]

Originally Posted by autoxgalant72

I figured the stock valve train wouldnt be up to the task, but what about the intake design and that sort of stuff, about how high can it spin until the bottom end starts to complain. Would Stage 2 cams, springs, and retainers be sufficient to spin 7500, valvetrain wise. Will it continue making power as it spins higher with the proper mods.

Can you list out everything you mean when you say full headwork.

I've been thinking a lot about this as well. I was actually talking to Len at BLE, and he said they were going from the 6G74 motor back to the 6G72 to spin it up higher in their drag car. We can supply the BLE solid lash adjusters, and we can get the cams & valve springs through RPW, or we can have custom valves/retainers made through Ferrea although you will pay a premium.

Bottom line, we've all seen the dyno charts on these engines, and we know the stock VE falls on it's face past 6K. You need to open it up and definitely make it easier to bring a higher volume of air in from the throttle body all the way to the valves.

This is something I definitely wanted to work on either through a customer or on our 3G this year.

[autoxgalant72]

So would the RPW reworked mani provide enough volume to do this or is that just going to move the restriction to the lower intake and heads. I know they where talking about having it available to put a holley twin 58mm throttle body on it from an ls1. I know they sell tbs that instead of the twin butterfly's they have a single oval one thats the size of the entire opening.

I would love to build my car N/A as its alot more managable and easier to work on for autocross purposses.

I think if the intake restriction can be isolated and eliminated a 100 hp per liter 6G is possible, I may be all wrong in the engines entire build being able to withstand alot of rpms but it may be worth a shot for those who are racing in fields that either restrict against turbo's or just dont want all the plumbing and lag of a turbo.

Dont get me wrong, I love turbo's but a 1 second lag on a 13 second straight is a big deal.

[Tearstone]

Originally Posted by autoxgalant72

So would the RPW reworked mani provide enough volume to do this or is that just going to move the restriction to the lower intake and heads. I know they where talking about having it available to put a holley twin 58mm throttle body on it from an ls1. I know they sell tbs that instead of the twin butterfly's they have a single oval one thats the size of the entire opening.

I would love to build my car N/A as its alot more managable and easier to work on for autocross purposses.

I think if the intake restriction can be isolated and eliminated a 100 hp per liter 6G is possible, I may be all wrong in the engines entire build being able to withstand alot of rpms but it may be worth a shot for those who are racing in fields that either restrict against turbo's or just dont want all the plumbing and lag of a turbo.

Dont get me wrong, I love turbo's but a 1 second lag on a 13 second straight is a big deal.

The RPW manifold introduces the velocity stacks and a reworked chamber, just that the manifold is ported. But the 6G74 manifold alone (before the RPW rework) is a great improvement over the stock 6G72 manifold we have now in of itself.

I really do not think it is out of the question to see 250whp out of a N/A 3G, but the first key is the cams for sure.

On the note about the turbo, keep in mind that with a properly sized turbo you get very little lag, but on a stock engine that would be stressful. I've taken my Eclipse AutoX before, and it's nice to boost through the straights.

[autoxgalant72]

I know exactly what your saying about turbo's, I love them, its just with what I'm doing with autox going turbo would just be more work than I'm willing to do and not in the right direction I need to go. With a high rpm engine I wont need to shift as often, and wont have any lag, or an exhaust turbine to worry about heat on. As it is I've bypassed the coolent lines to the throttle body, nothing in the way of power gains per say, but heat soak is definatly deminished, and has actually helped my times maintain consistancy.

Cams is definatly the key ingerediant to this project, personally I dont even really care for a smooth idle, basically unplug the brake booster line and thats what I would love the car to idle like, so I wont see any problems with going to a higher rpm cam. I would love to see a power band with a decent AUC between 4-4500rpm and 75-8000 rpm.

What do you think the limit of the internals would be?

[MattS00]

I've wondered if it would be possible to put a 6g72 crank in a 6g74 and use custom pistons(higher wrist pin location) and rods to give a better rod ratio. It would allow for higher revs. However, I think the cam and valve train is more of a limiting factor. Especially the rockers for the exhaust valves that are shaped like a y to open two valves with one cam lobe.

[6G72Duster]

IIRC some of the DOHC 3KGT people push their rev limiter up to 8K+, but im not sure which versions/years of the 3KGT had the Nitrile treated crank. on the subject of teh Y rockers, would they be prone to fatigue failure at upper RPMs? Also, thinking on the high revving honda/toyota motors, i think those run a shim on bucket valvetrain vs a rocker arm setup. if the rocker arms prove to be a weak link, then like the V8s newer stronger ones would be necessary. i would think that we might hit around 8000 maybe 8500 rpms as an upper limit, but i think that the displacement and rotating assembly weight are going to keep us from the 9000+ range. one of the reasons the hondas/toyotas rev so easily is that (the ones i can think of) they MAYBE top out at 2L, the real rev happy ones are 1.6L or 1.8L (toyota's 4AGE {1.6}, 3SG(T)E {2.0}, 2ZZGE {1.8}. Honda's B16A and esp B16B, the B18 series as well)

just on a personal note, the 6G72 seems to be a fairly rev happy engine for a v6 unlike some others like a GM 3.1 or some of the ford v6s

[Bitter]

what about intake/head work for the heads going on the 6g74? i can imagine that i'd need to breathe a little better considering its using the same heads as the 6g72.

[Tearstone]

Originally Posted by fasteclipse00

I've wondered if it would be possible to put a 6g72 crank in a 6g74 and use custom pistons(higher wrist pin location) and rods to give a better rod ratio. It would allow for higher revs. However, I think the cam and valve train is more of a limiting factor. Especially the rockers for the exhaust valves that are shaped like a y to open two valves with one cam lobe.

Nah the main size is different, and besides I don't think you would have to get custom made rods on top of that even if the mains did fit alone.

[thePNO95]

Does the 6G72 have smaller mains than the 6G74? In that case, it's possible to use spacers (obviously custom, of couse). In the small-block Ford world, the 351C has smaller mains than the 351W. Smaller mains = less surface area to put heat in the oil = better for high RPM running. So there are main bearing spacers you can buy to run the smaller Cleveland sized mains in a Windsor block. They're a proven product used in some pretty big power race motors. http://www.performanceparts.ford.com...tKeyField=2993

So the custom setup mentioned above (to get the better rod ratio) would also have better high RPM oiling if the 72's mains are smaller than the 74's - a nice side benefit!

I think it would be pretty cool to see a good N/A build up spinning 7000-7500rpm. I love the sound of a V-6, even with the stock RPM limitations - imagine how it would sound if you could wind it out another 1000rpm or more!! Mmmmm....

PNO

[Tearstone]

Does anyone have a stock crank lying around they can measure, I sold my spare one to Greg. I have a 74 I can measure to give you guys the right dimensions.

[stilwrks]

as Russ knows, i'm in the process of having RPW NA 1 cams installed in my completly PnP'd intake system!
once that is buttoned up it gets dyno'd!
will post results but don't expect anything great because doing this doesn't show good dyno results & that goes along with the car being over 300lbs lighter, its a no show. am sure the butt dyno will be a big diff.

[autoxgalant72]

I totally wouldnt expect our engine to spin in the 9k rpm range like a honda, its just not set up for it. I think the rocker arms are a good point that may need to be adressed, at what rpm will they start to fatigue fast enough for them to quickly fail, and is it a possablity to get stronger ones made. I think even with boosted motors another 1k rpms of rev would most certainly be bennoficial.

How much would the RPW springs be able to ward off valve float at the higher rpms, would they be sufficient for 7500. What about available cams, would the stage 2 be able to feed the engine up that high or would thier stage 3 be needed.

[Tearstone]

Originally Posted by autoxgalant72

How much would the RPW springs be able to ward off valve float at the higher rpms, would they be sufficient for 7500. What about available cams, would the stage 2 be able to feed the engine up that high or would thier stage 3 be needed.

I would have to query Dave with more details on that, although he's confident that these motors can spin up higher than 6K with their parts. But I want to get more specific details though for you.

[PharmEcis]

Who needs to measure the crank when we can pull that info straight out of Alldata. Russ, I told you to send me a drive so I could load you up a copy.

Main journals on the 72 are 60mm.

Surprise surprise, the main journal diameter on the 74 is also 60mm. Houston I think we have ignition!

Hrm, what's even more surprising is the fact that according to Alldata the 72 and 74 cranks are for all intents and purposes identical. Both have main journals of 60mm and both have rod journals of 50mm.

Monday or Tuesday I'll be finally picking my 74 up from the machine shop so I'll check everything.

[PharmEcis]

Son of a bitch, for the 3.5L it's pulling up a 3.0L diagram. The information above is bunk. Digging into the right charts as I type.

[Bitter]

lol alldata is shit, i prefer mitchel or getting it right from the horses mouth when possible.

[PharmEcis]

Ok, found the right charts. All journals still match sizes. There is also no apparent greater height found in the cylinder block. The heights are identical according to Alldata.

The idea of building a 3.5L motor w/ a 72 crank would give a displacement of 3.1L that should be able to be rev'd to high hell and back.

Oh boy I think this next week is going to be interesting once I get some parts in my hands.

[PharmEcis]

Originally Posted by Bitter

lol alldata is shit, i prefer mitchel or getting it right from the horses mouth when possible.

I have both systems and Alldata is far superior to mitchell in just about every way. This is the first time I've had Alldata give me bad info, all I had to do was pull up a different year and I got the right info.

[Tearstone]

Originally Posted by PharmEcis

Ok, found the right charts. All journals still match sizes. There is also no apparent greater height found in the cylinder block. The heights are identical according to Alldata.

The idea of building a 3.5L motor w/ a 72 crank would give a displacement of 3.1L that should be able to be rev'd to high hell and back.

Oh boy I think this next week is going to be interesting once I get some parts in my hands.

The main journals on the crank are 64mm.

The rod journals are definitely a different size. Below: 74 on the left, 72 on the right.

[thePNO95]

Originally Posted by Tearstone

The main journals on the crank are 64mm.

The rod journals are definitely a different size. Below: 74 on the left, 72 on the right.

I did some more googling after I made my post last night and found this - http://www.carolinadsm.com/forum/sho...d.php?p=139652 - scroll down to the table in post #10. I don't know who boriqueno96 is, so for all I know he's full of it, but his table shows the 60mm vs. 64mm that Tearstone gave. It also shows a 58mm rod journal for the 6G74 vs. 50mm for the 6G72. Last but not least, it shows a different deck height, too - 228mm vs. 210mm (which this other website confirms - or at least it confirms there's an 18mm difference.)

The 4th column of his table is certainly intriguing - a 4.0L stroker it appears. Perhaps that's been discussed in this forum somewhere already? I'll have to search.

PNO

[Bitter]

Originally Posted by PharmEcis

I have both systems and Alldata is far superior to mitchell in just about every way. This is the first time I've had Alldata give me bad info, all I had to do was pull up a different year and I got the right info.

i've always had better luck with ondemand5 from mitchel, atleast for electrical diagrams.

[Dragon34x]

this link should help you guys. . . .

http://msw.kiausa.com/viewer/default.asp?group=shop

even though its specs for the DOHC v6 6G74, the block specs are still the same

Search: Sedona (GQ). . . .2002-2005. . . . .Engine. . . .3.5L

[Bitter]

does the dohc head flow the same as the sohc head?

[Dragon34x]

i wouldnt know

[PharmEcis]

Hrm Alldata is failing me.

[Bitter]

Originally Posted by Dragon34x

i wouldnt know

lol, go ask and find out. on the 6g72 diamante we got at school the DOHC head looks a good bit wider than the SOHC head, which makes me wonder if the DOHC head has different valve angles and different port angles than the SOHC head?

[Dragon34x]

I'd assume it has longer duration/lift then the SOHC head, as the DOHC 74 in the montero puts out higher numbers then the Diamante. I've been trying to find this info. . . all i've found was what i posted above (da link!)

[ARACHNO]

What exactly is the advantage of converting to DOHC from a SOHC??

[Bitter]

Originally Posted by ARACHNO

What exactly is the advantage of converting to DOHC from a SOHC??

well i think that the dohc heads wont clear the firewall, but the advantage would be that you can dial in intake and exhaust cams seperatly and iirc the cams act directly on the valves, no arms to break.

[Mitsiman]

The 6G72 engine is more than capable of revving. The main limitation has always been the cylinder head and intake manifold. We fitted the 6G72 DOHC Mivec engine into a locla magna with a Haltech system, running a twin runner inlet manifold and man, this thing would rev to 8000 rpm all day I htink we have a 8500 rpm limiter on it.

Point being the cylinder head is the main limiation. Good porting work, good camshaft and a must is some more compression. Then you would be able to have it revving above 7000 rpm quite easily.

The mixing of the 72/74 cranks won't really work they run different bearing designs etc, you could possibly do it.

The other way to go as well is to overbore the 3.0 engine and get some larger pistons in it. This way you can increase the Bore size relative to stroke, and make it more revvable.

[Bitter]

what i'd like to know is, how will all of this affect the torque curve?

[autoxgalant72]

Originally Posted by Mitsiman

The 6G72 engine is more than capable of revving. The main limitation has always been the cylinder head and intake manifold. We fitted the 6G72 DOHC Mivec engine into a locla magna with a Haltech system, running a twin runner inlet manifold and man, this thing would rev to 8000 rpm all day I htink we have a 8500 rpm limiter on it.

Point being the cylinder head is the main limiation. Good porting work, good camshaft and a must is some more compression. Then you would be able to have it revving above 7000 rpm quite easily.

Thats exactly the type of info I was looking for. What kind of parts list would you prescribe to make the engine turn like that.

[01SpyderGT]

Originally Posted by Mitsiman

Point being the cylinder head is the main limiation. Good porting work, good camshaft and a must is some more compression. Then you would be able to have it revving above 7000 rpm quite easily.

What about raising the rev's on an FI engine? You want lower compression in an FI instance, right, so how much can you raise the limit with portwork and cams?

[tracer bullet]

FYI, for the 3.0 Eclipse, the service manual says:

Crankshaft Journal Outside Diameter = 60mm (2.4in)
Crankshaft Pin Outside Diameter = 50mm (2.0in)

For what that's worth.

(I generally think "rods and mains", not sure what "pin" is, or which journals the "journal" refers to. Also, calling it "60mm" sort of scares me, I'd rather see "60.000" or something like that, especially when you see "2.4" and "2.0" inches next to that - which is close, but not exactly right.)

HTH - Probably didn't. Best off with actual cranks and calibrated micrometers.

[6G72Duster]

found this in looking into rod ratios and the like. did a darn good job IMO, cleared things up for me

http://e30m3performance.com/tech_art...ratio/kin2.htm

even has DL-able spreadsheets

[Bitter]

pin just another term for the rod journal on the crank, back in the day some cranks weren't machined as one piece, but were pressed together and the rod journals were actually pins.

[repforenzo]

Originally Posted by Bitter

what i'd like to know is, how will all of this affect the torque curve?

Just getting the engine to rev higher may not change the torque curve much. Camming it out for the higher rpms, you find you even loose some low end, but gain it in the hp up top.

The big jump in torque come from displacement, coupled with camming and compression. And then you add FI...

[ridemlow03]

Originally Posted by repforenzo

Just getting the engine to rev higher may not change the torque curve much. Camming it out for the higher rpms, you find you even loose some low end, but gain it in the hp up top.

The big jump in torque come from displacement, coupled with camming and compression. And then you add FI...

Reving up higher is not always the best thing to do. Ive went against alot of hondas that had swaps and everything but the only thing they lacked was torque. If you can pull a person off the line than you will have a better chance of winning. Think about it, the old muscle cars redlined really low but the reason no import can beat them is because the displacement. If you want a quick car, build it up more for torque than horsepower.

[mozart4898]

IIRC Kris (Clipse3GT) has his redline set at 7K with his SDS and I believe he's got an entirely stock valvetrain. He says (and I've seen it) that it pulls like a sonofabitch all the way up to the redline - in fact I believe the tuners said they thought it would still make power up to at least 7.5K. Of course that's a boosted situation but the valvetrain holds up apparently.

On a side note it's nice to see another serious discussion of the N/A capabilities of this engine...I agree that I think there's potential. Russ, if I had money, I'd be more than willing to work with you on an N/A build to see what we could come up with.

[autoxgalant72]

Originally Posted by ridemlow03

Reving up higher is not always the best thing to do. Ive went against alot of hondas that had swaps and everything but the only thing they lacked was torque. If you can pull a person off the line than you will have a better chance of winning. Think about it, the old muscle cars redlined really low but the reason no import can beat them is because the displacement. If you want a quick car, build it up more for torque than horsepower.

Thats only half right, basically a larger engine will make power lower than a smaller engine, the advantage of that is when revs are limited or your being required to tow. Not all big engines will make power low either. I have a good friend with a '94 ranger drag truck with a 302 and a full race cam. It doesnt make any power below 3000 rpm, and starts coming into real power at about 4200. What he does is launch with a stall at 3500 rpm, which allows him to launch just enough below his power that he doesnt spin, but close enough that it doesnt bog and gets to it quickly. The truck spins to 6800 with a pushrod V8. The old Hemi's can spin to 7k without damage in stock form, and I know other people who are pushing pushrod motors to 8k and beyond. My little 3.0 can turn some of these motors backwards under 2500 rpm, yet I have no hope of keeping up with them in a drag race.

It would be an enormous bennofit to be able to make power from 4k-8k with our motors, and even with a loss of low end, we're driving fwd cars, you cant throw too much torque at the front wheels before they break loose, so that might make the cars easier to launch. Not to mention if existing, milder, cam grinds will carry the motor up to these rpms. Those with automatic/sporty transmissions should really be excited as the broader powerband will make the 1st 2nd shift alot less crappy. I know for autox this will allow me to stay in gear longer, and have to shift less. I can loose a little low end to do this as launching at 3-4k wont impact my times with any real exessive spin.

I believe the key to makeing power with this motor in an N/A format is to raise the redline, as the volumetric efficiency increases with rpm as well as the sheer airflow, which is why the turbo/supercharger solution is so easy, you slap fi on there and it breathes like a larger motor. Daily I work with 2 strokes that make in upwards of 10-14 hp per cubic inch N/A, the key to this output along with the nitromethane fuel they use is thier ability to hold very high rpms, as high as 40-50k rpms.

[repforenzo]

I enjoy seeing these conversations that explore the possibilities. However, I do think a little practicality has to be taken into account. All discussions of n/a builds, usually come to the conclusion of not being as cost effective as going FI on the stock engine (with the addition of forged pistons being the exception). That being said, I'd love to see someone try winding the 6g72 up.

Finally, you can use the flash to change the rev limit and tune up to 7000 rpms. Any higher, you may then have to go to a stand alone unit to tune effectively past 7000 rpms. (You obviously want to make sure the valve train or other engine components are built for that increase in rev limit).

[JoeKamel]

FWIW, is this actually worth it for auto/sporty owners? I mean the tranny is computer controlled, and I don't know if anyone's ever looked to see if the shift it done at what the rev limiter is set at or if the ecu tells it to shift based on something else. If there's a different setting for the shiftpoints that we can't change, auto/sporty owners might as well put the money elsewhere.

[cyberbrain]

clipse3gt takes his 6g72 up to 7250. Granted he is FI and only has forged pistons from my understanding. He is running a 5 speed though.

[autoxgalant72]

If you can adjust shift points with ecu flash, I'll be checking into this shortly, before my auto is totally blown. With ecu flash its possible to raise the rev limiter which should allow you to tune all the way up to that point, not having my information loaded up to play with it yet, but I'll be looking into it when I do.

I understand for most an FI build is more cost effective, but with my racing autocross outright power is not as important as trying to broaden my powerband to keep from shifting as often. Also for those looking to gain power with FI letting the engine spin higher is an option as well. It all depends on where power just drops off.

[Dragon34x]

Originally Posted by cyberbrain

clipse3gt takes his 6g72 up to 7250. Granted he is FI and only has forged pistons from my understanding. He is running a 5 speed though.

I've taken mine up to 7k several times. . . for the hell of it. Seems to hold up just fine

[repforenzo]

Originally Posted by Dragon34x

I've taken mine up to 7k several times. . . for the hell of it. Seems to hold up just fine

Did you change your rev limit?

Mitsu may have built in some safety tolerances that may let you go up to 7000 rpm, but I wouldn't rev up that high "...for the hell of it."

[Dragon34x]

Originally Posted by repforenzo

Did you change your rev limit?

Mitsu may have built in some safety tolerances that may let you go up to 7000 rpm, but I wouldn't rev up that high "...for the hell of it."

Ecuflash. . . set the rev limiter to 7,100 rpms. the guys over at 3si can and have rev'd they're stock engines up in excess of 7k rpms, our springs are nearly identicle as the ones they use (or atleast the 3kgt SL. . .near same spring weight resistance)

I dont mind testing the limits of an engine. . . if it breaks. . . it breaks. Time to fix. So yea. . .

[repforenzo]

Originally Posted by Dragon34x

Ecuflash. . . set the rev limiter to 7,100 rpms. the guys over at 3si can and have rev'd they're stock engines up in excess of 7k rpms, our springs are nearly identicle as the ones they use (or atleast the 3kgt SL. . .near same spring weight resistance)

I dont mind testing the limits of an engine. . . if it breaks. . . it breaks. Time to fix. So yea. . .

Ok, If you are willing to be the 'rev-limit beta tester', ever consider to take it to a dyno and get the hp/torque curve up to 7000 rpms?

[Dragon34x]

hmmmm, good idea .. . but im poor at the moment lol. Unless u guys wanna pay for the dyno haha

[thePNO95]

Originally Posted by ridemlow03

Reving up higher is not always the best thing to do. Ive went against alot of hondas that had swaps and everything but the only thing they lacked was torque. If you can pull a person off the line than you will have a better chance of winning. Think about it, the old muscle cars redlined really low but the reason no import can beat them is because the displacement. If you want a quick car, build it up more for torque than horsepower.

I'm just guessing, but I imagine the Honda swaps were making similar HP to you, so it makes sense that your wider torque band would prevail. But a torque-less S2000 would eat an Eclipse for lunch, so torque isn't the only part of equation (as autoxgalant72 said). Your last statement is an oversimplification and simply isn't correct. Torque, HP, and gearing are all going to determine how quick a car is. If making these cars quick was simply a matter of optimizing torque then we wouldn't be having this discussion about how to raise the rev limit and improve breathing at the top end.

I would say the stock 6G72 is pretty much optimized for torque, as are most OEM powertrains sold to the masses - John Q Public wants low end torque because when he drives to and from work he's not taking the car to the rev limiter away from every stop light. If we could extend that nice low end torque so that the car made more power up high, though, all the better!

Originally Posted by autoxgalant72

Not all big engines will make power low either. I have a good friend with a '94 ranger drag truck with a 302 and a full race cam.

Ahhhh, "full race cam" makes me laugh every time I read it. As opposed to a "half race cam"?

Here's a big engine that doesn't just make power down low: http://www.corner-carvers.com/forums...&postcount=102. That's a 434ci aluminum Ford small block (based on an aluminum 351W block), naturally aspirated, that spins to 7000rpm and power is still climbing. It's a road course car, so it'll do that all day long. The valvetrain would support going even higher, but for longevity's sake he keeps it to 7000rpm.

Originally Posted by autoxgalant72

The truck spins to 6800 with a pushrod V8. The old Hemi's can spin to 7k without damage in stock form, and I know other people who are pushing pushrod motors to 8k and beyond. My little 3.0 can turn some of these motors backwards under 2500 rpm, yet I have no hope of keeping up with them in a drag race.

There are plenty of guys running SBFs and SBCs turning 8krpm and beyond. Pushrods suck, blah blah blah, but the Nextel Cup guys are turning nearly 10krpm for hours on end with their small block pushrod V-8s. (Obviously race engines are a whole different subject, just pointing out a readily available example...)

Basically, there's no reason the 6g72/74 can't be modified to turn higher RPMs and made good power up high.

Originally Posted by autoxgalant72

It would be an enormous benefit to be able to make power from 4k-8k with our motors, and even with a loss of low end, we're driving fwd cars, you cant throw too much torque at the front wheels before they break loose, so that might make the cars easier to launch. Not to mention if existing, milder, cam grinds will carry the motor up to these rpms. Those with automatic/sporty transmissions should really be excited as the broader powerband will make the 1st 2nd shift alot less crappy. I know for autox this will allow me to stay in gear longer, and have to shift less. I can loose a little low end to do this as launching at 3-4k wont impact my times with any real exessive spin.

I think that's a pretty good point about FWD trying to put lots of low end torque to the ground. Sacrificing a little torque down low really wouldn't be that big an issue.

Originally Posted by autoxgalant72

I believe the key to making power with this motor in an N/A format is to raise the redline, as the volumetric efficiency increases with rpm as well as the sheer airflow...

With the stock setup the VE certainly isn't increasing with RPM. The torque curve will follow VE, and the torque curve peters out pretty early. All the basic N/A tuning needs to be done to the 6G72/74 (cams, head porting, freer-flowing exhaust and intake) if you want to take advantage of that higher rev limit. I think you know that, I just wanted to spell it out so others don't get the wrong idea.

More RPMs and the breathing ability to support those RPMs would certainly wake up one of these engines. As an example, Subaru introduced the EZ30R H-6 in 2005. 3.0L 6-cylinder, 10.7:1 CR, 250hp @ 6600rpm and 217ftlb @ 4200rpm. Note that peak torque is pretty comparable to the 6G72 (205ftlb @ 4500rpm for the 9:1 GT, 205ftlb @ 3750rpm for the 10:1 GTS), but the ability to spin to higher RPMs means the Subaru engine makes more power. It uses cam trickery (variable lift and timing) to help that upper RPM breathing - obviously the variable part isn't an option for the 6g72/74, but some RPW N/A cams I'm sure would wake the 6g72/74 up nicely.

Originally Posted by repforenzo

I enjoy seeing these conversations that explore the possibilities. However, I do think a little practicality has to be taken into account. All discussions of n/a builds, usually come to the conclusion of not being as cost effective as going FI on the stock engine (with the addition of forged pistons being the exception).

There's something to be said for taking a different path. I've kept my Subaru N/A because I like to play with it and experiment with stuff that other people haven't. I could easily have made more power with it had I done a turbo swap, but that doesn't interest me. For much the same reason, I'd love to convince my brother to do a N/A build with a 6g72/74. Unfortunately, he and his car are in England right now, so there's not much chance of that happening in the short term.

PNO

[6G72Duster]

i contacted CP pistons the other day and got some information about moving the wrist pin up on a 72 piston. if we went with a 1mm overbore to a 92mm piston, the wrist pin can be moved up .075" (1.905mm). this would only increase the rod ratio from 1.855 to 1.880. not enough of a gain in my opinion to warrant an overbore, new pistons and con rods.

now i was thinking, that if the 6G72 crank mains are 60mm and the 6G74 mains are 64, what about 2mm oversized bearings? tihs would compensate for the 4mm diameter difference on the mains. now apply the same concept to the con rods. the 6G74 con rods have a diameter of 58mm, vs the 6G72's 50mm, i dont know if they make 4mm oversized bearings, but if they do, that would allow the use of the 6G74 con rods (153mm long) on the 6G72 stroke length of 76mm to give a rod ratio of 2.013 instead of the 6G72 ratio of 1.855 or the 6G74 ratio of 1.732 and a displacement of 3.1L as stated previously.

granted i have no idea if this is possible, but conceptually it makes sense to me...

[mozart4898]

Jesus...this is getting technical

I only had one simple question to ask in relation to all the talk of using parts from the 6G74 - I thought I'd heard in the past that if you increase the stroke, you're generally cutting back on your ability to rev to higher RPMs? If I'm understanding what all is being said here (at least somewhat), why would you want to stroke the '72 with '74 parts AND also rev it higher? Isn't that counterproductive (well, maybe not counterproductive, but "not a good idea")? Or are we talking not that much of a difference that would warrant real concern with an upgraded valvetrain? (And here again, I thought the problem had to do more with the internal balancing of the engine - the crankshaft and rods being perfectly balanced in order to turn north of 7K RPMs when the pistons are actually being moved further away from the center of the circle...)

Personally I'm not following why, if you want to up the displacement, you wouldn't just drop the whole 6G74 in and then go to RPW town on the head so you could rev higher with a stage 2 or 3 cam.

[6G72Duster]

we arnt really talking about stroking the 72, but utilizing the 74 block to get a longer ROD into the motor so the rod ratio goes up, which increases revability. the 3.1L comes from the increased bore of the 74, but the stroke is the 72 stroke. the rod length would go from 141mm to 153mm.

[thePNO95]

A 1.855:1 rod/stroke ratio is already pretty high, there's certainly no need to go higher than that to rev to 8000rpm. There are plenty of engines out there, both OEM and "built", that have rod/stroke ratios in the 1.5-1.7:1 range that are perfectly happy revving to the moon.

PNO

[6G72Duster]

hmmm, didnt know that, but ive heard that some of the REAL high revving engines run a rod ratio of up to or past 2.2:1. Wouldnt the higher rod ratio of the hybrid allow someone to rev to 9000 or higher? that would help with power output as HP=(tq*rpm)/5252 IIRC

[thePNO95]

Originally Posted by 6G72Duster

hmmm, didnt know that, but ive heard that some of the REAL high revving engines run a rod ratio of up to or past 2.2:1. Wouldnt the higher rod ratio of the hybrid allow someone to rev to 9000 or higher?

The S2000s 9000rpm motor has a 1.82:1 rod/stroke ratio (153mm rods, 84mm stroke). There may be other high-revving Honda motors that have an even higher R/S ratio, I dunno. I can't see any need to go higher than that in a street engine.

Originally Posted by 6G72Duster

that would help with power output as HP=(tq*rpm)/5252 IIRC

Sure, but you have to draw the line somewhere. If you design the engine to make useful power at 8500-9000rpm it's gonna be an absolute dog until 4 or 5000rpm (just pulling numbers out of my ass.) With the right gearing it might be OK to drive on the street, but if you're working with stock gearing I think it would be pretty brutal. Just one of those compromises...

PNO

[autoxgalant72]

I'd like a cam that is designed to optimize torque and horsepower from 4k-8k, 3k-4k rpm is launchable and if your not making as much torque it would maintain drivability while making for a longer powerband and certainly more output given the proper mods are made to make use of the rpms.

I'd like to know more about the rocker arms, at what frequency will they fail, and how stiff of a spring will they handle.