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Exhaust porting (technical question)

11K views 125 replies 17 participants last post by  trebortheahole 
#1 ·
To those who have had their SOHC heads ported:

So I've taken on the task of porting and polishing my heads... a bit of a DIY job. The intake side is a no-brainer, but I'm wondering about the exhaust side. What I've done on my one of my junk heads is gasket matched the exhaust port. This opens it up quite a bit, but I'm just wondering if it's too much, even for a turbo appication.






Should I reduce the size of the exhaust ports relative to that one when I go to do my actual heads or do you think I'm perfect with gasket matching them?
 
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#2 ·
You need to know the difference between volume and velocity. Many people make the mistake of hogging out their ports thinking that this will lead to an increase in power. It is extremely FALSE. You are looking for a higher velocity which in turn increases flow. You should concentrate on the following areas:

#1 and most important! VALVE JOB. A proper valve job can pick up over 30% in flow if done correctly.

Bowl area.

Combustion area, unshroud the valves and clean up any casting defects.

For the ports themselves, do not worry about enlarging them. Remove the casting marks and make them smooth. Smooth surfaces = better flow w/ less disruption of the flow of the gases rushing past them.

DO NOT KNIFE EDGE YOUR EXHAUST PORT DIVIDER. If you knife edge it, it will eventually deteriorate and break apart.
 
#3 · (Edited)
I expected a lot better advice from you... no need to talk down to me. I understand flow dynamics a lot better than your average shadetree mechanic. I have my bachelor's, I have a full-time technical job, this is my hobby. These heads already have a 3-angle valve job. My goal is to approach the same exhaust flow characteristics as the 3S DOHC heads. The exhaust ports are tiny on these heads, there must be some potential gain from enlarging the openings. And that's not knife-edged, it's merely smoothed. No different than the photos of 3SX's ported/polished heads I've been using as an example.

Like I said before, this is for a pretty hefty turbo setup (relative to what anybody else has ever done). If you'd take the time to actually look at what I'm trying to accomplish (500+ WHP) before giving me advice, it may help. There's no way you're going to make 500 WHP on the stock exhaust ports. They're TINY compared to the 3S DOHC heads.

Anybody else?
 
#7 ·
I expected a lot better advice from you... no need to talk down to me. I understand flow dynamics a lot better than your average shadetree mechanic.

If you'd take the time to actually look at what I'm trying to accomplish (500+ WHP) before giving me advice, it may help.
First off I wasn't talking down to you. I have no idea what your respective knowledge or skill level is. Do I know you have spent a lot of money on your car? Yes.

500WHP on what kind of dyno? It makes a huge difference because cars that make 500 on our dyno make 600 else where. A car that made 592WHP on someone else's dyno made 480 on ours.

I meant no disrespect by my post. For the most part, 90% of the members of this forum are not technically inclined. My post was not just for you but also for others reading this thread. Your ports are not knife edged. I never said they were. I SIMPLY STATED TO NOT KNIFE EDGE THEM because that is one of the worst mistakes people can do when porting besides hogging out the ports.

Do I know how to obtain better flow out of the 6G72 heads? Why yes I do. Am I going to share all that information, such as the actual angles of the cut for the valve job? Nope sure won't. I'll be releasing built heads for this platform soon so I'm not going to undercut myself like that.

I gave you good general information about how to port these heads. And no, I don't keep up with what hardly ANYONE does on this forum. Why? Because I'm too busy.

whew... this is going to be a hot thread...subscribed
There is no need for this thread to get out of hand. I think there was a simple miscommunication. Not every thread I am involved in has to resort to name calling. :lol:
 
#4 ·
Here is what I'm talking about with the 3S DOHC exhaust ports:

http://www.3sx.com/store/catalog/heads-3sx-port-polish-valves-springs-17-300.jpg

and the exhaust runners are a whopping 5mm wider (stock measures 50mm, these measure 55mm) and 3mm taller (stock 31mm, ported 34mm).
And they're doing that to exhaust ports that all ALREADY significantly larger than our own. Please explain to me why you think it would be a bad idea to port the SOHC exhaust ports to match the gasket.
 
#9 ·
I'm porting my own heads on an engine I've built myself from the ground up... you can assume I know what I'm doing.

And for dyno numbers... Well, you can expect me to put down 450-475 WHP at 4400' uncorrected on a DYNO-mite dyno that reads about the same after corrections as a Dynapak at sea level. It's pretty consistent. I'm not about BS-ing numbers. She put down 374 on a conservative tune when I was stage 3 SDS with custom ignition,blah blah blah.

I'm going to try gasket matching these heads to the exhaust side. I'm going to closely look at the cross sectional area of the 3SX ported heads and try to accomplish my own variation on my own. We'll see how it goes. I do know I will lose low end, but it will definitely make up for it on the top end. Remember, I'm just opening up the port entrance and tapering it down over an inch.
 
#11 ·
Just because you build your own engines doesn't mean you know what you are doing. The first engine I built 10 years ago ... God how I screwed that one up. It actually still runs but I had someone give me a hand and we basically had to redo EVERYTHING as I was putting lube on the back side of bearings etc. :lol:

Today... I don't think there is any doubt I can build an engine. My best one so far is a 2.1L that has made 660AWHP w/ a touch of C16 in the mix and 633AWHP on pump gas.

Warm&SCSI... I don't know you and you haven't proven anything to me so I'm not going to assume anything about your skill level hence my previous post. For the future, I will remember that you are above average compared to the rest of the 3G population.
 
#13 ·
I've actually bought stuff from you personally, so whatever.

But thanks for all of the AWESOME advice. God this forum is useless with all of this fucking bickering. Closest analogue I can think of for Club3G is a middle school playground, but whatever. Went off on my own and came up with the following conclusion. I hope this helps anybody who is actually looking to improve upon the flow characteristics of these heads:
WarmAndSCSI said:
Well that solves that mystery. General rule of thumb is that you need about 2 mm of lip between the exhaust manifold and exhaust port to prevent exhaust gas reversion. Stock manifolds are 42 mm in diameter, stock exhaust ports are 38 mm in diameter. There's your 2 mm of lip right there. Question is should I port the exhaust manifolds to match the slightly larger 46 mm gasket at the same time? That would let me safely increase the exhaust port opening to 42 mm, which is still 4 mm over stock!

What's nice is I have a 42 mm exhaust gasket to scribe the head with as well as a 46 mm gasket to do the exhaust manifolds. I think this is the plan.
 
#18 ·
Your right, this thread went to shit in the 3rd post when you got your feelings hurt about what Pharm posted (which was not bad). Pharm is one of the few and only guys on here I would listen to when it comes to stuff like this about our heads and you told him off as soon as you got the chance.
 
#20 ·
How is sharing some unique information with these SOBs over here crying and whining? DO NOT HAVE 6G72 CRANKSHAFTS TURNED OR WELDED. EVER.

God I swear this forum is the largest group of keyboard warriors and douchebags I've ever seen in my entire life.

I didn't get my feeling hurt by Greg. I got minorly pissed because he wasted his time typing out a bunch of shit I already knew and didn't really answer my question. Get your fucking facts straight. I guess it's kind of hard since not a single person who has posted in this thread minus Greg knows what the fuck we're even talking about. Most of you have about a snowball's chance in hell with actually wrapping your heads around these concepts. Piss off, I'm done here.

And if Greg is the BEST you guys have to offer, then definitely fuck this forum. I know more knowledgeable and experienced people in real life.
 
#23 ·
I tried keeping it on topic. Every post I made had something relevant in it until the douchebag keyboard warriors showed up.

But I already gave my personal best opinion on this. Widen the exhaust manifold ports to 46 mm and then port the exhaust ports to 42 mm. Slightly closer to being able to provide the same flow as the 3S DOHC heads. Go do your own research, make your own conclusions. This thread wasn't about head porting in general, it was asking a specific question. If you want to have a head p&p thread, then start one. As far as I'm concerned, this thread can be locked. I have my answer.
 
#25 ·
This thread makes me :lol: for various reasons.

WarmAndSCSI: You might have purchased something from me. I actually believe it was the old XTECH turbo piping. Honestly, I've sold so many used parts that I have a hard time of keeping track of who bought what and when. Forgive me if I don't recall specifics at any given time. I also don't post much and barely read what is going on with other members. I'm too busy prepping REAL race cars for events so we can win more championships.

Back to the topic of the size of the exhaust ports, from the limited testing the company doing this for me has performed, it has become apparent that the majority of the gains is simply in the valve job. To note, it's quite specific about the particular angles used in the valve job. I sent them 4 cores two weeks ago to play with and I should have final results in the next month or so. Pricing for those interested will be released then.

I really do want to see what kind of gains you get from your DIY port job. Are you going to stick them on a flow bench when they are done? Honestly, it's the only way you know if what you have done will positively or negatively impact your performance.

In regards to cut or welded cranks... I would NEVER use any crank that didn't check out perfectly to spec in any sort of performance build for ANY engine, not just the 6G72. For a stock rebuild in a car I'm going to sell... Maybe.

What cams do you intend on running? Life and duration is going to have a significant effect on the type of porting you are doing in regards to flow and the stock cams aren't going to get you anywhere near your goal.
 
#28 ·
That is a spare head... like I said right in the first post. I've got 3 more where it came from.

Anyway.

Like I said I'm going to try out 42 mm diameter on the port which gets somewhere about the same cross sectional area as the stock 3S DOHC heads. And I really do believe the stock cams can support about 475 WHP after this basic port and polish. We shall see. I honestly don't think anybody has been to the point where they had a 500 WHP capable turbo, a fully built engine, and decided "hey, let's turn the boost up until we hit 500 WHP." Easier said than done, but I'm been screwing around with this platform for a while, and on more than just my own car.
 
#29 ·
Keep it up, I want to here how this turn out. It be cool to use one of your spare heads to have three different ports on it to test flow difenences. I guess you'd be stuck with the largest port job to reuse the head, though.
 
#30 · (Edited)
Figured I'd update this thread with photos of the progress on one head. Starting at 0 all the way to having the combustion chambers polished. All of this done with a kit from Standard Abrasives and a pneumatic die grinder on a pressure regulated compressor.
























 
#33 ·
Nope, sorry, these heads are just for me. I have neither the time nor the experience to give people the impression I could port heads for others. What I did was a very basic port+polish and should provide some decent gains, but I couldn't guarantee anything. I'm not even having these flow-benched because that's very time-consuming and I need my car back before the race season is over here in Utah.
 
#35 ·
Any gains I'm going to see will have to be empirically derived based upon the kind of power I've seen this setup (stock cams) produce under boost. Really I'm not doing this to show any gains, just to make a 450 WHP goal more of a reality. It's going to be a challenge with stock cams due to the restrictive exhaust side (you have to understand that under 1 bar of boost, you're basically forcing the cylinder to expel 2x the volume of air out the exhaust ports - once pressure equalizes between the exhaust runner and the cylinder, the combustion gases are no longer pressurized like they would be on the intake side). Stock exhaust lift and duration really cannot support this. I'm guessing under 20+ PSI, I'll just be grazing the 450 WHP mark.
 
#36 ·
I get what you are saying. I am running the Stage II Ripp Kit, with only the bottom end built and the top end completely stock. I was thinking about doing this, putting a set of RPW Stage II or III SC Cams (before they sale out) VR4 Head Gaskets and ARP Head studs. Then up the boost as much as possible.
 
#48 ·
Like Greg has said before, a 2.5" downpipe is more than enough to support 600 WHP.

Sure, you'll gain a few HP by moving to a 3.0" downpipe, but if you look at the turbine housing on your turbo, it probably tapers down to 2.5" anyway. My GT3582R is a straight 2.5" bore basically. All you're doing is eliminating a little bit of back pressure, but you're not going to open up a window of extra WHP just by upgrading your downpipe unless you're making some serious fucking power.
 
#49 ·
Actually, my turbine housing has a 3" outlet. I realize that upgrading the pipe to 3" is not going to give me a shit load of power but it is going to give me a decent amount. Also, a 3" down pipe won't hurt when running 25-30psi on race gas of course. ;)
 
#52 ·
What is your POINT? I'm seeing nothing that hasn't been discussed already.

And you can put whatever turbine housing you want on a GT turbo. I have a 4-bolt T31 housing that is bored to 2.5" whereas they're usually 2.25" or so on T3/T4 hybrid turbos. At least they were on my two T3/60-1's.
 
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