Mitsubishi Eclipse 3G Club banner

1 - 20 of 23 Posts

·
Pee in the sink
Joined
·
6,220 Posts
Discussion Starter · #1 ·
Ive been curious about this. Why is it that most raise the C/R when going SDS stage 2 or 3. All my conventional turbo wisdom tells me that I should lower C/R to compansate for the extra boost, but its different with the SDS, why? CAN you run a stage 2 or 3 SDS with a lower C/R, say 8:8:1 or so?
 

·
RIPPed
Joined
·
547 Posts
All my turbo knowledge from DSMs tell me the same. But then again, I was dealing with a different engine then so I don't know. Can someone shed some light?
 

·
Rooaaaaaaarrrrrrr
Joined
·
987 Posts
I know that it has been discussed before, but the reason being, (if I remember correctly), is because with the supercharger the air is not being heated up near as much. That is why you can run a higher CR piston and get away with it. I have an 03 GTS. If I were to put a supercharger on it w/o lowering the compression it would be like running the Stage 2 pulley when I am only running the Stage 1 because the air coming in is cooler and can be compressed more.
Hope that makes sense, if not Im sure someone else will come along and explain it better.
 

·
Vortech > VTEC
Joined
·
1,450 Posts
I just did a search for compression SDS and came up with very little.

So that said, what would the ideal compression for an SDS system be. (considering that I would like an equal balance between a system that produces power and engine longevity).
 

·
Pee in the sink
Joined
·
6,220 Posts
Discussion Starter · #6 ·
Hmmmm. Well why not lower the C/R on the SDS and up the boost even more. Theoretically if your are running a Stage 3 SDS @ 18lbs @ 10:1, couldnt you run the same set up to 23-26 lbs at 8:8:1? Im not sure how that would affect the gains though. Would it just end up canceling each other out?

For example (theoretically):

400whp @ 18psi @ 10:1 C/R
=
400whp @ ~25psi @ 8:8:1 C/R

:dunno:
 

·
Vortech > VTEC
Joined
·
1,450 Posts
stylez said:
Hmmmm. Well why not lower the C/R on the SDS and up the boost even more. Theoretically if your are running a Stage 3 SDS @ 18lbs @ 10:1, couldnt you run the same set up to 23-26 lbs at 8:8:1? Im not sure how that would affect the gains though. Would it just end up canceling each other out?

For example (theoretically):

400whp @ 18psi @ 10:1 C/R
=
400whp @ ~25psi @ 8:8:1 C/R

:dunno:
That's the kind of thing I want to know. And as I mentioned before, for example, the two 400whp you posted...which would be more likely to shorten the life of the engine?

:dunno:
 

·
New daddy
Joined
·
3,352 Posts
We have a modern 4 valve head with a hemi-sperical dome, it can handle the higher compression ratio with the boost without detonation. Also, the higher compression ratio pistons have quence areas on the edges of the pistion to force the air and gas towards the spark plug, making a more uniform burn of the mixture. The quence areas greatly reduce the chance of detonation. The lower compression or dished pistons do not come close enough to the head to push the air/fuel into the center of the chamber.
 

·
Got Boost? Me either
Joined
·
5,789 Posts
what is the C/R of the pistons on a stock '03 GT??
 

·
New daddy
Joined
·
3,352 Posts
Low compression and high compression pistons with properly designed quench areas. You can see where the piston matches the shape of the flat area of the cylinder head. This pushes the air/fuel into the center of the combustion chamber, which makes for on nice smooth burning flame front.



Typical dished piston, with no quench areas. This creates inconsistant mixing of the air and fuel. The flame front could start to burn, then almost go out, and then find another pocket of air and fuel.

 

·
Vortech > VTEC
Joined
·
1,450 Posts
fasteclipse00 said:
We have a modern 4 valve head with a hemi-sperical dome, it can handle the higher compression ratio with the boost without detonation. Also, the higher compression ratio pistons have quence areas on the edges of the pistion to force the air and gas towards the spark plug, making a more uniform burn of the mixture. The quence areas greatly reduce the chance of detonation. The lower compression or dished pistons do not come close enough to the head to push the air/fuel into the center of the chamber.
Ok, so that being said, what would be the optimal compression ratio to boost the horsepower of an SDS engine, without causing undue stress which would shorten it's lifespan considerably?
 

·
New daddy
Joined
·
3,352 Posts
slavetothemuzic said:
Ok, so that being said, what would be the optimal compression ratio to boost the horsepower of an SDS engine, without causing undue stress which would shorten it's lifespan considerably?
If you had 5 bags of microwave popcorn, what would be the best nuking time to pop the most kernels without burning the popcorn?














Which means, I really don't know. I think most people are running between 9:1 and 10.8:1.
 

·
Kampfbereit
Joined
·
17,852 Posts
slavetothemuzic said:
Ok, so that being said, what would be the optimal compression ratio to boost the horsepower of an SDS engine, without causing undue stress which would shorten it's lifespan considerably?
It comes down to how far you are willing to go on pump gas (93 octane or 91 octane for Californians). Without going into to much detail since I have explained this concept on the board a few times already. Tuning an engine to run under a given setup requires a balance of Octane, A/F Ratio, Timing/Spark, Compression and Mixture Temp.

Quick example, a SDS or Turbo if mixture temp charge, a/f ratio is proper, timing/spark is adaquate.were the same would go further on 8.8:1 on 93 pump gas than per say 11:1 pistons.

This is the definition of Octane:

This is a measure of a fuel's resistance to detonation (See Detonation). The higher the number, the better the fuel. Typical unleaded regular octane ratings range from 86 to 88. Premium grade unleaded fuels start around 89 and go as high as 93 or 94.
There are 4 cyl drag racing cars that are boosting 36 lbs of boost with 12.5:1 pistons but no way in the world with all the greatest tuning that you could run 93 octane pump gas on that.

Personally, I'm going to go with 8.8:1 pistons to be able to go as far as possible on 93 octane without the aid of methynol injection (substitute for race gas + intercooling). Now, even with the aid of methynol injection I will be able to surpass other vehicles in the amount of boost I will be able to run on 93 with higher compression pistons.

BTW: 8.8:1 has been determined by piston manufacturers as the lowest pistons can be designed to without loosing more strength versus the benefits you would get in our engine.
 

·
Vortech > VTEC
Joined
·
1,450 Posts
Tearstone said:
It comes down to how far you are willing to go on pump gas (93 octane or 91 octane for Californians). Without going into to much detail since I have explained this concept on the board a few times already. Tuning an engine to run under a given setup requires a balance of Octane, A/F Ratio, Timing/Spark, Compression and Mixture Temp.

Quick example, a SDS or Turbo if mixture temp charge, a/f ratio is proper, timing/spark is adaquate.were the same would go further on 8.8:1 on 93 pump gas than per say 11:1 pistons.

This is the definition of Octane:



There are 4 cyl drag racing cars that are boosting 36 lbs of boost with 12.5:1 pistons but no way in the world with all the greatest tuning that you could run 93 octane pump gas on that.

Personally, I'm going to go with 8.8:1 pistons to be able to go as far as possible on 93 octane without the aid of methynol injection (substitute for race gas + intercooling). Now, even with the aid of methynol injection I will be able to surpass other vehicles in the amount of boost I will be able to run on 93 with higher compression pistons.

BTW: 8.8:1 has been determined by piston manufacturers as the lowest pistons can be designed to without loosing more strength versus the benefits you would get in our engine.
That really helps. It sounds like 9:1 is a pretty good number for a street engine on pump gas.
 

·
Registered
Joined
·
970 Posts
When I redo mine the next time I am thinking of doing the 10:1 combo. Also I have the custom pulley coming to try the 22 PSI so we will see ?

For most I recommend the 9:1 or 9.2:1 pistons.
 
1 - 20 of 23 Posts
Top