PDA

View Full Version : high compression pistons



sr20goofus
06-27-2006, 09:35 PM
This thread is to tlak about 10:1 compression, by means of using the correct pistons and not just milling the crap out of the head and running a thin head gasket. I already do that now to have 9.03:1 compression with my stock pistons, andmy timing is off a tiny bit because of it, i know this can be correct with cam gears, but doesnt that seem silly with a completely stock engine. I am hoping to hear from people running flat top pistons, or almost flat top pistons, which are supposed to raise compression to 11:1 i think, so you compensate with a thicker headgaset to bring it back down 1.8mm i believe, but then the timing will be off a tad again. So for the engin im building to go in my car for race saeson next year when i get my comp license, i need to find a way to get the desired 10:1 compression without compromising the timing of the crank to cams. Custom pistons are always an option also, according to Enthalpys spread sheet he made, i need a dish of about 5cc i think, whish is almost nothing.

this is a race engien, not daily driven, only for track use (road course) cooling issues are covered, reliability is covered. Just this compression issue im looking to get some good new ideas, fresh thoughts on the idea.

tErbobOOst
06-27-2006, 11:10 PM
You probably dont want to hear this but I have to ask...

Why would you up the compression on a turbo motor? You can get MUCH more power out of boost than compression, and if you are running a T28 low end power isnt a problem.

All you are doing is risking detonation and creating excessive heat that will be harder to dissipate on the track.

sr20goofus
06-27-2006, 11:36 PM
right now i run a T28, but this is going to be my race engine, so im going to upgrade to a gt2871R top mount setup all that jazz. My reasoning being that i want to run the turbo at a lower boost range....say 18psi max, and still get the desired horsepower, response and smooth powerband that would be so useful for comming out of a corner, a very late and quick spooling turbo will try to break the rear wheels loose, but a more linear power band will keep the car settled. The lower boost will help the turbo last and keep if from over working. The raised compression will increase the response of the engine so everything happens a little sooner and smoother. Im not going for HUGE horsepower or high boost, this is my way of keeping everything reliable. All the race teams i have talked to and engine builders say that a 10:1 boosted setup is very good for the racing that i do. A very conservitive yet meticulous tune to keep everything in check will make sure everything runs right, say AFR of 11.6-11.9 across the board.

_Def_
06-27-2006, 11:51 PM
Eh - your total power produced is going to be octane limited unless you have 100+ octane, plus the change in CR isn't a huge change in efficiency. I still think the turbine will ultimately hold you back like almost every other 2871R on an SR.

sr20goofus
06-28-2006, 12:32 AM
Eh - your total power produced is going to be octane limited unless you have 100+ octane, plus the change in CR isn't a huge change in efficiency. I still think the turbine will ultimately hold you back like almost every other 2871R on an SR.

i plan to just run C16 or 93 octant with methenol/water injection. The CR being raised is to help smooth out the power band down low, and im not oging for high HP...350 max for now, but its not the max power its teh power band im looking to get a specific way. The 2871R may be switched to a gt3071 top mount (v-band), nothing is final yet.

sr20xs
06-28-2006, 07:07 AM
running a thin head gasket. I already do that now to have 9.03:1 compression with my stock pistons, andmy timing is off a tiny bit because of it
Did you get the 9.03:1 comp with just a thinner head gasket? What thickness is it? Has the increase in comp to 9.03:1 had a significant effect on reponse, and worth putting up with the timing issues? I've been thinking of going this way myself on a road car but not sure if the results would be worth it.

sr20goofus
06-28-2006, 11:51 AM
Did you get the 9.03:1 comp with just a thinner head gasket? What thickness is it? Has the increase in comp to 9.03:1 had a significant effect on reponse, and worth putting up with the timing issues? I've been thinking of going this way myself on a road car but not sure if the results would be worth it.


I got 9.03:1 compression by milling the head 0.040" and using a 0.027" Cometic Headgasket. I used Enthalpy's calulator to figure it out, if anythgin the compresison is a tad higher then i think. The change helped the turbo spool sooner and snoother, top end power increased a tad also, first test drive and break in of the rebuilt motor was awesome compared to teh standard 14psi on a stock setup, old Dyno's and pulls were around 260whp at 14psi, new pulls at 14psi felt much quicker with the raised compression.

tErbobOOst
06-28-2006, 03:06 PM
18 psi is not low boost on a 2871.

It sounds like you know what your doing, but I think your power band will be just as good, and your engine will be much safer if you run 18psi on 9:1 compression with the correct supporting mods. A small A/R 2871 will spool just as good as your current T28, which should be almost non-existant lag.

Basically to me it just isnt worth the risk of running that much compression on a turbo motor for a road racing car. If it was drag ok, but for road racing your engine is subjected to sssooo much heat and a high compression ratio will make that problem worse.

I'll stop telling you how to build your car though :p