Hey guys, I'm in the process of building a 3.8 Stage 2 off center motor. I'm using a set of GN1's race ported, stroker, etc. My question is what compression should I be shooting for? I have heard people say 8:1, 8.5:1. and 9:1. This car is going to be a mostly street driven car but I do definately plan to hit the track and blast off high 9's, low 10's. Plan to run a BB70 and may be up to 30lbs of boost at the track. What compression would you run? Thanks ------Jeremy
What compression to run??
- Thread starter jdpolzin
- Start date
if you do this math, you can shoot for what you want:
correct me if i'm wrong somebody
for each 2 psi of boost = 1pt. compression.
if you start with 9:1 compression, add in 30 psi with that 70,compression would be at about 24:1... for sure there would be some factors in that equation, but should be in there somewhere.
I'm pretty sure a 70 bb would'nt come to life till after 22psi or so, so the higher compression you run, the less boost you will be able to crank up, not letting the turbo run at its full potential. Good luck though, sounds fun. I'm sure some more guys like Don Wang will chime in on this subject. Don has a fast car from what I hear
correct me if i'm wrong somebody
for each 2 psi of boost = 1pt. compression.
if you start with 9:1 compression, add in 30 psi with that 70,compression would be at about 24:1... for sure there would be some factors in that equation, but should be in there somewhere.
I'm pretty sure a 70 bb would'nt come to life till after 22psi or so, so the higher compression you run, the less boost you will be able to crank up, not letting the turbo run at its full potential. Good luck though, sounds fun. I'm sure some more guys like Don Wang will chime in on this subject. Don has a fast car from what I hear
HMMM. good point! How are the other folks running 9:1 or 9.5:1 compression and still utilizing larger turbos? So, stick with 8:1? Originally I thought 8.5:1 would be great but after thinking about that comparison of the turbo to compression, I think I would be better off at 8:1. Thanks for your help guys!
Thanks for your help guys!If you're looking for a very slightly crisper throttle responce off-boost, the higher CR will help. And I mean very slightly, if noticeable at all. Once on boost, the CR is controlled by your boost level. If you want a higher dynamic CR, crank up the boost. If you start out with a higher static CR, you will have to carefully limit your max boost level. Not to say you won't have to limit your boost at some level with 8 to one, it will just be less touchy. Unless you're running an aftermarket ECU, stick with 8 to one. It will mean less trips to the chip man for timing and fuel changes.
OK good call! I am planning to go with FAST in the future but I do like the idea of having a safe compression rating! I am going to have to match that turbo and cam with the stall converter as well. Do you guys have any favorites? I would think a 3200 converter would work well with the turbo? What brands do you guys like? I like the Billet 9/11 from what I have heard about it but I know some of the serious guys like the Art Carrs. Any suggestions?
forcefed3.8's
Buckeye Bullet
- Joined
- May 26, 2002
The larger cam will have lower cylinder pressures too, which will lessen detonation with the higher compression. I am running 9:1, but have not tried pump gas, and I have only run 22 psi on 110 so far, so I am unsure about boost limits with octane.
I haven't calculated this out, so others are welcome to throw in an opinion, but I don't think durations under 220@.050" are going to affect cylinder pressures drastically, especially with, practically zero or negative overlap that the popular cams have. The big duration figures (230-250 with mild overlaps) will definitely affect cylinder pressures. If I'm off base, please educate me. We're all here to learn.
This is fairly accurate from what ive experienced as well as what ive heard form other builders. The pressures dont seem to change too much until you start running a lot of overlap which is uncommon with the hydraulic rollers that are typically run. Fwiw the cranking compression on my engine is 180-185psi at 9.0-1 with the relatively small comp 212-212.
Im at 9.0-1 with a T72 p-trim. I couldnt get anymore than 26 psi with the wastegate configuration i had for the dyno numbers or i would have gone to 29 and thats with the factory ecm and stock MAFS. Numbers will increase a lot with the 70 GTQ and a FAST. Most of the TSM guys are running 9.0-1 or more. An increase in compression will increase hp and tq everywhere in the curve on and off of boost. Not just at peak. I will also increase exhaust gas energy. On another note it will increase throttle response and fuel economy with the same cam profile which are desireable in a street driven car.HMMM. good point! How are the other folks running 9:1 or 9.5:1 compression and still utilizing larger turbos? So, stick with 8:1? Originally I thought 8.5:1 would be great but after thinking about that comparison of the turbo to compression, I think I would be better off at 8:1.
All very good points. If you're on top of your tuneup, 9.0 to one is the ticket. If you just want to do 10s, and your tuning skills are not top notch, 8 to one can get you there.
