High compression = heat

Heat = Detonation or the need for higher octane.

 

imsorry but how come people are running 15 psi on stock motors and with a good tune they are serviving?????

with forged internals it should be a walk in the park! if a stock motor can doit with thin heads and weaker pistons and rods, why the hell cant a forged motor take it???

does the outside air temp really have that much effect??? meaning you have to drop compresion over 2 point?????

Chris.

 

IMO, it is all related to CI's, turbo size and how much power you want to make.

 

100% Pump gas combos that I tune that have been together for some time under boost:
D1SC 17 psi, 8:1 Compression, 383ci, 697/641 HP/Tq, 2.5 years (mine )
D1SC 15 psi, 9:1 Compression, 346ci, 711 HP/600 HP/Tq, 2ish years
D1SC 10 psi, 10:1 Compression, 346ci, 500ish/500ish HP/Tq, 2ish years
TT 12 psi, 9:1 Compression, 346ci, 550ish/550ish HP/Tq, 1.5ish years

There are more, but you get the general feel for what works.

 

Why don't you add engine displacement to the list Kevin...

I'm thinking my new combo will hit 700rwhp on 93 pump, but higher than that will require race gas.

 

All of that is fine, but you have to watch out for detonation or Dieseling. If you get too much compression, you get too much heat in the cylinder and you get instantaneous combustion. That is bad for your motor if it fires in the wrong order. If the set-up can be tuned to avoid detonation, then you will be fine. But many of these set-ups are using lower compression to avoid detonation and allow you to run pump gas.

 

On just 91 octane my limit for boost was 11psi when my motor was stock, but that 11psi was in winter/fall. In summer I was limited to 8psi. This was in las vegas summer 115 temps, even 8 psi would knock ocassionally. I tested pure meth out one day to find the knock limit. Was able to get to 15psi on the t76 on 91 and it didn't knock. Once the motor would heat soak and coolent temps got to 220+ the car was done and would knock at 5psi.

General rule of thumb is don't beat on the car when it's hot out.

Now that i'm 8.7:1 I'm able to run 15psi on 91 even in our 100+ temps. I wish my compression was 8.0:1 though, more boost on 91... or at least more safety margin

 

I think 8 to 1 is too low personally. You would need to run a lot more boost to achieve desired horsepower. More boost is also more heat. Most have to remember not everyone is building a race car. If the Ford guys can tune their cars safely and monitor for issues why cant we? Since people like to give examples lets give some more. Say I am on meth and timing is the same. Which car would make more RWHP on pump gas? 15psi at 9.5:1 or 15psi at 8.5:1? I would think you would need a lot of timing to get 8.5:1 to even come close. Now use the same example, but use C16. Not too mention the turbo would spool faster and you will have more under the curve with a lil more compression. You would also get better gas mileage with more compression. Which car would make the most power at the same boost level? The key will be tuning with the correct supporting hardware.

9.5:1 is not high compression folks.

 

Funny to see someone talk about gas mileage and C16 in the same sentence. You know it costs $9 a gallon right.

 

I thihnk compression shoudl depend on what u plan on doin with the car. If ur lookin to have a all out no limits race car then a little higher on teh CR aint bad cause ull always be guzzlin C16. If ur lookin to have a mostly street car and gget all teh power u can outta it with pump gas then ull have to go a little lower on teh CR. Im goin 8.5 on mine in a 408 cause i wanna get as mch pwer as i can for the street. GL guys.

John

 

Come on PSJ. You just blew over everything else I said and went straight to the C16. I never said I wanted to drive all the way from Florida to California on C16. Actually C16 should only be used for racing IMO and that is what the majority use it for. Higher compression with a good tune will give you better gas mileage than lower compression on pump gas with a good tune.

 

My old Civic was 10.5:1 compression and I had no problems running 16-17 pounds of boost on the street. Why? Because it's a 3.2" bore motor that makes absolutely **** all for low end torque and has an ultra-efficient cylinder head. CYLINDER PRESSURE is ultimately going to limit what you can do for a given octane level, not boost pressure. A civic or S2000 at 17 psi is making a lot less torque and a lot less torque/cylinder than an LS1, so they can get away with more compression without detonation.

From all my datalogging on my Honda, DSM, and Turbo mustang, compression makes absolutely no difference in how the turbo spools, in terms of motor speed. In terms of actual time you might get a tiny advantage because of more horsepower to accelerate the car through the rev band to hit said RPM level to make boost faster, but the difference in my mustang between 11:1 when it still had the nitrous pistons in it and 8.3:1 with the low comp turbo pistons in it was not noticeable on the street. The 8.3:1 motor made as much power on ~8 psi as the 11:1 motor did on 6, which really isn't a huge difference if you ask me. The big difference? The 11:1 motor was knocking on pump gas past 8, whereas the 8.3:1 setup would take 17+ on pump gas no knock.

My new turbo motor is 414 inches (4.125 x 3.875) and 7.8:1 static compression. A healthy solid roller cam. 527 ft-lbs of torque at 3500 RPM's on an engine dyno. Now, you decide if a 408 really needs any more compression to make it lively on the street at low RPM's It makes 8 pounds of boost on a pair of 76mm turbos by 4000 RPM's on the street, 17 psi rolls in around 4500, and it will peg the 30 psi boost gauge before 5000. Those are about as big of a turbo as you can put on a motor for the street, and the low end is perfectly fine with my "dogged" compression. Only difference between my car and a 10:1 car? I fill mine up at Shell. My car will run ALL DAY in 100 degree heat boosting the ever loving **** out of it, I can dump another tank of 93 octane in it, and it won't skip a beat.

I'm all in favour of low compression, especially running a 400+ inch motor.

 

hummmmmm maybe low comp isn't so bad?? deffinatly got methinking, thanks guys.

so i guess high comp and high boost can be done but is harder to do than droping the comp and running a little more boost! so whatcomp would you guys recomend on a pump gas only strret 346 running 15psi??

thanks Chris.

 

I think the general consensus is on smaller CI motors rule of thumb is more compression.

and higher cubed motors can get away with lower compression. on a 346 at 15 psi id prob go 8.5-9.0 maybe a lil more but unless the tune was dead on id suggest using methonal. Plus I hope your talking about a forged motor as well.

 

8 rib D1SC Powered forged LS2-402 at 9.5:1 compression with 15psi @ 7000 RPM ,AFR 225's on 16* timing and 93 octane. HP Tuners 850rwhp.

 

I'm planning on 8.7:1 and 15psi, may be adding methanol since gas here is 91 crap, and it gets hot. I'll put around on the street at 10-12#, track 15+. Better safe than sorry, the 1:1 compression difference @ only 3% is not worth grenading the motor, or having to back off so much timing it isn't worthwhile.