jeepfun_tj

so im looking at a ls1 (5.7) street truck app dangerously close to 8.5 DCR.
Question is can i include elevation in my calculations and go off that to see what i can get by with???
the initial combo looks like this 4,700 lbs DCR 8.46:1 and 8.32:1 at elevation .
if i can just go off 8.32 i can breathe a little easier.
thoughts on minimum grades of gas?
thanks Tim

Bspeck82

Elevation is certainly a factor. Will need premium for sure though.

jeepfun_tj

thanks i updated DCR at altitude

Fast355

My SBC is 9:1 dynamic, moving a 6,200 lbs brick and I tow a 6,000 lbs travel trailer with it. Runs well on 91+ but really loves E85. If you have E85 around it would be the fuel of choice for that combination.

jeepfun_tj

thanks this is helping with my worries .now im starting to wonder why the 8.5 max dcr exists

Fast355

For people that do not know how to tune? Any boosted engine is well over 8.5 DCR. At 11:1 static with aluminum heads I had to pull 2° of timing compared to what I ran at 9.4:1 with iron heads. The aluminum head engine is 9:1 dynamic and the cast iron head was 8:1. 2° less timing is nothing compared to the power output of 1 point more dynamic compression. On E85 it made peak power at 32° @ 2,400 rpm. On 91 octane I was at 26° there. Once over peak torque the timing values narrowed. Higher compression adds efficiency at cruising speeds which is why the OEM manufacturers are doing it now. My 2011 M56s was like ~12:1 static from the factory and ran 22° of WOT timing on 91 octane but still managed ~26+ mpg in a 4,600 lbs car capable of a 12.2 @ 112 in the 1/4 on P245/45R20s. With the VVT and VVEL systems on that car it was as high as 11:1 DCR at times. At cruising speed and light load it ran as much as 60° of timing advance even at points it had 11:1 DCR. Nissan was literally trying to squeeze every bit of power out of the engine at light throttle, hence the MPG.

The newer 355 hp 5.3L runs on 87 and is ~11:1. My mom has a newer Titan that has a slightly detuned version of the ~420 hp 5.6L engine that was in my M56S. It has 11.2:1 static compression with right at 400 hp and it was tuned to tow 9,600 lbs on 87 octane. That truck gets 22-23 mpg highway. The more compression you can get away with the better your mileage and power will be.

jeepfun_tj

Great insight! the examples of Your iron head combo vs aluminum vs a stock set-up was really helpful! more and more people keep going with higher and higher and higher DCR on the street. i kind o wonder why so many tuners used to set a hard limit @ 8.5 DCR with premium gas?
Is it over time ideas are expanded or is it differences with setups or maybe ECM's are faster and reluctors have more teeth etc
THOUGHTS? I only bring it up as im running a 98' ls1 with a 24 tooth wheel
much thanks to everyone

jeepfun_tj

im thinking the varriable rate cam might allow the compression in the last 2 examples

bobcratch

that, and direct injection. pretty hard for the engine to detonate if you inject the fuel after compressing the air

Che70velle

iTrader: (6)

This.