BadAndy

I'm still pretty new to the boost game, and I need a new motor, as many who are new to the boost game often do. Since I can change my CR and lower it at this time, I'm considering doing that. People usually just decrease CR and pulley down? How does this affect the way the car drives? Forged 347 with a vortech. I was running 7lbs on a stock motor. If I was to drop the CR down to something like 9.0:1 and pulley down, what would the car feel like? If it opens up a hole in the low end power I don't think I would want to do that.

novaflash2002

iTrader: (51)

the more oxygen that you can introduce with fuel, the more power you will make. But it is a fine edged sword. the most compression that you can get with the most oxygen, fuel and timming will make the most power. its a game of getting the most with the most timming, and no detonation that is the hardest.

ive seen turbo cars with 8.0:1 clear up to 10.5:1 (and im sure there are pleanty with more) with 15psi each. if you take precautions such as meth, or cryogen you can run the high compression. if not... you have to compromise with less timming than the other with lower compression.

BadAndy

Right, I understand that the balance must be maintained, but I'm interested in how changing the cr this way affects the driving/powerband characteristics.

dschmittie1

iTrader: (64)

Is it a forged motor? I would never go down to 9.0:1 CR on a forged motor. all of my motors have been 9.6:1 or higher. also my next question would be what fuel and are you gona spray Meth?

Although a buddy of mine has 10:1 CR forged motor that gets about 13psi on 93 and no Meth with no problems

87silverbullet

iTrader: (11)

Yeah maybe up there by you with low humidity and decent temps, unlike down here with high humidity and temps staying 95*+ during the summer months. he might be ok with that psi with a little meth down here.

BadAndy

Forged 347, 93 octane, no meth.

This is not a "how much boost can I run" thread.

stevieturbo

For only 7psi, there probably isnt much point lowering the CR unless you have no intercooler or something and using crap fuel.

But I'd be surprised if you could actually feel much difference going from say 10.0:1 to 9.0:1

Yes it is a reasonable drop, but on a 6 litre V8...it would take more than that to feel sloppy.

Mine's around 9:1, and it means with decent intercooling I can easily run 20psi safely on pump fuel only, or I can add meth for extra performance.
There may be a very small performance hit...but I'd sooner take one that's safe and goes fast, than one that is right on the edge and doesnt really go any faster.

BadAndy

What's the real motivation behind the process? If I had an engine making 600hp at 10:1 with 7lbs, I could drop the cr and add boost to make the same power, but what's the point? Is it just a more efficient way to make power?

stevieturbo

No. I clearly said if you were only using 7psi there would be no point. And no you wouldnt make the same power.

With a lower CR you may make slightly less power, but really it s only slightly. But you will add a HUGE margin of safety with your tuning. So again it depends on many factors. Altitude, temperature, fuel used, your engine build ( stock pistons are always on borrowed time with boost )

With a higher compression, fuel dependant, tuning dependant, effective chargecooling dependant, there is potential for more power for any given boost level. But the harder you push all of those things then it becomes a very fine line between engine survival, and engine destruction.

With lower compression it will be more forgiving, you can use a lower grade fuel etc etc and still make very good power and with a wider/safer tuning window before the engine might die.
Perhaps ultimately not as high as a HC build....or the simple fact it allows you to run more boost more easily and most important more reilably....it could just as easily make more power IF you decide to go down that road.