Pressure and Horsepower
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TECH Resident
Joined: Sep 2009
Posts: 859
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From: Frozen Tundra, WI
Pressure and Horsepower Would a 5% increase in cranking pressure translate to a 5% increase in horsepower and torque?? If you have a set-up that generates 180 psi and change the gasket or mill the heads to achieve 189 psi, on a 500 hp set-up, would you get 525 hp??
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Joined: Aug 2012
Posts: 41
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From: South East MI
I would guess that there would definitely be a positive correlation between the two. Whether it's linear or directly related or whatever, I don't know. I would guess that the percentages would translate, like you say, but I don't know if a 5% pressure increase would yield a full 5% torque increase.
Cranking pressure isn't what matters. Effectively all you did was change the compression ratio a half a point or so. Depends on how much is milled, head gasket thickness, ect.
People usually claim anywhere between 2-4% increase in Hp per 1 point of compression change. You're looking at a 1-2% increase at most.
People usually claim anywhere between 2-4% increase in Hp per 1 point of compression change. You're looking at a 1-2% increase at most.
TECH Apprentice
Joined: Dec 2008
Posts: 372
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From: Kinston North Carolina
Theres a point of diminishing returns with compression gains. Going from 8 to 9:1 compression will yield bigger gains than say 13 to 14:1. So no they is no direct relation on static compression. As far as cranking psi goes typically either a static compression increase or intake closing point is the 2 big factors. You would either have to move intake centerline or try a different lobe profile to change it. In the case that its a solid valvetrain then valve lash can be used to alter cranking psi. I have seen engines on our dyno that were under cammed that responded well to tightening the lash or in other cases where it was over cammed it responded well to loosening the lash. Heck in some engines the lash change made unmeasurable differences. So no there isn't a direct relation. To me any sort of measurement of cylinder pressure, static compression, or dynamic compression isn't an exact science. However when you can accurately measure the volumetric efficiency of the motor then it would seem that you could more accurately figure the actual compression in a running engine based off of how full the cyl actually is verses what it is assuming 100% VE. That's just the way it seems to me but I've been wrong before.
i would disagree about measurement of cylinder pressure being an exact science- it can be or i believe it is if you use a pressure transducer and monitor cylinder pressure on a running engine.
http://en.wikipedia.org/wiki/Mean_effective_pressure
http://en.wikipedia.org/wiki/Mean_effective_pressure
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Joined: Dec 2010
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From: Maricopa, AZ
A 5% increase in cranking pressure will increase horsepower output, but by how much depends on so many other variables that it really isn't directly quantifiable.
For instance is the 5% increase from compression ratio change or valve timing events? Both can add power (depending on other supporting factors), but because they achieve the change in different way, the how much and where in the curve the power is added varies.
For instance is the 5% increase from compression ratio change or valve timing events? Both can add power (depending on other supporting factors), but because they achieve the change in different way, the how much and where in the curve the power is added varies.
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