chevynut
04-03-2023, 11:06 PM
I knew that cam timing can affect dynamic compression ratio and just googled this to see where my 502 actually was with the new cam...
"The actual cylinder pressure an engine sees is often referred to as dynamic compression, because (unlike the static built-in compression ratio) it changes dynamically according to camshaft variations. The most important of these variations is the intake closing point, because it extends beyond BDC into the compression stroke. Closing the intake later aids top-end power at the expense of low-end torque. Down low, where the engine is most likely to detonate, the late intake closure bleeds off cylinder pressure, effectively dropping the dynamic compression ratio.
Rod length and crankshaft stroke also play into the dynamic compression equation because they alter the piston's position within the cylinder at the intake-valve closure point. How all this comes together is explained by KB Pistons' John Erb: "Consider a 10:1 Chevy with a 3.48-inch stroke, 6-inch center-to-center rods, and a very hot race cam that closes the intake at 90 degrees after bottom dead center (ABDC). This engine will think it is running with 6.17:1 compression and will be happy with 80-octane fuel. As a general rule, the best available pump gas will work with an 8.0:1 dynamic compression ratio. To get 8.0:1 with the preceding rod, stroke, and cam intake closing event, you would need about a 13.2:1 static ratio."
So this got me curious. First I wanted to find out what my actual static compression was after changing head gaskets. GM specs the 502 at 9.6:1 static compression with a .039" composite head gasket. I used .027" Cometic gaskets to bump the static compression a bit. David Vizard told me it would be about 9.7:1 with that gasket. With a 4.47" bore and 4.00" stroke I get 62.772 CI cylinder displacement. At a 9.6:1 compression that implies that the compressed volume is 6.539 CI. The .012" thinner gasket reduces this compressed volume by 0.188 CI to 6.351 CI. The resulting compression ratio is then 9.88:1 which is higher than Vizard told me it would be. I guess a little more compression ratio won't hurt, I think. :eek:
I plugged in my engine parameters into this calculator:
https://uempistons.com/p-28-effective-compression-ratio-calculator
My static CR is 9.88:1
Stroke is 4"
Rod length is 6.135"
ICA is 36 degrees ABDC (measured)
I get a dynamic CR of 8.679:1.
That's higher than the 8:1 recommended for pump gas. And David Vizard actually wanted me to change pistons or mill the heads to get to 10.25:1 static compression. The Motor Trend article I quoted above goes on to say:
"Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines. An advanced, high-tech engine can go a point or so higher-but even with the latest high-tech electronically managed engines, actually shooting for a 10:1 dynamic ratio is way too high. "
Looks like I'll be running premium fuel. :p
"The actual cylinder pressure an engine sees is often referred to as dynamic compression, because (unlike the static built-in compression ratio) it changes dynamically according to camshaft variations. The most important of these variations is the intake closing point, because it extends beyond BDC into the compression stroke. Closing the intake later aids top-end power at the expense of low-end torque. Down low, where the engine is most likely to detonate, the late intake closure bleeds off cylinder pressure, effectively dropping the dynamic compression ratio.
Rod length and crankshaft stroke also play into the dynamic compression equation because they alter the piston's position within the cylinder at the intake-valve closure point. How all this comes together is explained by KB Pistons' John Erb: "Consider a 10:1 Chevy with a 3.48-inch stroke, 6-inch center-to-center rods, and a very hot race cam that closes the intake at 90 degrees after bottom dead center (ABDC). This engine will think it is running with 6.17:1 compression and will be happy with 80-octane fuel. As a general rule, the best available pump gas will work with an 8.0:1 dynamic compression ratio. To get 8.0:1 with the preceding rod, stroke, and cam intake closing event, you would need about a 13.2:1 static ratio."
So this got me curious. First I wanted to find out what my actual static compression was after changing head gaskets. GM specs the 502 at 9.6:1 static compression with a .039" composite head gasket. I used .027" Cometic gaskets to bump the static compression a bit. David Vizard told me it would be about 9.7:1 with that gasket. With a 4.47" bore and 4.00" stroke I get 62.772 CI cylinder displacement. At a 9.6:1 compression that implies that the compressed volume is 6.539 CI. The .012" thinner gasket reduces this compressed volume by 0.188 CI to 6.351 CI. The resulting compression ratio is then 9.88:1 which is higher than Vizard told me it would be. I guess a little more compression ratio won't hurt, I think. :eek:
I plugged in my engine parameters into this calculator:
https://uempistons.com/p-28-effective-compression-ratio-calculator
My static CR is 9.88:1
Stroke is 4"
Rod length is 6.135"
ICA is 36 degrees ABDC (measured)
I get a dynamic CR of 8.679:1.
That's higher than the 8:1 recommended for pump gas. And David Vizard actually wanted me to change pistons or mill the heads to get to 10.25:1 static compression. The Motor Trend article I quoted above goes on to say:
"Erb's 8.0:1 dynamic guideline is for classic, old-school, all-iron engines. An advanced, high-tech engine can go a point or so higher-but even with the latest high-tech electronically managed engines, actually shooting for a 10:1 dynamic ratio is way too high. "
Looks like I'll be running premium fuel. :p