How much does bore size effect compression ratio? Like .030 over, how much would that lower it. Or head gasket thickness like going from .040 to .060. I'm building a 383 and trying to figure where I need to be.
Hot rod magazine shows i'm at 10.8:1 with stock bore and .040 head gasket. I'm trying to figure where I need to be for pump gas.
The chart shows a 4.0 bore with a 3.75 stroke and 64cc heads and a .040 head gasket. I'm 4.030 bore. Will that get me under 10:1 or do I need .060 head gasket too. I'm assuming that I need to be under 10:1. I'm putting a 260,260 /.440,.440 cam in it.
[This message has been edited by hercimer01 (edited 04-01-2020).]
Generally speaking the larger bore with the same head and gasket will increase compression slightly, not reduce it.
What compression you can run with pump gas really depends on camshaft (dynamic compression), heads (aluminum or cast iron), and fuel injection or carb. The other thing to keep in mind is that the gas stations are really upping the price on 91-93 octane. I remember when it was $0.10 from 87 to 89 and another $0.10 from 89 to 91. E85 is nearly 1.00 per gallon cheaper than 91-93 anymore, so you might want to thing about running E85 and then you could run 12.0 or higher compression.
Generally speaking the larger bore with the same head and gasket will increase compression slightly, not reduce it.
What compression you can run with pump gas really depends on camshaft (dynamic compression), heads (aluminum or cast iron), and fuel injection or carb. The other thing to keep in mind is that the gas stations are really upping the price on 91-93 octane. I remember when it was $0.10 from 87 to 89 and another $0.10 from 89 to 91. E85 is nearly 1.00 per gallon cheaper than 91-93 anymore, so you might want to thing about running E85 and then you could run 12.0 or higher compression.
This is an interesting idea. I wonder what would need to be done physically to allow for an E85 tune. I have a pretty modified 2.8, now 3.2 with max over bore, bigger fuel injectors, a comp cam.. And a chip to manage the new fuel requirements. I wonder what I would need to do to get an e85 tune. I saw it did wonders for the FRS/BRZ gaining something like 30hp to it's anemic 200hp engine and eliminating it's notorious early torque valley.
IMO, head gasket thickness should be sized for piston-to-head clearance, not necessarily for compression ratio. I say this, because there is a "goldilocks zone" for piston-to-head clearance. If the clearance gets too tight, you run the risk of engine damage. If it's too loose, the engine will run like crap. The ideal piston-to-head clearance for a SBC with iron/steel rods is 0.040".
Also IMO, compression ratio should be adjusted via combustion chamber volume and piston dish.
My suggestion would be to use the following procedure:
1) Find out how much the cylinders need to be bored to clean them up. 2) Find out what head gasket thickness will get you to 0.040" piston-to-head clearance 3) Find out the combustion chamber size in your heads 4) Input all the above data into a compression ratio calculator. Play with piston dish size until you get the desired compression ratio. Order new pistons accordingly. If you're boring out the block, you'll be buying new pistons anyway. So no big loss.
Best of luck!
Edit to add: That's a pretty tame camshaft. I'm not sure what compression ratio the mfgr recommends for it, but I'm guessing around 9.0-9.5?
[This message has been edited by Blacktree (edited 04-02-2020).]
As Fieroguru said, increasing bore size in almost all engines, will cause the compression ratio to go up, with all other things being equal. that being said, the difference between a 4.000 bore and a 4.030 or even .060 bore is pretty negligible, I ran some numbers,
4.000 bore 4.1" head gasket diameter .040 gasket thickness, 64cc heads 10cc dishes in the pistons 0 decked
result, 10.34:1
changing the bore to 4.060 resulted in 10.62:1, if you were already on the high end of what you wanted for the engine, I would say it becomes more important, but at any kind of normal level, not really.
also, don't bore the engine anymore than required to get the cylinder walls in spec, anything more is a waste.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
I want to chime in here- Have you considered your Quench measurement? Quench is the gap between your piston top and the bottom of the head...If it is under .050 you have good quench....But even less can be good depending on top rpm.
I have a Ford 460 in my CVX-20 Jet boat.....I had the block decked so that the piston tops out at the top of the block- My Quench measurement is .040" (Head gasket thickness)....I am running 9:1 compression and after 13 years pulled the plugs to see how they were doing- Had two buddies check them...Put them back in. The engine turns a max of 5000 rpm.
If your over-bore lowers the compression enough, you can have the top of the block cut to get this situation- You also need pistons with a flat surface...I have diagrams but at the moment can't post pics....(Help Cliff!!!!) (I miss spoke; Over bore will raise the comp ratio) (Me and my big (Dumb) mouth)
Quench; Flat top of piston comes close to flat bottom of head, creating turbulence at TDC to mix fuel/air mixture PLUS flame front at ignition, improving combustion efficiency and allowing higher compression ratios and leaner mixtures>>>(Why the LS/LT V8 engines get better MPG and have better power than Toyota and Nissan four valve V8s)
[This message has been edited by cvxjet (edited 04-02-2020).]
I want to chime in here- Have you considered your Quench measurement? Quench is the gap between your piston top and the bottom of the head...If it is under .050 you have good quench....But even less can be good depending on top rpm.
I have a Ford 460 in my CVX-20 Jet boat.....I had the block decked so that the piston tops out at the top of the block- My Quench measurement is .040" (Head gasket thickness)....I am running 9:1 compression and after 13 years pulled the plugs to see how they were doing- Had two buddies check them...Put them back in. The engine turns a max of 5000 rpm.
If your over-bore lowers the compression enough, you can have the top of the block cut to get this situation- You also need pistons with a flat surface...I have diagrams but at the moment can't post pics....(Help Cliff!!!!)
Quench; Flat top of piston comes close to flat bottom of head, creating turbulence at TDC to mix fuel/air mixture PLUS flame front at ignition, improving combustion efficiency and allowing higher compression ratios and leaner mixtures>>>(Why the LS/LT V8 engines get better MPG and have better power than Toyota and Nissan four valve V8s)
OVER BORE WILL NOT LOWER COMPRESSION,
the ONLY application I can think of that it has even the most remote possibility of boring reducing compression is the Mark I Chevy big block, where the head was canted relative to the bores and combustion took place IN THE BORE because the head doesn't have a combustion chamber.
4 cam engines also have quench, attributing one parameter of the engines design to fuel economy of drastically different vehicles is hardly scientific.
------------------ "I am not what you so glibly call to be a civilized man. I have broken with society for reasons which I alone am able to appreciate. I am therefore not subject to it's stupid laws, and I ask you to never allude to them in my presence again."
Rats! I believe you are right....I did all my engine stuff back 13 years ago.....(That is a bad excuse...)
As far as 4 Valve vs 2 valve, most four valve engines cannot have much quench- no room in the combustion chamber because of the FOUR valves- They can have swirl but it is not as effective as the 2 valve TDC Quench.
Bottom line; Latest Camaro SS vs Latest Mustang GT- they are within 50 lbs of each other, and both had manual tranys. The Camaro has both quicker acceleration and better MPG. Ford had to bring out a turbo 6 in their F150 to compete with the GM LT V8s- The Coyote sucks gas compared to the LTs....
I run into this all the time..."2 valves are better unless you are artificially limited in displacement"......"No way! Four valves are the FUTURE!!! They are better in every way....Why, they are high tech- the Formula one cars all use them so they MUST be the best!!!!"
Ford can't GIVE their Coyote engine away to boat manufacturers- without multiple gears, the four valve deficiency shows up even worse....
Thanks everyone so far for the input. Man my head is just swimming in all these numbers. The image poster is down for now but I'll try to run the numbers. They came from this. https://butlerperformance.c...sion-calculator.html
If I go to 19cc pistons with .025 head gasket, that's 9.57:1
The 260,260/.440,.440 cam 110 LSA (CL12-206-16) is meant for towing. That's where I am with the 10.37:1 CR for now. Also I plan to go with 1.6 rockers.
If I do the 19cc pistons to get to 9.57:1 could I run 87 octane or mid grade?
[This message has been edited by hercimer01 (edited 04-03-2020).]
Originally posted by hercimer01: If I do the 19cc pistons to get to 9.57:1 could I run 87 octane or mid grade?
I think mid-grade would be OK; 87 octane... maybe? You could play it safe with the spark timing. Or if the engine has a knock sensor, you should be OK.
