Getting confused about compression ratio
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Joined: May 2015
Posts: 405
Getting confused about compression ratio
Neep help understanding compression ratio. I know it cant be answered in one post. Just trying to get an idea of what it is and what would fit with my build
The build im working on: 403 with edelbrock aluminum heads, Arias forged pistons, stock crank, stock rods, roller cam, and roller valvetrain.
Want to get 400hp/450tq on regular pump gas
Thanks
The build im working on: 403 with edelbrock aluminum heads, Arias forged pistons, stock crank, stock rods, roller cam, and roller valvetrain.
Want to get 400hp/450tq on regular pump gas
Thanks
Registered User
Joined: Sep 2011
Posts: 2,148
From: Plano, TX
The math is just basic algebra, but it involves cylinders, so I usually go to one of many websites with a CR calculator.
The question you probably meant to ask is what's the guidance around CR for new builds.
Like everything else with an engine build, you can't change one thing by itself. The CR has to be matched to the rest of the engine or it'll run poorly. The most important drivers for the preferred CR are head material, cam profile, and the octane of the gas you'll use.
Generally, higher CR gives higher performance, but is also more sensitive to detonation.
Iron heads start having detonation problems around 9:1 on pump gas (many variables, folks will jump in to tell me about how their 12:1 iron headed engine ran perfect on 87 pump gas. Fine.) Aluminum heads are generally good about a point higher - around 10:1. Things are totally different for E85 or race gas.
Higher compression ratios need higher octane to prevent detonation.
The cam profile greatly impacts dynamic cylinder pressures which greatly impacts liklihood of detonation. Obviously the ignition timing also greatly impacts detonation.
The manufacturers started going to very low CRs (8:1 and lower!) starting in the early 70's because it's easier to reduce emissions on a low-CR combination, but performance suffers.
The general goal is to get the most CR that will lead to combustion at the perfect moment with the fuel you plan to run.
You already have all the parts, so the only thing you can change is to shave the heads or change head gaskets. Changing head gaskets does impact the CR, but not by a whole lot. Hopefully you got the block zero decked or know how deep the piston is in the hole - some 403 blocks are horribly tall and pistons horribly short, which can knock almost a point off the CR.
For the 403, the common combos are:
flat top piston with big block heads
dished piston with small block heads
Getting that backwards leads to a very bad combination.
Looks like you have the former, so now you just have to verify that the cam grind works well with a ~10:1 build and work on the ignition curve when it's together.
The question you probably meant to ask is what's the guidance around CR for new builds.
Like everything else with an engine build, you can't change one thing by itself. The CR has to be matched to the rest of the engine or it'll run poorly. The most important drivers for the preferred CR are head material, cam profile, and the octane of the gas you'll use.
Generally, higher CR gives higher performance, but is also more sensitive to detonation.
Iron heads start having detonation problems around 9:1 on pump gas (many variables, folks will jump in to tell me about how their 12:1 iron headed engine ran perfect on 87 pump gas. Fine.) Aluminum heads are generally good about a point higher - around 10:1. Things are totally different for E85 or race gas.
Higher compression ratios need higher octane to prevent detonation.
The cam profile greatly impacts dynamic cylinder pressures which greatly impacts liklihood of detonation. Obviously the ignition timing also greatly impacts detonation.
The manufacturers started going to very low CRs (8:1 and lower!) starting in the early 70's because it's easier to reduce emissions on a low-CR combination, but performance suffers.
The general goal is to get the most CR that will lead to combustion at the perfect moment with the fuel you plan to run.
You already have all the parts, so the only thing you can change is to shave the heads or change head gaskets. Changing head gaskets does impact the CR, but not by a whole lot. Hopefully you got the block zero decked or know how deep the piston is in the hole - some 403 blocks are horribly tall and pistons horribly short, which can knock almost a point off the CR.
For the 403, the common combos are:
flat top piston with big block heads
dished piston with small block heads
Getting that backwards leads to a very bad combination.
Looks like you have the former, so now you just have to verify that the cam grind works well with a ~10:1 build and work on the ignition curve when it's together.
Registered User
Joined: Sep 2017
Posts: 5,219
From: St. Paul Minnesota
Cylinder head cc's, head gasket thickness, deck clearance, piston dish all can affect compression ratio.
Registered User
Joined: May 2008
Posts: 339
From: SE MI
CR calculator Here is a compression ratio calculator. Plug you data in and get the compression ratio. You can vary various factors to see how they impact the static CR.
Registered User
Joined: Jan 2010
Posts: 6,105
From: central Indiana
Fine, I’ll be the first to brag about my compression ratio. 😎
My compression calculates at 11.2 to one. Obviously, premium fuel only, aluminum heads, the under-the-bumper air scoops, and a kick *** cooling system. I have never heard detonation at the usual 32-34 degrees of timing, and the car runs the same numbers at the Dragstrip with race gas. That suggests to me the engine isn’t octane sensitive the way the car is set up now. The plugs look good, and the engine has 12 years of hard use and still going strong. You can run more than the old school suggestions of 9:1, you just need to pay close attention to what your doing and all the details.
My compression calculates at 11.2 to one. Obviously, premium fuel only, aluminum heads, the under-the-bumper air scoops, and a kick *** cooling system. I have never heard detonation at the usual 32-34 degrees of timing, and the car runs the same numbers at the Dragstrip with race gas. That suggests to me the engine isn’t octane sensitive the way the car is set up now. The plugs look good, and the engine has 12 years of hard use and still going strong. You can run more than the old school suggestions of 9:1, you just need to pay close attention to what your doing and all the details.
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