Matching cam and compression ratio

[Sagaris]

How do you determine what compression ratios work well with different cams? Is there a formula out there for this? What happens if the compression ratio and cam are not matched well?
I intend on increasing the compression ratio on my M20B25 to something that will still safely operate with the stock camshaft or something just slightly more aggressive and 91 octane (with 91 octane 17# injector markD chip).

What kind of compression ratio increase might I be looking at?

[nando]

there are calculators out there for this, but basically you're shooting for your dynamic compression ratio to be in a certain range. I can't remember off the top of my head, but here are some general rules of thumb for the schrick cams available for the M20:

272 - 9:1-10:1
284/272 - 10:1
288 - 10:1-11:1
304 - 11:1-12:1

there's some wiggle room (i've used the 284/272 on a stock B25 and it worked fine) but too low of a dynamic CR and your low end will suck, too high and you will run into detonation issues before you reach MBT (minimum best timing - the lowest advance you can use for the most torque output).

[abit]

pretty similar question from me

I'm building m20b30 stroker, I have to shave off material from block to make CR suitable for my NA engine.

on what CR I should aim for? So I can drive it daily with 95octane gas, It won't detonate etc.

camshaft is 285/285 degrees with adjustable cam gear, ported head, etc

I will use flat top pistons (m54b25), as I know, not the best idea, cause it may cause detonation on m20's, but for me there is no other options.

thanks already

[ahlem]

Are you sure you will have clearance between the valves and the tops of your pistons?

[digger]

10:1 tops with 95RON, you;ll need to retune ignition timing etc as it will be prone to knock

yeah might need to add some reliefs in pistons

[ForcedFirebird]

there are calculators out there for this, but basically you're shooting for your dynamic compression ratio to be in a certain range. I can't remember off the top of my head, but here are some general rules of thumb for the schrick cams available for the M20:

272 - 9:1-10:1
284/272 - 10:1
288 - 10:1-11:1
304 - 11:1-12:1

there's some wiggle room (i've used the 284/272 on a stock B25 and it worked fine) but too low of a dynamic CR and your low end will suck, too high and you will run into detonation issues before you reach MBT (minimum best timing - the lowest advance you can use for the most torque output).

I use MBT as "max brake torque" when dyno tuning. Usually a couple degrees less that max TQ is ideal.

[abit]

ok, thanks.

yeah, we already made pockets for valves.

management retune will be one of the first things on to do list after first starts.