Inliners International Forums General Discussion Hi Performance Compression ratio for 250

I finally have all my parts gathered (posted in another thread, all good stuff) and am getting ready to drop it at the machine shop. I haven't decided how high to go with compression ratio yet. I would like to run pump gas (92). Wondering how high others have gone and maintained some streetability. Thinking 10.5:1 is pretty safe, but would like to go a little higher. Opinions?

One point of reference:

My latest build is about 9.4 static compression, 3.935 bore, forged shallow dish pistons, Comp 252 cam, about .030 inch squish clearance, 87 octane no ethanol. I've played with the ignition initial lead and mechanical advance enough to know that it is close to spark knock. I wish I could add 2-4 more degrees initial but I can't without knocking.

The change I would like to make is a larger camshaft like a Comp 260 or similar. More duration will bleed off some cylinder pressure at low speed where I get the knock. I could run higher octane fuel but I like running 87. Again, just one reference point. Draw from it what you will.

10.5:1 is fine to use on 92 octane, it will depend on what camshaft you want to use & keep your quench tight.

For example, a camshaft w/an advertized duration of 204 @ .050 on a 114 lobe center & running 10.5:1 is not going to cut it.

You could run it, but your timing will need to be on the conservative side & light throttle.

I cant say for sure ,but in general, I believe the 292 will tend to detonate more than a 250?
Correct me if I am wrong guys.

MBHD

Camshaft 290 duration advertised .577 lift, solid flat tappet. Zero deck. Trying to decide how much to take off head.

I would aim for 10 to 1 for the street. Your cam probably has a 230-240dur @ .050 lift. You do not want to make a ping monster. Especially with those pretty pistons. Total timing around 34-36 degrees.

What is the actual duration @ .050? advertized duration does not mean much.

What is the lobe separation?

I have run 12.0:1 compression w/side draft DCOEs w/a soild camshaft approx duration of 234 in 238 exhaust on a 112-114 lobe center. Cranking compression of 220 -230 PSI.
This was my daily driver on 91 or 92 octane.
Actual deck was positve of .002" -.004" range. .038 gasket thickness.

MBHD

He does not know the real specs, it came in the engine.

I would think Tlowe would tell you what chamber CC's you would need to get 10.5:1 & even more compression if you desire.

You bought his pistons, correct?

MBHD

His pistons, yes, and valves,springs, guides, rockers and lumps. I wish the engine was clear so people could see all the cool parts, like jewelry for cars.