Real Compression Ratio Daily driver

[nwticerider]

I have been reading this forum and a few others and the 10.5 to 1 compression ratio just keeps getting thrown around all the time.

Who is actually driving around with this daily and where do you get the gas.

A flat top piston in a sbc at 10.5 to 1 detonates continuously. Are people just not calulating it correctly or is the gas that much better where they live?

With aluminum heads and sofisticated ECM's it can be made to live. But with a carb and HEI how is it done?

That is my rant for the day.

[F-BIRD'88]

We have good 94 octane gas here.
I try to warn people not to try to exceed 10.5:1 cr.
Yes on some cars it is touch and go even with good 94 octane gas. I had a 350 w flat tops and milled down 882 heads shim gasket .it was 10.33:1 checked.
It was touchy, especially on our 92 octane. Our 93 and 94 was fine. Dialing in the spark plug heat range made it good. On this one non projected tip slightly cooler Rv8c plugs and no more thsn 36 deg max brought it around.

I like to target 10:1 to 10.5:1 cr for my street strip pump gas stuff w 94 octane. And recomend so.
Any small gain of pushing it higher does not pay off beyond this as risk of detonation greatly increases.

On a dyno with cold air cold water etc you can get away with higher compression (sometimes)(briefly) but the street is a different environment. Conditions are just not ideal...
At 11:1 true you are really pushing your luck.
My stuff has to last ...Not rich....

[F-BIRD'88]

Another: same short block but 062 gm vortec heads .038" gasket
10.07:1 35 -36 deg.. Normal RS12yc plugs this thing would not knock. 91-92-93 or on 94. Never tried 87 octane. Same car.... Less cr very different heads.

Same car and short block again ported 4416 heads milled doen to 51cc, shim gasket
12:68:1 cr checked. Ran on 110 octane unleaded.
Ran ok on 94 octane for light street driving but only if spark timing was retarded back to 27 deg btdc.
RV8c plugs again...
Loved the 110 octane unleaded....

I could go on and on... I stick by my recomendation...
Notice no mention of cam... In the end the cam makes very very little difference. You cannot fix excessive CR by using a excessive cam.

[Orr89rocz]

I dont know why people bother with carbs on a daily driven or serious street car

[cjperformance]

Comp ratio V's what fuel you use can vary a lot with valve timing and how the chamber and quench are set up.
One 10.5:1 engine can be out of control while another can be perfectly happy.

[randy331]

Comp ratio V's what fuel you use can vary a lot with valve timing and how the chamber and quench are set up.
One 10.5:1 engine can be out of control while another can be perfectly happy.

This ^, and I'll add some heads get can be ran at higher comp with everything else the same.

My daily driver work truck is just over 10-1 comp and ran on nothing but 87 fuel. But it has vortec heads and tight quench and a few things to help.
But it has a small cam so the "dynamic comp" is high,
Keep in mind it has nothing but stock cooling system and it does work. Tows trailers etc and drives around in any weather from below zero to over 100*.

Lots of heads can't do that.

Randy

[BradH]

10.8 CR BB Mopar with aluminum heads & .040" quench ran on 93 E10 fuel no problem. Not a daily driver, but got regular street use in addition to track time.

[PackardV8]

Blocking the exhaust heat riser and adding a cold air intake can make all the difference on hot days and hard pulls.

jack vines

[F-BIRD'88]

Blocking the exhaust heat riser and adding a cold air intake can make all the difference on hot days and hard pulls.

jack vines

I agree there. On my 882 (crap) heads the heat riser waS blocked at the manifild but not in the. Exhaust ports.
I hate those twined center egr passages. On my worked 305 ho (crap too) heads i fill with auminum.

Vortecs. ..... Much better chamber (and ports) no egr passages..... Cl!early much better sbc head technology..

[Geoff2]

The induction system is a huge player. My 455 powered GTO has factory iron heads, CR is 9.8-9.9:1. With our 98 octane fuel, i had to be veeeeeery careful with the dist curve/timing to try & avoid detonation. Had three different 4bbl intake manifolds as well as different 4 bbl carbs, all would detonate at some point, light throttle detonation. Worse on a hot day.

With quad Weber induction, Individual runners,I can run 91 octane & it has NEVER detonated, & nothing I do to the timing can make it detonate, even on the hottest of hot days. Have had several cams in the engine, makes no difference, does not detonate. Full weight car, 3.08 axle.

[Truckedup]

I think maximum compression ratio primarily depends on combustion chamber and intake port design and cam timing..

[peejay]

Blocking the exhaust heat riser and adding a cold air intake can make all the difference on hot days and hard pulls.

jack vines

It'll also kill any fuel economy you might have gained by running higher compression.

[JDR Performance]

I've always found it easy to do with a good engine combination, decent gas, a good carb and a properly set up distributor. Other people's results might differ.

[JDR Performance]

At one time there was a chart posted showing different blends of pump gas by state.

[Orr89rocz]

i could go on and on... I stick by my recomendation...
Notice no mention of cam... In the end the cam makes very very little difference. You cannot fix excessive CR by using a excessive cam.

Didnt some cam manufacturers offer cams that were designed to bleed off compression of the 60’s muscle cars during the 70’s and 80’s when octane levels dropped and they removed lead from fuel? Guys were stuck with high comp stuff that couldnt run so cams came out with specs that bleed off cyl pressure.

Thats kinda where the dynamic compression ratio thing came from i think.


I had good succes with flat top -5 cc relief pistons in a zero deck 383 sbc with .041 gaskets and 65 cc chambers from afr 195’s. Thats 11:1 just about and ran fine as a summer driver with 93 oct but it was batch fire efi. Great driving car but loud

My stock 6.2 L9H sierra is factory 10.5 to 1 and tiny cam with variable valve timing. Fine on premium fuel. Towing rated too. Efi and head design i believe helps tremendously