Static compression and dynamic compression.

[Ziegelsteinfaust]

As my engine dies I am building in my head different power plants, and got deep into the weeds last night. When I couldn't sleep at 12am.

I was using a dynamic compression calculator on a 5.7 LT1 imaginary build to gain insight why my 383 is not operating the way I want with 91 octane. My dynamic compression combined with static is to high for a carb, and dizzy set up.

So I played with it, and entered numbers for a cam I would likely use.

My total compression number dropped from 11.1 to 8.9. Numbers maybe wrong. I am starting to get presbyopia here, and reading phone screens gets hard when I am tired.

Would that hypothetical engine thus run on 91 octane gas?

Or is this theoretical information you have to know how to interpret otherwise you end up in a worse situation?

[mattfranklin]

Theoretically 11:1 gives you more power *and* fuel economy.
It's like free money.

The only catch is that your octane or knock resistance must support that.

As you know, late intake valve closure as it helps lower your peak pressure and lowers your dynamic compression ratio.

The personal data I have was taken over 30 years ago as part of a masters thesis focusing on engine knock.
Engine: 350 Chevy
Cam: stock
Compression ratio: 10:1, from cc measurements of combustion chamber and piston top
Fuel: regular, premium, and 100LL aviation fuel
Speed: 2400 and 2800 rpm on the dyno

With premium and 100LL I could run full spark timing sweeps to find MBT (max brake torque) without any trace of engine knock. It did knock with regular around best timing.

Note that this was with a clean combustion chamber, free from any carbon deposits. Typically, new or rebuilt engines continue to build carbon deposits up until ~15k miles. As the deposits build up, the engine's appetite for octane increases by several points. You can use a combustion chamber cleaner like Techron Concentrate to get you back closer to new.

That was in grad school, 30+ years ago.

Fast forward to a few years ago when I built a clone of a '70 LT-1 for my '70 C10...
I was so tempted to go to 11:1 like a true clone would be. I know the mechanical cam was fairly aggressive, and the late intake closure would help lower my dynamic compression ratio. Still, I didn't want to risk being stuck with having to severely retard the timing in case my estimates were off. So I took the safe path and went with 10:1. I know I could have done more, given my cam, but I didn't want to redo things.

If it were me, I'd go 10:1 again. If I were retired and had lots of time to rebuild the engine a couple of time a year, or wanted to fiddle with the complexity of water injection, I'd probably go 11:1.

How's that?

[Ziegelsteinfaust]

Not simple enough. I am a idiot. Who over revved the engine a tad last night doing a break drag burnout for fun.

[vince1]

I like mattfranklin's explanation. I'd be happy with 10:1 considering my 79 engine has about 8.5:1. The truck runs fine though for what I want out of it.

BTW what happened to your engine during the brake drag burnout and I don't think you're an idiot. Have you got a link to that compression calculator?

[Ziegelsteinfaust]

https://www.gofastmath.com/Compressi...tio-Calculator

Is one I have used a few times and seems to work. I do not know enough yet of the technicalities, but yesterday I call Comp Cams. They recommended a cam that should work well when I get my newer LT1 Friday. If nothing is wrong with it.

I hit 6700 rpm before I knew it, and the blow by started getting worse. I have a breather on the valve cover, and you could see smoke rolling out the front of the hood. When I pulled over to get gas. It was coming from the breather, and occasionally it still does it when I drive.

[mattfranklin]

Sorry. I actually like simple things, too.

How's this:
10:1 ok if you dump in Techron Concentrate every 3 months.
And be a little careful with timing (listen for spark knock if your exhaust is quiet enough).

Originally Posted by Ziegelsteinfaust

Not simple enough. I am a idiot. Who over revved the engine a tad last night doing a break drag burnout for fun.

[Ziegelsteinfaust]

Originally Posted by mattfranklin

Sorry. I actually like simple things, too.

How's this:
10:1 ok if you dump in Techron Concentrate every 3 months.
And be a little careful with timing (listen for spark knock if your exhaust is quiet enough).

The engine is 10.7-1 I think. Definitely above 10.5-1. The guy who had the engine built has passed so I can not see the paperwork anymore. Plus it has a smog legal cam with 1.6 rockers. It was in a 1992 Corvette. So when I am cruising at 2000 rpm, and give it some gas. You can hear it ping pretty well. If I am at 3000 rpm there is no issues.

I am going to look at a pull out LT1 Friday morning with 80K miles. So I am looking at a cam that will bleed off enough pressure so it will work with 91.

[mattfranklin]

More food for thought...
https://www.motortrend.com/how-to/hr...p-compression/

I think I'd rather see a dyno test with Richard Holdener or the guys from Engine Masters (Westech), but here's what I've seen so far in a quick search. I'm not sure I'd stand fully behind all of the extreme statements and numbers in the MT article, but theoretically they are in the right direction. Again, rather see a Holdener dyno test.

Also remember that when you go with a wilder cam the idle will be worse (fun fun and nasty) and low end torque will be lower so you'll have to rev it higher to to get into the zone where the cam is designed to shine.