Compression for 91 Octane gasoline ?

[Monza355]

I'm finishing putting together a 355 Chevy for a heavy car (4000 lbs) with tall gears 3.08:1 with 31" tires. TH-350 transmission with stock stall converter.
The old engine is tired and will be replaced with a new rebuild piece. It must run on 91 Octane gasoline or lower.

The engine get's used under moderate to heavy load often.
What i have read is that it's better to have a tight quench 0.035"-0.045" for detonation resistant motoring on pump gas.

My question is about going for higher compression (9.4:1) with 0.041" quench or lower compression (8.9:1) with 0.066" quench or what some say no quench !
The engine will have a mild torque cam.

Comp cams X4250H
Duration Advertised: 250° Intake / 258° Exhaust
Duration @ .050'' Lift: 206° Intake / 214° Exhaust
Valve Lift w/1.5 Rockers: .432'' Intake / .453'' Exhaust
Lobe Separation Angle: 111°

The engine will have Dart 165 cc Iron heads (72cc chamber). flat top hypereutectic piston. Performer Intake manifold with a good Q-jet on top. and I will probably have the heat riser open because of the cold climate during the winter.
For now it will also have exhaust manifolds and dual exhaust 2.5"

Should I be worried about this engine detonating on 91 Octane ?

Should i go with larger camshaft to work with the increased compression ratio ?
My friend has an old Crower cam called Torque Beast and is sold by many company's
Duration @ .050'' Lift: 214° Intake / 224° Exhaust
Valve Lift w/1.5 Rockers: .442'' Intake / .465'' Exhaust
Lobe Separation Angle: 112°

just looking for improvements over the tired stock engine with more torque and horsepower and trouble free motoring on 91 Oct pump gas (or lower octane) under heavy load.

[David Redszus]

It must run on 91 Octane gasoline or lower.

Why must it run on low octane gas?

The engine get's used under moderate to heavy load often.

Those are the worst operating conditions for knock.

My question is about going for higher compression (9.4:1) with 0.041" quench or lower compression (8.9:1) with 0.066" quench or what some say no quench !

Higher compression produces more compression heat which will increase the laminar flame speed and could lead to knock. A tight quench distance will increase squish velocity and mitigate against knock. Low compression and slow squish velocity will produce a lazy engine that will need timing to wake it up.

[pdq67]

The 214/224 cam is a one step up from the great old Performer cam at 204/214.

You probably don't want to hear this, but imho, the old Chevy -929 cam is made for this.

Spec's are -- – 195/202, 112/108, .390"/.410".

Summit's equivalent is their SUM 1101 cam spec's -- 270/194/278/204, 112/108(?), .398"/.421" lift.

pdq67

[user-612937456]

Is a similar spec aluminum head out of the question it would be safer with the 9.4-1. or would you consider a different piston and still keep your quench tight you might want to shave the heads .030 to keep the 8.9-1 compression http://www.summitracing.com/parts/slp-h ... /chevrolet

[Monza355]

I want to use these dart heads and 91 octane is what's available at any station where I live.

[David Redszus]

I want to use these dart heads and 91 octane is what's available at any station where I live.

Then you might consider adding a knock sensor to help set your ignition timing correctly.
No one can predict the detonation properties of pump gas; it will vary considerably from day to day.
At what altitude do you reside? And drive?

[Monza355]

Normal altitude is around 400-600 ft but can go as high as 6000 ft in the highlands !

Would it be a good move to run the tight quench higher compression ratio with the larger camshaft to fight the possible detonation. Even though it will give a little low end power ?

[David Redszus]

Normal altitude is around 400-600 ft but can go as high as 6000 ft in the highlands !

Would it be a good move to run the tight quench higher compression ratio with the larger camshaft to fight the possible detonation. Even though it will give a little low end power ?

A longer duration camshaft will reduce the dynamic compression ratio and compression temperature. You could either add squish velocity or timing to get the power back.

[pdq67]

You said -- "a heavy car (4000 lbs) with tall gears 3.08:1 with 31" tires."

The best way this will work is to gently roll it out of the hole until your engine approaches it's torque peak, then nail it.

Now this is with a M-20 and 3.31's along with big tires, and even then, you WILL eventually get a clutch. Been there, done that with my 406 and great big L-60's!

An automatic's converter might "slip" enough to allow you more leeway, I don't know though, sorry, me being a stick guy.

pdq67