bensbaby

What's the 91 pump gas max compression before it starts to knock
I have a 383 with alum heads and hsri
I have 10.5 -1 now
I thought I could get it to 11 but I'm not getting racing fuel
I really want 91 gas
It's all we have around here
Let me know
Ty

Orr89RocZ

iTrader: (20)

Lot of variables here. Depends on variety of things from how good the induction is, altitude, use of the vehicle, weight, chamber dynamics of the head, cam size, etc

i would say alum heads and cooler coolant temps and proper fuel distribution, with decent cam you would be ok at 10.5. What really matters is cranking compression. Small cam at 10.5 will crank higher pressure than big cam since the valve is closed longer with small cam. I would say no more than 170-180 psi would be safe, but hard to say. Some go high as200. I would want atleast a 224 deg cam in a 383, maybe closer to 230. But have good timing control, pull timing out at max torque rpm as that is highest cyl pressure

bensbaby

Cranking I have 205 psi now every cylinder and I have a Chevy performance lt1 hot cam by no means huge cam just big for tpi
I have .525 lift
And it knocked 1 time like a bastard when I had reg fuel at 3000 so I let off
Now I run 91 and zero problems
But I was just curious if I could shave the heads down to get 11-1 for more power

bensbaby

I had my chip burnt to a tuner guy to all my stuff
I have 30lb inj to

Orr89RocZ

iTrader: (20)

Kinda surprised it cranks that high with that cam but thats a good sealed engine

you wont get much goin from a true 10.5 to 11.

I would verify tune data with a wideband and a dyno if you wanna go for more power

NoEmissions84TA

iTrader: (1)

With no other changes, the general rule is 4% HP increase for 1 full point in compression.
You are talking about a 1/2 point, or 2%. Is this 2% really worth it?
And you will make LESS HP if you have to retard the timing because of detonation issues.

AlkyIROC

Increasing the compression ratio builds torque.

BTU energy of gasoline is the same in 87 octane as it is in 92 octane.

Octane is the fuels resistance to detonation. If the engine design (see comments above) does not cause knock/detonation with a lower level of octane, it won't make any more power with a higher level of octane. Using a higher level of octane when not required just makes your wallet lighter.

Unless it's excessive, you probably won't hear the spark knock. A knock sensor and a scan tool are the best ways to see how much knock is detected. Under load, if the ECM detects knock, it will retard the timing to reduce or eliminate the knock. This causes a lack of power. People think that going to a higher level of octane means they're making more power but in reality, the ECM just isn't retarding the timing as much so the engine isn't de-rating.

ironwill

I just can't see a sbc running 11:1 on today's ****-poor gasoline without eventually damaging itself; I'm a bit surprised you're not already experiencing knock with your current setup. If you're not satisfied with the power your engine has now, your best path will be to go to a modern platform with a good controller.

As already mentioned, such a small increase in compression---even if the engine could tolerate it---would only produce minimal power increase, and then there's the question of decreased valve-to-piston clearance to address.

Orr89RocZ

iTrader: (20)

More cam would be better for power lol

Fast355

iTrader: (2)

My target compression for the aluminum head 383 going into my Express van is 11:1. Dynamic compression ratio calculates out to 8.2:1. I am using modern engine controls, aka coil per cylinder 24 setup, 0411 PCM and dual knock sensors. My cooling system never runs hotter than about 178*F. Not worried at all about hurting the engine. Will be pulling my 6,000 lbs travel trailer with it even.

The 383 TPI I had in my old G20 van years ago had cast iron vortec heads at 10.5:1. No issue with detonation on 91 octane even towing 7,500 lbs.

NoEmissions84TA

iTrader: (1)

Because you obviously have those combos dialed in.