How much octane for compression ratio ?????

943 Views 16 Replies 5 Participants Last post by aquaforce, Jun 3, 2008

aquaforce

Discussion Starter · Jun 3, 2008 (Edited)

I am at a decission point and don't know which way to go. More compression is good for power but high on octane requirement. What I am wanting to build is something that will run on pump gas preferably 89 and I want all the compression I can get with this limitation. So the question is how much compression can I have.

I have "thunderbolt" ignition so that is a spark limitation. I have aluminum heads so that helps. Stainless marine exhaust. This is a 495 bbc stroker. Fully ported and unshrouded heads.

Can I run 9.6 to 1???? 9.3 to 1????? 9.1 to 1?????

These are options with my piston and gasket combos.

Does anyone have a chart for comp ratio to octane requirement???

If I wanted to push the envelope and went 92 octane then how much comp?????

Help me out please.

1 - 17 of 17 Posts

t500hps

· #2 · Jun 3, 2008

When my 540's were built we came up at about 9.75.....Dyno runs were made with 89 and 93 but I have since found out that was non-ethanol based fuels. Motors ran fine on 89 to just over 4,000 rpms then started to knock....with 93 they ran all the way to 5,600 without issue.

500 EFI's converted to 540's, Iron Eagle ported heads, 9.7 comp, monoblade intake, cam a take-off of the 741 but a little "larger"/longer duration. (630HP, 700TQ fully dressed on the dyno)

noise pollution

· #3 · Jun 3, 2008 (Edited)

My motor is close to t500hps. I have a 540 na with 9.8 compression running 89 octane. Iron eagle heads.

aquaforce

Discussion Starter · #4 · Jun 3, 2008

Your iron heads would detonate before my aluminum heads as I understand it but you have knock sensors and better ignition control where I don't with this thunderbolt system.

I wonder it this trade off of poor ignition but aluminum heads would let me run 9.6:1 on maybe 92 octane?

aquaforce

Discussion Starter · #5 · Jun 3, 2008

noise pollution said:
I have a 540 na with 9.8 compression running 89 octane.

What ignition? Aluminum heads?

noise pollution

· #6 · Jun 3, 2008

aquaforce said:
What ignition? Aluminum heads?

A little slow.

Thunder bolt IV. Ign. Custom carb. 741 cam.

noise pollution

· #7 · Jun 3, 2008

Flat top piston. Pistons .010 in the hole. .027 cometic head gaskets.

aquaforce

Discussion Starter · #8 · Jun 3, 2008

noise pollution said:
A little slow.
Thunder bolt IV. Ign. Custom carb. 741 cam.

I didn't see the previous post........got it.

aquaforce

Discussion Starter · #9 · Jun 3, 2008

Do you think aluminum heads would run cool enough chambers for me to run 89 octane with 9.6:1??

noise pollution

· #10 · Jun 3, 2008

aquaforce said:
I didn't see the previous post........got it.

I was slow on the typing.

noise pollution

· #11 · Jun 3, 2008

aquaforce said:
Do you think aluminum heads would run cool enough chambers for me to run 89 octane with 9.6:1??

I'm no expert. Aluminum heads should let run higher comp. Dissipates the heat better.

aquaforce

Discussion Starter · #12 · Jun 3, 2008

noise pollution said:
I'm no expert. Aluminum heads should let run higher comp. Dissipates the heat better.

That is the way I understand it too. If you can run 9.8:1 with iron heads I should be ok at 9.6:1 then.

I have pop up pistons to make it work. Started out flat but couldn't get enough comp. 8.4:1 The flat pistons would be better for building power but low compression defeats the purpose. :unhappy1:

If I drop in the set I have now it will be setting at 9.605
That should give Bob something to build my cam from :bigsmile:

Tinkerer

· #13 · Jun 3, 2008

I am running 10 to 1 on a 509 with Dart aluminum heads.
I can burn 89 with no spark knock.
The pistons are flush with the block and I am running Felpro head gaskets. Jim V did all his tricks to the heads.
I also am running 741 cams and Lightnings.
I like to run on the edge.
I run EGT guages.

aquaforce

Discussion Starter · #14 · Jun 3, 2008

That gives me a lot more wiggle room. Thanks. :bigsmile:

Tinkerer

· #15 · Jun 3, 2008

My engines are pretty trick - there isn't anything stock other than the ignition and that will change soon.
I have spent a LOT of time and $ building these engines right.

Rattlesnake Jake

· #16 · Jun 3, 2008

509 with AFRs, 315 runners 119 chambers, JE 9.6:1 with a 7cc dome, EFI running 89 octane.

aquaforce

Discussion Starter · #17 · Jun 3, 2008

Rattlesnake Jake said:
509 with AFRs, 315 runners 119 chambers, JE 9.6:1 with a 7cc dome, EFI running 89 octane.

Is that EFI system distributorless?
The knock sensor helps a lot but I don't have that.

I should be ok at 9.6:1. I can always use a little more octane if needed. At lease I have room to work with.

My engine is aluminum heads 121cc chambers 320cc runners 2.250 int 2.19 exh, pistons 18cc dome .020 in the hole, 4.250 stroke, 4.310 bore, all forged components at 9.605:1
Bob Madera is going to set up my cam.

1 - 17 of 17 Posts

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