I have just built a new slant 6 powered race car for my son. The car has a true 11.00-1 compression ratio with a healthy camshaft. His budget is not.......as high as mine. His question is what octane rating does his car require with this much compression ? I have looked all through my books, but could not find the answer. I have it someplace but I can't find it.

HELP



Dave M

Octane requirement at 11:1 is going to be well into "race gas" territory.

How high can go with my compression ratio on 91 octane?
Right now I'm at about 9:1 (I shaved .060 off of the head).
That's the highest pump gas available out here in good old California.

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"Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of Him to whom we must give account."

There's really no hard-and-fast octane vs. compression chart that tells you "You have to use such-and-such octane with thus-and-thus compression". There are a lot of factors that go into it. Static compression ratio is only one. Camshaft characteristics, combustion chamber particulars, spark plug particulars, operating altitude, spark advance curves...

There are street-driven '60s engines (Volvos, not \6s) happily running on street gasoline (high-test, not regular) at 10.5 or higher compression, but they've been designed from the start to do so, and their combustion chambers are a great deal more ping-resistant than ours.

OK
Is there anyone out there running on pump gas with higher than 9:1 compression? If so, what cam are you running? What octane? I think up to 94 is available outside of California.

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"Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of Him to whom we must give account."

The only generally-available 94 octane fuel is Sunoco's alcohol-adulterated pisswater. In most states, 92 is the highest. In a few, 93 is available. At high altitude, usually only 91.

Since this is a race car I can buy racing fuel here in town. Price is $ 4.50 per gallon and it is 110 octane. I was thinking about trying a 50/50 mix. This should be around 101 when mixed with pump grade 92 octane. We have NO exhaust system so I will not be able to hear any pinging. The total timing will be set around 32* to start with and we will adjust from their.

Dave M

Dave,

Speaking from personal experience (and other data), you can run on pump premium with the right timing/induction, assuming you have a long enough duration cam.

Many people think that compression ratio is the key point, but really it is cylinder pressure. What are your cyl pressure readings with a comp gauge?

I have run as much as 11.1:1 on my 68 Dart on 91-93 octane for years with no ill effects. Big cams bleed off cylinder pressure. Getting the timing curve right (26-28 deg total mech max, 15 or so initial) is essential.

Have you mapped the timing curve? I would say you need in the neighborhood of 240 deg @0.050" duration or probably more. I have found that a 250 @0.050" cam will behave quite well with high comp and run 91-93 octane fine.

I should also say that almost all the racers use 30 deg or less total timing for best power. For my motors on pump gas, about 28 gives best power.

Hope this helps,

Lou

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Dave, I have 10.8-1 in my long rod 232. It runs very well on 91, and I have seen very little gain if any at all from anything higher. I have even raced it on 87, but it will slow down a tenth.

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Thanks Guys for the good information

The only way I have to check compression is with fluid and a buret. That's how I came up with the 11.0-1 I also did a cranking compression check and had 190 lb The cam in question is a custom Crane 258* intake with .560 lift the exhaust is 264* with .563 lift. This was a flat tappet camshaft in my son's engine.

My race engine is a 13.8-1 and had a cranking compression of 290 lb. The cam was a roller made by Isky 260* intake with .640 lift and exhaust with 270* and .643 lift. This engine I run on alochol so I don't worry about the gas octane rating. My engine likes the timing around 34* with the alochol. I tried 32* but the car slowed down a little.

Now my son got the bright idea to run alochol too. That way he wouldn't have to buy any fuel at all ! "just use dad's"

Dave,

With 190lb cranking pressure, you are on the edge, but pump premium should work fine. You may gain a little power with a race gas mix and more timing, but you can find a max timing that will work well on street gas.

I had 210lb on one of my 68 Dart combos and it lived on 91-92 octane in SoCal for a couple of years. My 64 Dart has around 180-190 right now and works beautifully.

Lou

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Home of Slant6-powered fun machines since 1988