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Old 12-20-2007, 01:07 PM   #1 (permalink)
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Highest comp ratio for regular gas?

What is the highest compression ratio I can run on a 351W and still use regular gas without detonation?
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Old 12-20-2007, 03:29 PM   #2 (permalink)
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Re: Highest comp ratio for regular gas?

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Originally Posted by zigmont View Post
What is the highest compression ratio I can run on a 351W and still use regular gas without detonation?
I'm assuming you mean (regular) unleaded.

That being the case I would say about 8.5:1 depending on timing. I did a bunch of research a few years back and the conclusion was that a good ballpark is to take the octane rating and directly correlate it to compression ratio.

Example. 87 octane = 8.7:1, 89 octane = 8.9.1, 92 octane = 9.2:1, etc.

Not exact science but a good guideline that will keep you safe with alot of advance in your timing. With aluminum heads you can step up about a half a point. I have a 1989 F250 with a 460. I bumped the timing up to 12* initial and have to run 92 octane to avoid detonation. It's only 8.5:1, so it all depends.

I know others will chime in as well, this is just the info that I was able to put together. I am running 10:1 with 92 octane in my 302.

It's actually quite simple to make your own "race" gas though. Just have to buy 55 gal drums of xylene, or toluene and store at your house.
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Old 12-20-2007, 03:59 PM   #3 (permalink)
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Re: Highest comp ratio for regular gas?

Many factors can affect this..What are the specs of the engine?..I'm running 10.7 on my engine but I have aluminium heads...The camshaft will play a big part as well.
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Old 12-20-2007, 04:54 PM   #4 (permalink)
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Re: Highest comp ratio for regular gas?

I have been restoring a 65 Fastback for the last 3 years, and am nearly finished. I hope to have it on the road by spring.
I am running a .060 over 351W. GT 40 cast iron heads with screw in studs and guide plates. 1.94 intake and 1.50 exhaust. Approx 9.2 compression ratio. Comp cams Magnum 270H camshaft with 224/224 duration at .050, and .500/.500 lift with 1.6 roller tip rockers. RPM range 1800-5800.
Dual plane aluminum intake manifold with a 600 CFM Edelbrock performer carburetor.
Modified Hi Po exhaust manifolds through 2 1/4" mandrel bent exhaust system and Flowmaster Super 40 mufflers.
This is coupled to a Z-spec T5 with 2.95 1st gear and an 8 inch 3.23 posi rear.
I was hoping that I wouldn't have to use 92 octane, but I certainly do not want any detonation.
How many degrees of initial advance should I use?

Last edited by zigmont; 12-20-2007 at 05:07 PM.
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Old 12-20-2007, 05:04 PM   #5 (permalink)
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Re: Highest comp ratio for regular gas?

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I'm running 10.7 on my engine but I have aluminium heads...The camshaft will play a big part as well.
frdnut, are you running 87 octane with 10.7:1?

I would think you'd be running 92, 93, or 94 whatever is available in Canada.

I am assuming what he meant was the lowest octane unleaded. (cheapest)
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Old 12-20-2007, 05:10 PM   #6 (permalink)
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Re: Highest comp ratio for regular gas?

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Originally Posted by zigmont View Post
I was hoping that I wouldn't have to use 92 octane, but I certainly do not want any detonation.
How many degrees of initialo advance would you recommend?
My initial is set at 14 + 24 = 38 total.

I would run 92 to break it all in so you are sure there are no issues. That cam doen't have very much overlap, which means it won't bleed of cylinder pressure like a higher overlap cam. Aka less compression you can run.

So I would get it broken in. Tuned up nice and then see if you can back it down to 89 without detonation. You could probably detune the advance to "make" it run on 89, but you would definetly lose performance.
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Old 12-20-2007, 05:10 PM   #7 (permalink)
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Re: Highest comp ratio for regular gas?

I'd start with 12-15 initial timing, as far as 87 octane theres so many factors, I've seen 11.1 comp used with 87 octane but a long rod motor,alum heads,extensive use of eng coatings etc...if you had good alum heads you could do it, besides all the other factors,piston shape,piston to deck ht,comb chamber shape,efi or carb play big factors on how much comp can be run on pump gas.
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Old 12-20-2007, 05:12 PM   #8 (permalink)
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Re: Highest comp ratio for regular gas?

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Originally Posted by cccdjb View Post
frdnut, are you running 87 octane with 10.7:1?

I would think you'd be running 92, 93, or 94 whatever is available in Canada.

I am assuming what he meant was the lowest octane unleaded. (cheapest)
OOPs I missed the "regular" gas part assuming he meant "pump" gas..Yes I'm running Sunoco gold 94 octane in the stang..
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Old 12-20-2007, 07:22 PM   #9 (permalink)
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Re: Highest comp ratio for regular gas?

Like mentioned, heads and cam play a big part.
I run a 408 with alloy heads and decent cam with 11.7 comp and use 98 pump fuel, but have run regular unleaded a couple of times in a pinch and was fine.
Timing is 20 initial and 30 total.
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Old 12-20-2007, 08:56 PM   #10 (permalink)
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Re: Highest comp ratio for regular gas?

The new 4V engine races on 91 unleaded Conoco with 10.5:1, timing locked at 36.
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Old 12-20-2007, 09:08 PM   #11 (permalink)
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Re: Highest comp ratio for regular gas?

My machinist says you should run the higher octane in any motor you own unless it specifies not to. Even your air cools.
Reason being, cleaner, better all around performance, better gas mileage and in the long run you will save money.
Some of the guys in the refinery will tell you the same thing. Premium fuel gets a lot more attention when processed so I've been told.
I'm sorta inclined to believe them.
But I'm also cheap when it comes to buying gasoline. I'll drive out of my way to save one penny per gallon which is costing more in the long run. I'm kinda stupid that way.
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Old 12-20-2007, 09:18 PM   #12 (permalink)
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Re: Highest comp ratio for regular gas?

Higher octane = slower burn.... which does not help with any of the reasons you mentioned.

I run 11:1 on 91 octane... as mentioned above camshaft and chamber efficiency play a large part.
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Old 12-20-2007, 09:38 PM   #13 (permalink)
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Re: Highest comp ratio for regular gas?

I run 91 or 92 Octane with my 11:1 motor too.
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Old 12-20-2007, 10:53 PM   #14 (permalink)
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Re: Highest comp ratio for regular gas?

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Originally Posted by gork1rogues View Post
Higher octane = slower burn.... which does not help with any of the reasons you mentioned.

I run 11:1 on 91 octane... as mentioned above camshaft and chamber efficiency play a large part.
Well, I don't know where you are from or where you buy your gas, but the higher octane gasoline in Pennsylvania is of a much better quality. Most of our oil comes out of Canada, but all of the local lower quality crude goes to producing the lower octane gasoline and would guess that it is the same everywhere.
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Old 12-21-2007, 04:07 AM   #15 (permalink)
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Re: Highest comp ratio for regular gas?

Every motor can be different and I do have a couple of concerns about yours.

How far is the piston down in the hole at TDC? If it is >.010 the motor may be prone to some detonation. I also have some concern about that older generation 270H cam as they were designed when higher octane fuel was still available.

My old combo (stroked 351w) had 9.8 compression (.020-.024 in the hole), and an equally old Comp 282S cam (230/230 at .521/.521) which needed 93 octane, even with aluminum heads. Initial timing had to be held at less than 12 degrees or else it would ping. The distributor could not be setup with an aggressive advance curve.

During the next tear down I zero decked the block (now 10.2:1) and added a more modern camshaft (mine is actually a custom grind) and now I can run on the street with 91 octane with no detonation. That is with fooling around too. I haven't tried anything lower, but it may work. The car gets Sunoco 93 octane when I go racing, just for safeties sake.
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Last edited by dennis111; 12-21-2007 at 04:30 AM.
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