Technical What's the practical limit for compression ratio on pump gas?

No automatics.
No rear gear under 3.73.
Distributor curve set for the vehicle.
Oh yeah, good gas!

 

Have you driven it yet with 93 octane ?

Just because someone put 11-1 pistons in doesn't mean you end up with that. It's usually lower.

If it pings on 93, swap to colder plugs, then start reducing base timing. Can also swap springs in distributor to bring the mechanical advance in slower, and swap to adjustable vacuum advance to dial it in.

 

I run 11.5:1 on an ally headed Windsor, 236/242 @.050 we get 98 octane on the pump down here though

 

I'm guessing you have the RON octane rating on that fuel, in the US they go by the PON scale instead and get lower numbers for the same fuel.

 

Yes RON rating, good point

 

They use the average of Motor and Research in the US. Old published numbers are kind of meaningless here.

 

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I can tell you first hand that if the engine is properly tuned and in good overall condition, that engine will run fine on 91 octane.

 

You are in tall cotton if you're only problem is a cantankerous 302!

 

I built one of those DZ engines and restored the 69 Z28 to put it in. It ran fine on high test pump gas.

 

yeah it should run on premium pump gas, I would still toss octane booster in it. My LT-1 out of a 70 Z/28 in my 33 needs booster in it with its 12 to 1 compression. My 327 with a 30/30 cam but flat top pistons and 1.94 camel humps can run on premium just fine.

 

I seem to remember, a new higher compression engine wouldn't ping with a given fuel, but after it got some miles on it with carbon buildup the rattling would start.

 

I think this is a valid question. Does anyone know?

.bjb

 

We use the R+M/2 method in the US, also known as AKI.

98RON is about 93AKI.

 

Yes, it would, but E85 has a radically different stoichiometric ratio than E0 or E10.

To achieve the correct AF ratio, you need 50% more fuel.

You would need a completely different fuel system, one permanently dedicated to E85

 

The engine I built for my Austin gasser is a 350 SBC bored .030"over, using the old camel hump 2.02" heads with the small combustion chamber. The shop that did the machine work told me I might have issues with even premium gas as it's close to 11:1 c.r., and they were right. I tried adjusting timing, but then it just got poor performance. So I run the crappy 92 octane gas, and buy VP Racing Octanium to add to it. The VP additive is the best octane booster I've found, and in my 16 gallon fuel cell it takes about 4-5 oz. added to around 12 gal. fill up to make it run great. It's not cheap at around $22 a quart, but since it takes so little it's not that much extra for a fill up.

 

This.
A high static ratio along long duration cam with later intake closing will lower the DYNAMIC ratio to a point of being able to run a lower octane fuel. Regardless of head material, or type of induction.

 

If it doesn't have custom internals (long rods) it can be tuned for 93-4 pump premium. I'd be more concerned with getting a little lead in the tank once in a while. Some racers sell off their end of the season race gas. Or, if you have a pal that races kick in for 10 or 15 gallons and have it on hand. You just need some tossed in now n then. We even do that to big heavy classics, give em 2 gals of 110 just for lead and lubricity.

 

I think it will be fine on premium fuel.

I ran this same short block in my sprint cars. It was a good buy from a Chevy dealer at around $1500.00 (mid 70's price). It came with 11:1 pistons, forged crank, pink rods, Clevite 77 bearings and the old Duntov cam. I would sell the cam, open up the side clearance on the rods, install my roller cam, a set of the angle plug heads, Hilborn injection, and drop it in the car. Before the wings became a thing, this was a winning combination.

 

My OT 68 GTO with stock 10.75 compression runs good on pump 93 premium. When I lived in California, it wasn't as happy with 91. Still ran fine, but would rattle a bit on first throttle tip in when cruising until the vacuum advance pulled the timing back.

So I agree, just run it and see what you find.

 

Yep! That long duration cam in your example with lots of overlap would also lower the reading you get on your compression gauge. My aluminum head SBC 383 will show 220 psi with one cam. I changed to a cam with more duration and lift, and the compression reading dropped to 190 with no change to the calculated CR of 11.34:1. Either cam runs fine on 93.

 

What Squirrel said, just run it for a little while to see how it works.
Besides the engine itself, it will make a difference what car it's in... is it a lightweight 1930 Model A, some heavyweight 1960s luxury boat, or something in between.

 

I can add a gallon or so of E85 to a tank of fuel with no noticeable effect on driveability, but a definite reduction of detonation (351 Ford, also one of the four cylinder cars in the fleet). This is basically a free octane booster.

 

i worked near a gas station that sold VP gas in two grades. loved watching the muscle cars come through on friday for their "fix"!

 

Mine has 12:1 compression, but it's computer controled...

 

One Beechcraft facility would sell it to us when I had stock cars. I'd bring 2 5-gallon cans up to the Shell truck, pay the man in the office. Not sure what the story is now. It was $1 a gallon cheaper over CAM2 in 1984. My current 355 is close to 10:1 with aluminum heads, I run 93 octane and add a can of VP additive at a 1/2 a tank level that brings it to 97-98.

 

Around 1971 to maybe 1980, we had a place that was open on a few nights a week, it was a little shack on a corner that sold CAM2. Cars would be lined up waiting. I know of one place with a high-octane pump, there are others but not that close. The last time I bought some, 3 years ago, it was just over $10/gallon.

 

Does Sunoco still make 94 octane unleaded fuel????.....

 

Hello,

When my brother and I were starting our first 1940 Willys Coupe build, we asked several local Long Beach drag racers and well known shops for details. We were ready to purchase a 283 SBC motor and it was listed as 10.5 to one, Jahns pistons. It was a long block and we wanted to know if it was a good starting point.

The main question was if it was going to be a dual purpose street/strip hot rod motor. Yes… so, they all gave their approval, Reath Automotive and Joe Mailliard, etc. At the time, a smaller shop had one for sale and we needed information from the experts.
An inspiration in 1959 stock class racing, but a red blur (far left) inspired us to move on to the next adventure.

We ended up with the 283 SBC long block motor and purchased some Chevy truck heads, Isky Cam and also a 6 Stromberg carb manifold and carbs. Everyone said the 6 two barrels would work on the street and be the best for drag racing. So, now, we spent the next several weeks putting the first puzzle together. The results were pretty good for our first time build for the B/Gas Class. But, we were a full second off of the record setting K.S. Pittman’s red Willys Coupe.

On the street, it ran fine and if we did not have a 2.5 gallon moon tank inside of the cab, we could have gone farther in our street adventures. As it were, we had to stop for gas a few times during our street driving nights.

Jnaki

Within a few months, we decided to go bigger and got a hold of a new 671 supercharger and Isky-Gilmer blower drive components. So, the new build had everything one could want with our budget. But, the one thing we were told was that the pistons were now rated 8 to one for the supercharged motor. Same gas from the Standard/Chevron gas station and it ran quite well on the street, again. The Moon gas tank really limited how far we could go on full tank.

At the drags, we started winning races and turned in some great times, closer to actually being considered a “threat” in the class. We assembled the motor carefully and was a great running finished product. .20 sec off of the national record for C/Gas was our best time up to the point of departure...

Note: After our adventure came to a halt, we moved out of Long Beach and come to find out, our young neighbors just bought a z 28 from the factory. It came standard set up with higher compression for smooth running. But, the technology was 9 years later. It was a powerful car. Ever see a Rhodesian Rdigeback dog sitting in the driver’s seat? It looked awesome as he could have been driving. Ha! But, his normal place was in the small back seat.

 

When I built the last engine (off topic) the machinist that was helping me dial in the compression ratio clued me in on effective CR VS. Static.

Effective is what the engine actually sees with other variables factored in such as cam lift and rod length. Turns out the motor I was building didn’t have as high of an effective CR as static and ran fine on pump gas.

There are calculators online for Effective CR but you need to know all the details of the build.

 

My 289 is .30 over, aluminum heads, comp cam XE274H cam, and close to 11:1 ( 10.8:1 ) and I run premium 93 octane with no issues, car doesn’t ping, knock, etc…still wish I could find some Turbo Blue at the pump like we used to have BITD, I remember paying 3.99 a gallon for it and thinking “wow”..last time we put some in another car I had it was 9.99 a gallon and I was suspect of the fuel because I asssuwmd it had been sitting in that tank for awhile..Wife loves the smell of race gas…