Here’s the Rambler’s engine compartment and with its 283 cubic inch small block Chevy V8. (Image/Jeff Smith)

I am an old school pepper tree mechanic, and not quite in tune with 12:1 compression ratio small block Chevys. I have a 1962 Rambler Classic with a 283 with 12:1 compression ratio. It originally had a points distributor and an engine-driven fuel pump. To update this engine with better performance, I installed an HEI. This was an improvement, but at cruising speed, if you nailed the throttle, it would begin to speed up then fall on its face and spit and sputter and eventually die.

I did some research and found out that high compression motors required greater spark and timed intervals on the cylinder firing for performance at higher engine speeds. Also I am trying to run pump premium gas without octane boosters. Am I all wet? Or trying to make a sleeper out of a motor that can not fit the requirements?

Any advice or reference materials for me to review would be very helpful and appreciated. Thank you for your time and vast knowledge.


Right away, I think it will be exceedingly difficult to run any engine with a true 12:1 static compression ratio on pump gas. Current pump gas is between 91 and 93 AKI (anti-knock index) and a 12:1 engine is going to need a minimum of 96 to 100 octane to operate without detonation. If you really want to run this engine on the street as a “sleeper,” I would suggest going straight to E85 fuel. It’s far less expensive than race gas and will offer more than sufficient octane to feed this engine.

One way to get an idea of how much static compression you have, run a cranking compression test. A normal 9.5:1 or 10:1 compression engine will have around 170 to 180 psi. With a 12:1 engine, this would have to be up to around 210 psi of cranking pressure. That’s just a guess because we don’t know your cam timing numbers, which can drastically affect your cranking compression numbers.

Converting to E85 Fuel

E85 will require a dedicated E85 style carburetor that has increased fuel volume capacity in order to handle the roughly 25 to 30 percent more fuel required to run E85. This is because ethanol produces roughly 25 to 30 percent less heat per pound of fuel compared to gasoline. So to make the same power, you must combust the additional fuel.

Rather than purchase a dedicated E85 carburetor, which isn’t a bad idea, you will need to convert to a Holley carb just so you can make easier conversions for the fuel. You might be able to get away with just converting the metering blocks to E85. Quick Fuel Carburetors offers an E85 metering block conversion kit that flow more fuel. We’ve tried this on a larger displacement small-block but ran into a situation where the boosters became the limitation to running more fuel as we were up into the high 90 jet sizes and still seeing the engine run lean.

However, with your small 283 cubic inch engine, it’s possible you could get away with converting a typical 4150 style Holley like a 750 cfm carb with these metering blocks. Another idea is to maintain your gasoline carburetor but mix E85 and pump premium to achieve a 50/50 mix. This will increase the octane rating of the fuel up to around 96 to possible 97 octane and that might be enough to keep this engine out of detonation. This will require significantly rejetting the carburetor and even then there might not be sufficient fuel flow with that Edelbrock carb with the single inlet. It would be best to convert to a dual inlet Holley style carb when mixing fuel.

With big domed pistons to achieve this compression, this engine will want more timing—perhaps as much as 40-42 degrees. This will not help the problems with detonation.

Octane Boosters as an Alternative?

Most traditional octane boosters will not help this situation very much. There are a couple of new octane boosters on the market that claim you can run high compression but I have no personal experience, although my friends at Tuned Port Induction Specialties (TPIS) say they have experimented with a product called Race Gas that seems to work for them. You can buy this directly through Summit Racing.

This stuff is about one dollar per once and their mixing instructions say they can raise the octane of pump gas by 4 numbers (say from 91 to 95) with 2 ounces per gallon. So that adds $20 to the price of 10 gallons of gasoline. If the price is, let’s say, $4.00 per gallon for premium—the total would be $60 for 10 gallons of gas.

You can see how that can get expensive pretty quickly.

Conversely, at the time of this article, E85 fuel is down around $2.40 per gallon in the Midwest (I’m not sure where you live) and around $2.75 per gallon the west coast. I think that it would be far cheaper to use the E85 even after factoring in the price of the conversion. All of this will change based on how much you drive the car. The more you drive, the better the deal is for the mixed fuel.

Don’t Forget the Fuel Pump

Changing to E85 will also require a dedicated fuel pump designed to run ethanol. While there may be mechanical pumps out there that are compatible, the easiest way to do this would be with a large electric aftermarket pump from companies like Aeromotive or Holley to name a few.

Look for pumps that are able to be used with E85.

Not all pumps are compatible. With a small engine like the 283, this won’t need a monster output pump.

Diagnosing the Engine Stumble

Without much input it’s difficult to diagnose why your engine falters so badly at higher engine speeds. It could be either ignition or fuel delivery. I’m leaning toward a fuel delivery problem where the engine just runs out of fuel, but without more details it’s difficult to diagnose.

What this really requires is more diagnosis. For example, to try to figure out why the engine falters, I’d suggest running a fuel pressure gauge where you can monitor the pressure at higher engine speeds under load. Don’t run a live fuel line into the interior of the car—that’s not safe.

Instead, if you have a mechanical gauge, just tape it to the windshield and run the car. If the pressure falls below 3 psi, that could be your problem. A minimum of 4 psi is necessary to maintain the proper float level in the carburetor.

Of course, this could also be traced to ignition related issues. First thing I would do is make sure you have the HEI connected to a proper 12-volt source. Older cars like your Rambler used points that demanded a resistor be placed in the circuit that feeds power to the coil. Points generally operate at less than 6 volts instead of near 14 volts of the charging system. If the HEI does not see at least 12-volts, it could suffer a situation like you are describing.

Hopefully we’ve offered some insight into both the fuel you need and a diagnosis on the high-speed issues. Let us know what you discover.

Parts List

Race Gas is a octane booster that our friends tell us seems to work really well. We have not tried it but for a short-use situation like a pure drag car it might be beneficial. (Image/Summit Racing)

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.