Running a 340HP on Pump Gas

[Scott V.]

I found a rebuilt 340HP on line but before I spend a lot of money I would like to hear others experience running an 11.5 compression motor on today's pump gas (detonation, timing, additives etc.). I am sure this has been discussed before on the forum but I have searched and cannot find a recent direct answer to this question. Any input/direction appreciated. Thanks

[Joe L.]

Re: Running a 340HP on Pump Gas

I found a rebuilt 340HP on line but before I spend a lot of money I would like to hear others experience running an 11.5 compression motor on today's pump gas (detonation, timing, additives etc.). I am sure this has been discussed before on the forum but I have searched and cannot find a recent direct answer to this question. Any input/direction appreciated. Thanks

Scott-------


How do you know the compression is 11.5:1? Is that the CALCULATED actual compression ratio or is it "what the seller SAYS it is? The original compression of this engine was advertised as 11:1 and, likely, the real compression ratio was a half point lower.

[Jimmy G.]

Re: Running a 340HP on Pump Gas

Either way It may run but you will not be happy Neither will the engine

[Leif A.]

Re: Running a 340HP on Pump Gas

I agree, that unless you can substantially prove that the comp ration is 11.5:1, it most likely isn't. Probably closer to 10.5:1. Properly tuned, your car will run just fine on todays 93 octane ethanol or non-ethanol fuel. My L79 which I had rebuilt with domed pistons and is very close if not 11:1 runs perfectly fine on 93 octane with ethanol. Would probably run fine on 90 octane but I just feel better running a bit more octane. Key again is going to be a proper tune, including a correctly mapped distributor, timing, manifold vacuum for your vacuum cannister, properly set-up carburetor not to mention correct heat range spark plugs, points, condenser and wires. Oh, and a known good ignition coil.

[Scott V.]

Re: Running a 340HP on Pump Gas

Thanks you all for your responses. Joe that's a good question. The seller has indicated he replaced the factory 11 to 1 pistons (not 11.5 to 1) with 30 over factory replacements, so I was just going by the information provided. If the actual compression is closer to 10.5 to 1 that would make things better. I have reached out to him to see what head gasket he used and will update the post when he responds. This will be a factor as well.

[Michael J.]

Re: Running a 340HP on Pump Gas

What altitude are you at? That makes a big difference.

[Edward J.]

Re: Running a 340HP on Pump Gas

Some point in the 63 model year Gm added another steel shim gasket, I am sure that lowered the compression ratio, my March car 327/340 had two head gaskets installed when I did a overhaul. And it will run fine on ethanol fuel, with a occasional long crank on hot start.

[Duke W.]

Re: Running a 340HP on Pump Gas

The advertised CR of the '62-'63 327/340 and 360 HP engines is 11.25:1. However this value is highly dependent on deck height, which increased as the broach tools wore, and could be up to half a point lower than advertised.

But there's more. Due to customer detonation complaints both the 340 and 360 HP engines were "double gasketed" beginning in mid-'62, and this continued through the end of '63 production. The additional .018" thick steel shim gasket knocked the CR down about half a point, so as built these engines were in the range of 10.25 to 10.75 averaging about 10.5.

Over the years most of these engines have been rebuilt, often with "low compression" pistons and usually with thick composition gaskets. As a result most owners really don't have any idea what the CR of their engine is. You can get an idea by measuring head gasket thickness with feeler gages at the corners of the head/block interface. You can also borescope a cylinder to determine if the OE type domed pistons are installed or something else.

The bottom line is that you have to test to determine the engine's octane appetite. Start by using the highest PON available, and if it doesn't detonate, let the tank get close to empty, add 2-3 gallons of the next lowest octane blend, and continue in this matter until you detect detonation, then go with the next higher grade.

It's likely that if you have 93 PON available the engine will operate detonation free on it and maybe a lower octane grade.

If you want to optimize the performance and fuel economy of any Corvette engine read my tuning seminar. Do a Web search including the quotes as follows"

"tuning vintage corvette engines for maximum performance and fuel economy".

With today's gasoline prices it will payback fast.

Duke

[Thomas S.]

[James G.]

Re: Running a 340HP on Pump Gas

.030 over with the same stroke raises your compression ratio by approx half a point if not mistaken.
The reason is you have the same stroke sweep of a larger volume cylinder.
The larger cylinder volume is compressed into basically the same volumetric area of the head.

[Duke W.]

Re: Running a 340HP on Pump Gas

A .030" overbore of a 340/360 HP 327 adds about 0.14 to the actual CR.

http://www.csgnetwork.com/compcalc.html

The additional .018" steel shim gasket reduced as-built CR by about 0.56, a bit more than I previously stated, but I was going from memory and haven't run the numbers for some time.

Duke

[James G.]

Re: Running a 340HP on Pump Gas

Thanks for the correction, I was off by a factor of better than 3.