435 HP running on pump gas

[Rick M.]

Hello everyone,
I need some advice on my 69 L71. Engine runs fine and is build to specs. However we have very low octane here in California. Was thinking about ways to use pump gas (92 octane) and wondering if anyone had any advice. I'm thinking about thicker head gaskets and wondering if anyone else has done this and what thickness they used. The problem is I don't know what true compression is because block/heads could have been milled/decked at some point. Of cours the other route would be retard timing. Any advice on pros/cons of each or should I just leave well enough alone and go with race fuel ($8 gallon in 5 gallon cans) or an octane boost.
Thanks,
Rick

[Joe L.]

Re: 435 HP running on pump gas

Hello everyone,
I need some advice on my 69 L71. Engine runs fine and is build to specs. However we have very low octane here in California. Was thinking about ways to use pump gas (92 octane) and wondering if anyone had any advice. I'm thinking about thicker head gaskets and wondering if anyone else has done this and what thickness they used. The problem is I don't know what true compression is because block/heads could have been milled/decked at some point. Of cours the other route would be retard timing. Any advice on pros/cons of each or should I just leave well enough alone and go with race fuel ($8 gallon in 5 gallon cans) or an octane boost.
Thanks,
Rick

Rick-----


I haven't seen 92 octane gasoline in California in years. The maximum I've seen at ordinary gas stations is 91 octane. Does your engine experience pre-ignition with 91 octane?

[Michael B.]

Re: 435 HP running on pump gas

My 400HP pings at 4-degrees on California gas with 10.25 to 1. I augment with a little aviation fuel when I take it out to the track.

Felpo makes a extra thick gasket they call a head saver gasket for those heads that were surfaced one too many times. 8180 SP-1 for the BBC.

I have a set that I plan on using if I ever get around to taking the heads off.

[Jaime G.]

Re: 435 HP running on pump gas

91 Octane gas is fine for all my cars except my fuelies.
However, the cars do ping and run hotter. I mix 5 gals.
112 Racing Fuel with 10 gals of 91 and the combo works just fine. I have a 65 425HP, a 64 365HP and a 66 425HP which all do well with this mix. the 62 and 65 Fuelies like non oxigenated non ethanol fuel so I run straight 112 and they do just fine. If you can get Racing Fuel try it out.

[Rick M.]

Re: 435 HP running on pump gas

Thanks Joe (as always) Michael & Jaime,
I mistyped, 91 octane is the word of the day here in the socialist state of California. Joe, the answer to your question on pinging is; I don't know. I started the tear down/restoration of the car 50 miles after I bought it. I have no idea if it pings or if a timing adjustment would cure it (Michael). I used VP race fuel (Jaime) when the car was delivered but shortly afterward jacked it up for a leaking rear diff pumpkin...well you all know the rest. It turned into a total resto. I'm thinking thick gasket, which would lessen the timing adjustments. Unless someone has a better idea it's that or just stick to race fuel.

[Duke W.]

Re: 435 HP running on pump gas

... subject has been discussed a zillion times, including recently with John McRae's L-72. Since the engine is disassembled take all the measurements you need to compute the actual compression ratio with various head gaskets. Use John's measured piston volume. It is more accurate than the specs.

A reference to a good online calculator is in the archives (many times). I recommend no more that 10.25:1 true for SHP BBs, but in California it might be a good ideal to limit it to about 10:1.

When you reassemble the engine convert the ported vacuum advance to full time. This alone will help keep it out of detonation, it will run cooler, and get better fuel economy.

Duke

[Rick M.]

Re: 435 HP running on pump gas

Thanks Duke, will look at the archives. Appreciate the input.

[Joe L.]

Re: 435 HP running on pump gas

Thanks Duke, will look at the archives. Appreciate the input.

Rick-----


Here's a "clue" that I've mentioned previously: ALL current GM crate big blocks with cast iron heads are built with a compression ratio of 8.75:1 AND, even at that, they require 91 octane, minimum, fuel.

All crate big blocks with aluminum heads except the ZZ-572/720 are built with a maximum compression ratio of 9.6:1 and also require 91 octane, minimum, fuel. (In case you're wondering, the ZZ-572/720 is built with 12:1 compression ratio and requires 110 octane, minimum, racing fuel).

427, 454, and 502 cid engines are BIG BORE engines and are, thus, highly susceptible to pre-ignition.

[Rick M.]

Re: 435 HP running on pump gas

Thanks Joe and Duke,
I read John MaCrae's complete thread in the archives. Way more that I want to do. The engine ran great on 98 race fuel, never tried pump gas in it. My only thought is this. The engine's out of the car (chassis/frame resto). I've noticed head gasket seepage. Bottom end looks good, oil was clean. Some carbon build up on underside of valves but don't have the heads off yet to see the top. Once heads are off, will check surface (for flat) and ask smarter folks than I to have a look. Go with a thicker gasket (maybe .050 to .060) to lower compression. Block was decked so it shouldn't create problems with intake matching up. The goal here is only to have a shot at 91 octane for point to point normal driving. As I'm going to replace head gaskets anyway this seems to be logical and low risk.
Your thoughts?
Thanks,
Rick

[Ridge K.]

Re: 435 HP running on pump gas

Does anyone happen to know, if one mixes 10 gallons of 92 octane pump gasoline, with 10 gallons of 100 octane racing or aircraft gasoline, does it make a 96 octane blend? Is it that simple, or, a more complicated formula, with many variables?
Thanks in advance. Ridge.

[Joe L.]

Re: 435 HP running on pump gas

Does anyone happen to know, if one mixes 10 gallons of 92 octane pump gasoline, with 10 gallons of 100 octane racing or aircraft gasoline, does it make a 96 octane blend? Is it that simple, or, a more complicated formula, with many variables?
Thanks in advance. Ridge.

Ridge-----


Yes, it's that simple to compute the final octane rating of different "blends" of various octane fuels. In fact, some service stations, especially the old Sunoco which sold 5 or more different octane rating fuels, only had 2 different octane ratings "in the ground"---the pumps apportioned the right blend of the two to achieve the octane rating being purchased.

[Joe L.]

Re: 435 HP running on pump gas

Thanks Joe and Duke,
I read John MaCrae's complete thread in the archives. Way more that I want to do. The engine ran great on 98 race fuel, never tried pump gas in it. My only thought is this. The engine's out of the car (chassis/frame resto). I've noticed head gasket seepage. Bottom end looks good, oil was clean. Some carbon build up on underside of valves but don't have the heads off yet to see the top. Once heads are off, will check surface (for flat) and ask smarter folks than I to have a look. Go with a thicker gasket (maybe .050 to .060) to lower compression. Block was decked so it shouldn't create problems with intake matching up. The goal here is only to have a shot at 91 octane for point to point normal driving. As I'm going to replace head gaskets anyway this seems to be logical and low risk.
Your thoughts?
Thanks,
Rick

Rick-----


I DEFINITELY RECOMMEND going with the thickest head gaskets you can find. It might not be enough, by itself, to alleviate the problem but it will help. If one intends to DRIVE a Corvette on a more-or-less regular basis, one doesn't want to be put in a situation of needing to use expensive, not readily available, high octane fuel or the need to constantly add large quantities of expensive octane boosters. This sort of thing gets old FAST.

[Ridge K.]

Re: 435 HP running on pump gas

Ridge-----


Yes, it's that simple to compute the final octane rating of different "blends" of various octane fuels. In fact, some service stations, especially the old Sunoco which sold 5 or more different octane rating fuels, only had 2 different octane ratings "in the ground"---the pumps apportioned the right blend of the two to achieve the octane rating being purchased.

Thanks much, Joe.


Thanks again, Ridge.

[Duke W.]

Re: 435 HP running on pump gas

You have to know if the octane rating is RON, MON, or PON which is the arithmetic average of RON and MON, and the difference is typically in the range of 8-10. "Race gas" can use any of the above methods, so you have to do some homework to find out what you have. The dealer will probably though out some "octane number" but hasn't got the foggiest idea what it really means. Get a spec sheet from the fuel blender.

Avgas uses a different method, but it's close enough to MON to assume that.

Once you have appropriate estimates and converted everything to a single rating system of you choice you can use linear interpolation to get a good idea of what octane rating a blend has.

If that's not a good enough estmate for yourself, you can hire a test lab to test whatever blend you want to submit.

The trouble with the above is that few really know what minimum octane their engine needs, and because of octane sensitivity to things like inlet air temperature (less on a cool winter day on the highway than on a hot summer day in traffic) it often boils down to a matter of experimentation.

Start with a minimum amount of a high octane blend in pump premium, like 10 to 20 percent and keep increasing it until there is no detonation under worse case condtions. Whatever the octane number of the blend is what the engine needs.

Duke

[Ridge K.]

Re: 435 HP running on pump gas

You have to know if the octane rating is RON, MON, or PON which is the arithmetic average of RON and MON, and the difference is typically in the range of 8-10. "Race gas" can use any of the above methods, so you have to do some homework to find out what you have. The dealer will probably though out some "octane number" but hasn't got the foggiest idea what it really means. Get a spec sheet from the fuel blender.

Avgas uses a different method, but it's close enough to MON to assume that.

Once you have appropriate estimates and converted everything to a single rating system of you choice you can use linear interpolation to get a good idea of what octane rating a blend has.

If that's not a good enough estmate for yourself, you can hire a test lab to test whatever blend you want to submit.

The trouble with the above is that few really know what minimum octane their engine needs, and because of octane sensitivity to things like inlet air temperature (less on a cool winter day on the highway than on a hot summer day in traffic) it often boils down to a matter of experimentation.

Start with a minimum amount of a high octane blend in pump premium, like 10 to 20 percent and keep increasing it until there is no detonation under worse case condtions. Whatever the octane number of the blend is what the engine needs.

Duke

Thanks much, Duke. I'll do some research on what octane calculation method is used here in Oklahoma. Good idea of trial testing at 10%,...20%, etc.
Thanks again, Ridge.