I Just brought home a 1970 LT-1 coupe, and I have a question as to what type of fuel to run in the car. The previous owner had been running premium unleaded with STP lead substitute in the car, and I used the same to get the car home, and it ran wonderful. In the documentation I got with the car, I found a receipt from when the heads were rebuilt, and hardened seats were installed in 1987. My question is, even with hardened seats, with 11:1 compression, is even the best unleaded fuel good enough for this car? Is it wise to use a lead substitute/additive, even with hardened seats? There is a gas station in my area that has "100 Octane Racing Fuel" but it says that it is unleaded and has some ether additive or something in it to boost the octane numbers. Lastly, how about aviation fuel- is it worth it? Thanks!
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Re: LT-1 Fuel Requirements
if the engine is a true 11:1 you will need to mix the unleaded race gas with highest available pump gas. i would mix 1 gallon of race gas with 4 gallons of pump hi test. do not let anyone tell you that you can run 11:1 engines on unleaded pump hi test because the highest ratio you can get away with is about 9:1 with iron heads and 9.5:1 with aluminum heads. with electronic fuel injection which give good fuel distribution you can run up to 10.5:1 with aluminum heads and knock sensors. -
Re: LT-1 Fuel Requirements
if the engine is a true 11:1 you will need to mix the unleaded race gas with highest available pump gas. i would mix 1 gallon of race gas with 4 gallons of pump hi test. do not let anyone tell you that you can run 11:1 engines on unleaded pump hi test because the highest ratio you can get away with is about 9:1 with iron heads and 9.5:1 with aluminum heads. with electronic fuel injection which give good fuel distribution you can run up to 10.5:1 with aluminum heads and knock sensors.Comment
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Re: LT-1 Fuel Requirements
First off, this is not an opinion, just relaying my own experience on this matter.
I have the 1970 350/350 hp L46 motor. It also came with the 11:1 compression ratio. After much discussion with the local machine shop, and a Mechanical Engineer(an old California hotrodder), I had the hardened inserts installed in the exhaust valve seats, left the compression alone. The machine shop and Engineer were of the same opinion of a few articles I had read at the time, that the overlap of the 350hp cam would allow me to keep the 11:1 compression as long as I ran unleaded 92 octane or higher. I've been running that configuration for 12 years and 13,000 miles. Knocking has not been a problem, except for the one time I accidently added some 88 octane.
The cam profile on the LT-1 might also permit the use of 92 octane.
Maybe some of the more experienced folks here can offer their thoughts.
LT-1 owners? Duke?
Regards,
BobComment
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Re: LT-1 Fuel Requirements
First off, this is not an opinion, just relaying my own experience on this matter.
I have the 1970 350/350 hp L46 motor. It also came with the 11:1 compression ratio. After much discussion with the local machine shop, and a Mechanical Engineer(an old California hotrodder), I had the hardened inserts installed in the exhaust valve seats, left the compression alone. The machine shop and Engineer were of the same opinion of a few articles I had read at the time, that the overlap of the 350hp cam would allow me to keep the 11:1 compression as long as I ran unleaded 92 octane or higher. I've been running that configuration for 12 years and 13,000 miles. Knocking has not been a problem, except for the one time I accidently added some 88 octane.
The cam profile on the LT-1 might also permit the use of 92 octane.
Maybe some of the more experienced folks here can offer their thoughts.
LT-1 owners? Duke?
Regards,
BobComment
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Av Gas is a no no!!
Av gas should not be used wholly by itself. The gas is made for different conditions than exist with an automotive engine. This is a whole 'nother story.You can run unleaded premium in this engine, the valve seat wear will be negligible. I would re-curve the distributor and put an adjustable vacuum advance on it. You may be able to find the right non-adjustable advance with some searching and keep it looking original. You will also have to change the vacuum from ported to manifold vacuum or vice versa. I have run this setup in my LT1 for years with no problem. I only use the "race gas" when I know it's going to be run hard. Hope that helps...Comment
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Av Gas is a no no!!
Av gas should not be used wholly by itself. The gas is made for different conditions than exist with an automotive engine. This is a whole 'nother story.You can run unleaded premium in this engine, the valve seat wear will be negligible. I would re-curve the distributor and put an adjustable vacuum advance on it. You may be able to find the right non-adjustable advance with some searching and keep it looking original. You will also have to change the vacuum from ported to manifold vacuum or vice versa. I have run this setup in my LT1 for years with no problem. I only use the "race gas" when I know it's going to be run hard. Hope that helps...Comment
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Re: LT-1 Fuel Requirements
i have built race engines that were required to be a max of 10.5:1 and these engines have lots of over lap in the camshaft and you could not run them on pump hi test. we used 100% 100 octane no lead race gas to make max power on the dyno because pump hi test would not allow max spark advance. if you back up the timing on any engine you can run lower octane fuel but you are not making max power or milage.Comment
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Re: LT-1 Fuel Requirements
i have built race engines that were required to be a max of 10.5:1 and these engines have lots of over lap in the camshaft and you could not run them on pump hi test. we used 100% 100 octane no lead race gas to make max power on the dyno because pump hi test would not allow max spark advance. if you back up the timing on any engine you can run lower octane fuel but you are not making max power or milage.Comment
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Re: LT-1 Fuel Requirements
What has been working for me with my original owner 68 L79 327/350HP with 11:1 is just using 93 pump premium. I use no additives just pump gas. I don't lug the engine, drive it with enough rpm's in the gear selected. The owners manual states that in stop and go driving in traffic at speeds 30 and below use second gear, works for me.Comment
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Re: LT-1 Fuel Requirements
What has been working for me with my original owner 68 L79 327/350HP with 11:1 is just using 93 pump premium. I use no additives just pump gas. I don't lug the engine, drive it with enough rpm's in the gear selected. The owners manual states that in stop and go driving in traffic at speeds 30 and below use second gear, works for me.Comment
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Re: LT-1 Fuel Requirements
There are a lot of variables that affect the propensity to detonate - ambient pressure and temperature, actual inlet air temperature, actual CR of the specific engine, valve timing, the initial timing and rate of centrifugal advance, and how one drives the car.
Detonation is primarily a low rev phenomenon and the reason SHP engines have more compression is that their overlap killed some low speed dynamic comprssion, which allowed higher CRs. For example the cranking compression pressure of the '63 250 and 300 HP engines (10.5: 1 CR) is 160 psi, yet the SHP/FI engines with the Duntov cam were only speced at 150 psi despite their higher advertised CRs of 11.25:1. All the overlap of the Duntov cam killed some dynamic compression allowing them to run higher static compression ratios.
My suggestion to one who has recently acquired a SHP engined Corvette is to let the fuel go down to near empty, then add about three gallons of the highest pump premium available to you. Run the car until you determine the level of detonation, if any. If you don't have significant detonation place another three gallons in and continue the test. Keep adding three gallons until you are satisfied that the engine operates satisfactorily.
If you do experience some detonation there are two routes you can take. First try adding a gallon of Unocal 100 octane unleaded. Keep notes as to the miniumu blend of 100 unleaded that will allow detonation free operation. IMO it's also okay to use low percentage blends of avgas. If you don't have cold starting problems there should be no other ill affects. Avgas typically has lower vapor pressure than automotive gasoline, so it might be a little harder to start in cold weather.
The second strategy is to "tune" the ignition advance by reducing the amount of initial timing and slowing the centrifugal. Total WOT timing can be maintained by increasing the amount of centrifugal advance in the distributor. I also recommend full manifold vacuum to the vacuum can, as the more advance you run at idle, the lower the EGT and combustion chamber boundary surface temps, which will reduce the tendency to detonate. Also the vacuum can should provide full full advance at a vacuum level less than your idle vacuum, so the vacuum advance is fully deployed at idle, but it should not be more than 16 degrees. This is not always the case with some engines and some emission controlled engines with TCS have too much vacuum advance. The '65 SHP/FI can is ideal for all SHP engines.
I don't believe valve recession is a serious problem if you have your valves adjusted properly and use the car for "normal driving", which would only include short bursts of WOT high rev operation. On the other hand, even though it is against the law to use leaded fuel on the street, I don't think you're going to cause and environmental disaster by running a blend of 25 percent low lead Avgas or leaded racing fuel.
Everyone basically has to come up with a fuel blend/advance curve combination that is comfortable/convenient for them that allows the engine to operate without signficant detonation. There is no one "right" answer.
DukeComment
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Re: LT-1 Fuel Requirements
There are a lot of variables that affect the propensity to detonate - ambient pressure and temperature, actual inlet air temperature, actual CR of the specific engine, valve timing, the initial timing and rate of centrifugal advance, and how one drives the car.
Detonation is primarily a low rev phenomenon and the reason SHP engines have more compression is that their overlap killed some low speed dynamic comprssion, which allowed higher CRs. For example the cranking compression pressure of the '63 250 and 300 HP engines (10.5: 1 CR) is 160 psi, yet the SHP/FI engines with the Duntov cam were only speced at 150 psi despite their higher advertised CRs of 11.25:1. All the overlap of the Duntov cam killed some dynamic compression allowing them to run higher static compression ratios.
My suggestion to one who has recently acquired a SHP engined Corvette is to let the fuel go down to near empty, then add about three gallons of the highest pump premium available to you. Run the car until you determine the level of detonation, if any. If you don't have significant detonation place another three gallons in and continue the test. Keep adding three gallons until you are satisfied that the engine operates satisfactorily.
If you do experience some detonation there are two routes you can take. First try adding a gallon of Unocal 100 octane unleaded. Keep notes as to the miniumu blend of 100 unleaded that will allow detonation free operation. IMO it's also okay to use low percentage blends of avgas. If you don't have cold starting problems there should be no other ill affects. Avgas typically has lower vapor pressure than automotive gasoline, so it might be a little harder to start in cold weather.
The second strategy is to "tune" the ignition advance by reducing the amount of initial timing and slowing the centrifugal. Total WOT timing can be maintained by increasing the amount of centrifugal advance in the distributor. I also recommend full manifold vacuum to the vacuum can, as the more advance you run at idle, the lower the EGT and combustion chamber boundary surface temps, which will reduce the tendency to detonate. Also the vacuum can should provide full full advance at a vacuum level less than your idle vacuum, so the vacuum advance is fully deployed at idle, but it should not be more than 16 degrees. This is not always the case with some engines and some emission controlled engines with TCS have too much vacuum advance. The '65 SHP/FI can is ideal for all SHP engines.
I don't believe valve recession is a serious problem if you have your valves adjusted properly and use the car for "normal driving", which would only include short bursts of WOT high rev operation. On the other hand, even though it is against the law to use leaded fuel on the street, I don't think you're going to cause and environmental disaster by running a blend of 25 percent low lead Avgas or leaded racing fuel.
Everyone basically has to come up with a fuel blend/advance curve combination that is comfortable/convenient for them that allows the engine to operate without signficant detonation. There is no one "right" answer.
DukeComment
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Re: LT-1 Fuel Requirements
Before I put the car away, I topped the tank off with 10 gallons of 93 octane unleaded, and 4 oz. lead substitute (this is before I found the machinists receipt). I took the car out on a long stretch of Judiciary-ignored pavement and poured the coal to her. I never got past 4-5 grand, but it literally screamed through the gears and the motor never skipped a beat. I do not plan on doing that very often(unless the need arises...!) but I want to keep the engine in top shape, obviously. Until I pull the valve covers off, I will not know for sure if it does indeed have new seats in it....hopefully I'll get to it soon.
Anyway, thank you all very much for the advice...I'll play around with different blends and see what happens!Comment
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Re: LT-1 Fuel Requirements
Before I put the car away, I topped the tank off with 10 gallons of 93 octane unleaded, and 4 oz. lead substitute (this is before I found the machinists receipt). I took the car out on a long stretch of Judiciary-ignored pavement and poured the coal to her. I never got past 4-5 grand, but it literally screamed through the gears and the motor never skipped a beat. I do not plan on doing that very often(unless the need arises...!) but I want to keep the engine in top shape, obviously. Until I pull the valve covers off, I will not know for sure if it does indeed have new seats in it....hopefully I'll get to it soon.
Anyway, thank you all very much for the advice...I'll play around with different blends and see what happens!Comment
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