Oh man, what a difference!
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 857
Likes: 0
From: York Pa.
Oh man, what a difference!
Having had to put 93 ocatane in the vette the last few times, it started carrying, knocking etc. Ran fine on 94, but I wanted to bump it a little since it has all the 93 in it. So I got 5 gallons of 100 octane and topped it with 94. The frown that was on my face from the $4 a gallon was quickly removed. It never ran on 94 like it does now. Holy cow what a difference. Not just a noticible difference, but a significant diffence. Heck if I wouldn't have know better, I'd have swore someone slipped a bigger cam in there. Bad news is I was getting disgusted enough with the car that I was going to put it on e-bay. Not really thinking of that at the moment. :D Classic
Le Mans Master
Joined: Jan 2002
Posts: 8,913
Likes: 283
Re: Oh man, what a difference! (Classicvette63)
I have run many mixtures of 100LL (avgas) and I am now experimenting with a 25% avgas and 75% 91 octane pump gas.
With 100% avgas the car seems a tad sluggish on the low end and cranks a bit longer when starting. And everyone behind you for a ten block area knows what you are burning.
With a 50% mixture (I ran last summer) I ran the initial timing up to 15* BTDC and had great results across the range.
I am running a 25% mixture and have the initial at 15* and no detonation.
So my experience is more is not better in this case. I think with some carb calibrations a 100% mixture would be acceptable but on the expensive side if you had to purchase it. I am fortunate that I work at an airport and there is usually a 55-gallon drum with sump fuel in it. Since fuel taken out of an aircraft cannot go back into it, they just give it to me.
If you are seeking to purchase avgas and have had difficulty in buying it, find a small airport with a pay at the pump unit. Talk to the guy that runs the FBO and tell then your intentions. They may let you pull your car up to the pump, or bring some gas cans. 2 - 5 gallon cans would yield 40 gallons of serviceable fuel.
Mark
With 100% avgas the car seems a tad sluggish on the low end and cranks a bit longer when starting. And everyone behind you for a ten block area knows what you are burning.
With a 50% mixture (I ran last summer) I ran the initial timing up to 15* BTDC and had great results across the range.
I am running a 25% mixture and have the initial at 15* and no detonation.
So my experience is more is not better in this case. I think with some carb calibrations a 100% mixture would be acceptable but on the expensive side if you had to purchase it. I am fortunate that I work at an airport and there is usually a 55-gallon drum with sump fuel in it. Since fuel taken out of an aircraft cannot go back into it, they just give it to me.
If you are seeking to purchase avgas and have had difficulty in buying it, find a small airport with a pay at the pump unit. Talk to the guy that runs the FBO and tell then your intentions. They may let you pull your car up to the pump, or bring some gas cans. 2 - 5 gallon cans would yield 40 gallons of serviceable fuel.
Mark
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,270
Re: Oh man, what a difference! (ghostrider20)
Avgas has a lower vapor pressure to prevent vapor lock at altitude, so this will make it a bit harder to start with OE choke calibration, and might also result in poorer idle and low speed throttle response due to its lower volatility.
This effect will be particularly pronounced if you have blocked the heat riser.
A mixture of about 25 percent 100LL avgas and pump premium should result in sufficient octane to prevent detonation on most Corvette engines if they have no more than OE static and dynamic CR. The octane number of the blend is difficult to calculate. Automobile gasoline octane rating is the arithmetic average of the Researach and Motor testing methods. The avgas rating is simlar, but not quite the same as the Motor method. A 25 percent blend of 100LL avgas and 93 PON premiun and the small amount of lead probably yield about 100 Research octane, which is what fed the beasts back in the sixties.
Duke
This effect will be particularly pronounced if you have blocked the heat riser.
A mixture of about 25 percent 100LL avgas and pump premium should result in sufficient octane to prevent detonation on most Corvette engines if they have no more than OE static and dynamic CR. The octane number of the blend is difficult to calculate. Automobile gasoline octane rating is the arithmetic average of the Researach and Motor testing methods. The avgas rating is simlar, but not quite the same as the Motor method. A 25 percent blend of 100LL avgas and 93 PON premiun and the small amount of lead probably yield about 100 Research octane, which is what fed the beasts back in the sixties.
Duke
Safety Car
Joined: Feb 1999
Posts: 3,504
Likes: 4
From: Mexico Beach FL
Re: Oh man, what a difference! (Classicvette63)
The last time I put Turbo-Blue gas in mine, I had the same results.
But, since I can't find it anymore, I got the Crane Cams adjustable vacuum advance kit and intalled it -- heavier springs on the mechanical advance and an adjustable vacuum advance can. This slightly detunes the vehicle, but at least I don't hear any clatter.
I try to keep my '64 as close to original as possible, but I have put a PCV valve and changed the timing -- I'm very happy with the results. I'll keep the original parts in case I ever want it totally original.
Tom Piper
But, since I can't find it anymore, I got the Crane Cams adjustable vacuum advance kit and intalled it -- heavier springs on the mechanical advance and an adjustable vacuum advance can. This slightly detunes the vehicle, but at least I don't hear any clatter.
I try to keep my '64 as close to original as possible, but I have put a PCV valve and changed the timing -- I'm very happy with the results. I'll keep the original parts in case I ever want it totally original.
Tom Piper
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 857
Likes: 0
From: York Pa.
Re: Oh man, what a difference! (Tom Piper)
The 100 octane was Gt race fuel, sold at the Sunoco station. I will switch to blending some avgas, just didn't have time yesterday to run up to the airport. I think I'll just buy a 55 gallon drum of it. Hopefully that should last all summer. I didn't realize there would that big of a difference between 93 and 94 octane and certainly not the huge diff between 94 and whatever the blend came out to be. Since the nostalgia craze is in full swing and everything that was old is new again, maybe they'll bring back 260. :D Classic
Le Mans Master
Joined: Jan 2002
Posts: 8,913
Likes: 283
Re: Oh man, what a difference! (Classicvette63)
On a side note, 9* BTDC was as high as I could go on the timing without detonation with pump gas, which is not to bad. I think 10* is the factory setting.
Pump gas in my area is limited to 92 octane and is 10% Ethanol. :crazy:
Mark
[Modified by ghostrider20, 12:51 PM 4/20/2004]
Pump gas in my area is limited to 92 octane and is 10% Ethanol. :crazy:
Mark
[Modified by ghostrider20, 12:51 PM 4/20/2004]
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Race Director
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Re: Oh man, what a difference! (ghostrider20)
The nominal OE spec for 64-65 mechanical lifter cam engines is 10 degrees with a range of 10-14. The maximum centrifugal spec is 24@2350, so total WOT timing will be in the range of 34-38. I don't think there is any benefit to going over 14 initial. SBs usually provide high rev peak power in the range of 36-38 degrees and more timing will just increase detonation, not power.
Also, it's typical that retarding the timing 3 degrees from ideal will only reduce power by about one percent, so there's really no point in pushing the timing to the absolute detonation limit.
The '65 L-79 spec calls for less intial timing and the centrifugal advance is slower, but has more total advance. Because the L-79 has a higher dynamic compression ratio that the 30-30 cam engiens, it can't take full WOT timing until higher up in the rev range.
If you experience detonation on a roadtrip and don't have easy access to avgas or race gas, go to a hardware store and buy a gallon or two of toluene or xylene. Ten to twenty percent of these two hydrocarbons in pump premium should keep the engine out of detonation, without having to severely retard the timing
Duke
[Modified by SWCDuke, 1:13 PM 4/20/2004]
Also, it's typical that retarding the timing 3 degrees from ideal will only reduce power by about one percent, so there's really no point in pushing the timing to the absolute detonation limit.
The '65 L-79 spec calls for less intial timing and the centrifugal advance is slower, but has more total advance. Because the L-79 has a higher dynamic compression ratio that the 30-30 cam engiens, it can't take full WOT timing until higher up in the rev range.
If you experience detonation on a roadtrip and don't have easy access to avgas or race gas, go to a hardware store and buy a gallon or two of toluene or xylene. Ten to twenty percent of these two hydrocarbons in pump premium should keep the engine out of detonation, without having to severely retard the timing
Duke
[Modified by SWCDuke, 1:13 PM 4/20/2004]