My 66 427/425 has been redone to factory specs of 11:1 comp. The engine builder recommends using only Cam 2 leaded racing fuel 110+ octane. Does anyone use the Lead Supreme 130 product in their motors or have any advice otherwise? Thanks Joe Pearson.
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Re: 66' 427 - 425HP fuel requirements
Where the heck are you going to find that? I'm not sure but most guys use pump gas. The only place that sells that kind of gas in CA is a racing fuel supplier, and it's almost $8-10 a gallon with a 50 gallon minimum.
Kevin -
Re: 66' 427 - 425HP fuel requirements
A lot of guys are running SHP/FI SBs on unleaded pump premium with maybe some minor timing adjustments. You can try this on your L-72, but BBs have a bit more octane appetite than SBs.
Try running pump premium and see what happens. If you get minor detonation, some timing map adjustments might eliminate it. You should determine you current igntion advance map and compare it to OE so you know your starting point and whether it has been modified.
If you hear detonation and suitable timing map changes won't cure it, you're going to have to come up with your own custom fuel blend. Straight racing gasoline should not be necessary, but you might have to blend some 100 octane low lead avgas or race gas with pump premium - maybe to a maximum of 25-30 percent.
DukeeeComment
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PS
Converting the OE "ported" vacuum advance to full manifold vacuum advance will help mitigate detonation. You can tap into the choke vacuum break line for a full manifold vacuum source.
DukeComment
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Re: 66' 427 - 425HP fuel requirements
Joe,
A few years ago, I drove my 66 425 HP car all the way from Illinois to Florida and I used what was available at the gas stations along the way with no additional additives. The car has the original 11-1 C/R pistons.
For the trip, I backed the timing off about two degrees (34 deg total) and inserted a small steel ball in the vacuum advance line so the unit was inoperative. The only time I ever heard it rattle was under a heavy load but for just cruising, there was absolutely no problem.
For around town and high performance, you will need some type of additive but I don't know much about them. Hope someone can supply some info on these additives.
MichaelComment
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Re: 66' 427 - 425HP fuel requirements
JOE,
I'VE BEEN USING JACK PODELLS OCTANE ADDITIVE FOR A COUPLE OF YEARS WITH NO PROBLEMS.I HAVEN'T USED IT SINCE LAST FALL SO I DON'T RECALL THE RATIOS USED TO ACHIEVE DIFFERENT OCTANE LEVELS. HE ADVERTISES IN THE DRIVELINE AND HEMMINGS.ALSO SELLS THE STUFF AT CORVETTES @ CARLISLE.Comment
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Re: 66' 427 - 425HP fuel requirements
Unocal/Union 76 sells 100 octane unleaded along with the usual gas at some of their retail locations. Darned if I can find it, but they have a comprehensive list on a web site. Last I bought any, it was around $4.50 per gallon, but it's probably more now. Best I could find on the web was at 76.com, then look at locations, then click on the area code button and it lists products sold at that particular location. I know there's better location info somewhere on the web. 100 octane unleaded should work well with 11:1 compression.
RogerComment
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Re: 66' 427 - 425HP fuel requirements
Maybe something has changed, but I did find this link:
The gas I'm talking about is listed as Competition 100. They have a link to a station locator, then you select the state, then go from there. I checked a place here in Phoenix that sells their 100 octane and it does show on their location listing as "Racing fuel Y" (yes).
RogerComment
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Re: 66' 427 - 425HP Thanks Roger
Thanks Roger, found one not too far from me.Comment
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Re: 66' 427 - 425HP fuel requirements
Disabling the vacuum advance is ill-advised!
DukeComment
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Re: 66' 427 - 425HP fuel requirements
Duke,
I've messed with the vac advance on that car for 20 years and finally gave up on it. The addition of about 25% racing fuel was the only thing that ever helped. In 1983, I completely rebuilt the motor and I know everything inside is dead on correct, including chamber volumes and cam timing etc.
The car came to me with an auto parts vac advance unit that was probably made for a small block as the unit produced about 16 deg of timing change. I installed the proper unit for a 425 HP which, as I remember, supplied around 12 degrees. If the engine temp was low or it was a cool day, it wasn't as bad but when the engine temp reached normal, the rattling was severe at certain throttle openings. Traveling at 45 MPH up a hill with a hot engine was the worst. It was right at that point where the adv unit was adding timing and the only way to eliminate the pinging would be to accelerate, dropping the vac signal/timing or accelerate harder to drop the vac advance completely.
I've gone over the distributor mechanical advance numbers several times and made sure the "map" was correct. It is a correct original distributor shaft and cam so the timing came out almost exactly right for the "093" distributor.
A few years later, I brazed part of the adv slot in the adv unit, limiting the total number of degrees to nine instead of the original 12. This seemed to help slightly but it didn't even come close to eliminating the pinging. I finally plugged the line and it's been that way for 15 years now.
All of the usual items have been checked or replaced. The Holley carburetor is functioning properly so there's not a lean problem. In fact, there have been several different Holley carburetors on it over the years including a new #4779 and I know there isn't a lean condition with this carburetor.
It's just a typical 65-67 big block ping that I'm sure all of these cars have using todays pump fuel instead of the 100+ octane they were designed for. I suppose most have now been converted to side exhaust so the owners don't hear the rattle but if the motors are built to original factory spec's., they're rattling! There's another 66 425 HP car (without side exhaust!!!) about a mile from my house and the owner has the exact same problem.
My 63 FI car is just the exact opposite. I'm pretty sure I could use 25% karosene in that car and it wouldn't ping. I've never heard it ping once in all the years I've had it.Comment
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Re: 66' 427 - 425HP fuel requirements
An overaggresive vacuum advance can cause part throttle detonation, and the L-72 is between the rock and hard place, because it needs the vacuum advance to provide full advance at 2" less than idle vacuum. Based on conversations with L-72 owners the engine will pull about 14" at idle, so a 12" can should work from this standpoint.
I believe the OE L-72 can part number ended with "360" and this was stamped on the can along with the maximum advance. The vacuum advance specs I have from Corvette News are 0@6", 15@12", and the current NAPA/Echlin replacement that most closely matches this is the VC1765 (marked "B20").
One of the unusual features of the L-72 is that it has "ported" vacuum advance. I believe this was implemented to meet CA emissions, and since the L-72 was a low volume option, the CA version was sold in all 50 states. (Note that CA L-72 was the only Corvette engine NOT to require a AIR system for CA.) Apparently the tailpipe standards were met with only ported vacuum advance, which increased idle and low speed EGT, but also tended to make them run hot. Ported vacuum advance will also increase the tendency to detonate because it heats up the combustion chamber boundary surface temperatures, so low speed driving followed by moderate acceleration can cause detonation.
One key point is to convert the ported vacuum advance to full manifold vacuum advance by tapping into the choke vacuum break line or the tapped manifold nipple behind the carburetor. Several L-72 owners have even installed the more aggresive VC1810 can (0@4", 16@8") and report good operation, but I don't necessarily know what fuel blend they are using.
Full manifold vacuum advance will improve idle quality/stability and fuel economy. A VC1765 vacuum can costs about ten bucks. I think it would be worth the money and effort for you to install one and connect it to full manifold vacuum. You might find that overall engine operation is better with no greater tendency to detonate than now, maybe even less.
DukeComment
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Re: 66' 427 - 425HP fuel requirements
i would try to open the PVR about 10% area and see what happens.Comment
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