POTENTIAL OVERHEAT ON BORED '68 L71

[Jack H.]

Octane may no be the only consideration....

Had a 'nightmare' '69 tri-power at the shop last year. Was built 100% to factory spec with fresh, flow-bench tested 3x2 carbs. On the street it did 'funnies' (died under WOT, wouldn't climb above 5000 rpm, Etc.). All the kings horses/men suggested fault isolation techniques and repair routes (including owner writing Corvette Fever and getting a real 'off the wall' published reply).

Finally, we put some SCIENCE to the job. An out of the way, 'mom/pop' vintage race shop up in the mountains was rented. Shop owner was formerly a Ford Engineering Project Mgr (remember the Skyliner?) and has a nice barrel dyno.

We put the car on the dyno, engaged trans, and took it to max RPM/WOT with engine/exhaust under monitor. Had to re-fixture the dyno because Shark nose interfered with standard forced convection feature of the dyno (insufficient air flow through the radiator with car not in motion), but finally got to where we could duplicate the problem(s) regularly....

Used TV cameras to probe/monitor venturi on all three carbs during dynamic run-up. BINGO! Regardless of octane/lead additive being used the fuel bowls were boiling at WOT/max BTU dissipation from the engine causing rather random spurts of fuel out the vent stack(s) and back down/into the venturi's.....

Wrapped fuel line(s) supplying carbs with wet towels (evaporative cooling) and made the problem(s) GO AWAY! We concluded the 'environmentally friendly' alcohol being mixed into todays stock pump gas had dropped the boiling point of the fuel in carb bowls and eliminated what design safety margin the original engine designers had built into the tripower setup....

[Dale Pearman]

And Now We Know

why some people cook their gasoline before using it.

Dale.

[Dale Pearman]

Don't EVER try that!

In addition to being illegal and polluting, the process is VERY Dangerous!

Dale.

[Clem Z.]

Re: Octane may no be the only consideration....

did you have the heat riser crossover in the intake manifold blocked? this will take a lot of heat out of the carbs plus gain entra HP. it also makes your side pipes nice and loud!!

[Jeff Burkey]

Re: tri-power set-up, or no...??

John: I have been advised that a requisite amount of manifold vacuum is needed for smooth idle off the center tri-power carb set-up, and cr is a function of how much vacuum is generated along with cam timing, etc. If there is say, less than 12" of vacuum, I may encounter rough idling difficulties. This is also a power brake assist car and the ONLY power option. This car came from the factory without the radio, so it's kind of "neat"...

[Clem Z.]

PS

make sure that the fuel line from the pump to the carbs is not touching the bottom rad hose. a lot of heat also is picked up by the fuel line from the tank to the pump being close to the exhaust in the engine compartment.. inline horizontal fuel filters will cause the same problem because they always have a air pocket at the top. i know some corvettes came this way but i have found that if the filter is mounted vertically there is no air pockets in the fuel because the filter is kept full of fuel.

[Jeff Burkey]

Re: Octane may no be the only consideration....

Clem: I guess the nitty/gritty and nuts/bolts of what I am trying to ascertain about this L71 motor is what cr is nominal/optimum for smooth operation/performance of trips...?? When Traco Engineering re-built this motor in 1977 they lowered the cr to 9.6: 1 and removed the 3X2's for a four barrel set-up and the car dynoed at over 430hp. The original owner, from whom I purchased the car, tells me they ( Traco) built the motor as a iron version of the L88. The current cam is a 1968 L88 camshaft and set at those valve timing events. My desire is to return the set-up to factory spec. and begin the process of Bowtie judging. Tks, Jeff B.
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[Clem Z.]

Re: tri-power set-up, or no...??

a low manifold vacuum may require that the butterflys be open too far and cause the idle transfer slot to be exposed too far and cause a rich idle condition. this can be cured by drilling small holes in the butterflys and closing down the butterfly opening. the transfer slot should only show about .030

[Clem Z.]

Re: Octane may no be the only consideration....

i guess you are going to change the cam and if so i do not think that the lower comp ratio will cause any problem.

[Jeff Burkey]

Re: Octane may no be the only consideration....

"Thanks, Clem!"

[Patrick H.]

Re: And Now We Know

Dale,

As soon as I read Jack's post, I KNEW what your response was going to say. I remember well your dissertations on cooking your gas previously...

Patrick

[Mike Baker]

tri-power? You bet!

Yes, I'm keeping the 3x2. I am not anticipating any issues with a reduction in vacuum. But, I see where you are going as the final compression ratio would be effected by the overlap in the cam, and the valve overlap does have alot to do with the manifold vacuum. I think at an idle cam design is going to have more of an effect on manifold vacuum than piston shape. (please correct me if I'm wrong)

Clem, commented in this thread about drilling holes in the butterfly to correct a poor idle situation. I had to do that to a small block Ford I had in high school. It had way to much overlap considering the rest of the motor.

I'm smarter now. I found someone I trust who builds engines for our old Vettes, Camaro's and some off shore speed boats. I ask lots of questions and get to learn something. I also have some assurance that it will turn out as expected. Internally it will not be as it was on the showroom floor, but it will run on todays gas and have enough power for me.