JL66REDCPE

I recently installed a DeWitts aluminum radiator in my 66 427/425. The radiator works great and keeps the engine operating temperature where it belongs. I am however noticing that the underhood temperatures after shutdown seem awfully high. I am experiencing percolation. Comment please on this and how I might control the heat soak.

SWCDuke

Have you converted from ported to full time vacuum advance?

Duke

JL66REDCPE

Duke - no I have not. Car is running great and not overheating. Last time I tried this car overheated. How would changing ported to full affect heat soak ??

Frankie the Fink

Its all the usual drill, try non-ethanol and/or racing gas, block intake runners (if you have them) spacers under the carb, etc,, etc. Many, many posts on the topic...

Perc and vapor lock are the evil twins that plague carbs in hot weather...

SWCDuke

There's no way it could overheat if you did the conversion properly. Your engine needs about 25-30 degrees total idle advance to minimize EGT and fuel flow to keep the engine turning over at idle. This is achieved with the combination of initial timing and full vacuum advance.

You can clearly see the effect by measuring manifold temperature at idle after a drive. With full time vacuum advance it should be no more than about 500F. If total idle advance is less than optimum, such as with ported vacuum advance, EGT will be higher as will the engine mass, and this increases "heat soak" after shutdown.

Your original ""360" VAC is 0@6", 12@12". Check it with a Mighty Vac. If it does not function properly, replace it with a B26,(NAPA VC-1765 or equivalent in another name brand) which is 16@12. If you have an OE equivalent cam it should pull about 14" at 900 idle, so the B26 passes the Two-Inch Rule.

Many have done this conversion on SHP big blocks and all but you reported lower operating temperatures, especially in hot weather traffic.

Duke

JL66REDCPE

Frankie -- I am already running avgas. It is not overheating - the problem is heat soak after shutdown

JL66REDCPE

Duke -- I am sure I hooked it up correctly. I am already running a vc1810 can. The problem is heat soak not necessarily engine temp.

Frankie the Fink

The point is the boiling point of the fuel....if, after 20 minutes of heat soak, the squirters start dribbling down the throttle bores to sit on the intake runners you are basically starting a flooded car... Eventually, the gas will evaporate, say in an hour, and the car will start normally.

"Shooting" the carb bowls with an I/R temp gun 20 minutes after a 'hot run' will tell the story...

bluestreak63

But has this always been a problem? Even when these cars were brand knew, people experienced percolation?

PAmotorman

fix the heat riser valve in the open position and block the heat riser cross over in the intake manifold with some .005 thick stainless steel shim stock

rongold

No, because back then gas DID NOT BOIL at 140 degrees like it does today.


RON

Critter1

If you noticed the under hood temps higher after the new radiator was installed, and after shut down, I would guess that the radiator is doing it's job and more heat is radiating from it. I wouldn't worry about it

My 66 425 HP does the same thing. Never goes over 210° but under hood temp goes up a lot after shut down.

(and I haven't had my vacuum advance hooked up for the last 30 years, but don't tell anyone)

bluestreak63

But he's running avgas, which I assume has a lot higher boiling point than 140, like the gas back in the 60's.

Frankie the Fink

Folks would often confuse perc with vapor lock 'back in the day'.
Vapor lock was much more prevalent back then. I can tell you that my Dad traded in many a car with the 'ole wooden clothes pins clamped to the fuel line as a cure for vapor lock. (Which did nothing BTW)

tbarb

Critter,

Can you explain to me the reasoning why you do not use the advantage of ported vacuum advance on your engine.

Are you always driving at WOT..

Critter1

There are good reasons why it's a good idea to have a correctly functioning vacuum advance system. There are also a few reasons why I don't have that system on my car.

Detonation can kill an engine. Even light detonation if prolonged. So light that it can't be heard. The kind of detonation that breaks top piston rings or ring lands. I'm sure everyone will agree with that. Although not much of a problem around town, it can destroy an engine on long trips.

In about 1983, I completely rebuilt the 425 HP 427 engine and also rebuilt or replaced everything that had to do with the cooling system. I kept the original 11-1 CR. Prepairing for the 1983 NCRS road trip that eventually ended in Copper Mtn Colorado, I knew that fuel quality along the way from Illinois to the Hoover dam was not going to be good. Also, the 4500 mile trip was mid August, the hottest time of the year.

Driving mile after mile at 70 MPH, it's likely that there will be at least light detonation hammering away for hours. That gets worse at part throttle and slight lean conditions. (power valve closed) Add a bit of vacuum advance and high air temp and detonation gets worse.
So, I knew I didn't have time to search for a vacuum advance unit that was more relaxed so, for the trip, I just plugged the one that was on the distributor. I knew that fuel mileage may suffer a bit but that wasn't an issue.

During the trip, especially driving through some of the canyons at 10 MPH behind other Corvettes, 98° ait temp, our car was one of the few that didn't overheat! In fact, several of the other members were quite surprised that we were one of the few that wasn't spitting coolant out of the overflow.

Also, an initial timing setting of 12° helped too. Yes, less than it would have been with direct manifold vacuum advance but enough to keep chamber temps down at idle.

After returning from the trip in 1983, I decided to leave the vacuum adv plugged and see how things went. For the 30+ years, it's been disconnected and I drive that car a LOT during our hot Florida summers. Still no issues but I do see a bit of an increase in engine temp now that the radiator is 34 years old.

Also, one of the main reasons why I disconnected the vacuum advance is because of the transistor ignition system. As the vacuum advance unit moves, it rotates the pick up coil and it's common for those small wires to break. And that's exactly why they break. Can you imaging having that happen in Nowhere, Oklahoma, 75 miles from anything?

Again, I agree that for most folks, it's a very good idea to have a correctly functioning vacuum advance system but I'm not most folks.

Some older Corvettes with fuel injection came from the factory with no vacuum advance at all and they didn't over heat when new. They did have an initial timing setting of at least 12° though.

If my old fat block 66 ever does start to run hot, I promise I will hook up the V.A. unit.

PAmotorman

I owned both a new 65 425 HP 396 and a new 66 427 450 HP and both I blocked the heat riser crossovers because for every 10 degrees you lower the inlet fuel mixture temp you gain 1% in HP so why would you not block the heat riser crossover ?? you lose the choke but these engines do not need a choke even in Pa. where I live. this was GM answer to keep heat from the float bowls

Critter1

Yup, when I rebuilt the engine in 1983, I used the little stainless plates to block that passage and cut out most of the flapper in the heat riser.

A 425 HP certainly doesn't need that to warm the manifold and carburetor. I even added the under the GM manifold shield that keeps hot oil off of the manifold.

Frankie the Fink

The good news with percolation is that its pretty much binary...if you can get carb bowl temps just a couple of degrees below the boiling point - you'll be OK..

MikeM

There is a rumor that seems to persist on this forum that a choke is required to start the engine.