AZDoug

I had originally set up my EFI based an a furmula of static advance+ theoretical centrifulagal advance+theoretical vacuum adavnce throughout teh RPM range and at different manifold vacuums.

What I ended up with was around 46 degrees total at 22-24" vacuum, 44 degrees at 20", etc, over 2500 RPM. The engine cruises at 24" at 2000 RPM, and 20" at 3000 RPM on the highway at about 75 MPH.

It was hard to even get the temp gauge to reach halfway even after several miles of driving.

However, timing at 1200-2000 RPM, at same high vac was too advanced, even though it was much lower as the theoretical centrifugal advance was less and less between 2500 and 1200 RPM until we only had vac and intial at 1200 RPM.

Problem was low speed bucking/surging in higher gears, the person operating the computer while I was driving took out a bunch of timing at high manifold vac (over 20") below 2000 RPM. Bucking stopped.

HOWEVER, he also took out a bunch of timing at high manifold vac over 2500 RPM, such that total high vac timing was maybe 30-34 degrees.

Next day, engine ran OVER halfway on the temp gauge, and got there quickly.

Easy enough to fix, I just added timing back in at high vac, and over heating problem went away. It didn't really over heat, but it was running at about 205 instead of 180.

So, pay attention to your timing, and if you have a car that runs at higher engine vacs, make sure your vac advance works.

Doug

Matt Gruber

thanks for the report!
i must of asked 20 guys what their low speed cruising vac was(with a cam bigger than mine) to determine if my power valve would open at 40mph, and nobody knew see, my pv opens at 13", so it seems u stay way above that even at 1200.

AZDoug

Back when i was in the stone age and used carburetors, i once drove around with a vacuum gauge in the car to check operating vac under different conditions, that is how i selected my PV rating.

I ended up with a 9.5 Primary and IIRC, 6.5 secondary, as often vac would briefly drop to 11-12 or so during moderate accel, but not so much that PV help was needed.

The vacuum on my motor is probably a lot different from what other motors see.

Doug

Matt Gruber

i did try EFI, but, after it sat for a year, the injectors were plugged up. VERY EXPENSIVE to fix.
So i prefer cheap stone age $28 carb rebuild kits.
.
Your vac. does seem high, but i believe it is helpful.

Plasticman

Yes, but Doug's engine is very large (427 SBC) compared with original sized engines. So I would expect his vacuum to be higher.

Plasticman

Matt Gruber

Doug
have u done any more tuning or r u happy with the way it runs now?

1Sweet66

Good timing on this question ( sorry for the pun ) but it does bring up a question that I have about my bone stock 66/ 327/4sp. I can idle all day in 80* weather and the engine barely gets above 190 or so but running down the road at 60mph on that same day I see the temp rise into the 210-215 (sometimes higher) range. When I pull off the highway and get back to stop and go traffic it comes back down.
Any ideas? Now keep it simple, I looked for "ignition timing for dummies" at the book store and they were sold out.
I'm thinking it's a timing issue, not a rad since it's at highway speeds. Timing at idle is set at factory specs and yes I checked it with the vac line disconnected and plugged. What should I look for when I get back into it this spring? Rad is newer and cleaned out inside, seems like good air flow, new hoses with spring in lower hose, 50/50 mix, no ping in the motor, never overheated.

67L36Driver

Yup, you have more air flow thru the rad at highway speed, but you also have more load on the engine.

If possible, plug in a direct reading temperature guage (the kind with a bourbon tube) and drive it while watching the temp. I'd suspect the inst. cluster guage accuracy.

1Sweet66

Thanks, sounds good since it looks like the guages are original and the sending unit might as well be too.
Do you, or anyone know if there is a way of checking the sending unit with an OHM meter? Hot/cold values?

JohnZ

Those are classic symptoms of "not enough radiator". Only two things really matter for cooling - the radiator's heat rejection capability, and the airflow through it (assuming components are OK and the timing map is set up properly).

Since airflow isn't an issue at highway speed, that leaves the radiator; exactly what type of radiator do you have? Original aluminum? Replacement copper/brass?

AZDoug

Only tuning has been in the cruise RPM areas, for A/F ratios and a little timing tweaking here and there for fuel economy. Shooting for 15.5 in cruise, 12.6 is dialed in for acceleration.

Won't make it to the dyno for some more WOT timing experimentation for 3-4 weeks, I want to try bumpimg teh WOT up 1, then 2 degrees as we pulled timing out in 3 degree increments to get best TQ curve last time, also try more advance over 5000 RPM.

Doug

1Sweet66

Hey JohnZ, always respected your expertise on Corvettes. Stepped out of the office for a bit.
It's a Harrison aluminum stock type rad. When I stated that it was "cleaned" out I didn't take it to a shop and have it boiled, I just flushed it out really good with the garden hose last year when I had it out of the car. Maybe there a restriction inside and I just don't know it.

bweaver999

Check out the FAQ above, I added a lot of timing discussion threads in the overheating area. Look at JohnZ's timing 101 and Lar's tuning info.

If the rad is "newer" it could be the vacuum or mechanical advance. If you have a dial back timing light, you should be able to see what it does with everything connected and increase the RPM.