VAC Ported to Manifold Vacuum Modification Results

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results



Update on ignition timing tune: Running 100 octane avgas (e10 only gas here in Mass), I went aggressive and took a stab, setting initial at 10 degrees for a total of 40 at WOT. Pleasantly surprised with no detonation and performance was excellent with noticeable low end improvement.

Next up was to bring in the centrifugal advance curve as soon as possible, on the ragged edge of detonation. Using the Mr. Gasket advance spring kit, I experimented with different springs / combos, eventually going with 2 silver springs. The car has never run so well.

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

The OE max centrifugal is 30 @ 3800. So with the two silver springs installed, at what RPM is it all in?

The aviation octane method yields similar numbers as the Motor method, and given the typical 8-10 sensitivity the RON would be about 108-110, and the PON 104-105, so assuming the CR is no higher than what it left Tonawanda at, it should tolerate a very aggressive centrifugal curve!

Duke

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, my son's off at college so no one here to check tach while I check timing. Guess I need one of those fancy timing lights with built-in tach.

On to fuel now. In your paper you mention Chevy set the cars up to run rich for sea level etc. I'm in eastern Mass which is basically sea level. Do you recommend I leave the jets / power valve stock or experiment going leaner? Thanks, Chris

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

There's no one around my place to help out either, but I can usually conscript a neighbor, especially the ones with the '75 Bronco and '77 L-48 Corvette that I help work on once in a while. Usually the helper can operate the throttle and record the data, and I'm still really interested in how your centrifugal is currently set up, so when you find a helper, dig up this thread and post the data.

Most Holley carbs have fixed power jets, and the power valve simply opens at a specific manifold vacuum level, typically 6.5" Hg, and usually the power mixture is fine. It often starts out overrich, less than 12:1, and leans out as revs increase. As long as it does not lean out beyond 13.5:1 at the redline you're good to go, and the best way to determine is some chassis dyno pulls to the redline with a wide band O2 sensor attached to an exhaust pipe.

As far as the cruise mixture goes, there are two types of Holleys. One has replaceable main jets and the other has a "metering plate" with fixed orifices. On the former its easy to alter the cruise mixture by replacing the jets, and IIRC each jet size changes the fuel flow by about 3-5 percent.

I'm not sure what type your center carb is, but if it has replaceable jets, it's easy to try one size smaller, then test. You keep going smaller until you pick up some lean surge or a lack of crisp response to small throttle changes, then go back to the next higher size. If the lean surge/throttle response issue is just barely noticeable, richening the idle mixture might eliminate it.

For the metering plate type Holley you have to either precision drill the metering orifices, or maybe Holley offers metering plates with different orifice sizes. Of course, once you drill an OE metering plate and it doesn't work out you can't go back.

I'll take a WAG and say that at sea level you have about a 50/50 chance of one size smaller jet providing satisfactory performance with the leaner cruise mixture, but it's a pretty easy thing to do, especially if the center carb has replaceable jets, and you like to experiment.

I recall seeing a table that lists the actual diameter of the Holley jet numbers (It's probably on the Holley Web site), and for small changes, fuel flow will change leaner/richer about the same as the decrease/increase in area size.

Duke

[Timothy B.]

Re: VAC Ported to Manifold Vacuum Modification Results

IMO, the best way to proceed if you want to experiment is by leaning the idle circuit (curb idle discharge holes and transfer slot), that's the circuit most active at cruise rpm's on the street. It's not as simple as changing jets and you may end up doing that also but it's a great learning experience (smaller IFR/larger IAB).

There are some pretty smart people here and on other forums and it will be like a disease if you get hooked on carburetor tuning.

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results

Ok Duke, my helper came home for the weekend and we determined that centrifugal is all in by 2500 rpm. On a recent 90 degree day I thought I detected some ever so slight detonation. Recent cruise on an 80 degree day I didn't detect any however. Chris

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

That sounds good. A "little" detonation in worst case conditions iidicates that the spark advance map if just about optimum.

You can always "drive around" a little transient detonation by using less throttle and more revs by shifting or cruising as required in a lower gear.

Duke