Corellian Corvette

I'm trying to get my engine back to original and running smoothly. Replaced some gaskets, tightened some fittings, q-jet tune-up, replaced some hoses and slowly the engine idle and performance is getting better. Timing is at 18 initial, all in (36) around 2500 - 3000 RPM.

Now I'm looking at my Vacumm Advance situation. Right now there appears to be some adjustable V/A on, B31 is a number I don't identify in Lars' tuning guides - I think it may be a crane unit.

I haven't checked how much advance it's adding, but i'm probably going to pitch it.

My engine doesn't have great vacuum at idle - only about 10deg @ 650 RPM. I don't know why. I don't believe it's any sort of vacuum leak - it's either a tired engine, or some sort of cam.

I haven't checked while on the road but idle testing shows it get up to about 15 - 17. Let's assume that's all it pulls.

I think the correct unit for my '73 is a B26 which is an 8deg all in around 11-13Hg. That appears to be a med to fast ramp.

However, I'm thinking I would like to run full manifold vacuum, to get the advance up at idle and smooth it out a little. Therefore, I would want to run something that's all-in under 10Hg - the most aggressive can is the B28, an 8 deg unit that's all-in around 8Hg.

What's the better overall choice, and what's the pro's/con's of advancing that fast?

Should I choose the factory ramp for my system and run ported vaccum? Or run full advance for better idle?

I drive on roads w/ traffic pretty regularly. I do have some hills, etc. and I don't really race or drag the car.

Thanks!

Peterbuilt

Hi CC, Lars is the authority on this but I've learned a lot from him. My 74 is a stock 350 and I use it as a daily driver. My vette produces 20" of vacuum at idle and I have a stock distributor. At my last tune up on 01-15-08 I set the initial timing at 8* btdc and with the vacuum can connected I got 24* at idle. The centrifugal gives me an additional 24* at 3000 RPM so I get 48* at cruise.
I believe our cars origionally used ported vacuum for better emissions, I use manifold vacuum. It seems that there are many combinations of vacuum cans and centrifugal weights that can be used to get the desired and best results. I'm sure that Lars will reply and get you (and maybe me) the best combination of parts to use. PG.

BarryK

10"Hg of vacuum at idle IS on the low side. First thing I'd do to check everywhere to make sure you don't have a vacuum leak anywhere although if you have a big cam in there it is possible vacuum is that low but better check just to be safe before anything else.

Step two would be make sure your idle fuel mixture is correct because if that needs adjusting it can affect your idle vacuum.
Connect a vacuum gauge up to a full manifold vacuum source and adjust idle fuel mixture screw(s) to achieve the highest steady vacuum reading.

Once these two steps are done reconnect vacuum gauge to a full manifold vacuum source and read vacuum at idle.
For simplicity, lets say your vacuum at idle really is at the 10"Hg you stated. The rule of thumb is that you want to use a Vacuum Advance Cannister (VAC) that is pegged out (advance is all in) at a point 2"Hg BELOW what your vacuum level is so in this case you want a VAC that has the advance all in by 8"Hg which is the VC1810 (B28) VAC as you have already determined. This is assuming you have a points distributor. If you have an HEI unit it's a different unit (VC1862 or AR31).

Keep in mind that the amount of advance on VAC units is expressed in distributor degrees which is HALF of crank degrees so the 8º of advance the unit lists it adds to the timing is really 16º as measured at the crank on your timing light.

As already mentioned, ported vacuum was used SOLELY for emissions reasons so for the best idle characteristics, performance, fuel economy, and lower operating temps you do want to use the VAC connected to full manifold vacuum.

If you do the first two steps and find that your vacuum at idle does increase, simply remember the rule of thumb that states that you want to use a VAC that is all in 2"Hg below what your idle vacuum level is.

TopGunn

Just for info..... the Crane unit does not adjust the amount of vacuum, it allows for the adjustment of the rate at which the vacuum gets applied to the distributer.