Hey Folks, I drive it 120 miles round trip to the track, timing locked at 30 degrees. Many engine builders are wary of it, but Ive read (Richard Ehrenberg) that not having it washes down the rings, and I suspect it's why my exhaust stinks so much while driving and after. I don't yet know who manufactures one that will clear an Indy Legend head, but I'd like to know what experience is out there. (fuel injection isnt in the budget right now, multiple projects lol)
Thanks, scott

I went a few rounds with Eberg on this topic at Carlisle years ago and we kind of agreed that for a pure stock engine sure it's a good idea. But for most engines with modifications like a cam and moderate compression you need to just customize a simple distributor with mechanical advance will do the job.
Gus

Well you must have just worn Eberg out to have him concede on this topic because unless I'm mistaken, he's always touted the benefits vacuum advance would have on NASCAR and other race engines.

I run vacuum advance on my 10sec street car. Works well, gets great mileage, and the plugs always look great. Just requires the distributor to be set up correctly and you need a adjustable vacuum advance to limit the total.

hi
why would you not use vac adv on a steet car ?

you are just wastin gas on any cruse time .

only people that i know that dont use it dont undestand it

Every street engine will benefit from properly set up vacuum advance using non-ported manifold vacuum. Smoother, cleaner idle, better mileage, cooler running, longer life. You can plug it for racing. Drag race only engines should not have it.
I'm not saying nobody gets their car to run good on the street without vacuum advance, but you can make it run better and more efficiently at part throttle with it. Most of the time on the street is part throttle.

Have to completely agree with that statement...no reason NOT to use a vac advance, it has a number of benefits and literally is one of those freebies that we can take advantage of.

At part throttle, the air/fuel mixture in the combustion chamber is very sparse, so it burns much slower than the piston travels away from it. For efficient use of the fuel to make power, the maximum flame spread and cylinder pressure needs to occur near the beginning of the downward movement of the piston on the power stroke when it has the most mechanical advantage and before the exhaust valve starts to open. With the slow burning part throttle charge, it needs a bigger head start to be maxed out in the sweet spot. Vacuum advance gives it the head start it needs, so the piston is not past 90 degrees atdc when the flame front is at its peak. Ever try to pedal hard on a bike when the pedal is past 90 degrees? And the exhaust valve on a performance mill will start opening +/- 70 degrees before bottom dead center. Lack of advance will send part of that still expanding charge out the exhaust.

I've tried several different pump gas street motors both ways and really didn't see a performance gain with using the vacuum advance
I don't track the MPG either or drive them more than 50 miles in a single day usually
I do work on my carbs and distributors to get the mechanical advance and AFR the way I want them also
I like to see between 14 to 18 BTDC at idle on the timing and between 34 and 36 degrees BTDC above 2000 RPM, 12.8 to 13.5 AFR at WOT and between 14.2 to 15.3 AFR at light part throttle cruise with the heat range spark plug that motor wants to stay clean at idle and at WOT

hemi Joe has this correct, when setting up a street/strip engine always run some form of advance. On the dyno, with a part/ light throttle engine load, as soon as you start adding advance you will see the BSFC number start dropping like a rock. (BSFC measures how efficient the engine is. A lower number is more efficient)

How much to add, that is going to be trial and error. I wouldn't worry about "washing" out your rings or cylinder walls, you are under light load and the fuel is still being burned, just going out the exhaust.

Joe

I am working on a 440 six pack with with the Promax metering block. It does not have the fitting in the metering block for the normal vacuum line for the distributor. Now that port on a stock metering block is ported, correct? Meaning it does not get vacuum at idle but at throttle, correct? So, I should run the vacuum line straight to manifold vacuum, right. I read on one of the Corvette forums and they were saying the first Tri powers had the vacuum advance straight to the manifold vacuum.

Thanks Everybody, I appreciate all the input. Now I have to find one that will fit, at least it's safe to call Indy these days lol

yes, manifold vacuum is what you want. I have seen it both ways, but my opinion is off manifold would be best.

the problem with a vacuum advance on a performance street/strip car is that people don't understand them and don't know how to tune one. typically it seems to be human nature to dislike what they don't understand. i use vacuum advances on both my cars and would never take them off; unless i was using the car in some sort of competition where the advance wouldn't work anyway. i made learning to tune for a vacuum advance a priority.

Absolutely, positively for damn sure you NEED vacuum advance.

You also need to run a curve. Not sure why you only have 30 total, because that seems really low. Plus if you have ANY sort of ignition box, the timing WILL retard with RPM, so you actually will have LESS than 30 total with the distributor locked out and RPM retard.

So get a curve in it and add vacuum advance.

When you run the timing that far retarded you get a TON of heat way too late in the power stroke. On blow down, when the exhaust valve opens you still have combustion happening and all that heat goes into the exhaust valves, the exhaust port and out the exhaust.

That’s not only a power killer it’s a parts killer. So yeah, you are wasting fuel and power and I’d bet everything I have, everything you have and a bunch of stuff neither of us have that your exhaust gas temperatures are through the roof.

I used to see it all the time when I was tuning at the track and guys were logging EGT’s. The EGT would be where they thought it should be, or some other hero told them the EGT was too high, so they start adding fuel (usually wrong) and pulling timing (usually wrong) and the EGT would keep on climbing.

Then when you tell them they are doing it backwards, all the guys who’ve never looked at a data log ever tell them why they need to go to a colder plug ([censored]??????) and get the timing down some more.

Get some timing in it. And get a curve. And vacuum advance.

If there was a like button I’d hit that thing like a lab rat hitting the button for more meth until it killed me!!!!

To NOT need or benefit from vac advance, someone would need to be able to explain what they did to the engine to no longer require it, if we all agree that the OEM application of it is correct and useful.

Possible things people might say are modern shape combustion chambers, quench pistons, computerized engine controls....essentially trying to tell Hemi_Joel that the control he explained that is needed because of inefficiencies in the engine are combated in some new way.

I'm not saying that at all, just laying out the argument someone would need to make because what he said is still true with a big cam and more compression...

Adding, let alone tuning vacuum advance is just about the same as choosing and tuning a carb for most guys. It is just over their head.

For a car that cruises at light load, steady throttle why wouldn't you want vacuum advance? What could possibly be the down side? I agree that on some combos the advantages could be modest but there would still be advantages. It would seem that the biggest hurdle to widespread use of vacuum advance would be the misunderstanding of it's benefits and the lack of ability to tune it.

I use a hand held vacuum pump to test and tune my vacuum advance. You can hear the difference in the exhaust note as you add advance at cruise. I find that finding a good amount of advance is not difficult at all. The hardest part is making the necessary changes to the advance stop.

It would be nice if there were some more definitive data on this. Maybe an episode of Engine Masters?

For engines close to stock tune (any size, but mild cam) full vacuum to the distributor works well.
As the cam approaches 240 degrees @ .050", full vacuum works at idle, but as you open the throttle vacuum quickly dies, which reduces RPM, etc.
Ported fixes this, since idle is based solely on initial advance it's completely stable until a later throttle position.
You can even fab a new port position by inserting a small tube right through the casting to a point (generally slightly higher to delay vacuum) than the original, which will not add vacuum until slightly higher throttle disc angle. Hint: doesn't need to be horizontal.
Try both, keep the one that works best, cap the other (don't blank it), it may help the next time you change your tune-up.

Yes to the first question and a big NO to the second! The distributor vac advance needs ported NOT manifold vacuum. Manifold vac is high when the throttle blades are closed giving max advance until you open them up then it drops to nothing. This is backwards of the way it's supposed to work. Ported vac is higher during open throttle and nothing when it's closed. I can't comment on what GM did but this is the way Chrysler works. And to make it short the advantage in a street motor is it will apply vac advance before the rpm's get high enough to apply the mechanical advance. For a strip motor it doesn't make sense because as soon as the light turns it's full throttle and the rpms are up where the mechanical advance kicks in.