[64speed]

What are the pros and cons of iit? should it be run on a street car? My builder says no but he is a hardcore racer crew chief and is always looking for max performance

[b-man]

A street engine should have and will benefit from a vacuum advance.

High performance engines aren’t always running at wide open throttle, when you’re cruising down the highway with the throttle barely open vacuum advance kicks in to give you better gas mileage (yes that matters) and cooler engine temps.

It’s nice that your builder put together a sound long block, but clearly he’s more into racing than street driving.

Run the vacuum distributor and move forward. Get it tuned for the type of driving you plan to do, your builder’s job is finished and now it’s your turn to get it exactly the way you need it to be.

[Gach]

Here’s thing about vacuum advance you want to make sure it doesn’t keep Advancing, when they say all in by 2500 rpm’s that’s your total timing. So in other words if you bring it up to say 3500 rpm’s and it keeps advancing say up to 50 degrees then your going to have over heating issues and possibly detonation issues. I’m sure he’s checked that out for you, but won’t hurt to ask.

[64speed]

He told me it was where it needed to be but again he’s a racer.and I don’t want to sacrifice some mileage for absolute top end cause that’s not how the car is being used.

[b-man]

Originally Posted by 64speed

He told me it was where it needed to be but again he’s a racer.and I don’t want to sacrifice some mileage for absolute top end cause that’s not how the car is being used.

Exactly.

[70GS455]

You know when you're at WOT the vac advance goes away, leaving you with just mechanical advance. So in that case, it's just like not having it

Sent from my SM-S901U using Tapatalk

[Kenth]

Originally Posted by 64speed

What are the pros and cons of iit? should it be run on a street car? My builder says no but he is a hardcore racer crew chief and is always looking for max performance

Here is the reason why hundreds of millions of cars leave the factory with an ignition vacuum advance (and why you should not listen to some "racer" who don´t understand the concept for street usage):

VACUUM SPARK ADVANCE

Along with the mechanical spark advance, another method is used to control ignition spark under load. This is commonly called the vacuum operated spark advance unit. The vacuum advance unit supplements the mechanical advance, however, it operates differently as it varies spark timing in relation to engine load. Engine load can best be determined by manifold vacuum so consequently, the vacuum advance unit mounted on the distributor is connected directly to engine manifold vacuum.
Although spark timing is not a direct function of the carburetor, it is used on some applications to control the vacuum operated advance unit on the distributor during the idle and off-idle range. On these units a calibrated port is located in the throttle bore just above the throttle valves. It is connected by a vacuum line directly to the distributor vacuum advance unit.
In the curb idle position the throttle valve is below the spark port so consequently, no vacuum is applied to the advance unit and the spark advance remains in the retarded position. As the throttle valve is opened and engine speed increases the throttle valve moves past the spark port so that manifold vacuum below the throttle valve is applied directly to the vacuum advance unit. Timed spark advance is used on applications where engine design demands retarded spark for smooth idle and performance improvements at low speeds.
On other units the vacuum advance line may be connected directly to the manifold or carburetor bore below the throttle valve. In this case the engine will idle with full spark advance. The full advance spark at idle is used on engine applications where it is not detrimental to engine idle, and improves engine cooling. When setting ignition timing, the vacuum advance line should always be removed, especially on engines using full spark advance at idle as initial ignition timing will be set retarded if this is not done. The end result will be very poor fuel economy. To be sure, always disconnect the vacuum advance line to the manifold or carburetor when setting initial engine spark timing.

[Formulajones]

Absolutely run a vacuum advance. I even use them on more race type engines that are street driven.

You just need to find someone knowledgeable to set it up properly for you. That's rare these days but well worth the effort for better all around drivability and fuel mileage.

Worth mentioning again since this seems to be a highly misunderstood part of ignition timing, the vacuum advance is inop under wide open throttle conditions and has no affect on WOT timing events.

[1funride]

Lean mixture at idle or part throttle conditions burns slower and needs more time for the flame front to travel, as such increased timing is necessary.

[Gach]

I am assuming his engine guy not because he’s a racer, but because he knows his customer well, and that leads back to his turning ability. In other words if a vacuum advance it isn’t working properly could course more issues then it’s worth. The other question is, is it hook up to ported vacuum or manifold vacuum. Should he run and adjustable vacuum advance. If so who’s going to Dial that all in. The argument has always been you’ll get better gas mileage if everything is working properly. But someone with little experience could end up chasing his tail only to find out The vacuum advance isn’t working properly.

The real question here is exactly how much gas mileage is going to improve by running vacuum advance. I’d like to hear some answers to that. How much you going to pick up, vs what your going to loose. Is it worth telling him to go back to his guy and demand that he sets it up for vacuum advance.

[Formulajones]

I see substantial gains with a proper working vacuum advance, usually in the 3-4 mpg catagory, sometimes more.

All my classic stuff here gets upper teens for mpg without an overdrive, and low 20's for mpg with an overdrive. Basically better than most of your SUV's running around today, and most of that is attributed to a proper timing curve with a functioning vacuum advance. Some of it is carb tuning as well and a total engine package that is happy.

Like I said earlier, worth while to get a vacuum advance curve working and dialed in, but finding the person that knows how to do it is the challenge. Each car will be a little different in what it wants and someone has to be willing to spend the time with it.

There are other benefits other than mpg though. Overall drivability is improved. You'll find that carb tuning comes much easier, especially in the transition areas, and in some cases you can lean the carb a bit with the added vacuum advance timing. In fact most guys blaming the carb for drivability issues that I've been involved with, I find it's actually the timing curve causing the issues and the carb was fine, other than they now have it so far out of wack chasing their tail. I also find that the engine temps stay a bit cooler as well with the added vacuum advance timing.

[mgarblik]

I like vacuum and mechanical advance both on a street engine. The primary benefit in my experience is sharper light throttle response and better gas mileage, like 10-15%. As mentioned, at heavy throttle, less than 4-6" typically, vacuum advance goes away so your actual timing is initial + mechanical. I like to limit vacuum advance to 10-12" maximum and do this with braze closing the slot to create a stop. Of course maximum total advance would be higher engine speed, and low load like going down a hill. In this max situation, I don't like over 48-50 degrees. It would be very rare to see this in real operation. Typically, properly tuned, you would be at 40-45 degrees, light throttle. Tons of variables based on engine mechanical package, every engine is different. Taking vacuum advance out of the equation is simpler for your engine builder and safer. However you are giving up some driveability and gas mileage.

[Gach]

I’m not disagreeing with you, but we know what’s involved in getting it correct. If he’s willing to pay the dollars because it’s time consuming. The Q’jets had it coming off base of carburetor in back, they worked great ! I see he’s running a Holley carburetor. To be honest I’ve never seen that much of a gain in miles per gallon. But I’m not doubting it’s possible on a well tune Carburetor and vacuum advance.

[b-man]

I gained 1.7 mpg (from 8 mpg to 9.7 mpg) on my low-compression 8.6:1 455 that had a fairly large cam (HC-03 244/252 @ .050). I went from a locked factory points distributor (set at 38 degrees, high altitude track) that I used for bracket racing to a reworked factory points distributor with both mechanical and vacuum advance.

So that’s about a 21% increase from a simple distributor swap, well worth it. I was using manifold vacuum not ported vacuum off the 1977 800 Quadrajet carb built by Cliff Ruggles.

Link to thread regarding the distributor swap:

https://forums.maxperformanceinc.com...d.php?t=554067

My engine was fine for brackets but hardly a great combo for street in the sense of efficiency. Nevertheless I was able to realize a substantial gain in mpg and driveability by simply changing the distributor, I’m not any great tuner but I can handle a timing light.

[Gach]

HC 03 that was a good cam, that’s good amount of pick up mpg wise.

[OG68]

When I bought my 70 El Camino SS 396 it was getting 8.5 mpg on the highway. Did a basic tune-up and found the 90's era MSD distributor plastic advance plate was broken to the canister rod. Installed a factory distributor with a Pertronics III and set the vacuum advance curve to a max 49*, from a base 14*. All in by 3000 RPM which coincidently was at crusing (72) speed. Mild cam, Q-Jet engine ran so well on 91 octane I didn't want to mess with it any further. End result averaged 13 mpg

[dataway]

Nice thing about installing vacuum advance is .... if you want to disable it for some reason, pull off a hose and put a cap on the port ... done, no longer have vacuum advance.

Kind of like the whole ... better to have a gun and not need it, than need it and not have it.

[BILL BOWMAN1]

Since we are on the subject. I always thought I got lousy mileage.
65 2+2
421 + 60 288 HR 670 race port head
Qjet, HEI with vacuum adv.
4 spd 3:73 27” tire
Just putting around town and back roads I never seen above 9 mpg.
Had several carb setups, all about same mpg.

[mgarblik]

That gas mileage is pretty bad for sure. I had a 66 2+2 421 HO that was basically stock with a 4-speed and 3.73:1 gear. The tri-power was very economical given the vehicle. I got 12-12.5 in city driving and 14-15 MPG straight highway at 2700-2800 RPM, highway speeds. A properly tuned Q-jet should get equal to a tiny bit better MPG. My guess is primary jetting/metering rods/power piston spring are not perfectly happy. Engines love to run a little rich, but mileage suffers. Possibly you could consult with Cliff Ruggles and get a better suited primary mixture. Another alternative would be to weld on two oxygen sensor bungs to the head pipes and monitor the Air/Fuel ratio while driving. This way you could really dial in the ratio for better mileage without guessing.

[BILL BOWMAN1]

That sounds like a great idea. I’ll pick up a unit a jegs and tinker with it. Thanks.