Sorry to burst you bubble, but vaccum advance was engineered and added to automotive engines in the dark ages right after WWII to use ported vacuum. In fact the lines used where steel and used fittngs instead of hoses like today. It's the way the engines came from the manufactures with it. I can not think of one that came with manifold vacuum to vacuum advance. So how can you claim to like vacuum advance when you don't like how it was designed to work, how can you like it? FYI, with the ported vacuum port in the carb throttle bore, if it is in the base plate where the throttle plates are (it should be), then the air velocity will have dropped from the high speed thru the narrow venturi and it will will have at the same velocity and vacuum as the manifold when the throttles plates are open. I checked one of my 1850 Holleys and guess what? The ported vacuum port is in the base plate not the carb body. This has been gone thru many times on this forum and a manifold connection to the vacuum advance is a crutch for a cam to large or a maladjusted carb. If what you advocate was true, don't you think the manufactures would use it. True performance engines use no vacum advance, mechanical advance only.

 

Bear, the first vacuum control dizzy that I recall was the late 40s-early50s flathead V8, it had a vacuum BRAKE to retard against what the centrifugal was doing, and it hooked up to manifold vacuum. So you're wrong again. Yes, I remember the steel lines on vacuum advance, several of my mid 60s cars and trucks had them. there was a short rubber section so the dizzy's static time could be adjusted. The tune up guides of the time contained instructions on how to set the timing with a strobe light. First, one was supposed to plug the vacuum line so that the vacuum can wouldn't affect timing while you wer trying to set initial. Ported wouldn't have needed this precaution, so, once again, you're wrong. Just because you 'can't think of any' doesn't mean there weren't lots of them. Your poor memory doesn't cancel out history. Get out some of your tuneup guides and read the procedures. Have Bernoulli's principles been repealed and noone told me ? Somehow, I kind of doubt that. You haven't tried the test as I suggested, have you ? Afraid of what you might see ? And what is this nonsense about a 'true' performance engine not using vacuum advance ? Honestly, where are you getting these ideas ? I'm thinking of engines like a Boss 302, a Boss 351, a 429 thunderjet, a 390 GT or a 428CJ, all performance engines, I would consider, and all came with vacuum advance. The only two applications I can imagine where vacuum advance is not helpful would be a stationary engine running a pump or a dragrace only engine. EVERYone else should use it, performance or not. DinosaurFan

 

Gee, I know ported vacuum shouldn't NEED to be disconnected and plugged when initial timing is set, BUT every set of instructions I've ever seen (and that's a bunch over the years) for setting initial timing says to disconnect and plug the vacuum line. So seems your pipe dream theory once again doesn't hold water with the facts. Never seen a vacuum advance unit on a NASCAR motor or any roundy-round car motors I've ever worked on, not counting the drag motors or boat motors. But I also worked on hundreds of factory/street motors and I never saw one without a vacuum unit connected to the ported vacuum. I think your imagination needs some work.

Oh yeah, your performance engine list (it's actually a street tuned performance engine list) doesn't include any real performance engines. Try the 406, the 427s or the SOHC427. Those were real race motors, not streetized race motors.

 

How many times will we have this same, stupid argument?

 

I tried to stop it early, but he just kept pushing his theory of altered reality.

 

Kind of makes you wonder, doesn't it?

I've made clear my position in the past, which was based on sound physics. For me and my motor, it seems to work great. Others dont see it that way, and that's fine. I'll run what I want, they can run what they want. Whatever floats your boat!

I'm done arguing about it.

 

I enjoy technical topics such as this one, and hope the discussion continues.

If I might add my two cents, my understanding is that vacuum advance is strictly for fuel economy, and an engine runs very well without it but gets poorer mileage.

One thing I don't think has been mentioned, is that timing advance isn't needed at idle nor during initial accelleration from a stop. This is why ported vacuum is used, because it can be easily controlled by the throttle plate movement. As the vehicle starts moving and more timing is needed, then the mechanical advance comes in, and timing is also increased by the vacuum advance under light engine loads, up to a point of about 2500 rpm or until the distributor timing movement is maxed out.

The reason for plugging the vacuum advance when setting initial timing is because of the relationship between the idle speed and timing advance. If the curb idle is set too high, the carb vacuum port will be uncovered and give increased vacuum timing, which is not wanted when setting curb idle. Plugging the vacuum line just decreases the chance of error.

Some late model (cough) Chevy engines use manifold vacuum for timing advance, but they have a solenoid to close the vacuum at idle. This is a much more complicated system, IMO, than using ported vacuum.

In my earlier post, I'm using manifold vacuum timing until after warmup, then ported vacuum takes over (through the PVS). The manifold vacuum gives a faster curb idle during warm-up and driveability is loads better, at least in my case. This is the way Ford designed it on the 78's, although most people have long ago re-routed the vacuum line direct from the carb to the distributor.

Regards,
TheLoneRanger

 

Will a motor running a Comp 292H would it have good vacuum? What should the vacuum reading be? And were would you check it at? The ported side or the full side?

 

I'm no expert, believe me, but ...

My cam was a 290/290 advertised duration, so I'm assuming it's somewhat like the Comp 292H.

It was hard to get my 390 to generate more than 11 inches of vacuum at 900RPM. My cam was 292/292 adv, 230/230@.050, and .554/.554" of lift. It was degreed in, and then, if I recall correctly, advanced 4 degrees for low-end torque. (I really should go out and find out, it's on a stand in my garage). The entire cam card is in one of my galleries.

At 900-1000RPM, with 20 degrees advance at idle (yeah, 20 degrees), it idled pretty decent but lopey (with an MSD 6al) and generated 11 inches of vacuum.

How I got that 20 degrees advance at idle, and used a stock starter to turn it over with, I won't go into It generates too much controversy.

 

The 292H is 292 adv, 244/244@.50 and .560/.560 lift. It was degreed in and was at 1 degree advance. I'm not 100% sure on advance yet, haven't got the timing light out yet. Hopefully I can do it this weekend. What should I expect at idle? 30 to 35 advance is what you want at 3000rpm right?

 

Remember a couple of replies ago I got hammered on advancing a cam 2 degrees for chain stretch and general wear by 10,000 miles and how can I reply 100's of miles away? Here again a cam was bumped 4 degrees advanced. At 4 degrees unless it's a fat cam the dynamic could well be above street gas zone as the BMEP will be increased more. Then again he didn't just install and run but built a motor with all components working together in harmony not just thrown together by what a speed shop idiot recommended. Mention FE and they give you that blank stupid look 98% of the time.
Cam advance is our friend with these heavy trucks unless you encounter detonation or building a Pro Stock truck. A dyno program is of great help these days.

 

I guess I am just a ding dong when it comes to all these different terms. BMEP, advanced or retarted and how to use these dyno programs. I should have the "NUT" name at the end of my name. I can put stuff together with my eyes closed, but when it come to the fancy stuff I'm lost.

 

I believe there is more to just assembling a motor, you must have all the correct parts be it internal or external add ons working in harmony helping each other from air filter to tailpipe tip. Rewards are a efficient high hp and tq motor that will not eat itself up. From what you said so far it sounds like you need to research a little deeper into motors, at 53 i'm still learning.

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Yes you are absolutly right!! I do need to do alot more studying. I am use to tearing a motor apart and fixing a problem then putting it back together. You know just a regular mechanic. Never been to a mechanic school, just learning as I go. That's the reason I LOVE THIS SITE!!! Many nice people willing to help a dumb $&@ like me. Most of the info on the motor I just built came from people like you and Rusty70f100. Both of you have been helping me since the begining. I'm only 25 so I am still a young pup with lots to learn. So thanks for all the help now and in the past.

 

We all started with zero mechanical knowledge at sometime, it's those that never progress even with many years are the ones that become an attorney or mayor. You'll catch on quick, the faster the less it costs in screw ups (don't ask).

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