Hi c_creations, All,


Different combos have different needs, and different owners have different knowledge levels.



A racecar is driven at WOT and under those conditions there is no vacuum, so the canister is nothing but excess weight.

Regards, Norm

Maybe im wrong, but doesnt the amount of vacuum an engine pulls inrease as the engine speed increases?

Andrew

Spending too much time on ROP?

Manifold vacuum, no.
Ported vacuum, only to a point. If it were true, centrifugal advance wouldn't be needed.

I'll write an explanation, when I have a couple of hours (or more) to compose it.

Problem is, I can't write "white papers". If someone could post something about the basics of centrifugal and vacuum advance systems, it would give me something to work with.

Regards, Norm

without that vacuum my engine dogs when you press on the accelerator, or this is what they shop guy told me. if that vacuum isnt there, how would you fix that? advance the dist to counter it? then change the idle on the carb to counter the higher rpm?

If I paid 150 bucks for him to tell me what to do, I would do what he said.

This was written about VWs. As far as it goes, it's accurate and applies to this subject.

Regards, Norm

I guess so. I will research this topic more tonite and post my findings so we can discuss.

Andrew

This morning I remembered the Holley "vacuum secondary" carb.

It would seem to contradict my statement, but, in fact, only adds to the confusion.

Regards, Norm

Update- I havent had time to look up stuff as of now. I will try again tonite.

Andrew

Hi Norm, all,

Ok it took some thinking, but I think that you are right.

In a gasoline engine, the intake stroke produces a partial vacuum in the air intake, right? So the function of the throttle butterfly is to restrict air going into the engine, thus producing a deeper vacuum at idle due to the restriction. Similarly, if you put your had over a vacuum cleaner, it sucks harder. At idle the butterflys are closed, so they make the engine suck more to get fuel. That is why engines with little to no compression won't idle, and why they will get hot and die. The vacuum secondaries on a carb will work because you will have the primaries all the way open and the vacuum that is still there from half of the intake being closed off pulls the secondary open. That is also why you can get different diaphragms for the secondaries so that you can account for differencers in cams. (bigger cam=less vacuum) So therefore, as engine speed increases, manifold vacuum will decrease. I also remember that in the 50's some auto manufacturers used vacuum operated winshield wipers that would quit working at full throttle because of the lack of vacuum. My Grandpa just reminded me of that. So hopefully that clears up the confusion.

Andrew

Hi andrewk, All,

Thanks for giving me something to work off. I have it in my word processor, and will work on it as I get time.

Your discription of manifold vacuum is mostly correct.

Ported vacuum is from a different source and is different than manifold vacuum.

Third source is Holley only and is for its own vac secondary, which is not as simple as the Q jet, AFB, and Thermo quad.


They work fine in both my Coupes. In most ways they are better than electric wipers.

I’ll elaborate when I can.

Regards, Norm

Ok thanks, that answered my question.

Hi andrewk, All

I just caught your typo. I knew the concept and didn't look at the numbers til tonight. Caught one of my own also.


Longer than we realized.

#5 and #7 are not companion cylinders. Should be 4-7 swap.

Since they are together physically as well as in the firing order, the theory is: After #5 is filled, there is not enough mixture left to properly fill #7.

The fix is: To change the cam to swap the firing strokes of #4 and #7 to separate #5 and #7.

Trouble is: The new firing order is 18736542 and the imaginary problem has been shifted to #4 and #2.

Or you could swap #5 and #8, and fire #8 right after #6. Same results.

A professional engine builder that knows about the swap, would also know it doesn't work as intended. A rebuilder wouldn't understand it, so wouldn't do it. A cam grinder would advise against doing it.

I always change wires one at a time. On a new assembly I usually check the order three or more times and I still have been known to F**k up. If it doesn't start on the first revolution, I know what I did wrong.

I edited my typo.

Regards, Norm

I also just realized that... OOPS! This is a good discussion. I look forward to your vacuum explanation. Sorry for the confusion, just goes to show that double checking your info in a post should be done. I always forget to proofread... Silly kids anyway:lol:

Hi andrewk, All,

Found a good reference site to speed things up.
http://www.innerauto.com/
Material is very basic, generic, and mostly pre ’73, but it provides a foundation for the good stuff.
And so far, it seems to be accurate.


Yes.


Yes.
http://www.innerauto.com/Automotive_Def ... rtance_Of/


I don’t believe it sucks harder, but the statement is a good illustration.


Closed butterflies are at idle. More fuel = more power, RPM, or both.
http://www.innerauto.com/Automotive_Def ... s/Venturi/


I don’t understand this one. You can’t have combustion without compression. Compression = heat.


With only the primaries open there might still be some manifold vacuum, but how do all 4 barrels stay open with no vacuum.
Never thought about it until yesterday morning.


You have the right idea, except springs are changed, not diaphragms. Lighter springs to open the secondaries quicker for lighter cars, high stall, and/or lower gears. Cam is not as much of a factor.


It’s a combo of butterfly opening, engine speed and load on the engine. A light throttle at higher engine speeds will show high vacuum and the most fuel efficiency. Full throttle at low speeds will show no vacuum.


http://www.innerauto.com/Automotive_Def ... cuum_Pump/
As used on all my 394" and earlier Iron Olds engines.


Not even a start. Ported vacuum is next. Then the relationship between vacuum and centrifugal advance, and the economizer valve, and how the combination can be tweaked for more power and/or fuel ecomomy.

For now I'll be spending a few days at http://www.innerauto.com/ to see what I can learn.

Regards, Norm

That sounds like a good idea, Me too!