10" of Vacuum at 700 rpms.

Skeeter55 · Aug 22, 2008

OK. Is 10" of vacuum at idle at 700rpms to low with a 4-speed. Ive tried to get as much vacuum with the use of my vacuum gauge with my Idle fuel mixture screw adjustments... NO VACUUM LEAKS

My initial timing is at 16* and my total advance timing is 34*, it is a BBC 454 Destrocked to a 427 with a #280s Comp Cams Solid Flat Tapit lift of 604.....

It could be that at the time of installing my Pro Magnum Roller Rockers i adjusted the lash .004 tighter as it was recommended from Comp Cams....

This is a street car and i just recently noticed that the brake pedal gets stiff once in a while when stopping after the installation of the Pro Magnum Rockers Roller.

I'm thinking if i raise the idle to 800-900rpms the vacuum then reads 12-13" or can i get a 1-2* more initial timing.

No big deal but what ever you professionals can come up with would be well appreciated. Thank Skip :thumbsup:

Gary L · Aug 22, 2008

Seems like 10" is OK. That is all I get at 900-1000 RPM. My power brakes work fine. But I am no professional.

Skeeter55 · Aug 22, 2008

Gary L said:
Seems like 10" is OK. That is all I get at 900-1000 RPM. My power brakes work fine. But I am no professional.

Are you running a 4-speed. Yes i see the M-20 How big of a cam are you running in a BB or SB.... Im just asking becuase i dont want to run the vacuum can but i may have to turn the idle up 100rpms or grab 1* more on timing.

fatblock · Aug 22, 2008

Skip..you can add a vac can to your mix and still retain your low idle vac #,s.Easy to install and the one way check valve will trap decel hi vac in the vac can and assist your booster at low engine speed.I would not jack up the idle speed with the 4 speed.This is a bandaid fix that uncovers the transfer slot and leads to engine run on/dieseling issues.

Skeeter55 · Aug 22, 2008

fatblock said:
Skip..you can add a vac can to your mix and still retain your low idle vac #,s.Easy to install and the one way check valve will trap decel hi vac in the vac can and assist your booster at low engine speed.I would not jack up the idle speed with the 4 speed.This is a bandaid fix that uncovers the transfer slot and leads to engine run on/dieseling issues.

That makes good sense and I'm glad I'm getting this good info... I understand on what you are saying about it being a bandaid, and i really don't wont to do that. But what dose the can look like, i hear it stick's out like a sore thumb.

fatblock · Aug 22, 2008

Skip..the vac cans that I have installed are flat black.There is nothing stopping you from hiding the tomato can sized reservoir..say under a fender.Do it right and the vac hoses will just become a mystery for prying eyes..but you will know that it is functional and solves your hard brake pedal issue with your less than stock oem vac #,s at idle speed.
FWIW..I have manual brakes and 5 hg at idle in gear with an auto.

Skeeter55 · Aug 22, 2008

I don't need the vacuum for the dizz as i have a mechanical advance dizz, i just need it for the brakes but possible tomorrow I'm gonna check my car settings one more time to squeeze 1"...

Skeeter55 · Aug 22, 2008

fatblock said:
Skip..the vac cans that I have installed are flat black.There is nothing stopping you from hiding the tomato can sized reservoir..say under a fender.Do it right and the vac hoses will just become a mystery for prying eyes..but you will know that it is functional and solves your hard brake pedal issue with your less than stock oem vac #,s at idle speed.
FWIW..I have manual brakes and 5 hg at idle in gear with an auto.

That sounds like the ticket... I'm going to look in to it more tomorrow. Thank you very much and i will keep you posted :thumbsup:.

Skeeter55 · Aug 22, 2008

Well I'm going to give it another shot today, with me trying to squeeze 1-2" more of vacuum

.. I'm also going to look into the vacuum can to see where i can hide it. I would feel safer with more vacuum do to the fact that my wife drives the car every once in a while and believe me she needs brakes :yes:.

If you guys can come up with anything else please let me know.
Skip :beers:

JimM · Aug 22, 2008

Skeeter55 said:
I don't need the vacuum for the dizz as i have a mechanical advance dizz,..

That would be the fix right there. If you had a vacuum advance cam putting in 10 more degrees at idle, you'd get at least 3 more inches of vacuum.

Alternatively, you could determine your "max real initial timing" as in how much advance you can run and still reliably crank the motor hot, dial that in, and then recurve your mechanical advance to match.

Are we really talking about the cause of the problem though?

Under what conditions are you running out of booster?
I'd think that if you were "using the car hard" that you'd also be using some downshifting and engine braking, and these would keep the vacuum up, way up, and keep the booster working.

If you're idling, you're sitting still... and you need neither brakes nor a power booster.

Skeeter55 · Aug 22, 2008

JimM said:
That would be the fix right there. If you had a vacuum advance cam putting in 10 more degrees at idle, you'd get at least 3 more inches of vacuum.

Alternatively, you could determine your "max real initial timing" as in how much advance you can run and still reliably crank the motor hot, dial that in, and then recurve your mechanical advance to match.

Are we really talking about the cause of the problem though?

Under what conditions are you running out of booster?
I'd think that if you were "using the car hard" that you'd also be using some downshifting and engine braking, and these would keep the vacuum up, way up, and keep the booster working.

If you're idling, you're sitting still... and you need neither brakes nor a power booster.

Thanks Jim... My dizz is set up and recurved with 16* at idle and total 34* the only thing i need vacuum for is the brake booster. I'm getting 10" at 700rpms do i need more, the car runs awesome and i don't want to spend the money on a new dizz, which i know would be a better dizz for street driving. But the car is very well tunned and after my install with the new Roll Rockers i adjusted the valve lash .004 tighter as Pro Comp recommended i could do. I was told by going tighter on the valve lash i could loose some vacuum...

So today I'm going to double check my idle screws again and timing, i had the carb off yesterday, i clean it and put a new bas gasket thinking i had a vacuum leak... No vacuum leak just a big cam.

What can you recommend i do now, but like i said i would like to keep my MSD mechanical dizz. Thank you Jim
Skip

Skeeter55 · Aug 22, 2008

Oh and my brake booster i notice yesterday coming to a stop the brake peddle got really hard as i was coming to a complete stop.

So would you recommend the vacuum cannister for this braking problem or look into something els.

Skeeter55 · Aug 22, 2008

Jim i sent you a PM.

JimM · Aug 22, 2008

got it Skip.

If you're running out of boost coming to a normal stop, then that needs to get fixed.

Easiest fix is a higher idle. 700 is kinda low with that cam anyway. 800 or 850 will get you more vacuum quick.

More initial will help too. If you have a degreed balancer you can experiment with that, see how much advance is practical at idle, and how much it helps. If it's a good thing, you recurve your dizzy. If it's not, no hamr no foul, just put it back.

I don't know that one of those lil softball size vacuum reserviors is gonna help. Those are for a/c systems, lil vacuum controlled flappers and stuff. The brake booster is allready a check valve vacuum can, 10 times the size of those lil balls.
The booster ALONE should hold enough vacuum power to fully depress the brakes 3 times.

Speaking of which, are you certain the check valve in your booster line is good?
It's the right angle thing where the hose connects to the booster. The more I think about this, the more it seems this check valve could be your only problem.

If you run the engine, rev it to pull some vacuum, and shut it off, you should be able to stomp and release the brakes 3 times, losing boost toward the bottom of the third stroke. If not, something is leaking, either the check valve or the booster itself.

Steptoe · Aug 22, 2008

Noticed this comment in your post

I'm thinking if i raise the idle to 800-900rpms the vacuum then reads 12-13" or can i get a 1-2* more initial timing.

u mean when u up the idle the timing increases?
If so it may meaqn the cent springs are too light but more likely at idle the vac canister is not all in ie the vac advance should be all in on manifiold vac about 1 to 2" below the idle...your may be all in at 12 to 14"

Skeeter55 · Aug 22, 2008

JimM said:
got it Skip.

If you're running out of boost coming to a normal stop, then that needs to get fixed.

Easiest fix is a higher idle. 700 is kinda low with that cam anyway. 800 or 850 will get you more vacuum quick.

More initial will help too. If you have a degreed balancer you can experiment with that, see how much advance is practical at idle, and how much it helps. If it's a good thing, you recurve your dizzy. If it's not, no hamr no foul, just put it back.

I don't know that one of those lil softball size vacuum reserviors is gonna help. Those are for a/c systems, lil vacuum controlled flappers and stuff. The brake booster is already a check valve vacuum can, 10 times the size of those Lil balls.
The booster ALONE should hold enough vacuum power to fully depress the brakes 3 times.

Speaking of which, are you certain the check valve in your booster line is good?
It's the right angle thing where the hose connects to the booster. The more I think about this, the more it seems this check valve could be your only problem.

If you run the engine, rev it to pull some vacuum, and shut it off, you should be able to stomp and release the brakes 3 times, losing boost toward the bottom of the third stroke. If not, something is leaking, either the check valve or the booster itself.

Jim i think this should be a sticky.... This is the best information for me and others to diagnose a low vacuum at boost.

I have to tell you, that i tried again to re-tune my carb but can only successfully get 10" of vacuum at 700rpms and still 16* timing at idle. So i went out most of the day to drive the car, the brakes seem to be better and they do work like you described above, but a couple of times yesterday as i was coming to a stop the peddle got really hard when it stopped. I get that i can turn the idle up to maybe 800rpms and i just may do that tomorrow.

I did take the check valve off the booster and you can hear the vacuum let loose out of the booster. So i know it was holding vacuum in the booster, but now that i read the above info I'm leaning toward the check valve may need to be replace it and i don't think my vacuum is all that important because i only need it for the brake booster.

Would this be acceptable for my car 10" of vacuum only for my brake booster.

Skeeter55 · Aug 22, 2008

Steptoe said:
Noticed this comment in your post

u mean when u up the idle the timing increases?
If so it may meaqn the cent springs are too light but more likely at idle the vac canister is not all in ie the vac advance should be all in on manifiold vac about 1 to 2" below the idle...your may be all in at 12 to 14"

Steptoe I'm going to try it just the way it is for now without the cannister. I'm going to check the check valve and possibly turn the idle up to 800rpms which will give me 12" of vacuum.

ace's68 · Aug 22, 2008

Interesting thread.
700rpm for a cam that only produces 10" of vacuum cam must be fairly large, 700rpm is purdy low for a larger cam...
I know people with big cams idle around 1,00-1,200rpm and still sound really lopey,
I have a friend who had a similar problem and the vac can killed his performance at the track, it would launch very well but then it would go to crap after the 60'.

Skeeter55 · Aug 22, 2008

ace's68 said:
Interesting thread.
700rpm for a cam that only produces 10" of vacuum cam must be fairly large, 700rpm is purdy low for a larger cam...
I know people with big cams idle around 1,00-1,200rpm and still sound really lopey,
I have a friend who had a similar problem and the vac can killed his performance at the track, it would launch very well but then it would go to crap after the 60'.

I'm going to check real quick on the rpms that they suggest from Comp Cams, its a 280s Solid 604 Lift Flat Tappet.

ace's68 · Aug 22, 2008

If you can keep the idle within a certain rpm, my hat goes off to you.
Seems that mine jumps from 1,000 to sometimes 1,400
Gotta love low vacuum and poor tuning...not.

10" of Vacuum at 700 rpms.

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Gary L

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fatblock

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