not enough vacuum for power breaks...

[joflaig]

Car is just about ready, but they're finding that there only seems to be only about 8" of vacuum and the 4-way discs breaks aren't working after being bled. Apparently, Wilwood says we need 18" of vacuum. So either the build didn't allow for enough vaccum or perhaps there is a leak somewhere. We're ordering a electric pump for now. And they are also looking for leaks, though it's been dynoed 2x already once on the stand and then tuned on the chassis dyno.

The build:
http://www.dodgecharger.com/forum/index.php/topic,36197.180.html

I know this is pretty common on a high-performance motor, but I'm not sure what kind of vacuum I should have expected to begin with.

[firefighter3931]

John, that cam should be making more than 8in at idle. I would check the timing to make sure you have enough base timing. With the MSD distributor you should be using the black bushing which limits the mechanical advance and allows for more spark lead at idle. 

The idle mixture screws will also affect vacuum and i'm assuming that this was dialed in on the dyno ? If the engine idle speed is set too low this will also cause low manifold vacuum....it should be ~850-900 in gear.

Barring all that it will be a vacuum leak of some kind....



Ron

[joflaig]

Hmm, Ron do you think it should be making as much as 18"?

[Charger-Bodie]

I think 18 is a lot to expect from any perf gine.

[joflaig]

I think 18 is a lot to expect from any perf gine.

Yeah, I really don't know what to expect. Like I said Wilwood said their discs need 18 ". Do people run with discs on all 4s with slightly less and get away with it?

[firefighter3931]

Hmm, Ron do you think it should be making as much as 18"?

Nope, that's asking a bit much, inmo

I think 18 is a lot to expect from any perf gine.

Yeah, I really don't know what to expect. Like I said Wilwood said their discs need 18 ". Do people run with discs on all 4s with slightly less and get away with it?

I haven't used a Willwood booster before but i know many (myself included) who've used a stock PB booster with as little as 12in of vac @ idle and it worked fine. 


Your cam is pretty conservative for a 512 ci build and it should be pulling at minimum 14in if not more.



Ron

[joflaig]

I just spoke with the shop again. They said that their vendor told them the Wilwood setup does need 18 inches so even if we figure it out it sounds like to have them work at their optimal I may still need the electric vacuum pump assist.

One other thing they mentioned was that it is idling at around 875, which is good, but also that the vacuum gauge bounces as you go up in rpm. They said one cause for that kind of thing could be one valve is not seating right. These are eddie heads. Is there another way to diagnose something like that?

Of all the people who have played with the motor, from the builder, to the resto shop to the guys at the dyno shop nobody has said they could hear anything off in the way it ran and nobody has told me going by any of dyno #s that there was something majorly wrong that they could detect. Am I being paranoid?

BTW, these were the results from the chassis dyno last week:

http://www.dodgecharger.com/forum/index.php/topic,48487.0.html

[firefighter3931]

One other thing they mentioned was that it is idling at around 875, which is good, but also that the vacuum gauge bounces as you go up in rpm. They said one cause for that kind of thing could be one valve is not seating right. These are eddie heads. Is there another way to diagnose something like that?

John, if you're concerned that there might be a valve seal issue the best way to look for that is a compression & leakdown test. I'm sure the engine is mechanically fine...probably just a fine tuning issue that needs to be worked out. The chassis dyno results look great....nice flat torque and horsepower curves. This is gonna be a blast to drive ! 



Ron

[Belgium R/T -68]

My GTX has Wilwood discbrakes and stock booster, poor brakes is the only problem I don't have with that car. 

//Per