Why is my manifold vacuum so low?

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Bill Crowell

Bill Crowell

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I've got a rebuilt LA 360 with stock 8.2:1 pistons due to smog compliance, but its "J" heads have been ported internally about as much as possible, as well as being match-ported to the Edelbrock Performer intake manifold. The cam is an MP P4452759 having an advertised 260 degrees of duration on intake and 268 degrees on exhaust, with some overlap (can't remember how much at the moment). My intake vacuum at idle is only 7", which doesn't operate the power brake booster very well at all. Is my intake manifold vacuum so poor due to all the porting I've done?
 
If your throttle plates are open to far you wont have any vacuum. Do you have holes in your throttle plates?
 
Your actual SCR is in the 7's. A said above degree the cam & adv it 4 deg with the 3 keyway timing set. You may want to consider another cam.
 
I know the SCR is low, but I think it's about 8.5:1 because the heads were milled and the block was decked. I don't want to change the basic engine tune because the speed shop that built the engine for me has it running nice and strong, even though it is a low-compression smog motor (forged pistons, dynamically balanced; nice engine). I remember now that the engine builder told me I was going to have a problem with the power brakes so he installed a Mancini Racing vacuum canister in line with the power brake booster. When I connect my vacuum gauge directly to manifold vacuum, it reads an average of only 7" of vacuum, but the outlet of the canister has 12" of vacuum because it stores the intake's peak vacuum figure. Twelve inches of HG is just barely enough to operate the power brake booster. After making a bunch of measurements with a vacuum gauge, I have determined that the reason my power brake assist crapped out was due to a bad vacuum booster. Thanks for all your help.
 
Why would you have a motor rebuild with 'good' (cam / pistons / porting) parts and still have it end up at an ineffecient smoglevel compression ratio?
Sound like a professionally 'mismatched' engine build.
 
You had to keep the OE SCR with the new forged pistons for smog compliance checks out there on the west coast? I would degree the cam/adv it 4 deg & as said work with the timing: set initial with the "vac gauge method". limit total (initial+slots) to 35 with the FBO $22 plate then toss the heavy spring with the elongated loop & sub in a mr gasket or MP light one in its place for a start & mix & match & you want to be under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day and any ping you can hear is WAY too much then plug in the vac can & same thing, staying under the pinging limit in everyday driving under varying load/RPM conditions & CCW with a 3/32 allen wrench slows the curve.
 
BigBlockMopar and Rapid Robert: since neither of you seems to have read the earlier posts in the thread, I am going to have to disregard your replies.
 
Bad porting can really hurt an engine. But so can an MP produced cam that wasn't degreed. I think your issue is less of the static ratio, and more of the tossed together and not blueprinted. Tuning will also be a big deal. the timing curve it's going to want will need to be set into any distributor you might be running. If you just dropped something in or used stock then that's hurting you too. Lastly is the carb tuning. Once the timing is set, then properly tune the carb. I'd expect a smooth idle around 750rpm, with at least 12-14" of vacuum if not more. Unless the porting sucked...

Edit - the booster was bad- makes sense. But vacuum is also an indicator of just how good the engine is really running. IMO, you're low, so potentially missing out on some power and economy (and emissions given your neighborhood). But you're the customer so if you're happy...lol.
 
You post asking for help. 15 posts later, you've not answered two vital questions, but rather decided to disagree with two posters.

Are you really lookin for help here, or just trying to be stupid?

Was the camshaft degreed in?

Yes ____ No_____


Where is the initial timing set?

I don't know. ______ X degrees BTDC. ________
 
pishta said:
No, not possible.
Click to expand...
Maybe he danced the grinder across the seats.
Do a leak down, valve job might be chattered. That will hurt it in a short bit, go downhill quick..
Most of the mp cams I've used needed them retarded, 7-9 degrees. So if it was put in ...say the old 4 degree adv.....it could be wayyyy off
 
Sorry, too many self-professed "know-it-alls" on here. If you guys want to display your insecurity, do it with somebody else, please.
 
Bill Crowell said:
Sorry, too many self-professed "know-it-alls" on here. If you guys want to display your insecurity, do it with somebody else, please.
Click to expand...

We're trying to help YOU. You asked for help. Some very important questions were asked and as of yet, they are unanswered. YOU are the problem here, no one else.

I don't give a crap, to tell you the truth. My vehicles run well. People take time out of their day to help you and all you can do is be an ***.

Classy man. Real classy.
 
Last edited by a moderator:
Bill Crowell said:
Sorry, too many self-professed "know-it-alls" on here. If you guys want to display your insecurity, do it with somebody else, please.
Click to expand...

I'll make sure to communicate your feelings to the Automotive Society of Engineers. They said I was certified in among other things Engine Performance. So this is all their fault. Enjoy your new-found brakes.
 
Bill Crowell said:
Sorry, too many self-professed "know-it-alls" on here. If you guys want to display your insecurity, do it with somebody else, please.
Click to expand...


If you want help, just tell us the basics. The first thing you need to know is where the cam is installed.

If you don't have a degree wheel and dial indicator, you can pull the number 1 valve cover off and roll the engine over to overlap on number 1 cylinder. Then look at the valves. If both valves are off the seat the same amount, the cam is straight up. If the intake is open further the cam is advanced. If the exhaust is open more than the cam is retarded.

A compression yes and leak down would also be good to know.
 
Problem with most engine builders is they put the ignition in and run it on the dyno for best max hp. That has little to do with how the engine runs at idle or in gear.

As moper mentioned it should have a lot more vacuum at 750rpm than it currently provides. Lots of times that is a symptom of not enough idle timing. If they timed it using a "stock" setting, that is the first thing to change.
 
Bill doesn't know if the cam was degreed or not. Probably doesnt know what the timing is set at, either. Hell, he might even live at >3500 ft elevation.

Every MP cam I've degreed was more than 3 degrees off. The guys that have helped you here are top notch and combined, actually do know-it-all. Try contacting your builder with that attitude and see how much info you get from them.
 
The OP sounds like one of those guys that pays for everything to be done, learns a few "car guy" terms but has a hard time understanding them. This could explain why he cannot understand or answer the basic questions.
Diamond Springs is below 1700 ft above sea level, I live about 30 minutes from there.
Bill: If you want help, don't be a dick to people that take the time to TRY to help.
Sure, sometimes you'll get responses that are NOT helpful but you get what you PAY for right? Everyone is a volunteer here...Nobody is paid to help you out.
If someone asks you to check this or that, they are doing so because it may reveal the cause of your problem.
* Initial timing has a great effect on idle vacuum. Check it.
* Drilled throttle plates lower the vacuum. Answer the question ....Are yours drilled?
* Did you degree the cam? Camshafts that are not in their optimal position will negatively effect performance.
I recall the words of a wise man I once knew. He didn't like to be bothered for stupid ****. He would often say..."Help me help you. This meant ....Don't make me do everything for you.
 
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