70newport

Hey everyone,

I am running a 383, bored 30 over, Keith black 162 pistons, comp cams 23-223-4 cam, stock crank, e-street 84cc heads, 9.2:1 compression ratio, edelbrock performer intake, roller rockers, Holley 650 crm carb.

The engine runs great but I only make about 9 inch of vacuum so my power disk brakes struggle. I ran a vacuum booster for 2 summers but it crapped out and I figure now is the time to actually address this issue. According to comp cams and edelbrock they both say I should make 12-15 on that setup. I can’t find a leak anywhere.

any thought could be helpful.

thanks,

Gary

Archer

Gary -

That cam should give you at least 12" at idle.
If there's no leak, then it's timing.
If timing seems correct, then think timing chain slip or cam degree being off.

Archer

70newport

thanks archer. I will triple check for vacuum leaks and go a little more insane playing with timing. Would love to not have to rip this engine apart again but that’s life. Thanks.

Kuvasz101

Power brakes struggling and low vacuum. Just a thought here. Maybe reverse thinking but could the booster be leaking causing bad braking and then the low vacuum. Plug it off and see what it pulls. It's worth the 5 min check. PCVs are often overlooked that cause huge vacuum leaks.

Kuvasz101

Looking at your cam's specs, I overlooked cams I've used on 383/340 engines that turned out poor vacuum. I just recently took out a Comp Cam 21-224-4 from my 383 with 274/286 duration with 488/491 lift that produced low vacuum with poor idle and lots of fumes. I also used a Crane in a 340 engine 280/288 duration with 460/450 lift that produced bad vacuum. In fact, the power brakes in the 340 didn't work at idle. Love Chryslers, but they are a bit finicky with engine mods. I've been keeping things more stock than ever now with the bad gas out there. I recently used the stock Magnum Road Runner spec cam for the 383/440 268/274 duration 450/458 lift in my 383 Dart. I love it!!

RacerHog

Yep, You can "Cam" yourself rite out of vacuum pretty fast.....

Stevem

Most folks don’t know that the overall amount of CCs above your pistons effects your amount of vacuum that you have.

Your original iron heads where 88ccs and now you have 84 CC heads, but are you running with a dish piston now ?

If so your total CC volume is larger and your vacuum has deminished from what it was stock, no less with the bigger cam now in the overall picture.

I would also bet that cam is retarted a bit due to normal chain stretch.

I always advance my cams 2 just to make up for this effect.

did you degree this cam before buttoning everything up?

you also really want a 10.2 to 10.5 compression with those Aluminum heads to make up for there very real level of heat loss taking place in the chambers over a iron head.

70newport

yeah the compression ratio was screwed up by the shop that built the bottom end and wouldn’t own up and fix their screw up. Was supposed to be built to 10:1.

starting to sound like I’m basically just screwed and need to go back to the drawing board or keep running a vacuum booster….

RacerHog

This may sound funny coming from a guy down south, lol...... BUT.... Not sure what engine shop you are dealing with? But have you stopped and talked to the the guys at "Bel Engine Service" down on 47 AVE?
Maybe they can straighten out your issue... Having Been to Castrol Raceway.. I known them as reputable in the past..

70newport

I havent talked to those guys at all but likely will now. I had the work done by a place called "the block shop". I was referred by someone saying they are the best around for Mopar.... I wish I had only sourced the machining and done everything else myself. Live and learn.

Thanks.