Do I have low vacuum?

[eagleone1983]

So here is what I've got. New engine just broke it in a week ago. It's a 383 with the comp cams xe275HL (duration 275/287, lift .525). I'm just learning about engines so I'm not sure what my vacuum should be but I'm only reading about 7.5 -8" and the needle is not steady. I have timing set a 18* initial. Did a compression test and that passed. I don't notice anything when driving down the road crazy with the engine, I figure my timing should be good but I guess I could try a little more advance.

Also the little amount of vacuum is causing me to not have power brakes. I don't know what other info I need to help you guys help me. If I had a vacuum leak somewhere say at the common intake lower corners would it be obvious?

 

[MikeyT]

Ok, that is a big cam for that engine for sure. Can you tell us any more about the build? A

Where do you have the gauge plugged into ?Also when running the vacuum gauge what is the needle doing? It will move a little like .5 to 1 psi with that cam, however you don't want big arching movements. My guess is you might have a leak in the intake area, go and spray a little wd 40 around the bolt holes and see if it cleans up a bit and increases in pressure. BTW You will need close to 12 psi for your power brakes unless you get a brake cannister in there at some point.

mike

 

[Meep-Meep]

That's low but pretty much normal for a big cam like that. What was your compression PSI?

 

[SwedCharger67]

Your vacuum reading sounds normal. I had about the same vacuum with the very similar XE274H in my 383.
My problem was the low compression, my engine had a nice sound but was powerless with that cam. I had 24 degrees initial timing with the distributor limited to another 12 degrees, i.e. 36 degrees in total.

 

[RustyRatRod]

I don't know. That's only 231 and 237 degrees duration @.050. Do you know what the static compression is? Also, what was the cylinder pressure? If it was 150 or less, you probably need to bump the timing up a tad more. You may end up needing to get a vacuum can.

 

[GrabberOrange69]

I don't know. That's only 231 and 237 degrees duration @.050. Do you know what the static compression is? Also, what was the cylinder pressure? If it was 150 or less, you probably need to bump the timing up a tad more. You may end up needing to get a vacuum can.

I have to agree with Rusty: 231/[email protected] isn't that radical. What was your static compression ratio when you assembled the engine? How many miles you have since the break-in - I'd get 300 miles then run a compression check as the rings should be pretty well seated. As a comparison, I ran a 413ci motor with [email protected] 110CL cam back in the day and it would idle at 850, no power brakes - plus I didn't have a stall converter (but it would have been nice). It did however have 11.1:1 CR....

 

[eric383]

Try a little more timing. If it rattles, back it off till it doesn't

 

[Northwest]

I get about 9Hg of vacuum with the XE295HL comp cams. .564 lift and 251/257 duration. - not enough for power brakes which I do have on the car.

I got this kit from SSBC as well as a vacuum canister which serves as a reservoir for vacuum.

I have about 1000 miles on the kit - and think it will be adequate. With the addition of a vacuum can which I mounted under the battery tray- I will get a good 2 stops, before the pump needs to run again.

Where reliability is an issue however - is the relay. I already had one fail where the small soldered contacts shook loose and therefore the pump motor wouldn't run. Poor quality solders was the primary cause. The pressure switch runs to the relay to turn the pump on when the vacuum runs low.


I have a new relay - but am thinking about putting in an emergency switch and guage where I can directly turn the motor on and bypass the relay.

It is a noisy dog though - not a problem for me since the car is too and no one really knows its running but me.

I have heard someone else on this site recommend a master power system.

 

[eagleone1983]

Ok, that is a big cam for that engine for sure. Can you tell us any more about the build? A

Where do you have the gauge plugged into ?Also when running the vacuum gauge what is the needle doing? It will move a little like .5 to 1 psi with that cam, however you don't want big arching movements. My guess is you might have a leak in the intake area, go and spray a little wd 40 around the bolt holes and see if it cleans up a bit and increases in pressure. BTW You will need close to 12 psi for your power brakes unless you get a brake cannister in there at some point.

mike

I have the gauge plugged into the small port on the front of my holley. The needle is bouncing around and do you mean it will move .5 - 1 inch? I'd say it's doing about that. I'll try that spraying trick, what am I supposed to be looking for when spraying?

 

[RustyRatRod]

Don't use anything flammable checking for vacuum leaks please. It's dangerous. Use water in a squirt bottle. You'll find it just as quick. I don't think it's a vacuum leak anyway. That needle is never going to be still with a camshaft that has 237 @.050. It just ain't gonna happen.

 

[eagleone1983]

Here are some more engine specs. The compression ratio is 9.651, it has an edelbrock alum performer RPM intake, alum stealth heads, has been bored 30 over stock, magnum roller rockers, schumaker tri-y headers, an FBO ignition system, and a holley 770 street avenger.

Right how I've got my idle set at about 850rpm. I only have maybe 20 miles on her since break in but I'm going to be putting more on her today. I've still got the break in oil in it if that matters at all. I can't remember what the compression check showed because someone else that was helping me out did it but he said it was good.

 

[RustyRatRod]

9.6 according to who and how? In other words, how did you arrive at that number? Was it actually MEASURED or did you simply take this spec and that spec and that spec and run it through a formula? Unless you used actual measurements it could be much lower. Especially with a Mopar given their known tall deck castings.

What I'm gettin at is, if it's an actual blueprinted and measured 9.6:1 then with that cam you cranking pressure should be relatively high. I would guess over 160.

 

[eagleone1983]

It came from my builder and looks like he just did as you say and ran it through a formula but I would imagine he spec'd everything himself and input those measurements into the formula but my faith in others working on my car is getting pretty thin.

 

[RustyRatRod]

It came from my builder and looks like he just did as you say and ran it through a formula but I would imagine he spec'd everything himself and input those measurements into the formula but my faith in others working on my car is getting pretty thin.

If you can give us the cranking PSI, I can tell you real quick how the number came to be.

 

[WP29440SE]

don't overlook the vacuum line from the intake to the booster. check around the fittings for leaks. a bad booster can also cause a vacuum leak.

 

[69Bee]

You also live in Utah. What is your altitude there? The vacuum readings usually thrown around are referenced to sea level. The higher your altitude, the lower your reading will be.

 

[eagleone1983]

Our altitude here is about 4300 ft. I haven't had time to look into the vacuum yet, I've been dealing with a leaky fuel sender to tank issue. But I got that solved by realigning the sender so back to the vacuum issue.

 

[eagleone1983]

Didn't really mess with things too much but did take her out for a drive. Noticed my secondaries don't seem to be opening. I have whatever springs come out of the box. Should i change them? Can this be related to my vacuum issue? I checked to make sure my throttle cable was set up correctly and it is.

 

[Meep-Meep]

Does the car make good power? Or is it a slug? The 383 I had in the 80's was 8-10 in-hg at idle and hauled ***. But my buddies 57 Chevy has 4 in-hg at idle (2" in gear) and he darn near can't drive it. His problem is a stock 325 HP 396 with some thumper Comp cams that someone talked him in to putting in. The point is if your car runs great with a little bump in the timing then don't worry about it. But if it can't get out of its own way or you need to have your full advance in at idle then it's time to put in a smaller cam.

 

[RustyRatRod]

I bet those secondaries are opening. That's the whole point of a vacuum secondary carburetor. When you have it tune to where you can feel them open, you've slowed it doen. It's when they open unnoticably that the car is fastest. It's all about a smooth transition.