Vacuum this low normal?

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340Duster247

340Duster247

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Hey guys. My 340 with a lunati cam 60405 is producing low vacuum like my old garbage thumper cam. I'm just curious if it is normal to get this low of vacuum.
In gear at idle ( around 900 rpm) I'm pulling 4.5-5" of vac.
Details of engine ect. Are:
Eddy heads.
Holley 750 street hp
Air gap.
3800 stall
Manual breaks.
No vacuum advance.

I've checked for a vacuum leak but haven't found anything.

Specs on camshaft
 

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I have a solid cam with almost identical specs in a 360 I pull about 7" at same rpm. Did you have the valve job checked or are the ootb Eddy's?
 
Its a good size cam, and its not going to have a ton of vacuum. Its been my experience to get around 7-8 with this cam in a 340.

Whats your timing at? At idle? They like a lot of advance at the lower rpm, 20* or so. Plus have you tuned the carb at idle with a vacuum gauge to get the best vacuum reading?

Anyhow, timing comes first. Get it in the 18-20* at idle and 34-36* total if you can
 
I'm 20 initial 34 total. Heads were checked before install by engine builder.
 
Mine is similar except I'm running the .557 solid mopar cam. Getting 8.5" at idle in gear, 10* initial timing and 35* total. What's your carb specs? How are your idle mixture screws adjusted?
 
Does it run good? I had a cam that I thought I should have more vacuum with, what I did was make sure the cyl your testing is on the compression stroke pull the plug and pressurize the cyl and hear for anything leaking into the intake or exhaust, I found a couple intake valves that needed a touch up.
 
10.5:1 compression. Valves are good. Just checked those last month. Runs good and strong. Does not like being over heated at all, got stuck in traffic last night got to 210 for longer then I wanted and engine started to shake at idle like a dog sh$ting razor blades.

Idle mixture screws are set for the best vacuum possible.

The engine has always shook a lil more then I wanted but when it was overheated I could feel it ...got bad.
Other then that, at runs strong , and smooth.
 
242 and 252 @ .050. Yeah. It's gonna have low vacuum. You cannot really compare it to a solid. A solid with the same .050" specs will act like it has 10* less duration @ .050.
 
Well, higher compression= better vacuum, Check. 20* timing at idle, more vacuum, Check. Carb set to Highest vacuum reading, Check. Something don't seem right here. It should have more, but I guess its as good as it gets. Are you sure the gauge is right? No leaks anywhere?

One more thing that pops into mind, where are you taking the reading from? Intake or carb? If from carb, are you sure its not a "ported" vacuum port? Just asking.
 
If I had to guess at this point I would say low initial timing. It's worth double checking with a known good timing light.
 
I agree with the others that it's a little low. You say that it's 4.5/5.0 "in gear". How is it out of gear? Even though that converter is spec'd in the high 3's, could it actually be a little tight down low? Then there's the degreeing in.....is it right?

Plus, I've personally found that some Holley's like a little richer A/F adjustment at idle with big cams then the highest idle vacuum reading.....like a 1/16th to 1/8th turn or so.
 
Was the cam degreed? Was the compression figured or assumed? It's a big cam for what you're running it in, so low vacuum and shakes should be expected. 210° is not hot BTW. 230-240 is getting to be a problem.
 
I know it's not over hot ...... But the car didn't like it at all.

The Compression was figured by the engine builder, not a guess. Came in @ 10.3:1.
 
MRL Performance said:
Well, higher compression= better vacuum, Check. 20* timing at idle, more vacuum, Check. Carb set to Highest vacuum reading, Check. Something don't seem right here. It should have more, but I guess its as good as it gets. Are you sure the gauge is right? No leaks anywhere?

One more thing that pops into mind, where are you taking the reading from? Intake or carb? If from carb, are you sure its not a "ported" vacuum port? Just asking.
Click to expand...

The Gauge is a autometer ultra lite Hooked up to the carberator full manifold vacuum port.
I'm gonna double check for leaks tonight.
 
As far as the degreeing in I left that totally up to the engine builder. There pretty we'll known in town and I'm sure they did it the way it should be. But I have no idea exactly
 
peppers said:
227/241 @.050 Will vacuum be a problem for p/brakes? Thanks
Click to expand...

With the cam degreed in and enough initial timing, I think you'll be fine. I've run intake duration at .050 in the 230's without a problem. They produced somewhere between 9 & 11" of vacuum. If you idled long enough and pumped them repeatedly the pedal would get somewhat hard, but during normal driving conditions, it was not a problem.
 
Checked my vacuum in park today. 6.5" and drops to 4.5-5" in gear.
Checked for vacuum leaks and couldn't find anything.
 
I would try adding more initial and see if it helps. If the car starts afterwords when hot back off the mechanical and let her eat.
 
Sounds like it just may be the nature of the beast :).

Only thing I can add is, make sure if you're setting the timing in neutral.....guessing around 1000/1100rpm's, that you're not into the "curve" at that point. If so, when the rpm's drop in gear, the initial will drop also and cause a lower vacuum reading at that point. I know we're spitting hair's here, but if that's the case, you may pick up a little vacuum by delaying the mechanical curve to a little higher rpm using the 20* initial?

Regardless, run a 3.5" power valve, and enjoy the power that the cam can provide. :D
 
OldmanRick said:
Sounds like it just may be the nature of the beast :).

Only thing I can add is, make sure if you're setting the timing in neutral.....guessing around 1000/1100rpm's, that you're not into the "curve" at that point. If so, when the rpm's drop in gear, the initial will drop also and cause a lower vacuum reading at that point. I know we're spitting hair's here, but if that's the case, you may pick up a little vacuum by delaying the mechanical curve to a little higher rpm using the 20* initial?

Regardless, run a 3.5" power valve, and enjoy the power that the cam can provide. :D
Click to expand...

Good info. Wouldn't hurt to throw a timing light on it in gear vs neutral.
 
Does the rpm drop when it's put into gear? It sounds to me by the way the temp effects it, and the vacuum drops a bit, that it might be lean at idle.
 
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