• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Big Cam (to me) and Low Vacuum - Is this Correct?

70RoadRrunner

Well-Known Member
Local time
5:36 AM
Joined
Jun 14, 2020
Messages
84
Reaction score
45
Location
Illinois
Happy Sunday.

I have a relatively big cam:
300 Advertised
245 @ .050
.510 lift

When in park with engine idling @ 1000 RPM, I get 6-7 inHg.

In drive with engine idling around 800-900 RPM, I get around 3 inHg.

Does this seem correct to you guys?
 
Can't say. For comparison, 496 stroker w 230/236 duration at .050, .541 lift hydraulic roller, at 14° initial and 900 rpm, 12 inHg. At 20° initial and 900 rpm 14 inHg. Same idle, different initial timing.
 
Last edited:
Happy Sunday.

I have a relatively big cam:
300 Advertised
245 @ .050
.510 lift

When in park with engine idling @ 1000 RPM, I get 6-7 inHg.

In drive with engine idling around 800-900 RPM, I get around 3 inHg.

Does this seem correct to you guys?

First off, thats a lot of duration for only 510 lift....

Second yeah low vacuum with that much duration is common....
What is your base timing set at?

It needs at least 18 degrees, by the time you get to 250 @ .050 it's common to simply lock out timing so you have 34-38 degrees all the time... When that's done sometimes theres a retard or an ignition disable to assist starting...
 
IMO, yes, it’s low. Adjust your carb and distributor.
I like 18* initial. Depending on your head and quench, full mechanical advance on a tight quench aluminum head at a 36 degree range (give or take 2* ether way.) to start will be about right. Full vacuum advance limited to 52*, or less.

Your vacuum should rise a bit more.
 
If that is an old design('70's ish) 300/510 sure could have low vacuum. Timing & carb adjustment will help. Also cam installed timing will change that.
 
Thanks for the responses.

The cam came installed in the car.

The car starts and runs great.

I’m pretty sure the timing is already around 18 degrees initial - bounces around a little I will double check.

It is an MSD Pro-billet distributor so only mechanical advance no vacuum. I need to verify which springs are in it.
 
If you are running a PCV, eliminate it. Run with a breather to atmosphere on one side and hose to the air cleaner on the other. With that much duration and low vacuum, it is just a massive vacuum leak.
 
What size engine is it?
For some perspective--your cam is close to the MP 292/509 of legend, just with a tad slower lobe design maybe? The one I used in a 440 made 9"/hg at about 850-900 rpm with a Holley 850. It was the narrower LSA version. So you might start with some tuning experimentation as the dudes have mentioned and see what changes you get, if any, and go from there...but if you're happy with the performance I wouldn't get too far away from where it's at, unless you notice big gains.
 
Its a 383 with a Holley 770 Street Avenger.

The main reason I am asking about the vacuum is that I am wondering if the power valve in the carb is correctly sized. The stock size is 6.5 and from what I read, they should be sized for 1/2 the vacuum.

I will also look into changing the PCV to hook up to the air cleaner instead of carb.
 
Really - I thought where I am currently at is way low.

Cant imagine going any lower.

Do you think my power valve should be changed assuming I don't figure out a way to increase my vacuum?
 
The timing comment 66Satellite47 made was about installed cam timing, not ignition timing. That’s the number of crankshaft degrees past TDC that the intake reaches max lift. If that’s 4 degrees late your vacuum won’t be as good as you might otherwise expect.
 
Unfortunately, I do not know the timing of the installed cam - it was in the engine when I got the car.
 
To directly answer the PV question, yes you need to change it with those vac readings. You ought to be pig rich in gear at idle and Everytime you mildly take off.
I would put a 3.5 in it, then do the carb/timing back and forth optimization and see if it gets better. Certainly spray carb cleaner around the carb and intake gaskets to see if the rpm picks up from vac leak. Ensure all vac lines are in good shape - triple check all that.
 
Its a 383 with a Holley 770 Street Avenger.

The main reason I am asking about the vacuum is that I am wondering if the power valve in the carb is correctly sized. The stock size is 6.5 and from what I read, they should be sized for 1/2 the vacuum.

I will also look into changing the PCV to hook up to the air cleaner instead of carb.
Thanks, that's why I was asking...the smaller the engine, the 'bigger' a given cam will behave.
On the powervalve....the generic '1/2 the idle vacuum' method is just that...generic. The proper way is to hook up your vacuum gauge with a long piece of hose, so you can watch it in the cab, and go for a drive. Size it just under, maybe 2" less the reading at your typical cruise rpm. So- if you just sized it at half your idle, a 1.5, when you throttled it while cruising you'd get a nice lean bog until the vacuum dropped all the way down there. So for example If your cruise vacuum is,say, 10", start with an 8 and test.
The power valve adds fuel through the boosters (mains) and does not affect idle mixture. If you're not getting lean spots when hitting the go pedal it may be fine as is...
 
Which brings another question, how much have you needed to turn the idle speed adjustment up? With a big cam you can uncover the transfer slots and start drawing fuel from the main circuit rather than idle circuit. Do the idle mixture screws effect the engine? Can you kill the engine by adjusting them?
 
To directly answer the PV question, yes you need to change it with those vac readings. You ought to be pig rich in gear at idle and Everytime you mildly take off.
I would put a 3.5 in it, then do the carb/timing back and forth optimization and see if it gets better. Certainly spray carb cleaner around the carb and intake gaskets to see if the rpm picks up from vac leak. Ensure all vac lines are in good shape - triple check all that.

I have an A/F ratio meter and get below 10 regularly when jumping on it. I am running way rich.

I will double check that I do not have any vacuum leaks.
 
Thanks, that's why I was asking...the smaller the engine, the 'bigger' a given cam will behave.
On the powervalve....the generic '1/2 the idle vacuum' method is just that...generic. The proper way is to hook up your vacuum gauge with a long piece of hose, so you can watch it in the cab, and go for a drive. Size it just under, maybe 2" less the reading at your typical cruise rpm. So- if you just sized it at half your idle, a 1.5, when you throttled it while cruising you'd get a nice lean bog until the vacuum dropped all the way down there. So for example If your cruise vacuum is,say, 10", start with an 8 and test.
The power valve adds fuel through the boosters (mains) and does not affect idle mixture. If you're not getting lean spots when hitting the go pedal it may be fine as is...

That sounds like a good idea. I will try this.
 
Which brings another question, how much have you needed to turn the idle speed adjustment up? With a big cam you can uncover the transfer slots and start drawing fuel from the main circuit rather than idle circuit. Do the idle mixture screws effect the engine? Can you kill the engine by adjusting them?


The idle mixture screws do make a difference and I can kill the engine if I turn them all the way in. I have them set using a vacuum gauge.

I will recheck them after I check for vacuum leaks and disconnect the PCV valve. I will then start looking into the ignition timing.
 
Auto Transport Service
Back
Top