anybody ever take a vacuum reading of their 500 (or 472 even?) while its idling in gear. i need to know what lbs. of vacuum i should expect from a stock untouched 500 and it would be even more helpful if anyone knows what kind of vacuum i would pull in a 500 idling in gear with the performance cams out there.

thanks,
Nick J

I haven't measured my 472 ('70, 10:1), but a big engine with a smooth stock idle like that should be getting up around 20 inches of H20.
One inch of water translates to 0.036psi, so with a super-well tuned stock idle, maybe 0.75psi? Engine vacuum generally isn't measured in psi though.

I've gotta get a bit of wiring sorted out on my engine and then I'd really like to tune the idle and set it to full vacuum advance. I'll see if I can remember the vacuum number and post it.

Performance cams can drop vacuum down quite a bit. 15" of vacuum will give you a pretty nice sounding idle but still good torque for getting around town if you swap out the stock axle ratio. Down around 10" you really start sounding mean but power assist on the brakes is getting iffy, and you really need some gearing.
Just remember if you're putting in a performance cam to get a looser torque converter and keep the compression up. I still have a stock converter and kinda low compression in my Camaro and it is really hurting my idle vacuum because the engine has to work harder than it should have to to idle against the converter.

well my local division of circle track racing requires me to have 15lbs. of vacuum at idle in gear with foot on brake. im trying to find out what i should expect from my engine stock and how much cam i can get away with. id like to know if there are ways to boost that reading up also.

hmmm a caddy dirt track motor? never herd nor thought of that. how does it do on the dirt? what series do you run in is it a stock series or like a bomber serires? what kind of car you runin'?

asphalt not dirt...im no farmer. i dont really know yet the car is still being built. yes its basically a stock division. im putting the '74 500 in a '76 seville. im still in the fabricating stages now. just trying to find out what i can get away with.

That's gotta be 15" of vacuum, not 15#.
15# is vacuum is more than one atmosphere. That's some serious vacuum...

If you want to keep the vacuum up, try these:

Keep the LSA wider on the cam. Narrow LSA's have more overlap, so less vacuum

Loose torque converter so the engine has less load at idle.

Keep the compression up. You'll make more vaccum with 10:1 than 8.5:1 with the same cam

Put the vacuum advance on full manifold vacuum. It won't matter once you're racing, but at idle the extra advance will help a bunch.

Look into Rhoads lifters. They shorten up the duration at low rpm's with a fast bleed rate, but return to full duration at high rpm's.

hey there's nothin' wrong with dirt track. thast where a lot of big nascar and IRL drivers got thier start. if noone ever raced on a drit oval then NASCAR would not be around today thast for sure.

As a spectator, I'd MUCH rather go to a dirt 1/4 oval and watch the bombers, modifieds and World of Outlaws and NARCS run than on a paved track...not that I don't enjoy the hell out of them both... I grew up just outside of Chico, California, and havn't missed a Gold Cup ROC in many a year...Shane Scott, the Flyin' Hawaiian, was my man back when I was a kid and my dad would take me to the races...Now I'm a fan of Kinser in the Outlaws

Blue eyes is correct about everything ecept the Rhodes lifters
The quality has gone in the crapper and wont survive long on a performance cam.
We sell a lifter that we have built for us and they work well on pretty radical hydraulic cams that have 260+ duration and well over .600 lift

You can probably get away with around a 234@ .050 cam with a 115-116 deg lobe separation...maybe bigger
Compression will help also

Hey, it's been awhile, but I finally tuned the idle on my 472.

I'm pulling 18" of vacuum idling in gear. It's so smoooooth, and the low down response is so fun on gravel!

The setup is:
35 year-old 472. stock 10:1 compression. Haven't done a compression check
30* dwell on the points
9* base timing. Napa Echlin VC1810 vacuum cannister hooked to full manifold vacuum giving ~25* total advance at idle. (going to change the cannister, this one puts the advance in too soon for my liking)
650rpm idle speed.
Everything inside the carb is as I bought it.

The idle mixture screws are at 5 turns out, but I wouldn't just blindly copy that. I got this by tuning for best idle with a vac gauge and a low RPM tach. The usual procedure: adjust idle speed, turn mixture screws 1/2 turn one way and see if that improves the vacuum. If it does, reset the idle speed and turn the screws another 1/2 turn the same way. If turning the mixture screws hurts the vacuum, then turn them the other way, reset idle, and continue. Just keep doing that untill you can't improve the idle vacuum any more. Remember, it's gotta be in gear or else it'll be all off, so securely chock the wheels or get a friend.

Sucks to hear about the Rhoads lifters going downhill. I've heard alot of good things about them. Especially one guy with a 455 Poncho in a wagon pulling 11's or something like that with good street manners...