How Much Vacuum??

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mopowers

mopowers

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Any idea approximately how much vacuum you would expect from a 9.6:1 CR 340 with this cam?

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What's the big deal?
It will run your power brakes.
I have a 230/237/110 in a 360 that pulls 10/12 at 750. It's a stick car and will pull itself along on flat level pavement,down to 600 rpm easily enough with a 10.97 starter gear, if I pull a little timing out of it with my dash-mounted,dial-back timing gizmo. Initial idle timing is 14*, and I pull 7* out to do that,else it get's a little jumpy.At 600 you can finally hear the rumpidy-rumpidy......
But that cam, on the street, will want a lil more compression. With an ICA of 66*, your Dcr falls to 7.55/148psi cranking cylinder pressure. It'll be a wee-bit sluggish off-the-line, with 3.23s and a stock stall. And the power doesn't really get going until she's wound up pretty good.Amazingly enough 66* is the same ICA as the stock 340 cam; 268/276/114. But supposedly the stock 340s were "rated" at 10.5Scr,IIRC
So; what's on your mind?
 
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Thanks for the replies. The engine is already together and on the road. It runs great and seems to want quite a bit of initial timing. It's at 25* now, but seems to want more. It's got RHS heads and with 1.6 rockers, so theoretical lift is at .561". It idles great at 750, but only seems to pull 6" vacuum in gear at 750. I just thought that seemed a bit low. I don't have power brakes or anything, just wonder if 6" is all it has.
 
Check your lifter preload!

Or get a second vacuum gauge; 6" is not right and would/should be giving you tuning fits. And 16* initial should be plenty. Mine runs fine down to 12* and even lower.
Yeah guys crank 'em up to 25 all the time, and if you have an automatic with a 2200plus stall, that works.
But it don't work so good in a stick-car;on account of a stick car typically idles at 6mph@750rpm, and there is no TC to absorb the power pulses, so they go straight into the rear springs.And then the car starts to do this little annoying bunny-hopping, and in a bit more time, the entire chassis gets involved.But by backing off the initial timing, the pulses are weaker, less violent. And once you get a good tune into it, you find out that just because she likes 25*, she can be happy with 14* or even less. Mine "likes" 7*@600; the bunny-hopping being all but gone. I suppose I could put some SS springs back there, but then the pulsing would go into the chassis and tires. Besides, I don't want those springs.
Yeah so,back to the top^
 
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AJ/FormS said:
Check your lifter preload!

Or get a second vacuum gauge; 6" is not right and would/should be giving you tuning fits. And 16* initial should be plenty. Mine runs fine down to 12* and even lower.
Yeah guys crank 'em up to 25 all the time, and if you have an automatic with a 2200plus stall, that works.
But it don't work so good in a stick-car;on account of a stick car typically idles at 6mph@750rpm, and there is no TC to absorb the power pulses, so they go straight into the rear springs.And then the car starts to do this little annoying bunny-hopping, and in a bit more time, the entire chassis gets involved.But by backing off the initial timing, the pulses are weaker, less violent. And once you get a good tune into it, you find out that just because she likes 25*, she can be happy with 14* or even less. Mine "likes" 7*@600; the bunny-hopping being all but gone. I suppose I could put some SS springs back there, but then the pulsing would go into the chassis and tires. Besides, I don't want those springs.
Yeah so,back to the top^
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I'm not sure how the lifter preload would've changed. I set each at 1/2" turn past zero lash and the motor hardly has any miles on it. Doesn't pop or make any unusual rattles either. I've tried initial timing from 10 all the way to 25. It definitely runs and starts better with more timing. I guess I can bump it back down to 15* and go from there.

nm9stheham said:
What PCV valve is in there?
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Ya know, I'm not even sure. It was on the 340 when I got it. I'm assuming it's stock, but obviously I could be wrong. Should I try disconnecting it?
 
Yes, for a test, plug the hose, and see where the vacuum goes to. If it is a stock unit, then it will be wide open at that vacuum level, thinking you are at heavy cruise mode with that level of vacuum. (Well, it doesn't really THINK LOL....) PCV's are designed to shut down to a lower flow level when the vacuum rises above a certain level, so that it does not interfere with idle operation. I have found that the stock PCv's open up to cruise flow at around 14-15" of manifold vacuum so that is the reason to think this is wide open right now at your low idle vacuum.

I really would expect several inches more idle vacuum; my son's 340 runs 14-15" at 850 RPM with a Crane Z268 cam. It has a wider LSA so I would expect yours to be a bit lower.
 
That's an interesting thought. I'll have to unplug the PCV and see what happens. I'm gonna have to wait until tomorrow for the weather to cooperate. Just thinking out loud, but can I test the PCV with a hand vacuum pump?
 
No, The PCV hole is huge, to a manual pump.It takes 6 or 8 pistons, idling at 600 to 750plus rpm, to run the pintle-valve up to the top where air-flow is minimum.
It is an adjustable flow device.A controlled,calibrated, vacuum leak.
I use it as an idle-air bypass device, to help set-up the T-port sync with a minimum of secondary cracking . But I always start with a new one,lol.
Even a 230* cam wants quite a bit of bypass air.

Going back to the lifter preload,
we (on FABO)have read about guys that have assemble their engines with the pushrods bottomed in the lifter, and the valves actually off the seat.Even if only a couple of the valves fail to close all the way, the cylinder pressure leak back into the intake manifold wreaks havoc in there.
We have also seen jumped timing chains, mis-timed cams,and machining off enough to where the intake won't seal on the underside of the ports.
1/2turn preload, correctly done,works very well.
 
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mopowers said:
Thanks for the replies. The engine is already together and on the road. It runs great and seems to want quite a bit of initial timing. It's at 25* now, but seems to want more. It's got RHS heads and with 1.6 rockers, so theoretical lift is at .561". It idles great at 750, but only seems to pull 6" vacuum in gear at 750. I just thought that seemed a bit low. I don't have power brakes or anything, just wonder if 6" is all it has.
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Hang on; 750 in N/P and also 750 in gear?
There's something wrong here. It can't be both, unless there's something wrong with the TC or the tach.There is usually about 50 to 150rpm drop from N/P to D/L. And A 750rpm unloaded idle, usually bangs pretty hard going into gear.
 
AJ/FormS said:
Hang on; 750 in N/P and also 750 in gear?
There's something wrong here. It can't be both, unless there's something wrong with the TC or the tach.There is usually about 50 to 150rpm drop from N/P to D/L. And A 750rpm unloaded idle, usually bangs pretty hard going into gear.
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There is almost no RPM change when put in gear at 750rpm. The converter is a dynamic 9.5". It may drop a tiny bit but still stays above 700. I'd call it 725 maybe, but that's just splitting hairs.
 
Silly me, I guess I shoulda asked,back in the first post.
Ok so if your TC is that loose, it makes sense.
In that case, back to post #8.
And while the PCV is out; plug the holes in the valvecovers and put a vacuum gauge on the dipstick tube.Any vacuum there now, has to be coming from the intake manifold. So hopefully you won't have any there. Rev it up to 2200 and wait about 5 to 10 seconds.
 
I think I have that cam in my 340 (I'll confirm later tonight) and I get 15 at 750 curb idle.
 
With it getting dark so damn early, I haven't had a chance to get out and tinker on this issue. I'll keep ya updated when I get a chance to get my hands dirty.

AlV said:
I think I have that cam in my 340 (I'll confirm later tonight) and I get 15 at 750 curb idle.
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I'd appreciate it. Thanks! The cam was spec'd by Brian at IMM. I love the cam. Just gotta figure out this minor low vacuum issue. I'm sure it's probably something little.
 
After trying most of everything already mentioned, turns out the gauge I was using was bad. I'm now at 10" vacuum at 750rpm in gear at 20* initial. Still have a bit of tuning to do on the carb, but it's getting closer. Thanks for the help guys.
 
mopowers said:
After trying most of everything already mentioned, turns out the gauge I was using was bad. I'm now at 10" vacuum at 750rpm in gear at 20* initial. Still have a bit of tuning to do on the carb, but it's getting closer. Thanks for the help guys.
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At least you figured it out.
 
AJ/FormS said:
post#5 ,Nov 18
IMO vacuum gauges are like hockey sticks. You can try about 5 of them, and no two work the same.
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Hockey sticks? WTH is a hockey stick? They all look the same to me.


Now, if you'd have said baseball bat...that I get. Must be the language barrier again!
 
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