Low Vacuum Reading Question?

[Anonymous]

Just curious....

What causes low vacuum readings?

My engine has always pulled only 8-10 inches of vacuum ever since I installed my new cam 3 years ago.

I degreed it and installed it straight up.

Nothing is physically wrong with the engine and I'm not going to be tearing into it to fix this.

Just wondering why? Isnt 16-18 inches of vacuum normal on a small six?

Thanks,

Doug

 

[JackFish]

I've got a steady 14 on mine.

 

[addo]

What's your CR, and what are the hot/cold cranking pressures?

I'm suspecting the cam is just bleeding off compression due to its duration and overlap.

 

[falcon60]

I agree with addo. My 10:1 compression 200 with FSPP 264 cam advanced 4 degrees idles with 9"-10" vacuum at 800 rpm. It's just the nature of the beast with the overlap these cams are designed with. But they sure do make power.......
Mine has 190# cranking psi,so it's not lack of compression-just the effects of overlap at low speeds.

Terry (glad I don't need power brakes)

 

[Anonymous]

Not sure what the cold/hot cranking compression is...never checked it

According to Dennis & Davids calculator I'm running 10.5:1 compression.

Also running the 274 H cam from Mike so I'm sure its bleeding off a lot of compression.

I'm glad I dont need power brakes either!

Later,

Doug

 

[Lazy JW]

Yup, lots of overlap bleeds off compression at low speeds, hence the low vacuum. That all changes at high rpm though 8)

My 300 with the Reed Cams Economaster cam pulls about 22" at idle. It has substantially less overlap than stock.
Joe

 

[Anonymous]

Overlap has no effect on compression.
Before you give up and consider you're stuck with it, sometimes a nice increase in vacuum can be had by simply increasing the initial spark setting (and removing the same amount from the auto-advance curve).
Example:
Initial 10 + 20 mechanical might give 10" vacuum.
Going to 15 + 15 (same total) may give 14", etc.

 

[Lazy JW]

Overlap has no effect on compression...

It doesn't affect static compression ratio but it jolly well allows pressure to bleed off at low speeds. If you say that has no affect..... :roll:
Joe

 

[Anonymous]

And where does the cylinder pressure go with both valves closed?

 

[CZLN6]

Howdy All:

The difference is static compression as compared to dynamic compression.

Static compression is a mechanical ratio comparison of the volume of the combustion chamber and cylinder with the piston at bottom center, to the volume of the combustion chamber and cylinder with the piston at TDC. This ratio assumes that both the intake and the exhaust valves will be closed.

But, both valves, in reality are not closed through the compression and combustion cycles. The intake valve opens before TDC on the exhaust stroke and remains open after BDC, while the piston starts up on the compression stroke. And the exhaust valve opens before BDC on the combustion down stroke and remains open after TDC on the exhaust stroke.

Static CR is a mathimatical ratio that does not take in valve timing.
Manifold Vacuum definitely goes down as cam timing becomes more agressive. The generality that you lose bottom end power to gain upper rpm power with a performance cam is true for this very reason.

Doug- 8 - 10 inches seem low, even with the cam you're running. I've got a Comp 260 and am at 5,000 ft elevation and I get 15 -16 inches. You'd better look into this a little further.

Adios, David

 

[wallaka]

Everything I've read says the intake valve closing event is the most critical factor in compression pressure. The compression ratio is the second, and overlap a distant third. The fact that high overlap cams also have the intake closing later could be the reason that overlap is thought to cause low compression pressures.

 

[falcon60]

Everything I've read says the intake valve closing event is the most critical factor in compression pressure. The compression ratio is the second, and overlap a distant third. The fact that high overlap cams also have the intake closing later could be the reason that overlap is thought to cause low compression pressures.

Correct-compression begins when the intake valve closes and is not affected by overlap. The thing that affects idle and vacuum is a side effect of a higher-overlap cam.....reversion of exhaust gases into the intake charge at lower engine speeds.With both valves open and very little air velocity due to low throttle opening,some exiting exhaust is pushed back through the opening intake valve,dirtying up the charge.This makes for the uneven,loping idle and lower vacuum. Kitabel is correct in stating that overlap is not the cause of lower compression-it's the later intake closing event.

Terry

 

[gtm1086]

Glad to see I'm not the only one with lower than expected Vacuum. I see 8 to 10" running a dual pattern cam. Ran it straight up and now have degreed it and running 2Degrees advance. No improvement. No vacuum leaks in the system.

Doug what power valve and high speed jets are you running in your Holley.

Gary

 

[mugsy]

My old tired 200 ran a nice steady 21 in. Hg. After the rebuild and the bigger cam I get about 13 in Hg. I, like Doug, am glad that the car doesn't have power brakes.

one of my other project cars is '98 Neon with a dual over head cams. I rebuilt that engine and installed adjustable cam gears. I started the car and it ran for crap, I had installed larger, longer duration cams and had milled the head. I started to playing with the intake closing and found that with more advance on the intake the better it idled. As stated earlier, the intake closing point greatly effects the idle quality and vacuum generated.

 

[Lazy JW]

Everything I've read says the intake valve closing event is the most critical factor in compression pressure. The compression ratio is the second, and overlap a distant third. The fact that high overlap cams also have the intake closing later could be the reason that overlap is thought to cause low compression pressures.

Correct-compression begins when the intake valve closes and is not affected by overlap. The thing that affects idle and vacuum is a side effect of a higher-overlap cam.....reversion of exhaust gases into the intake charge at lower engine speeds.With both valves open and very little air velocity due to low throttle opening,some exiting exhaust is pushed back through the opening intake valve,dirtying up the charge.This makes for the uneven,loping idle and lower vacuum. Kitabel is correct in stating that overlap is not the cause of lower compression-it's the later intake closing event.

Terry

The major factor is indeed the closing point of the intake. However, at LOW speeds, the long overlap causes lower volumetric efficiency, hence there is less air/fuel mixture to compress. Result: lower dynamic compression.

I have done a bit of minor research within my limited resources on low-speed torque engines. The older tractor and gasoline powered truck engines that ran at low speeds had some interesting cam timing. My 1941 John Deere Model "A" tractor runs at a rated speed of 975 rpm. The exhaust valve is FULLY closed for several degrees before the intake even starts to move. The intake begins moving at about 18º ATDC. This engine will idle so low you can read the writing on the flywheel.
Joe

 

[xctasy]

Just curious....

What causes low vacuum readings?

My engine has always pulled only 8-10 inches of vacuum ever since I installed my new cam 3 years ago.

I degreed it and installed it straight up.

Nothing is physically wrong with the engine and I'm not going to be tearing into it to fix this.

Just wondering why? Isnt 16-18 inches of vacuum normal on a small six?

Thanks,

Doug

Addo is on to it. To fix it idle Hg, you have to do a cold cranking compression tests on all six cylinders, and then adjust the advance or retard untill you get it to 180 to 185 psi. Cold is something of a misnomer. Run it, drop the plugs, and test it (make sure your plugs are still connected to your electronic ignition or you'll mangle your MSD!)

With that sorted, you can then forget about degreeing the cam. Hot Rod ran an article on it a while back, 2003 on something. The internal combustion engine is limited by detonation, and if you get the CCC right, you can lean on it hard and still have it work ok on the street. If you have to retard it too much, you have a cam too radical to get good idle Hg.

Most cams work best hooked up straight up. When the engine is being run in, you then tend to tune the engine by retarding the cam, and then redo fuel mixtures and run it another 2500 miles, and then find that the cam runs wrong...and that heads up was the right on at the begining!. The rings and rope seal are all bedding in, and even the lifter bleed down or clearances change. Everything is related. I'd just do the cold cranking compression, and do it each couple of months. I'd say vacuum will go up over timne, as Fords take a long time to settle in, especailly with a high lift cam.

My mate had a CC280 on his 71 Ford Falcon 351c 2V with 4-bbl intake, and he still got 22 mpg on the highway with less than 12 "Hg vaccum. Around town, it dropped to 16, which he was still happy with.

 

[Anonymous]

Glad to see I'm not the only one with lower than expected Vacuum. I see 8 to 10" running a dual pattern cam. Ran it straight up and now have degreed it and running 2Degrees advance. No improvement. No vacuum leaks in the system.

Doug what power valve and high speed jets are you running in your Holley.

Gary

Gary,

I'm running a 6.5 power valve and #67 jets. Going to sort out the jets and timing when the local track opens up in a couple of weeks.

Later,

Doug

 

[Anonymous]

Howdy All:

The difference is static compression as compared to dynamic compression.

Static compression is a mechanical ratio comparison of the volume of the combustion chamber and cylinder with the piston at bottom center, to the volume of the combustion chamber and cylinder with the piston at TDC. This ratio assumes that both the intake and the exhaust valves will be closed.

But, both valves, in reality are not closed through the compression and combustion cycles. The intake valve opens before TDC on the exhaust stroke and remains open after BDC, while the piston starts up on the compression stroke. And the exhaust valve opens before BDC on the combustion down stroke and remains open after TDC on the exhaust stroke.

Static CR is a mathimatical ratio that does not take in valve timing.
Manifold Vacuum definitely goes down as cam timing becomes more agressive. The generality that you lose bottom end power to gain upper rpm power with a performance cam is true for this very reason.

Doug- 8 - 10 inches seem low, even with the cam you're running. I've got a Comp 260 and am at 5,000 ft elevation and I get 15 -16 inches. You'd better look into this a little further.

Adios, David

David,

What should I look into? Cam is degreed straight up and matches the timing card.

Timing is set around 38-40 degrees total advance @ 2800 rpms

Not sure what else to check?

Thanks,

Doug

 

[Anonymous]

Just curious....

What causes low vacuum readings?

My engine has always pulled only 8-10 inches of vacuum ever since I installed my new cam 3 years ago.

I degreed it and installed it straight up.

Nothing is physically wrong with the engine and I'm not going to be tearing into it to fix this.

Just wondering why? Isnt 16-18 inches of vacuum normal on a small six?

Thanks,

Doug

Addo is on to it. To fix it idle Hg, you have to do a cold cranking compression tests on all six cylinders, and then adjust the advance or retard untill you get it to 180 to 185 psi. Cold is something of a misnomer. Run it, drop the plugs, and test it (make sure your plugs are still connected to your electronic ignition or you'll mangle your MSD!)

With that sorted, you can then forget about degreeing the cam. Hot Rod ran an article on it a while back, 2003 on something. The internal combustion engine is limited by detonation, and if you get the CCC right, you can lean on it hard and still have it work ok on the street. If you have to retard it too much, you have a cam too radical to get good idle Hg.

Most cams work best hooked up straight up. When the engine is being run in, you then tend to tune the engine by retarding the cam, and then redo fuel mixtures and run it another 2500 miles, and then find that the cam runs wrong...and that heads up was the right on at the begining!. The rings and rope seal are all bedding in, and even the lifter bleed down or clearances change. Everything is related. I'd just do the cold cranking compression, and do it each couple of months. I'd say vacuum will go up over timne, as Fords take a long time to settle in, especailly with a high lift cam.

My mate had a CC280 on his 71 Ford Falcon 351c 2V with 4-bbl intake, and he still got 22 mpg on the highway with less than 12 "Hg vaccum. Around town, it dropped to 16, which he was still happy with.

Deano,

If its the overlap or later intake closing event that bleeds off compression, how is adjusting my timing going to affect something thats mechanically built into the engine as the cam profile is?

I think I have the timing pretty much right on. Retard it and it looses power, advance it and it pings.

Thanks,

Doug

 

[xctasy]

I sent Jimbo65 on a wild goose-chase when his engine was just needing a long hard run in. We discussed cold cranking compression there.

The valve event timing is changed as you retard or advance the cam. I'm not going to go into what part of the event is most important, because cams are designed around a certain engine tune, and even little change in the chambers and quench and pistons will influence the way an engine behaves.

If you check the cold cranking compression, it will most likely affirm what timing is correct. There is 30 minutes in changing the cam timing if your quick, so to check +2, straight up and -2 is a good morning worth of messing about.

If you can get the CCC to 180-185psi, then your vaccum will be ideal. Detonation can be brought on by other simple issues, and the first corner stone is getting that sorted first. I trust the cam maker, but if your trying to get every ounce of performance out of it, that happens when detonation is least. That happens when a high compression engine is tuned for that elusive 180 psi.

I haven't been listening about the cam, but changing lash can help vacuum a little too. The recomended and actual lash can be varied to your advantage.

I won't promise to find the Hot Rod article, but it was one of the most compelling reads since War and Peace, and its mandatory for hot sixes, where the cam is the whole heartbeat of the engine.