cam advance and vacuum

[texas69bee]

If I advance the crank 1 notch on the gear or 2 degrees I will have advanced the cam 4 degrees. What affect does this have on vacuum?

 

[mopar 3 B]

First off there would have to be 180 teeth on the crank gear to move it 2° with one chain link. Second off cam advance or retard should be done at the cam to gear connection never by moving the gear alignment locations at the chain. As far is what affect advancing the cam will have on vacuum the cam spec's would need to be known.

 

[62 Dart Convertible]

First off there would have to be 180 teeth on the crank gear to move it 2° with one chain link. Second off cam advance or retard should be done at the cam to gear connection never by moving the gear alignment locations at the chain. As far is what affect advancing the cam will have on vacuum the cam spec's would need to be known.

I don't think texas69bee is suggesting that he will advance cam timing by jumping a cam chain tooth, rather the 2° advance keyway...

If the keyway is marked +2°, that should be +2° at the cam I'd think. You'll know for sure when you degree it in anyway. You do plan on doing that I hope?!

Generally speaking, advancing the cam, through closing the intake valve sooner, should increase engine vacuum (and low rpm torque at the expense of high rpm torque.)

 

[texas69bee]

62 Dart,You are partially correct in what I was trying to explain as well as the answer I was looking for with the vacuum. I have a gear on the CRANK that can be retarded or advanced, this should retard or advance the CAM as well. By advancing the CRANK 2 deg, shouldn't I get 4 deg of advance at the CAM

 

[lewtot184]

moving the crank forward/advance with the cam stationary will retard the valve timing. backing the crank up will advance the valve timing. 2 degrees may not make much difference for the work involved. before playing with the cam timing you must know if it has any advance ground into it and where it's at (installed centerline) now.

 

[Cranky]

Yup....where is the cam setting now? Are you looking for more vacuum and is the only reason for the change and what are the cam specs. Also, you need to know how much piston to valve clearance you have. If you are close now, just a small move in cam timing could put you in the danger zone....so, anytime you make an adjustment, you should be using a degree wheel and dial indicator setup.

 

[62 Dart Convertible]

By advancing the CRANK 2 deg, shouldn't I get 4 deg of advance at the CAM

OK, I've had a good think about this.

The answer is No.

Advancing the crank sprocket 2° will actually move the cam 1°, but cam degrees are not what are measured, checked or quoted.

Cam timing events are always measured, checked or quoted in crank degrees, not cam degrees. When degreeing in a cam, the degree wheel goes on the crank.

So whilst advancing the crank sprocket 2° will actually advance the cam 1° (because cam turns at half crank speed), in terms of crank timing, it's 2°.

I've degreed in enough cams to know this to be true.

 

[Cranky]

Isn't cam timing stated in crank degrees and not cam degrees? Example, if an intake valve opens at 15 degrees BTDC @ .050 lift would mean crank degrees. If it's recommended that a cam be set up say 4 degrees advanced, you would use the measurement on the degree wheel, correct? That's the way I've always done it....

 

[texas69bee]

62dart, after more thought and research on the internet you are correct. Cranky and Lewtot, this is the tail end of my engine build. You have commented before on my questions during the assembly. The motor is running but I'm not impressed with the vacuum. That's the reason for wanting to advance the cam. During my setup I did retard the crank 2 degrees. It sounds nice but the vacuum bounces and hangs at 11 around 700 rpm. Compcams says I should have 15 to 18. I will post a video when I get home.

 

[lewtot184]

Isn't cam timing stated in crank degrees and not cam degrees? Example, if an intake valve opens at 15 degrees BTDC @ .050 lift would mean crank degrees. If it's recommended that a cam be set up say 4 degrees advanced, you would use the measurement on the degree wheel, correct? That's the way I've always done it....

correct

- - - Updated - - -

trying playing with the ignition curve first.

 

[Meep-Meep]

Engine vacuum is largely dependent on the overlap so advancing the cam a couple of degrees may not give you what you are looking for. Advancing or retarding the cam will affect your cranking compression pressure.

 

[texas69bee]

So I said I was going to post a video but I can't figure out how to get it from my phone to the site. What it would show is at 750rpms the vacuum is at 11-13 as the rpms go to 2000 the vacuum will go to 19. My concern is what would cause the vacuum to fluctuate and be so low at idle. I was expecting it to be 18 at idle.

Engine Specs
383 .030 over
KB Pistons
440 source heads
5 angle valve job
pro magnum roller rockers(compcams)
Compcams xe268h http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=707&sb=2
promagnum lifters .002 preload
Stock carter avs carb recently redone by Harms
Proform distributor
Mopar Performance electronic ignition (orange box)

I've been playing with ignition timing, seems to run better as I continue to advance.

 

[lewtot184]

too much pre-load on the tappets can cause flucutuations, why those tappets? if the carb is stock, and i mean at stock specs, you'll have an issue with it.

 

[cdr]

as said by lewtot184,, ign timing,,,, you want about 20 to 25 initial . total 35 to 38 no vac advance so you will only have 15 mech advance in the dist.

 

[lewtot184]

that cam is small and with a tight lsa. there should be vacuum running out your ears. if the vacuum jumps up to 19" at 2000rpm then that ought to tell you something. between ignition re-curve and a serious idle mix richening you should be in very good shape. i know when i went after the air fuel mix on the avs on one of my 440's there was a very noticeable difference.

 

[texas69bee]

lewtot, this was my thinking when deciding on a lifter. There were 2 choices the 822-16 with a pre load of.040 and the 867-16 with a pre load of .002. With a pre load of .040*1.5(rocker arm ratio)=.060 or .002*1.5(rocker arm ratio)=.003
Take those #'s and subtract from the gross valve lift of my cam I end up with .480-.060=.420 or .480-.003=.477 so I decided on the lifter with the smallest pre load to keep the largest valve opening possible.
I will go back and check the pre load on the lifters, the pre load was set when cold but recommended when warm. I also failed to mention that the rpms fluctuate as the vacuum moves.

CDR

as said by lewtot184,, ign timing,,,, you want about 20 to 25 initial . total 35 to 38 no vac advance so you will only have 15 mech advance in the dist.

Serious, that much on initial timing?

As always thanks for the help!

 

[62 Dart Convertible]

Serious, that much on initial timing?

If you can without detonation, yes. More initial timing will improve your idle vacuum lots.

 

[cdr]

yes on the timing, but you have to limit the amount of mechanical advance in the distributor.

my stock 383 was at 24 initial & 37 total, my 512 is 25---34 NO VAC ADVANCE

 

[Meep-Meep]

I'm going to say that cam you have is producing the expected amount of idle vacuum. Advancing timing is good but shouldn't need much more than 15 initial if your compression ratio is between 9.0 and 10.0:1 and cranking pressure is 170-ish PSI. If you have 8:1 CR then it will be a pig on the low end and will like as much timing as you can throw at it.

 

[texas69bee]

I calculated compression to be at 9.5. Motor was decked, .004 below deck and heads are at 75cc