Cam/vacuum

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I had the 280/474 cam from mopar. Similiar in duration and size. Vacuum was good. All my accessories worked good. Only sometimes had a little trouble switching from defrost to a/c vent. But seldom was case. Power brakes also were fine.
 
Depending on engine size anywhere from 8-18. Kim

I agree with the kimster. It will have good vacuum. Make sure you have plenty of initial timing. You'll want a distributor that can limit the a mount of mechanical advance such as an MSD type, or you can use the Four Seconds Flat limiter plate if you're running a Mopar electronic distributor.

Also, "I" might advance that cam a little further than recommended. Maybe down around 104 or 102 ICL. That would also give a higher vacuum signal. Just my opinion.
 
Depending on engine size anywhere from 8-18.and how much initial advance u run. Kim
Ok, it’s in my 440 and I’m getting close to 9”@700rpm in park. Unfortunately the car isn’t drivable at this point so in park is the only running info I have. It seems to run and sound good(IMO) as for initial advance pardon my ignorance but I’m not exactly sure as to what your referring to. Base timing?

I agree with the kimster. It will have good vacuum. Make sure you have plenty of initial timing. You'll want a distributor that can limit the a mount of mechanical advance such as an MSD type, or you can use the Four Seconds Flat limiter plate if you're running a Mopar electronic distributor.

Also, "I" might advance that cam a little further than recommended. Maybe down around 104 or 102 ICL. That would also give a higher vacuum signal. Just my opinion.
I’m running FiTech fuel injection and it is controlling the timing. Using a locked out MSD distributor. Base timing of 10* btdc and full advance of 36* @2500rpm. And thanks for the replies, I’m always looking to further my understanding.
 
That's a hair lower than I'd expect.
For accessories... like air ducts, try one way vac valves, inline.
 
orange one
grays-shark-upright-vacuums-hv301-64_1000.jpg
 
Ok, it’s in my 440 and I’m getting close to 9”@700rpm in park. Unfortunately the car isn’t drivable at this point so in park is the only running info I have. It seems to run and sound good(IMO) as for initial advance pardon my ignorance but I’m not exactly sure as to what your referring to. Base timing?

I’m running FiTech fuel injection and it is controlling the timing. Using a locked out MSD distributor. Base timing of 10* btdc and full advance of 36* @2500rpm. And thanks for the replies, I’m always looking to further my understanding.

Experiment with your base timing. Since you have the FiTech all you need to do is type it into the hand held. Raise your base timing to 15 degrees and see what happens. Raise it to 20 degrees and see what happens. Raise it to 25 degrees and see what happens. Drive the car, see what happens. Pay attention to your vacuum gauge and re-adjust the idle setting as needed.
 
Nowhere near enough base timing. Cam needs 18-22 is my guess. Vacuum will come up with more base timing. It should have 12-13" at 800-850 rpm without much issue

Idle is low at 700 rpm too.
 
I agree with crackedback on this. You need to have a lot more initial timing. Computer controlled or not, it slap doesn't have enough.
 
Nowhere near enough base timing. Cam needs 18-22 is my guess. Vacuum will come up with more base timing. It should have 12-13" at 800-850 rpm without much issue

Idle is low at 700 rpm too.

Now my curiosity is really peaked, as I stated in my last post I will be playing with the timing this weekend, what I’d like to know is can you explain how you come to this conclusion? What in the cam specs or other info points the amount of timing you suggested? I’m not questioning you, I’m curious and would like to understand. I do know that low vacuum can be caused by retarded timing but never really understood why. @RustyRatRod your input here is welcomed also.
Also should the advance settings be changed too? Right now it’s 36*@2500rpm.
 
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It just does. Yes it's the cam specs and working on engines with that camshaft before. Leaner mixtures, which is what you want, require more timing to ignite. If you know that retarded timing may cause low vacuum signals why fight reality?

You are not timing a stock engine at this point and using anything related to timing events from the factory is useless/worthless. The factory also had to deal with emissions, which required RETARDED base timing to pass the testing levels. Hemi, six pack, 340's even 318 did not have sufficient base timing. Give the engine what it wants, it will tell you with timing and a vacuum gauge.

Do the test yourself and observe the results as AndyF and I suggested. There is a lot on the internet about the "whys" of this.

Leave the total timing where it is as long as detonation is not present.
 
I’m certainly not fighting it, just wanted to understand it. Thank you for your time, experience, and input, it is much appreciated.
 
IMO
for a streeter with a big street cam
By running more advance, giving the engine what she wants, you are lighting the fire early enough that the pressure spike gets an earlier transfer to the piston. Now all the fuel gets burned up in the chamber, and more of the pressure is transferred to the crank. So by the time the exhaust valve opens the pressure has fallen in there, to a lower level. Now,as the exhaust valve approaches closed,and during the overlap cycle, there will be a lessor pressure pulse leaking back into the intake,past the now-opening intake, which formerly reduced the vacuum in the intake. The more overlap your cam has, the lower the intake vacuum will be, and especially at the lowest idle speeds, because this allows more time(in degrees) at piston TDC(overlap), for the burned up exhaust to travel into the intake, when the intake valve pops open. As an example; the 292/292/108 Mopar cam has 76* overlap, just at advertised duration. Whereas a 268/276/110 might have only 52* at advertised; that works out to the larger cam having ~50% more overlap/ 50% more TIME,in degrees.
So your first defense is increased idle speed, and the second is increased idle-timing.
There is nothing wrong with a low-vacuum idle, if you can tune for it. which with a carb is not all that hard. But with EFI that depends on a MAP sensor, it gets tricky.
So like has been said, start with a higher rpm and more idle-timing, then as time goes by and you get a handle on your tune, you can experiment downwards.
How much idle-timig to run is somewhat dependent on your TC if automatic, and on the very low-rpm jumpiness if a standard trans.
Generally, with a big cam, and a regular mechanical advance distributor, you want to run a lot of idle timing, because this increases the low-rpm responsiveness. But with a low-stall TC or especially with a manual trans, the more powerful pressure pulses can make low SPEED driving a little jumpy.With a factoryD you are limited to a two-step curve at best.
With computer-controlled timing however, you can take care of all that with programming. You can advance or retard anywhere you want.
Say with a manual trans and 3.55s;
You can have a start retard to say 5*up to 450rpm, then have a smooth idle with 15/20*beginning at say 700, and cut back to say 8* at 900, so you can idle along at 7 or 8 mph, and not have the car start bucking like a wildhorse. Then you can bring it back in to say 28*@2800 for that low-rpm responsiveness. Then slow it down to 34* at 3400, so you can burn 87E10 with aluminum heads and stay out of detonation. Your options are limitless.
With an automatic, and a 2800 or better TC, you can just about do whatever you want cuz the TC will suck it up. But there is no good reason with a TC to run just 10* with a big cam; you're just sending a lot of unburned fuel out the exhaust; OOps that's carb-talk,lol. With EFI,and a MAP sensor you're just complicating the fuel tune.
IMO
 
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IMO
for a streeter with a big street cam
By running more advance, giving the engine what she wants, you are lighting the fire early enough that the pressure spike gets an earlier transfer to the piston. Now all the fuel gets burned up in the chamber, and more of the pressure is transferred to the crank. So by the time the exhaust valve opens the pressure has fallen in there, to a lower level. Now,as the exhaust valve approaches closed,and during the overlap cycle, there will be a lessor pressure pulse leaking back into the intake,past the now-opening intake, which formerly reduced the vacuum in the intake. The more overlap your cam has, the lower the intake vacuum will be, and especially at the lowest idle speeds, because this allows more time(in degrees) at piston TDC(overlap), for the burned up exhaust to travel into the intake, when the intake valve pops open. As an example; the 292/292/108 Mopar cam has 76* overlap, just at advertised duration. Whereas a 268/276/110 might have only 52* at advertised; that works out to the larger cam having ~50% more overlap/ 50% more TIME,in degrees.
So your first defense is increased idle speed, and the second is increased idle-timing.
There is nothing wrong with a low-vacuum idle, if you can tune for it. which with a carb is not all that hard. But with EFI that depends on a MAP sensor, it gets tricky.
So like has been said, start with a higher rpm and more idle-timing, then as time goes by and you get a handle on your tune, you can experiment downwards.
How much idle-timig to run is somewhat dependent on your TC if automatic, and on the very low-rpm jumpiness if a standard trans.
Generally, with a big cam, and a regular mechanical advance distributor, you want to run a lot of idle timing, because this increases the low-rpm responsiveness. But with a low-stall TC or especially with a manual trans, the more powerful pressure pulses can make low SPEED driving a little jumpy.With a factoryD you are limited to a two-step curve at best.
With computer-controlled timing however, you can take care of all that with programming. You can advance or retard anywhere you want.
Say with a manual trans and 3.55s;
You can have a start retard to say 5*up to 450rpm, then have a smooth idle with 15/20*beginning at say 700, and cut back to say 8* at 900, so you can idle along at 7 or 8 mph, and not have the car start bucking like a wildhorse. Then you can bring it back in to say 28*@2800 for that low-rpm responsiveness. Then slow it down to 34* at 3400, so you can burn 87E10 with aluminum heads and stay out of detonation. Your options are limitless.
With an automatic, and a 2800 or better TC, you can just about do whatever you want cuz the TC will suck it up. But there is no good reason with a TC to run just 10* with a big cam; you're just sending a lot of unburned fuel out the exhaust; OOps that's carb-talk,lol. With EFI,and a MAP sensor you're just complicating the fuel tune.
IMO
Thank you! I really appreciate the explanation. Not going to pretend I understood it completely, but it’s making more sense now that I’ve read it 3x! :lol:
I’m looking forward to playing around with it this weekend.
 
What timing do you have programmed in for cruise? If you have 36 at WOT then you should probably have something around 45 degrees at cruise. That will help the engine run smooth going down the freeway and will give you better mileage.
 
Yeah I had a hard time trying to make it comprehensible too! lol. I knew what I wanted to say, but I spent some time in editing to get as good as I could. The Key is in the overlap.
There is only one way to make more power with any given engine size and that is to put more fuel and air thru it. The fuel is easy.
There are only three ways to put substantially more air thru a given engine; more head flow,super charging, or more rpm.
More rpm by itself is not the answer; the engine has to process more air, and that requires ever increasing valve-opening times.
Your engine needs more and more intake/exhaust durations to make power at those ever increasing rpms. There are only a given number of degrees to work with; namely 720.To get what you need, you have to steal it from the compression and power degrees. And when that runs out, you have to start at the other end; and when you do that, the overlap grows and grows. Eventually it becomes a fifth cycle, and becomes a mini supercharger at mid to high rpm.
But in so doing,the cam almost destroys the engine's ability to idle at a street-normal sub-900 rpm, as the intake and exhaust get all mixed up in where they are supposed to be going.
So while a lot of overlap at higher rpm,the result of the long durations, can make a powerful engine, it is lousy at idling.
And while most SBM engines make best power at 34 to 36* of power timing, this is generally waaay to much at WOT/lower rpms. The use of hi-stall TCs has opened up the opportunity to give the engine the idle-timing it wants, yet not detonate itself to death at stall rpm.
As an example, on a distributor engine, you can often advance the idle timing to 40/50 degrees with accompanying ever increasing rpm;but with a lo-stall factory TC it might rattle like a diesel on take-off.
And it is possible to cruise 65mph with as much as 55/60* timing, but it better drop out like lightning when you go to pass someone.
 
What timing do you have programmed in for cruise? If you have 36 at WOT then you should probably have something around 45 degrees at cruise. That will help the engine run smooth going down the freeway and will give you better mileage.
I don’t remember off hand what I set it to. And seeing that I am unable to drive it at this point, i’ll Have to save these conversations for future reference. I will be playing with the timing and tuning this weekend. Looking forward to the tuning process. Thank you for all the input.
 
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