Hot Rod Forum banner
1 - 10 of 10 Posts

·
Registered
Joined
·
393 Posts
Discussion Starter · #1 · (Edited)
Hello everyone, I have a question about my manifold vacuum reading dropping a decent amount after a cam/intake swap.


My original set up was a comp 282s cam (235/235 @ .050" & 282/282 advertised, with .521"/.521" lift with 1.6 rockers, 110 LSA) and a RPM air gap intake with a 2 inch open spacer. With this set up I had about 15" Hg at 900 RPM in park and about 11" Hg at 700 in drive with my foot on the brake. Timing was set to 16° initial and 35° total all in by 2500 RPM.


I swapped out to an Isky z-25 cam (240/240 @ .050" & 278/278 advertised, with .512"/.512" lift with 1.6 rockers, 108 LSA) and a professional products single plane intake. With this set up my manifold vacuum reading dropped to about 10" Hg at 850 in park and about 6" Hg at 650 in drive with my foot on the brake. Timing had to be kicked up about 3° initial to get a decent idle RPM, so it's now set to 19° initial and 35° total all in by 2500 RPM.


The engine is still the same everywhere else, Holley 670 carb, same HEI ignition with a high volt coil, same heads/valve springs (Comp #986-16)/etc. It's a 355 with a SCR of 10.1:1.


After I broke in the new cam I changed the oil and took it for a test drive. It felt extremely lazy and when I was at low RPM in the transfer slots it would miss all over the place. I pulled some plugs and noticed with the old carb tune for the other cam and intake it was running nasty rich. So I ended up back down two jet sizes on each circuit to 69 jets on the primary and 76 jets on the secondary, dropped from a 7.5 power valve to a 5.5 power valve, and I had to drop my IFR from .030" to .028" to get the optimum vacuum/plug readings with the mixture screws at about 1.75 turns out. Now it runs a lot more crisply, but vacuum still will not get any higher.


Is this normal for a set up like this?? The cam is very similar in specs other than a little more duration at .050" and a tighter LSA, but not much.


Any Ideas on how to get a little more vacuum out of this at idle?
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #2 · (Edited)
Yeah that does make sense.. I just didn't expect to lose 5" Hg from it. I was thinking I'd lose maybe 3".. so that's normal then? So then how low do motors with huge aggressive cams even manage to make vacuum? For example a cam with 250°+ at .050" and a tight 106 LSA? That isky z-25 is small compared to some of the cams you can get.

What benefit would I get from less total timing? Or do you mean take out some total timing so I can get a few more degrees of initial?
 

·
Registered
Joined
·
4,338 Posts
Just to show a difference in vacuum reading. On my old short lived comp big mutha thumpr roller cam with 243/257 @ 50 duration and a 107 lsa with a 1000 rpm idle in park I had only 6 inches of vacuum and it was just too much cam and a headache driving with it and swapping it out to a lunati voodoo roller cam with only 231/239 @ 50 duration and a 110 lsa I now get 12.5 inches of vacuum and engine tuning was a little easier to tune and ran better.

Just like stated the bigger the cam and the tighter the lobe separation angle the less vacuum you will have and the more timing the engine will need in order to run and be able to run.
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #4 ·
Yes more inital timing and less over all with really large cams it normal to lock mech advance and not use vacum at all and just run full inital timing. Your cam shpuld not require this but may run a little faster like this. Gas milage will suffer with mech and vacumn advance.

Sounds like you got the tuning down sometime using the vac guage to set idle can bring back a litle vacum but i am assuming you have already gone this route.


Yeah I tuned the mixture screws with the vacuum gauge.


Man more initial timing?? Lol yeah I've read about guys locking out the dizzy, but man I couldn't imagine running more initial than 20°.. But it is worth a shot! I don't have stops in my mechanical advance, so I have to do the trial and error game with a stop screw. I have a hole tapped out for 1/4-20 threads in my dizzy so I use the allen head bolts and thread them in so it limits the amount of travel from the mechanical advance weights.. I adjust the amount of travel by shaving material away from the head of the bolt. It works great! Just a bit of a trial and error sorta deal.
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #5 ·
Just to show a difference in vacuum reading. On my old short lived comp big mutha thumpr roller cam with 243/257 @ 50 duration and a 107 lsa with a 1000 rpm idle in park I had only 6 inches of vacuum and it was just too much cam and a headache driving with it and swapping it out to a lunati voodoo roller cam with only 231/239 @ 50 duration and a 110 lsa I now get 12.5 inches of vacuum and engine tuning was a little easier to tune and ran better.

Just like stated the bigger the cam and the tighter the lobe separation angle the less vacuum you will have and the more timing the engine will need in order to run and be able to run.


Yeah that helps Eric... I actually found a thread on corvette forum where everyone would post their cam/engine set up and the vacuum they had, just for a kinda informational thing for everyone else to base off of.


Man it's crazy what a tighter LSA can do to vacuum lol.. I only went from 110 to 108 and a tid bit more duration at .050" and I lost 4 inches.


Even with your set up switch you gained twice the amount of vacuum!


I think next time I'll stick with a 110 LSA.. I just tried a 108 because supposedly the tighter LSA gives a higher peak power, but at the loss of a longer power curve.
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #6 ·
With vacum advanced hooked up you will have a lot more than 20 degree timing at idle. So more than 20 is not unherd of just be careful that you total does not crazy. Some mech advance can travel way beyond useful. Low compression motors can tolerate tons of timing so the factory used this to increase fuel milage. All this stuff is bad for high compression engines.

But it can lead to some hard start problems. If it turns slow make sure you use a ford starter relay kit.


Yeah makes sense.. but lucky for me I don't have vacuum advance because my Quick Fuel metering block doesn't have a vacuum fitting on it.. and I don't want full manifold vacuum because chances are that would be too much timing.


After 16° of initial timing I started having hard starting problems with my stock direct drive HUGE HEAVY starter.. so I changed out to a 1999 Tahoe starter which is a factory gear reduction starter. Works like a champ now.
 

·
More for Less Racer
Joined
·
20,442 Posts
The previous posts have got you lined out, but I'll add a few things....

If you were to install the cam 4, 6, 0r even 8° advanced (at a 98-102° intake lobe centerline) you could get some vacuum back, it's not the overlap that is leaching off your vacuum, it's the now later closing point on the intake lobe ...the result of adding duration and decreasing LSA at the same time. Moving the cam advanced will lower the peak power rpm point, which works if you want more low end punch.

If you were to add an adjustable vacuum advance can and Crane's adjustable vacuum advance limiter plate you could dial in a lower 10-12° amount of vacuum advance to be added and be able to set the amount of vacuum level needed to pull the can in, hooked to manifold vacuum your engine may like it, and it will help with 3-5 MPG. Manifold vacuum drops to zero when you go to WOT, so it can't over advance things when you are under full power. Dialed in to pull in at cruise rpms it can do wonders for keeping clean plugs and better mileage.
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #8 · (Edited)
The previous posts have got you lined out, but I'll add a few things....

If you were to install the cam 4, 6, 0r even 8° advanced (at a 98-102° intake lobe centerline) you could get some vacuum back, it's not the overlap that is leaching off your vacuum, it's the now later closing point on the intake lobe ...the result of adding duration and decreasing LSA at the same time. Moving the cam advanced will lower the peak power rpm point, which works if you want more low end punch.

If you were to add an adjustable vacuum advance can and Crane's adjustable vacuum advance limiter plate you could dial in a lower 10-12° amount of vacuum advance to be added and be able to set the amount of vacuum level needed to pull the can in, hooked to manifold vacuum your engine may like it, and it will help with 3-5 MPG. Manifold vacuum drops to zero when you go to WOT, so it can't over advance things when you are under full power. Dialed in to pull in at cruise rpms it can do wonders for keeping clean plugs and better mileage.


You know what? Thanks for bringing that up because I completely forgot that my dizzy has an adjustable vacuum canister on it. :drunk:


That will definitely help my idle a ton I'm sure.. I'll probably just dial it in so I get very little advance from it because I have such a quick mechanical curve.


And I've never tried advancing a cam before.. I've always kinda been a little weary of it. This Isky cam is supposed to make power from 2500 to 6500, so if I were to advance 4° (my timing chain only allows 4° advance or retard) how much would that affect the power curve?


Thank you, Eric lol.
 

·
Registered
Joined
·
393 Posts
Discussion Starter · #9 ·
Thanks a hcompton, I probably won't mess with the cam timing until I've tried EVERYTHING else first anyway. I'm gonna throw in a looser converter (3000 rpm) and try adding some vacuum advance. I have a feeling that'll give it better manners down low.
 

·
Registered
Joined
·
192 Posts
The vaccum advance can help with engine vaccum and idle quality. An engine at part throttle tends to have a leaner mixture than an engine at full throttle. The lean mixture needs more advance to burn properly.
Experiment some. 20 degrees initial and a 20 degree vaccum can will really smooth out an idle. If it pings under load at part throttle back off the adjustable advance. The Crane adjustable vaccum cans came with instructions on what to do.
 
1 - 10 of 10 Posts
Top