Too much cam...too little vacumn....

[waterboy1976]

Im loking at the vacumn reserve canisters from Summit racing. Wondering who out there has added one and what should I be concerned with. Is there a specific size? Is one TOO small or too big. Thanks.....

 

[1979toy]

I have one in my '79. I no longer use it since I have converted to a Hydro-Boost unit. The vacuum can worked OK for most uses but if you were in heavy traffic, like leaving a cruise in or on a poker run it would run out of vacuum on the second, third or forth use of the brakes. You never knew when it was going to happen and when it did it alays created a minor emergancy stomping on the brakes hard enough to get stopped. I had disconnected the unit before I changed to the hydraulic brakes due to the issue.
The vacuum unit only traps the vacuum that your engine makes. My car idled with 8-10 inches of vacuum and went up to 19 inches during coast down. So after running down the road at 40 MPH you coast down to enter a cruise site and the can fills up with the full 19 inches of vacuum. Each time that you hit the brakes the unit gives up some of that vacuum while the engine is only making 8-10 inches of vacuum while you cruise in to a parking spot. You may still have 15 inches of vacuum in the can when you start up to leave with the engine still not making more than 10 inches. Revving the engine and letting it coast down will help but is not always practicle. By the time that you've gotten yourself to the exit you've hit the brakes 3 times and you are down to 10 inches of vacuum in the can, that being all that the engine has put out. There is a major differance in pedal feel between 19 inches and 10 inches of vacuum.
Overall I would not do it again. I would try to maximize my vacuum through advancing or retarding the cam, or rocker clearance adjustments if you have a solid lifter cam.

 

[SG4206]

I agree with Steve ; I've had the same results.

 

[waterboy1976]

Ok..heres my specs on my cam what do you guys think..Will I have a big problem if I dont run a can.

Cam: .512 lift intak .544 exhaust Duration 308 Intake 318 exhaust
Intake: RPM Air Gap
Carb: Demon 650 (mech 2ndries)

Should I bother with a can? Thanks guys. This is one area Im trying to learn all I can..

 

[SG4206]

You realize this is a race cam , not really a good sreeet cam?
This has too much duration and probably a 108 or 110 degree lobe separation.
In my opinion ; you have to stay 285 degrees advertized or below on duration ; and stay with a 112-114 lobe separation; if you want the power brakes to work with a can.

It would be worth a call to Comp Cams or your favorite cam company and talk to them about this. They probably know the exact max. cam you can get away with.

 

[BacaBill]

I've got a Comp Cams Magnum 286 w/ 1.6 roller rockers & have NO vacuum problems. The problem I do have at the moment is that a few of the lobes have spun down & concaved the lifters. I'm pulling it out today, I'll try to post pics. It was an excellent cam while it lasted but didn't really "kick in" til it's rated 2200 rpm. I may move down to a 278 for a little more off the line torque.

 

[1979toy]

The cam that you are using is only going to work with a large displacement engine with a lot of static compression.
As an example the cam in my 12 to 1 static compression ratio 406 small block is a 300/310 on a 114 lobe center. I have 10 inches of vacuum at idle in gear. The reason that I can get away with such a high compression ratio and run on pump gas is because the cam bleeds off a bit of the compression. I have a 3200 stall torque converter and I suspect that the engine would like to have an even higher stall.

What are the rest of the specs on the cam and the engine that this cam is going in?

 

[waterboy1976]

the motor is a 350 .40 over
dart 2.02 heads 64cc chamber
block decked 20thou
heads decked 10 thou
1.6 roller rockers
total compression 10.32:1
rpm air gap
demon 650 (mech 2ndries)

 

[waterboy1976]

edelbrock told me at that setup I should have 10 to 12 at idle.....

 

[iron cross]

The headlights and wiper doors will open slow, but out side of those problems you shouldn`t have too much trouble. I sucessfully use taller solid lifter cams than that on the street.

 

[AKRAY4PLAY]

Rob,

what is the duration at .050"?? advertised duration can be tricky to judge due to ramp build angle. i am guessing it around 230/240 at .050". if so, that is a huge cam for a 350 on the street. with the single plane intake, that cam will be very soft on the bottom end of the rpm spectrum. what size are your intake runners on the heads?? Brian

 

[1979toy]

The specs on that cam are 234/244 @.050 on a 112 lobe center. It is a hydraulic lifter cam. Seat to seat it has 89 degrees of valve overlap!

You might want to download a program called Dynamic Compression ratio and run it with your numbers. I suspect that this cam will make VERY LITTLE dynamic compression. E-mail me at smericle@cox.net and I can provide you with the rest of the camshaft information that you will need to run the program. You can download a copy of the program from here http://cochise.uia.net/pkelley2/DynamicCR.html

For what it's worth, I looked at that camshaft for my 406 with 12 to 1 compression and decided that it was too much cam for my engine.

Could we steer you toward maybe a Comp XE274 or similer?

 

[waterboy1976]

heads are 180 cc runners and 64 cc comb chambers.....

so basically this cam they put in is not going to be good for me

also the RPM Air gap is a dual plane isnt it?

thanks for all your guys help....

cam lift is corrected at 510 and 544 with the 1.6 rockers.....

 

[Bender]

Dont they make electric vacum pumps?

 

[AKRAY4PLAY]

Rob,
my bad, i was thinking of the Victor air-gap manifolds, the RPM Air-gap is a high rise dual plane intake. do you have the motor installed already??? if not, i would take the advice already mentioned and look to a smaller cam unless you plan on driving at 6000 rpm all day. if the car is for racing only, you could probably live with the cam, but you'd have to turn 3500 to start getting the motor to make any power. as a daily driver, it will give you fits and run very doggy on the bottom rpm spectrum. Brian

 

[waterboy1976]

Does it matter that this is a 4 speed car or not?

Im starting to get ticked at Summit, Edelbrock, and my local speed shop who has been in business for like 50 years...

 

[1979toy]

All of the "experts" will tell you that a stick shift car can handle a bit more camshaft than an automatic equiped car but I really think that what matters more is the fact that your '73 is at least 500 pounds lighter than my '79.
If we were to line up at the drag strip you could launch at a higher RPM than me, however my torque converter is matched pretty well to the camshaft needs and I don't think that either of us has enough traction to give the manual clutch car an advantage. On the street you've got to nurse your engine in to the power band by slipping the clutch a lot to move from a stop while I can just press the throttle and move off, letting the torque converter do it's slipping while I sip a double latte from the local Starbuck's. Where the manual is a great advantage is down track or cruising. When everything is working right I still have some amount of torque converter slippage while you have your drivetrain locked up solid so your cruising RPM, and thus fuel mileage, will be better.
The bottom line is; it's just two different paths leading to the same destination.

 

[waterboy1976]

You drink a Latte' in your baby.....ohhhhhhhhhhh........

I have the DynoSim Version 4 results of this combo and will post the results as soon as I get a chance. Its pretty interesting.....

 

[JHL]

I had a similar set up and was running the Performer RPM cam and dispite what the Edelbrock guy told me on the street it was a dog to drive, completely destoyed the clutch and was useless anywhere except on a wide open road where you could keep the rpm up above 3,000 or at the track where you could use some revs off the line.

If I'm honest I turned a perfectly nice Vette into an undrivable dog !!!! I have changed the cam for a torquey Comp Cam grind, will replace the $1200 :cry clutch with something a bit more sensible and put a proper exhaust on it.

Not just a hard lesson but an expensive one !!!!

Good Luck

J.