Bbc Comp Cams 290hr vacuum

[rapidride2]

Buddy of mine has a 461 bbc with a comp 290 hydraulic roller.

With vacuum advance disconnected this engine only gets 4hg of vacuum. I was thinking 12+ with this cam.

Any idea on what vacuum should be for a cam like this?

Duration:290
Advertised Exhaust Duration:290
Intake Duration at .050 Inch Lift:230
Exhaust Duration at .050 Inch Lift:230
Intake Valve Lift:0.578
Exhaust Valve Lift:0.578
Lobe Separation:110
2500-6200 rpm

[rgalajda]

Have you rotated the distributor advancing until best idle ?

[rapidride2]

Have you rotated the distributor advancing until best idle ?

Had it up to 38 degrees at idle. But set it at 34 to be safe for now.

[rgalajda]

In my 454 is 227@.050 with 107 LSA HFT cam . Idle is 12" vacuum at 800 rpm in gear. 32 degrees advance. I think it is 14" vacuum in neutral 1000 rpm.

[mag2555]

Was the cam degreed in?

How much cranking compression is the motor now making?

With the motor fully warmed up and at idle does closing the choke down slowly cause a increase in rpm, or does the motor load up and stall like it should when there’s no vacuum leak?

[1980RS]

Restrict total timing in the mechanical part of the distributor so you can add more base timing. Try to shoot for 20° mechanical and then use 18°when setting your timing, vacuum should come up after that.

[F-BIRD'88]

To max the idle vacuum and minimize the vac drop when idling in gear with that cam you want to mod the diz mech adv curve. Limit mech adv travel to 10° .(stock is 20-24°).. This 10° limited curve allows 26° intial base idle timing and 36° at WOT.
Then use ported vaccuum advance.
It will idle in gear steady witout vacuum surging and the PB's will function fine.
This cam needs this modified mech adv curve with
very generous initial base idle timing.
Use medium tension advance springs for a stable progressive advance curve. The lighest fastest advance springs can be inconsistant.
Be sure your Big block starter motor has the end support brace bracket installed. If its missing get one.

If you have a very low stall torque converter consider swapping that for a higher stall converter. this cam likes a 2500 rpm stall ++. A 3500 stall converter is not too much. A low stall converter with less than 2000 stall is going to not like a cam like this.
(some people work around this by simply locking out the mech adv in the diz to max idle vacuum and eliminate the vac drop in gear with a very tight oem low stall converter)
Some other people might choose to swap out the camshaft for another grind with less overlap and duration for tamer idle and low rpm operation.
Re-grinding that cam is a possibility.

[Geoff2]

Something very wrong. Cam installed retarded? Very low CR?

[rgalajda]

Have you rotated the distributor advancing until best idle ?

Had it up to 38 degrees at idle. But set it at 34 to be safe for now.

Idle timing is already 34 degrees, There is no sense recurving the distributor until you find out why the vacuum is so low. Something is wrong. Are you sure it is only 4 inches vacuum. What is the compression ratio?

[bobmc]

start with the basics, is there a vacuum leak? are all of the cylinders running? is the gauge good?

[mag2555]

That's not a lot of duration @ .050" for that CID!

In my own experience with a Cam of that duration in a motor of over 54 CID less and10 to 1 comp I have seen far more vacuum, you must have other issue(s) going on.

As aside note and while Speaking of compression, the smaller the total CC volume you have above the piston top,the more vacuum a given Cam will produce.

[F-BIRD'88]

Possible slipped harmonic balancer.
This would cause retarded ignition timing that will make the idle manifold vacuum lower than expected with that camshaft.
Check for true TDC on the balancer using a piston stop.

[rapidride2]

This 3,530 1968 camaro was very recently purchased. Street strip car.

This engine was known to need some work going in.

461bbc
"Believed" to be about 9:1 compression. Will run on 89.
For now....Comp 290hr cam.
Holley single plane
4150 carb. Currently a small 570 cfm holley vs old 750cfm that was seriously lacking. We will install a proper carb later on

4.11 gear
Turbo 400.
3000 stall.
840 AND a 291 square port head....(yeah I know)

Did a compression test. #1,148...#3 150..#5 150.
.#7 150. #2 140...#4 140...#6...130...#8 142. Sprayed carb cleaner all around intake and carb base. No issues.

Car ran better when we switched from manifold to ported vacuum for vacuum advance. Still a total dog for what it is. Dragy says 14.80 e.t. Vacuum at idle is still a best of 4 to 5 hg.

Current base timing at idle is now at 40. Don't wanna raise too much more as starting will become an issue.

Still checking. Gonna be installing an msd distributor, coil soon.

Car ran better when we switched from manifold to ported vacuum for vacuum advance. Still a total dog for what it is. Dragy says 14.80 e.t. Vacuum at idle is still a best of 4 to 5 hg.

Current base timing at idle is now at 40. Don't wanna raise too much more as starting will become an issue.

Still checking for gremlins. Gonna be installing an msd distributor, coil soon.

[rapidride2]

Sorry for the late reply..

Enough money and too little time or plenty of time and not enough money. My job is demanding on my time to tune.

Plan on larger 750 to 1000 carb eventually.

Cam change eventually.

I think this car should be 11.60 to 12.40 car at the absolute slowest.

As of now its just about pathetic with the dragy.

[HQM383]

As F-bird said find tdc with a stop on cyl #1. Confirm balancer timing marks line up. If they do great, if not then make new marks. Then get your hands on a dial indicator.

Comp Cams lobe catalog says your cam should have .063” tapper lift @tdc. When tdc has been accurately confirmed then set dial indicator up on top of #1 pushrod. Check you have .063” lifter rise +/- .005” @tdc. If not