Idle problem

[Pat]

No.   The cap is just a pressure relief device and provides access to the coolant circuit.  It is designed to keep a certain pressure thereby raising coolant boiling point.

You've got a head scratcher, man.  I am sure you'll figure it out.

[dlphil10]

Would it idle a bit more rough if the compression is lower like observed. Should I go for a cam that closes the intake valve sooner in the cycle?

[Cliff Ruggles]

120 psi is really low for cranking pressure.  Did you degree the cam when it was installed?

That cam should make a lot more cylinder pressure in a 350 engine with 72cc heads on it......Cliff

[dlphil10]

I originally had bad 46 heads on the motor when it had the comp cam factory muscle and I fixed the 061 heads as a replacement. The mistake I made was not CCing the heads. However, even as the 041 cam went and wiped a single lobe, the same head combo had at most 150psi cranking pressure. Thats what is weird.

The current 60240 cam was installed and degreed at 110 degrees. Do you think 2 degrees off (ICL of 108) will make a large difference in cranking pressure?

I did find my throttle cable was not fully extended as I was using a different year intake/throttle cable setup. It was keeping me from full throtle by about 3/4-1" so I am wondering if rings are not seated. I have verified that cross hatching is still present on the cylinder walls and that the cam is in good shape.

Thoughts? I sure do appreciate it.

[Cliff Ruggles]

120 psi cranking pressure is pretty "low" for a N/A V-8 engine.  It indicates low static compression combined with late cam timing or cam too big for the application.

We like to see at least 150psi cranking pressure for these engines, most of the ones we build here are around 170-180psi.....Cliff

[dlphil10]

Thanks, I need to do some checking and verify some specs.

[dlphil10]

I pulled the motor and have checked it out. The heads are around 72CC-73CC. I did find that the pistons are around 0.041" in the hole. This is not the only issue, but I would say this is one of the main issues (lower compression and poor quench)

I found a 400 block that I think would be a better choice. I see D dished pistons in my future.

Thanks for the help, I appreciate it.

[Cliff Ruggles]

.041" in the holes with .040" or so head gaskets just will NOT work well.  Not only does that lower the compression nearly half a point, combustion efficiency is very poor, as you were finding out.

For our engines here, we shoot for .035" quench, and NEVER over .040" for any reason.

Finishing up a Pontiac 455 Super Duty engine in the shop this weekend, pick of deck height attached.....Cliff

[dlphil10]

Thats sweet, is the deck machined down to be flush with pistons with zero deck, or are there other ways?

What head gasket do you use to get 0.035" quench?

Given my beginner status, whats the max CR you would recommend to stay on top of the tune? I would like to reuse my heads on a 400, but still run pump gas. That would put me around 10.5:1 - 10.2:1. Im going to have to find a relatively large dish I feel. Is 9.5:1 still pushing it for a beginner wanting to play it safe?

Thanks for the information, I'm trying to do this right.

[429bbf]

not a pontiac guy ,i don't know what they like for compression . I'm a ford guy and they like about 10.7 or so and they run great on 91 pump gas with about 14 degree advance .maybe pontiacs just can't handle the compression (just joking )i don't want to be throw off the site. there are a lot of pontiac guys on here including the guru cliff.fwiw

[429bbf]

cliff nice engine. i built a bbf 20 plus years ago. the flattops stuck out of the block  about .010  i used 72cc heads and .035 gasket . it had over 220 cranking compression. a little much . everything went could until i pushed the valves up through the heads . the idiot i hired to do the heads didn't put hardened seats in but charged me for them , long story short it ran about 10,000 miles and started missing . shortly after it started . once the lifters pumped up the valves wouldn't close. i ran it on pump gas but it liked to knock a little. had to use octane booster. fwiw

[Cliff Ruggles]

We shoot for zero deck then use the stock Felpro head gaskets that crush at .039".

Hardened seat sure work well with this new fuel, nothing in it to keep the valves from receding into the heads like the old fuel with lead.....Cliff

[dlphil10]

I'm not sure the hone was good on the motor. It had a really random idle which I'm thinking had to be ignition because a vacuum gauge showed a steady 17 inHg at hot idle (~22inHg cruising). The decent vacuum also makes me think that cam wasn't too bad for this motor.

I had an original 350 2 barrel that ran great, just got a lot of miles on it. Its compression should have netted around 8.8:1 in terms of actual calculated SCR. I think decking a block 0.041" seems extreme so im having my builder take a solid 0.023" of the deck and go ahead and take it out to 0.040" pistons. Im also going to reuse the same cam as it seemed to be fine. This should put me around true 9:1 and around 7.1:1 DCR (around 63deg. intake closing). It might not be the most efficient or all out motor, but it should be close to efficient as a stock build, no? This should be good to get more done to the car and then go for the 400, 428, or 461.

Thoughts on how the carb should be tuned?

[Cliff Ruggles]

I would try it as-is and see how it works.  Should be pretty close for what you are doing.....Cliff

[dlphil10]

What power piston spring should I use? I'm guessing it'll make around 15inHg in gear. Dark blue? I think I may have the orange spring in it now, for whatever reason.