[Raven Blayde]

For those who remember the overheating, terrible performance and detonation issues I've ran into since my cam change (to the XE274H), I'm back at it and have some questions. One of the tech guys at Comp Cams wants me to do a cylinder pressure check and give him the results (which I will do). He believes that I may be having an issue with my cylinder pressure being too high and switching to this smaller cam would cause the pressure go up. ?????? I didn't want to look stupid on thier board by asking too many questions that I probably SHOULD already know, so I'll do it here in front of you guys. What on earth is he talking about? Remember, I'm a "Shadetree Mechanic", so I'm not quite following all this. I understand where bad rings, a cracked piston or such could cause low cylinder pressure (if it bleeds off or something), but how do you get too much cylinder pressure? I'm not getting it.

[Evil Stu]

Raven,

In a nutshell; the smaller cam closes the intake valve "sooner", i.e. with the piston further down in the hole. The piston then rises in the bore longer while both valves are closed (remember, no compression happens until the valves are closed). So cylinder pressure rises.

There's tons of stuff here on Dynamic vs. Static Compression and all the associated theory about valve events and under what circumstances you may want to move them and the potential consequences of such a move.

HTH,

Stu

[steve25]

A motors compression ratio and its cylinder pressure are do not go fully hand in hand.
In a typical 14 to 1 compression full race motor with a intake cam duration of [email protected]" we will see cranking compression readings of 200 psi or more.
In a street motor with a 9.5 compression you can see cylinder psi numbers of 180 which can be a problem with that much running pressure.
The main differnces between the pressure is how long the cam holds the valves open.
Your motor may not have 14 to 1 compression but the small cam is creating near the same cylinder pressure as if it did!
Other factors come into play here also, as in.
You may have installed the cam advanced by using the wrong key postion on a multi keyed crank gear.
You distributor could be providing too much timing.
your carbs jetting could be lean, or you could have a combination of two or three of these things making for your trouble.
I never saw your first post about this so it would be good to have more detail on the motor and the cars gearing and such.

[Half-Inch Stud]

To be "Closing too early" on a Compression test would equate to 285 to 300 PSI on a poked 455 with Flattops & Ram III heads. I know for sure.

Throttle plates held wide open, no air cleaner, 8 plugs out, compressions into the gauge.
================================================== =============
Because of that 1st-hand data, I honestly do not know what the ideal compression readings would be for a successful 1/4mile Street/Strip 455 running 11's or quicker on an economy rear gear.

[chiphead]

Intake closing affects cylinder pressure. The earlier you close the valves and seal the cylinder, the higher the dynamic compression. This can cause detonation.

I still think the cam needs degreed, and compression checked. You also need to triple check cooling system for blockages, misplaced gaskets, etc. Check timing curve too.

I'm surprised you have not done these things yet. What you're seeing is NOT normal, even if the cam is too small. I'm guessing you've got a few things wrong that are compounding to really mess with you.

[440GP69]

Yeah, Detonation can also be caused by The engine temp being too hot, especially cylinder head temperature, any sharp edges on the pistons,combustion chambers can glow red and cause pre ignition.
Dynamic Compression is your engines actual running compression ratio
Static compression ratio is your mechanical compression ratio ie pistons and cylinder head
Different Types of engines can run More Dynamic compression than others for example
My 5100lb 4x4 2006 Silverado 383 LS based stroker engine is 11.44:1 static compression and i'm running a 9.27:1 dynamic compression ratio 246psi cranking compression Over 610hp at 6500rpm on 92octane pump gas
Now There is a lot of supporting mods to do that LOL!
Your 274 has an advertised duration of 274 intake 280 exhaust i believe, your cams advertised duration is very short "small" even for a small pontiac v8
which means they are closing the intake valve too soon or at least not optimised for your combo, Factory Pontiac Cams had Huge Advertised durations of like 320* versus 274 with comparable duration at .050 and Pontiac would also widen the lobe separation angle to smooth out the idle, bleed off cylinder pressure and Build More Average Power across their usable rpm range,
I don't know your engine specs but i'd bet that cam is your culprit, either that or your timing is jacked to 40* by 2000rpm
To find out, set your timing back to 28-30* and try working your timing back in 1-2* at a Time, if it can only handle 25-28* its the cam Provided all else is ok IMO

[Cameano]

My son's 11:1 350 sbc had a small cam when he bought the car. '78 Z-28 Camaro with 48k on the clock, been in storage since the early 90's, nice 80's throwback. I was seeing 240-250 psi cranking pressures with it, and lots of pinging. We put in a .525" lift Lunati Voodoo cam, with a lot more duration and lift than the old Crower "Beast" grind. It still has 225psi or so cranking pressure, but with a good distributor recurve, limiting vacuum advance to 10° on the can, and good carb tuning, it's running really good now. Engine runs real cool, makes great power, and doesn't ping. It can be done. Stepping up with the cam helped out a bunch, but so did the bigger intake, Performer RPM vs. Performer.

[440GP69]

The Bigger intake helped because it slowed the air down in the low-mid range fwiw

[Raven Blayde]

I'm begining to understand now! Thanks for the great info here guys! I'll start back with the engine combo for those who don't remember or didn't read the posts before. 1972 400 (.030) with SpeedPro forged flat-top pistons, block-zero decked, CAT forged H-Beam rods, factory crank and rotating assembly balanced. Heads- 1968 72cc d-ports (actual cc was 86-giving me around 9.5:1 CR), heads ported to 240cfm by Kauffman Racing, #995 valvesprings (setup at a machine shop with correct installed height), stock length valves, stock length pushrods and 1.52 Comp Magnum roller tip rockers. Have tried setting the valves at both-1/2 turn and 1/4 turn past zero lash. Using a CAT "Quiet" gear drive with single key crank gear. Holley Street Dominator intake with Holley DP carb (has a ProForm 800cfm main body kit with 78/86 jets and no choke horn). Distributer- I sent my old one to Cliff Ruggles for a check. It had issues, so I've got one of his distributers in the car right now that we know is good. Changing distributers didn't fix it. Cooling system- 4 core aluminum radiator, 5 blade Super Duty clutch fan and stock waterpump. I've flushed the engine block and radiator several times now and have disassembled the waterpump. Checked the bearings and reassembled with new gaskets-No change. I can set the timing on this engine as low as 32 degrees total and it still overheats in just a few minutes. Haven't degreed the cam or checked the cylinder pressure yet for two reasons- #1-I don't have the equipment yet. #2- When I do get these things, I'll have to learn how to use them. But that's where I'm at so far. Car is back in the garage with the timing cover off. The timing dots on the gears are still aligned perfectly, so I had that part right anyway.

[Raven Blayde]

Borrowed a guage from my neighbor and we did the cylinder pressure check- All cylinders are between 150-170 psi.

[ta man]

So this engine overheats?You have a fan shroud?Can you confirm your clutch fan works?Thermostat?

[PontGuy]

Just read through all three of your threads- it would be a little easier to follow if they were combined. Just to keep everyone straight you checked the compression today with the engine cold because you have the timing cover off and in the middle of things, right? But 150-170 sounds very reasonable. I would check it again when you have everything buttoned back up again to make sure the actual numbers are not too far off from the cold reading.

I actually would have expected higher compression numbers with your combo, so maybe the cam key is cut way off and the cam is actually retarded? The only way to know for sure is to degree the cam.

Ignition timing can certainly have a big impact on performance, but I am having a hard time believing that it would make the engine run as hot as you are describing.

[ry57pont]

I once had a a motor that would overheat, after i tried EVERYTHING, an older member of the car club noticed that the v-belt i was using was bottomed out on the pulley, causing slippage. jsut a thought. - Ryan

[440GP69]

To check Cranking Compression as HIS said
Motor needs to be at operating temp
all Spark Plugs Out
Throttle wide open
Crank till you see 4-6 pulses
170 cold is High!
you wont know exactly what it is untill you do the above fwiw
As steve25 asked what is your cam degreed at or was it degreed? probably 5-6 degrees advanced
Personally i wouldn't build that combination with a iron headed 400 at 9.5:1, try 8.75:1-9.00:1!

[Raven Blayde]

I see someone brought back the original thread I started from way back, so to avoid confusion, I'll start posting on that one again. Sorry for the confusion guys.

[Half-Inch Stud]

TO make progress , you will need to get the Compression #'s for each CYL.
When the numbers all show to be 190PSI +/-30 then we will all know you got a proper cam-train, and the Bottom-End holds the problem (rod bearing worn out and possibly spun).