[Rholmes]

Can someone explain to me any relation between the amount of vacuum our Pontiac engines make versus it's c/r?I've noticed that certain cams require x amount of c/r,but offers low vacuum.
And how much compression ratio can an iron head pontiac tolerate without going into detonation?Or does this depend on carb,intake,cam and timing combo.Thanks for any helpful info.

[Rholmes]

Can someone explain to me any relation between the amount of vacuum our Pontiac engines make versus it's c/r?I've noticed that certain cams require x amount of c/r,but offers low vacuum.
And how much compression ratio can an iron head pontiac tolerate without going into detonation?Or does this depend on carb,intake,cam and timing combo.Thanks for any helpful info.

[Half-Inch Stud]

..can't wait to see the answer to this one. Here's my try:

Hmmm: low compression engines have less vacuum signal, less HP and torque than high compression.

Yet high compression engines tend to receive larger cam profiles, that reduces their low RPM efficiency, thus reducing the vacuum signal.

Safe ranges to work within:

Street Compression 9.0:1 to 9.8:1
Street/Strip Compression 9.5:1 to 10.5:1
Strip only: 10.5:1 to 13:1

Street Supercharger for looks, not much boost: 9:1 to 9.5:1
Street Supercharged for power: 8.0:1 to 8.5:1
anything else is ego stroking.

Strip Supercharged: 9:1 to 10:1

Nitrous...I dunno.

Rule #1 : what's your intended application?

H.I. Stud

"11.00/123MPH/1.50 60foot/29.5"/4.10:1/10"/472 #48/Flat HYD/DualQuad/Wenzler/3250Lbs
12.00/112MPH/1.61 60foot/26"/3.31:1/10"/472 #48/FlatHYD/Q-Jet/Torker/3650Lbs"

[Brian Baker]

I don't see where the compression ratio has anything to do with how weak or how strong the vacuum signal is. That 8:1 350 I had in my ole '78 T/A would pull 19" of vacuum at idle. I think cam timing and stroke have alot more to do with how strong or weak the vacuum signal is than the compression ratio does.

[Brian Baker]

Here's perhaps a more direct answer to your question about vacuum and compression ratios.

Cams with long duration valve events (ie - BIG cams) tend to have more overlap between the intake and exhaust valve events than those with smaller duration events. This can be lessened somewhat by widening the lobe seperation angle (often called the LSA or LDA), but there will still be a significant amount of overlap on the "bigger" cam. This overlap is a large contributor to the lower vacuum signal at low rpm's.

Overlap bleeds off cylinder pressure by allowing a portion of the incoming intake charge to bypass the cylinder alltogether and go right out the exhaust valve. To bring the cylinder pressure back up to a satisfactory level that makes power, the compression ratio is raised. This is why "bigger" duration cams are always recommended to be run with engines having a higher compression ratio. You can still make power with certain "bigger" cams and lower compression ratios (NHRA Stock class racers do it all the time with "cheater" cams), but the overgeneralizing consensus is that "bigger" duration cams need more compression ratio to make adequate power.

[Rholmes]

Brian and HIS thanks for the replies.I'm in the process of putting together a 0.30 overbore 400 with #16 heads,hyd roller(CC'S HR288),th400 and 390 gears.I have 2 friend's,one has a 70 GTO 400 with heavily ported #48 heads and "big" Erson cam that runs mid 7's 1/8 mile,the other friend has a 71 GTO 400 with ported 670's RAIV cam running low 8's.These times is not what's of interest but the fact they're doing it on PUMP GAS with about 40 deg. total timing,and NO dish pistons.The #48's were actually cut for 66cc chambers.So not being a physicist or engineer I'm thinking that maybe the "big" cams are "bleeding" off excess compression and making the engines lower octane tolerant.Am I going the wrong way in my thinking?

[Will]

Rholmes, you are correct that the larger cams are bleeding off cylinder pressure which helps to stave off detonation. There are a lot of tuning issues that need to be taken into account, as well as the quality of gas in your area of the country. Around here the best you can get is 91 octane camel piss that's often cut with ethanol which effectively makes the engine run leaner. I guess in some areas of the country 94 octane is available.

Altitude also plays a role. You can get away with higher compression ratios at higher altitudes since the air the engine is ingesting is less dense, thus less cylinder pressure develops than at sea level.

I'm going to go out on a limb and guess that your friend's cars aren't driven a whole lot on the street in stop & go traffic, or that you have some good gas available in your area, or that you are at a relatively high altitude. Gearing and converter selection also come into play, if the engine doesn't see a significant load below 4000 RPM then it will be more detonation resistant. Depending on various factors, about 2500 - 5000 RPM is the danger zone for detonation. If it's occurring at higher RPMs than that, you have something way out of whack in the setup or a REALLY high compression ratio.

[Brian Baker]

In addition to what Will said, although the overlap of a "big" cam is bleeding off cylinder pressure, it is less noticeable at higher rpm's. The higher the rpm, the less "time" for the cylinder to "bleed off" pressure. Ever noticed how a "big" cammed engine lopes at idle but smooths right out when you rev it up?

[78 GHOST]

Yeah, what those guys said. [img]/infopop/emoticons/icon_biggrin.gif[/img]

____________________________________
I work in high speed aluminum tubing

[Will]

Oh, forgot to point out one other important aspect of cam timing that plays a large role in the amount of cylinder pressure your engine develops - the intake valve closing point.

The later in the cycle the intake valve closes, the less cylinder pressure you develop at lower RPMs for the obvious reason that the mixture can't be compressed until the valves are closed. When you add duration to a cam, you generally end up with the intake valve closing later. You can also affect the intake closing point by moving the intake lobe centerline around. Advancing the cam makes the valve close sooner, thus more pressure, retarding it does the opposite. This is only at lower RPMs, at higher RPMs the effect is the opposite. That is, an advanced cam devlops less high RPM cylinder pressure as compared to the same cam with less advance or a retarded intake centerline.

You will notice that most of the famous factory cams from the '60s, like the 067, 068, 744 and 041 have relatively late intake centerlines, right around 110 - 113 degrees, and thus later intake closing points as compared to modern performance grinds. This is because the engineers knew about the relationship between cam timing and cylinder pressure and were designing these cams for best performance in 10:1 or higher compression engines. The 066 cam was the 2-bbl cam and generally came in lower compression engines, and it's intake centerline was right about 106.

[Dave A]

The dominant effect is indeed cam timing (and intake port/induction system design play a role too).

My theory on the 'direct' effect of Compression ratio on engine vacuum: higher CR does give you slightly higher vacuum. In order to increase compression ratio for a given engine, you must reduce the combustion chamber volume. The driver for creating in-cylinder vacuum to fill the cylinder is the piston moving down and changing the volume of the cylinder. The smaller the combustion chamber volume, the quicker the 'rate of change' of overall cylinder volume for a given engine at a given speed, and the stronger the vacuum signal. The higher the CR, the higher the overall 'expansion ratio' and the larger the overall 'percentage increase' in cylinder volume the cylinder sees as the piston moves from TDC to BDC.

Cam timing of course screws a lot of this up and is a stronger effect, so you see a lot of low vacuum, high CR engines and a lot of high vacuum, low CR engines running around.

Knock tendancy is affected by: static CR, cam timing, induction system, exhaust system, combustion chamber shape, bore size, piston speed, spark timing, fuel quality, fuel/air mix quality, cylinder head material, engine material, intake air temperature, coolant temperature, piston design and material, and more. Very hard to set a precise limit - but the above mentioned guidelines are good to start.

An engine's octane requirement could be defined as the minimum octane rating required to prevent an engine from being 'knock limited' in its tuning - i.e., you can advance spark, set fuel ratio, etc, to achieve maximum power and not see any detonation. It is generally better for power to actually pick a static CR a little on the low side, and make sure it's tuned right, than to have to back off timing because the static CR is too high!

[Half-Inch Stud]

I have been seriously trying to lower the compression-related ping on my RAM III headed motors for years.

MY enginework features timing backoff until ABOUT 2500rpm. Then total advance in by 3300RPM, or whatever the highway cruise RPM might be.

Difficult effort to gain strong idle vacuum with advanced timing, then obtain off-idle retard-timing, then re-advance timing vs RPM. I have failed in those efforts.

I really desire an electronic, programmable timing to replace the weights. Also, Vacuum Advance pod remains a ping-problem for off-idle response...Vac ADV pods respond too slow.

Thus far: medium springs, medium (139?) weights, and ball-bearing blocks to limit ADV weight travel has been my best HEI solution .

"11.00/123MPH/1.50 60foot/29.5"/4.10:1/10"/472 #48/Flat HYD/DualQuad/Wenzler/3250Lbs
12.00/112MPH/1.61 60foot/26"/3.31:1/10"/472 #48/FlatHYD/Q-Jet/Torker/3650Lbs"

[Brian Baker]

Dave, your first paragraph there on your "theory" was described in an axcellent manner. Very well put. I've never thought about it from that perspective, thanks for sharing that!