I have a question for the many gurus here on the HAMB. I think I understand cams, somewhat! Compared to stock, hot rod cams have higher lift, more duration , steeper ramps to achieve this and overlap because of longer duration giving the great sound associated with a hot cam. I understand static compression ratio and that the dynamic compression is a result of the engine running and is higher, usually! I have seen hot cams advertised that you “MUST” have , like 10.5 to one compression for this cam. Can anyone here elaborate on this? Bones
It means what the heads cc's, pistons + or - is per relief or dome, bore volume and head gasket volume all equate to with no reference to camshaft at all. A big cam simply speaking can/will bleed away from above sum of request of 10.5 to a lower compression #.
How can it bleed away compression, by putting more air and fuel into the cylinders? It looks like, to me it would raise the compression! Bones
By adjusting the camshaft overlap, Or how long the intake valve is open at ABDC. The longer the overlap the more it will bleed off dynamic compression. I guess that's why an engine with only 9.1 static compression with a long overlap cam could be a real dog.
A wild cam with long duration keeps the valves open longer, this is great when everything is moving fast at high RPMs but at idle and low speed allows the mixture to blow back out of the cylinder. This is why some cammed up motors have a lumpy idle, cylinder filling is uneven.
But if you're asking this question because you have a relatively low compression motor that you want to wake up a little, there are camshafts made specifically for that type of application.
Okay, I know this is OT but it might help explain. On new dual overhead cam engines they have variable cam timing on both cams on most engines. Too much cylinder pressure causes NOx, a binding of nitrogen and oxygen molecules which in turn causes brown smog so this is a no-no. To overcome this too high pressure they used to use exhaust gas recirculation or EGR for short which is just displacing some of the oxygen that goes into the cylinder with exhaust gas from the EGR valve into the intake manifold. Once variable cam timing came along the EGR valve went away because you could accomplish the same thing with valve overlap, or opening the exhaust and intake valve at the same time. The higher pressure (exhaust) would flow towards low pressure (intake) accomplishing the same thing as the EGR valve did thus lowering cylinder pressure during combustion. A big overlap cam does the same thing (holds both valves open for a duration of time) but at high RPM this overlap will cause the intake to fill more effectively, as the exhaust is flowing out at a high velocity it causes a low pressure which will draw the intake charge into the cylinder. At low RPM the flow isn't sufficient to cause this to happen and poor filling of the cylinder will cause running compression to be lower so mechanical compression (compression ratio) needs to be higher or as someone commented earlier the engine will be a real dog at low RPM. I am sorry to make this so long but I hope this helps. Mark
Back when we couldn't get good premium fuel, I believe it was Crower who made a cam for large displacement, high compression engines that made them basically a lower compression engine. They were using overlap to bleed off cylinder pressure. When I worked for the Dodge dealership, we traded for a 440 Ramcharger that the original owner had put 12.5:1 pistons in. It knocked badly on the premium fuel of the time. We installed one of the Crower cams and it never knocked again on premium. It didn't have as much low end power as it had previously, but it ran real good once you wound it up. Second cam story; I had an LTI short block engine for the sprint car. The guy I got it from ordered the wrong part number and got an 8.5:1 short block rather than an 11:1 setup. The engine actually ran pretty well and we ran it part of one season. I was dabbling in USAC and they limited CID to 355. At the time I was running a 377 engine and didn't have a fresh 350 short block. I did have the 8.5:1. I put it in the car and it ran okay, but didn't have any bottom end power. I was running a Racer Brown 42R cam and the last time I ran the 8.5:1 engine it had an Isky 505T. The Racer Brown had a little more lift and duration than the Isky. Swapped cams and regained bottom end power. The low compression engine didn't like the extra overlap.
Just to clarify, it is high temperature that results in the formation of NOx. Nitrogen has a high reaction temperature, so at low temps the NOx compound is not formed. EGR lowers the combustion temperature to reduce or prevent the formation of NOx. Now, having high cylinder pressure does result in elevated temps, so in the long run you're right, but it is temperature, not pressure, that causes NOx.
The biggest issue with cams that have long duration, high lift and close LSA's is they will result is a lot of unburned gas going out the tailpipe if compression ratio isn't high enough to cause good combustion. Any engine can accept those big cams, and sound good. But it's simply a waste to think you can cam up a lower compression engine and get anything more than a good lopey sound. Without the proper compression ratio to go with it, the engine will have less performance, and run stinkier than a smaller cam would.
Static compression is calculated with the piston at the bottom of the stroke. Dynamic is calculated with the piston already travelled up the bore some amount due to the overlap in the cam on the compression stroke. Dynamic is always less than static because there is less stroke in the equation. Less stroke, less compression. It doesn't "bleed off" compression. It just never makes it.
gofastmath has a static and dynamic compression calculator. For an engine that's capable of running on pump gas, you want to keep the dynamic compression at/below 8:1.
And the Poser cams are even worse, using an altered timing designed to make a lame engine sound "bad". There will be those who say their whatever engines are very strong with a Poser cam, and they can run fairly well, but they would still be stronger with a "real" performance cam. The King of the long duration, close LSA noisemaker cams is still the old C8AX-6250-D 330/600- designed for Superspeedway use with a Tunnel Port engine, so below 4K use was irrelevant. If you never heard one run in a 427 with a lot of squeeze, you won't understand- one shagnasty idlin' sumbitch, doesn't really get with it until about 4500, then goes to visit Alice Kramden
Dynamic compression ratio(DCR) is a made up number that will vary with Altitude, it is not an absolute value. There are different calculators for this that gives you different answers. Too keep it simple: -An engine with high DCR has high cranking compression pressure. -An engine with low DCR has low cranking compression pressure.
I find this one is most accurate. It accounts for gasket thickness and gasket bore. When I plug the numbers into it I get 8.51 to 1. right around perfect with aluminum heads and 94 octane fuel. I have around 170-175 psi cranking pressure. https://www.gofastmath.com/Compress...atic-And-Dynamic-Compression-Ratio-Calculator
None of those DCR calculators ask for your elevation. Altitude does not enter into the equation for determining DCR. It is a pure volume calculation that determines the amount of compression after the intake valve closes by calculating how far up the bore the piston is when the intake valve closes. Then figures the compression on the amount of stroke left. I have talked to 3 different well-known custom cam designers and none of them use DCR when designing a cam. They say the DCR just comes out where it comes out. My 383 has 11.03:1 SCR. The DCR is 9.5. Way above the recommended limits. I have no trouble at all. Runs on 93 octane. You gotta keep 'em tuned up!
I have read about some high compression engines running a supercharger, but using a camshaft with enough overlap to bleed some of the cylinder pressure off. Probably not a very streetable combination, but interesting to think about.
If you want a professional explanation for DCR, a "made up number", talk to Ted Eaton (eatonbalancing.com). He might even calculate that "made up number" for you. It is true that there are variables that the online calculators do not take into account; however, if you plug in the right numbers, they will give you a ballpark figure. Use a big cam/w a lot of duration and overlap, your DCR will drop.
That extra air and fuel as you put it bleeds out when both intake and exhaust valve are open at the same time during overlap caused by cam having greater duration. Will call this pic a stock cam. You'll notice no overlap. Now will call this pic a performance cam. Now look at over lap compared to above pic. This is how bleeding of cylinder pressure happens.
Very simply put, a "static" engine is a non-running engine. A "dynamic" engine is a running (in motion) engine. It doesn't matter what the compression ratio of a "static" engine is. If the engine isn't running (not in motion), there is no compression. There is a lot of information available regarding dynamic compression ratio, even though it's a "made up number." Put a big cam/w a lot of overlap in a low-compression engine, you won't be happy with the results.
you may want to look into a miller cycle engine we had a buick? traded in where i worked and all i could find out about it seems to be what you are talking about this was a factory engine
I think that is one of the biggest mistakes made by hot rodders. Bigger must be better can cost you HP. The same goes with carburetion. Usually the cam companies have a "select a cam" form you can fill out. It usually takes a few days, but I use them a lot to make sure I am not getting overly zealous myself.
"I understand static compression ratio and that the dynamic compression is a result of the engine running and is higher, usually!" You simply have that backwards. Actual dynamic compression is usually "lower" than static compression. Static compression is just a calculated number derived from dividing the total volume (swept volume and combustion chamber volume) by the combustion chamber volume and does not include any cam timing bleed off. Dynamic pressure includes cam bleed off and can be measured in psi with a compression gauge.
Thanks guys for all this info. This was one item that I hadn’t thought out . I got my answer and the little light bulb came on about the third of fourth reply! Lol You have to have a higher static compression with a large cam to compensate for some of the compression sneaking out ( bleeding) when both valves are open. I always thought that that really only happened at low rpm and went away at high rpm. The effect, not the actual overlap, therefore not needing higher compression. Great discussion. Bones
Altitude has a large impact on Compression! If your race track is located at 5000 feet you can run a higher Static compression ratio than if you were at sea level. Why? Because your Dynamic compression ratio is a point lower at 5000 feet than at sea level! This can easily be tested by checking the cranking compression pressure. It can be up to 30PSI lower at 5000 feet than at sea level!
At higher rpms, the exhaust is leaving the chambers at such a high rate, it creates a vacuum as it leaves. When the intake valve is open during OVER LAP, the vacuum from the exhaust pulls the fresh fuel mixture into the cylinders. A really good tuned engine can use this to fill the cylinder up to or even over 100%. So over lap is a good thing when used correctly. You need cylinder heads that flow lots of air, and exhaust the is tuned for the pulses correctly for it all to work right. It's not that you must have higher compression for the higher lift cams, but if you want the cam to work to it's fullest, compression will be needed.
During overlap the exhaust valve is on it's way shut and the intake on it's way open. It's later on, when the intake valve is closing further up the cylinder from BDC on the compression stroke. As Jimmy Six said, advancing the cam will close the the intake valve sooner and help bottom end power. And, as Joe H said, the whole point of this is to raise VE above 100%, otherwise might as well have no overlap and close intake at BDC.
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