Is it possible to determine the amount of compression lost with high performance camshafts?
Either an exact way to calculate or a rule of thumb would be nice.
That way if you want to end up with a final compression ratio of 9.5:1 and your camshaft would lose one full point of compressioin you could actually buy pistons for a 10.5:1.
Maybe I am crazy but it sure seems like this would help on engine building.
Camshafts have nothing to do with compression unless it is REALLY off and the intake valve, or exhaust valve is still open while the piston is beginning the compression stroke.
You will not see any real valve overlap, or compression loss until you have a cam in the range of 300 degs. or so. Or if the centerline on the cam is crap.
Please make it more clear as to what you are asking for.
newchevyman,
You are correct high performance cams will bleed off cylinder pressure. What this is called is DCR (dynamic compression ratio). I am on my way out the door right now I will post a link to a DCR calculator when I get back. It is very helpful when trying to push the limits of pump gas.
This is also the reason bigger cams require more compression to make power. Most guys don't take DCR into account. You don't start making cylinder pressure until both valves are closed. What the DCR takes into account is the timing of the valves, rod length, etc... Meaning if you have a 3.75" stroke but the intake valve is open for .25" of the stroke you are only making compression with 3.50" of stroke (that was an over simplified example). I will return in a little while with details.
That's exactly why some guys can run 10:1 compression on pump gas because they have low dynamic compression with a big cam while a guy with 9:1 compression and a mild cam will ping constantly on pump gas.
A cams overlap will affect the readings you get on a compression test. However, that's no indication of cylinder pressures at operating RPMs, which is what matters, regardless of static compression ratio.
In short, no you can't determine compression loss with a high performance camshaft cause there's no such thing. Regardless of what size cam you run, you're still gonna have the same static compression ratio (the cam will not affect this).
This information will help me plan my next engine build. The time to start buying is now and this one will be pretty expensive, at least compared to my past builds. I just want to squeeze all I can out of the pump gas that costs so much these days and really is pretty crappy.
Naaa.. in HUGE cams it will. In cams made for something like a dragster, having really high lift and high duration (example: 310x320) and crappy center line will cause pressure to bleed out the intake valve right after the combustion storke and cause pressure to bleed out the intake and exhaust valves as the compression stroke begins.
BUT cams for street use you will almost never see this problem.
Camshaft figures have everything to do with compression. Different cams build compression at certain RPMs, and the amount of pressure varies according to duration and valve timing.
You can estimate using the engine's DCR. Carb size and other sometimes overlooked variables can change your DCR as well, though they aren't included in the equation (a restriction can limit cylinder fill).
So, back to your question, a camshaft can loose compression. It can bleed off compression at an RPM where you do want it to, or where you don't.
When you hear people talk about a car with too much cam for the street what is usually happening is the cam is bleeding too much cylinder pressure at low RPMs. This causes an erratic idle and a lack of low end torque. This can however be useful to an extent, when you have a high compression engine and you want run optimum ignition timing without frequent risk of detonation, you can use a cam with a longer duration to bleed some of the excess pressure down until later in the RPMs.
On the other hand, you can do the opposite. You can use a cam with a shorter duration on an engine with lower compression to build more cylinder pressure down low. That way you can gain back some idle quality and some bottom end.
87442lover said:
Naaa.. in HUGE cams it will. In cams made for something like a dragster, having really high lift and high duration (example: 310x320) and crappy center line will cause pressure to bleed out the intake valve right after the combustion storke and cause pressure to bleed out the intake and exhaust valves as the compression stroke begins.
BUT cams for street use you will almost never see this problem.
I don't want to turn this into a major argument. If you don't understand or believe in DCR then don't worry about it. For those of us that do here is where to get some information http://cochise.uia.net/pkelley2/
You have to down load the calculator, it is VERY handy.
The most accurate to push the limits on pump gas is with your DCR compression, not by the static compression.
Just as willys36@aol.com said, this is how some of us are running higher compression ratios and running decent numbers on pump gas.
lluciano77, Is right on the money as well.
lift has nothing to do with it, it is all about valve timing/duration.
One thing is correct cams do not effect "static compression" that is because this is a measurement (cylinder volume compressed into a certain space, depending on stroke, etc...) This is assuming the both valves are closed.
With that said if you take the same engine lets say 10:1 with cam A, and do a compression test and get 210psi, install cam B and you could have 160psi. So how does that happen? (for those of you that say a cam makes no difference in cylinder pressure).
Just something to think about.
Like I said I don't want to argue, I won't change your minds (for those that don't believe) and you won't change mine.
Why are "effective" compression ratios higher with nitrous or forced induction? You can have a static compression of 8:1 and under boost have an effective compression of 16:1 or more ( I know we weren't talking forced induction). This all goes hand in hand.
I'm not wanting to argue either, because what you stated is simple fact, nothing to argue there or "Not believe" at all.
However, he was using terms of static compression ratios and I was telling him that means nothing.
If he's thinking that he can run a higher static ratio and just use a 'bigger cam' to mask it, he's partially right, at lower RPMs. Now take that same combination and run that motor where it's producing it's peak power with pump gas and you have a recipe for disaster. Another simple fact......
That has been stated also. That the cylinder pressure will come back to haunt you at higher RPMs.
I have an 11:1 SBC 400 with a 234o duration cam and iron heads, running on 91 octane. It bleeds down the cylinder pressure enough so that it doesn't detonate at lower RPMs. At higher RPMs there is less tendency for detonation and I have no problems either. My timing is at 34o total.
That pressure return at higher RPMs theory is possible, but don't forget that the engine is under less detonation sensitive load when it is at higher RPMs. There are too many variables to take in to account to try to predict if you are going to run into any problems with a cam and engine combo. Assuming that bleeding down cylinder pressure at lower RPMs is not safe in all cases would not be accurate.
Anybody tried to figure DCR using cranking pressure as a reference? I saw some numbers before -
180 psi=10:1
160psi=9:1
140psi=8:1 etc.
Also read that 180psi is roughly the limit for pump gas. Was wondering about a "lower limit" for effective cyl. pressure (most cars I've seen with less than 135psi have poor performance). When choosing a cam for my motor - used '76 350 w/stock flattop pistons and 882 (76cc chamber) heads - I picked the one that gave the highest cranking pressure (Crower 267HDP, 210/214 dur and .444 lift w/11 deg overlap), it gave me 150 psi (8.5:1 DCR?). Also tried a SSI 282 adv. dur 224@.050 and .465 lift for 120 psi and an Isky 270H 221@.050 and .465 lift for 135 psi. (The only one that had a cam card was the Crower). The Isky cam was strong above 3000 rpm and the SSI was a total loser in my motor. The Crower cam gave the best low end torque and revs well to 5500 (or 6000 if I spin the tires in 1st). Anyway I thought there was a connection to cyl. press./DCR, but haven't found a lot of info related to this. Hopefully someone here will know more about it. If it matters I am using a performer intake and 1406 carb, TH350, 2200 stall, 3.50 gear and 28" tall tires.
The rule of thumb is 10:1 static with iron heads. 8:0:1 DCR with iron heads, you can push it a little higher as long as other things are in order (8.25:1 or so), you must have a tight quench to pull this off.
With everything the same except using aluminum heads you can typically run 1 point higher 11:1 and 9:1 (DCR)
DCR should not be confused with static compression. If you have low compression and want to make the most of it I agree you want a cam that builds more DCR or cylinder pressure, but you can only take this so far before you will have detonation problems (quench is VERY important). To can have a 10.5:1 engine that runs fine on pump gas with a quench of .040", change the head gasket (thicker), get the compression down to say 10:1 with a quench of .050" the 10:1 motor would detonate easier/more than the 10.5:1 engine (I pulled the number out of thin air for the example).
Cranking compression doesnt mean much.My stock pickup truck had an 8:1 static compression ratio and made 185psi of cranking compression with the stock smog cam and only made 250 hp out of a 454.My race car has 12:1 compression,and only makes 95 psi of cranking compression with a 330 degree camshaft,yet it makes 750 hp out of 440 inches.The pickup truck runs fine on 87 ocatane,but the race motor needs at least 110 octane.
In all the years I have been playing with engines I have really only learned 2 things.
1. Long durations say 280+ degrees run crappy in anything under 9.5:1 compression.
2. Bigger displacement engines, for whatever reason, tend to tolerate longer durations better regardless of static compression. A 383 or 400 will idle smoother pull more vacuum and have better low to mid than a 350. Cam, heads, compression, and air flow being equal.
That DCR download site was cool that guy is really sharp.
Cam selection is still hit or miss. I've put together assy's before where I thought I matched everything up well just to find the power curve was not what I was hoping for. Other times I've hit it right on the money. Lately I have been having real good luck with Comp Extreme Energy HR cams.
>>>Camaroman,you are correct about the state of health of the engine by doing a comp test, when we used to hot lap fuel cars in the 70s and didnt have time to tear it apart, I always did a comp test to make sure that the rings still had a seal and the pistons werent holed. then I would do a leak test and make sure I had no more than 8% leak after a run. by the way the pressure was 100 -105 with 6.6-1 static.
You know you guys aren't telling me what I want to hear (lol) but that's ok. If I find anything to the contrary I'll be sure to post it here. Thanks for all the info. BTW this thread is the reason I applied for a membership on this site, hard to even find a discussion related to this topic.
The 750 horsepower figure I quoted was with no power adder,but I have a single stage plate nitrous system on that 440 incher to step it up even more.The engine is built to withstand the maximum nitrous setting of 400hp,{around 1150 hp total}but since I have the car geared for running on raw horsepower,there isnt much perfromance potential with more then about a 150 shot.Now if I swapped out my 4.88's for some 4.33's,I could jet up and really run some numbers,but at that setting I would get maybe 2 runs out of a full bottle,and at $40 a pop to fill them{$50 at the track},there is no money in shooting the car.
O.K.,so I embelished a little,the motor "only" made 730 hp on it's best dyno pull,but its close enough.
your cylinder pressure will also be increased by advancing the cam,which will lower your torque curve and give you more power on the bottom end.
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