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Dynamic Compression - What to aim for ?
As we all know Static compression assumes perfect piston rings (no leaks) and perfectly closed & sealed valves. If a valve sticks open, then we have no compression. Dynamic compression is the compression we have when the cam that we are using, has taken its effect. The more overlap the cam has the more Static compression we need to achieve the same Dynamic compression. Therefore, large cams = high static compression.
Programs like Performance Trends "Engine Analyzer Pro" are now able to predict Dynamic compression ratios with some considerable accuracy. In Engine Analyzer Pro's case the Dynamic compression ratio forecast is found having run "Calc HP" and looking under "General Engine Calculations" - "Theo. Crank Comprssn, PSI" Hughes are now suggesting a "Minimum Suggested Cylinder PSI" for their cams ..... http://www.hughesengines.com/cams/bb_heh.asp My questions are: 1) What PSI should we be aiming for on a street only motor (cruiser/daily driver) using 93 Octane gas? A) Cast Iron Heads? B) Aluminium Heads? 2) What is the maximum Dynamic compression 93 octane gas will stand before detonation? A) Cast Iron Heads? B) Aluminium Heads? |
#2
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well, therea a question! hughes is wanting certain crank compression so their cams wont be to big for the compression, crank compression is one way to tell.
a 10 to 1 engine, with a cam with 40 overlap should have something like 100 psi. put 120 of overlap in it, and ittl have like 65-75. the faster you spin it, th ehigher the crank compression will go. this has nothing to do with detonation once the overlap effect has gone, but, even a 12 to 1 engine with a 120 overlap cam wont have even 100psi crank pressure. this is a real bad way to look at compression. unless your looking solely for low end torque |
#3
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If what you say is correct I can put an HEH 1928 BL in my stock 8.2 CR 440 without any problem. Hughes say it's a no no! |
#4
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well, that cams a little too big, in my oppinion for your compression ratio, if you do the overlap math. youll be bleeding off a lot of compression at low rpm. your engine would run better with a milder cam anyway thats what hughes is trying to do, keep ya torkin! but, heres the other prob, how the hell do you know what compression pressure this cam will have in YOUR engine without installing it ya got me confused now.
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#5
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from lots, and i do mean lots of experience overcamming engines (all my buds want to cam th hell out of their engines) you are better off undercamming it, even though it wont sound as cool
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#6
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Hughes use this program to do the calculation BEFORE you build the engine and they use it to recommend suitable cams. I know that the HEH 1928 is too big, I was using it to make a point. The point is that you can now use software to design the engine - question is what are you aiming for in dynamic compression terms ............. how can the software help if you, if you dont recognize the perfect, simulated engine/dynamic compression |
#7
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#8
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My Thoughts are:
Dynamic compression is calculated by using the swept volume at the moment the intake vlave closes rather than the total swept volume of the cylinder. Longer duration cams have a later closing intake event. Advancing the cam will move the intake closing event earlier thus increasing the swept volume thus increasing the dynamic compression. Now to the real question, What is the maximum dynamic compression ratio that 93 octane can be used with. I had a 260 cranking psig engine that pinged unless I backed the timing off. This was a 9.2:1 Static Compression Ratio engine. The cam was a stocker advanced 6*. So I would guess that a dynamic ratio that gives 260 psig is just above maximum for that particular engine. (I think it was near 8:1) High compression helps bottom end torque. When the piston is at top dead center, the smaller the chamber volume, the smaller amount of used air remains in the cylinder. This means there is less used air/fuel in the next firing of the cylinder. Better idle quality and more torque are effects of high compression. As rpm comes up, the inertia tuning effect of the intake/exhaust system will increase the actual filling of the cylinder making the actual compression pressure even higher. The fuel octane needs to be sufficient for the time the cylinder is under the greatest heat and compression pressure. This will be near the peak torque output of the engine. My current enigne is "under comperssioned" for the cam I use. I choose to say under compressioned rather than over cammed because I need a cam that will rev and make power up the rpm range. |
#9
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So much for them using the program to determine that on their own... |
#10
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E, I agree. I have gotten the same response from them. It is the same kind of dillema as this: So, I can't get a job without experience, but I can't get experience without a job?
torch |
#11
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well, heres my idea on these computer dyno thingy's. i just dont think were smart enough to us ethem, and im confused. i have never had a problem building engines andf selecting cams, even without a computer program. so, i guess you can anyloise it to death, or just order your cam based on expereince, as far as the huges cranking pressure, i think these guys may sit behind the computer too long. they need to get out and experement hands on.
so many tings can effect cranking compression, even intake valve size, too confusing to me for a xtra 3 horsepower. |
#12
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#13
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It seems to me that if they want a certain cylinder pressure with a certain cam...and the cam is part of that final psi number they should be asking you for the other components that contribute to your final psi number (i.e.: head CC's, head gasket thickness, where's the piston at TDC). Or, am I on crack?
I've heard and read about hughes being touchy with their help. When I bought my cam from them, I read over their checklist of info they want you to supply them with. I answered all that I could and told them "I can't answer what I don't know." Example: I have to tell you my converter is stock, because given that the original motor was a 318 2 brl it won't be some hot piece. I also said that I'm not changing my valve springs (even if that makes me a fool). They said...O.K. and recommended a cam for me...and it was one size smaller than my intial choice to keep good vac. for my power brakes. So, my expeirience with them tells me that you tell them what you know and tell them to make their best choice with the info given...and they will. But, maybe that has changed. |
#14
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It really depends on who you talk to. They have some people that are real pains in the butt. They have others than are pretty sharp and can make an educated recommendation.
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#15
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To answer the original question, the most dynamic (or running) compression for a street motor on good gas with steel heads would be about 8:1. But thats probably pushing it, try to stay around 7.3-7.6. This will avoid detonation and still make decent power in most cases. Remember, no 2 engines are identical.
Dynamic comp. is far more important than static comp., because it is what the engine uses to make power with. Compression ratio assumes the intake valve is completely closed at BDC just as the piston starts to rise up the cylinder. And of course no engine has a cam that does that. Cylinder pressure would be more than in a diesel engine! That being said, you have to know what static comp. is so that you can determine what the dyn. will be once you've chosen the cam. This is why a 9:1 motor needs to close the intake way sooner than an 11.5:1 motor. I hope this is a simple enough explanation, I'm not trying to confuse anyone. Finally, if Hughes, or any other cam retailer can't advise what cam to sell you based on the info you allready have about your engine, GET THE CAM FROM SOMEONE ELSE!!!!! This is not the first time we've heard strange cam stories about these guys at Hughes. Sorry if I was too long on this one....djs |
#16
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But acording to the guy's at Perf' trends "Overlap has nothing to do with Dynamic Compression, only Intake Closing event". |
#17
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I agree that on a street motor 9.25 Iron & 10.25 Aly' is a good starting point. But only a starting point .............. Hence this debate about the vaule of modern software. |
#18
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2) Dumb question, Does "PSIG" = PSI ? Would an engine even with stuck valves and a 8.1 CR ever make 260 PSI cranking pressure? 3) Should that read as "High Dynamic compression helps bottom end torque"? If so then engines with smaller cams built for torque and low rpm use would want more Dynamic Compression - Hughes seems to want more "minimum PSI" as the cams get larger ! ? |
#19
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I know after a lot of emails to Hughes that they use Perf trends Eng Analyzer Pro to do the calculations for minimum PSI. |
#20
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#21
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"[Most people look at things too black and white. If you have too much dynamic CR, you can make up for it with running the engine richer, backing off spark, running the engine cooler, etc. I would look at what the EA Pro says for Knock Index at the various RPMs, and use that to predict Knock, rather than just Dyn CR.]" |
#22
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2) No change - They use Eng Analyzer Pro - So why cant we plan an engine and use our own copy? Problem is knowing what parameters to look for - Hence the question at the top of this list |
#23
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"I hope this is a simple enough explanation, I'm not trying to confuse anyone".............. OK we are with you so far, but could you quote your maximum cranking dynamic compression in PSI for both aly' and iron heads please. |
#24
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heres a suggestion, before trying to explain something like this, maybe you should explain camshaft theory and what overlap/closing event/duration/intensity and all that neat stuff, there is a lot of people out here that could benifit from it! |
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#26
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Overlap has no effect on running compression, only intake valve closing makes a difference. On a steel head motor try to have running compression about 7.6:1, around 7.9:1 for alum. heads. NOTE: these are ball park figures only! For a street motor with decent pump gas....................djs
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#27
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What does that equate to in PSI ? |
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