Compression Ratio

the right cam profile can make a huge difference in preventing detonationat low speeds, which is where you need it. what you want is a cam that has a longer overlap period than normal. usually this means either getting a bigger cam than you want, or getting the cam you want with narrower lobe centers(106 degree vs 110 degreee centers). the nice thing is that you can go with more overlap in your 408 than you could with a 351 due to the extra 57 cubes. dont go too big though otherwise you lose low end torque, so you might need to back the timing down a bit as well.

paintballtommy said:

how is it that they can pull that off running on california pump gas 91 octane and not detonate liek crazy? Timing? Cam profile?

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Probably becuase the DCR is alot lower than the Static CR. That is determined by the cam.

I'm no cam pro to say the least, but that seems awefully radical to me.

Go here:
http://kb-silvolite.com/calc.php?action=comp
You can determine dynamic compression by inputting rod length and cam timing. You want something around 8.5:1. 9:1 would be pushing it on 91 octane.

SoCalCruising said:

You want something around 8.5:1. 9:1 would be pushing it on 91 octane.

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That is what I have always heard about DCR..

One word ... computer!! If these guys were carb and dizzy.. the ping would be breaking pistons!! the computer will pull ign, fatten up the injectors... blah blah blah..

I am running a 1.6 with 15 PSI boost on a roots style s/c.. on 93 octain. when the egine starts into ping in high gear.. the computer pulls a lot of timing and goes rich!! thus the 200 WHP 1.6 can drive all day on pump gas...

Just me............................................

Thumper

Compression Ratio is far more comp0licated than anything said so far.

I assume we are talking about STATIC CR here, which is pretty much the standard when discussing engine specs. Static CR is also easily calculated with bore, stroke, deck height, and combustion chamber volume. Static CR is how much the piston would compress the mixture in the cylinder, if the valves were all closed from start to finish of compression stroke, i.e. no bleed off, no overlap.
Static CR is what the manufacturer lists in their engine specs.

DYNAMIC CR is the net value, remaining pressure after you factor in the pressure loss from open valves during compression stroke. It is always smaller than static CR. It is hard tro calculate, and usually the closest you can easily get is to measure actual compression pressure. Then you also have some pressure loss from blowby.
Let us discuss static CR from now on, since dynamic CR is more of a curiosity and not very useful for what we are doing.


There are a LOT of variables at play, when optimal static CR is being decided.

The cam profile is probably the single most important variable. Hotter cams bleed off more pressure, and therefore require higher CR to be able to make SAME actual pressure that the stock cam did. Many high performance engines usually go a little higher than stock actual pressure, so high CR numbers are not unusual.

Some things that affect CR choice:

Cam profile
Fuel octane
Deck height/squish area (large deck height and squish area promotes knock)
Combustion chamber shape (Some are knock prone, but can be improved)
Piston dome shape (some promote, some prevent detonation)
Altitude (very thin air in higher altitudes needs more CR)
Massive intake air restrictions (Some race classes require restrictor plates)
Knock control (ECU or aftermarker knock sensor/ign retard box)
Mixture control (programmable EFI is always more accurate than carburators)

Add to the list:
Intended use (! in some cases, you can use up all safety margins and run sky high compression for a short while, in certain race applications for example.. where longevity is not important and the rpms vs. load rarely stay in the knock prone region for long. In other words, make a decision to let it knock and ping under certain conditions, to gain more power in other conditions. If all you do is all out racing, it doesn't matter much how the engine behaves under 3000rpms. Or if it runs there at all. LOL)

Some engine designs are just not made for high compression, while some can tolerate much higher than stock pressures. Some can be significantly improved, while some are just plain doomed to being low efficiency tractor motors.


Jan

I have a tractor motor

If we had the means, we should all be talking dynamic CR, not static. Only DCR matters, for any application. That's why you have to adjust your thinking about SCR by factoring in the cam and application, etc. For any application, there ought to be an ideal DCR, right? If one had a nomogram, or such, that told us the ideal DCR for each application (road racing to towing), we could use that when selecting parts. For example, if we were targeting a torque peak at 2800 RPM (maybe an RV application?), we could choose the appropriate cam and then use that info to elect parts for the ideal DCR. Same for any other application. Right or wrong?

SoCal got it right, if we could eliminate all guesswork and know all variables spot on, we could use Dynamic Compression and get everything right, every time. We could make excel sheets with automatic calculators that give us every detail and allows us to play around with the variables we have control over, to show us what our Dynamic Compression is. (You'd still need to know how much dynamic pressure each cam profile and parts combo likes best).

Sadly, we'll have to make do with Static CR, because that is the only hard, undisputable dataset we have, assuming we can measure our parts adequately.

http://www.johnmaherracing.co.uk/enginecalc.htm

Use that to play with the abovementioned variables, and see how changes in one thing affects Static CR.


Now, since things are pretty much the way they are, we need to stick with static CR in our calculations. The most vital "variable" then is the engine builder's experience. With time, you learn which cam likes which static CR, under what kind of other circumstances. You can't beat experience.

If you are really deep into it, you will know to read new (to you) cam profiles and determine the best CR for that too, without ever assembling anything or running the engine on a dyno.

Even so, you always need to leave a little bit of safety margin, more for the street, less for racing (Where every HP counts and rebuilds are frequent).
Race engines are generally built to closer tolerances, more accurately, and with better tools, so you can safely (?) narrow your safety margin to harness the last mangy pony (no pun intended) hiding in there.

Race engine building is more of an exact science, stock rebuilds and mild street engines are easier because you can rely more on 'guesswork', just err on the safe side and call it safety margin

Then there are perfectionists who rebuild their lawnmowers to race tolerances, with computer assisted design and optimization, blueprinting and balancing etc... just because they are fascinated by the fact that they can Or they are too proud to half-ass anything they do. Or they are just bored out of their minds.

It gives you a nice warm feeling of accomplishment (and superiority) when you can tell your neighbor that your 5.0hp briggs&stratton is now actually a 5.025hp.

Sorry, I guess I got a little carried away

Jan

I agree that what the cam grinder wants to know is static CR, plus a bunch of other variables, and then he will choose a cam based on experience, as well as theory. I just think that the body of knowledge would be even more accurate if it was approached from the ideal dynamic CR perspective. But, what I think doesn't change the nature of things, which is that almost no one uses DCR as the target, but rather it is checked to make sure that reasonable timing may be used without pinging, given the available fuel.