Comp ratio vs cam duration

[Caveman]

I was reading an article that was doing an engine buildup story recently...

They referenced something to the effect of the motor would 'need more compression to work with a longer cam'...
I.e., that a cam of longer duration would only work better if paired with a higher compression ratio. That doesn't make sense to me.

I know that a bump up in Compression ratio will definitely boost power levels... but I would think they are more independent of each other.

How exactly does more squeeze benefit from longer duration?

And wouldn't a motor with less compression benefit just as much, given an equal amount of added duration?

[std]

Re: Comp ratio vs cam duration

Other people are way smarter then me about this then me, but the more duration you have the less dynamic compression you have because the intake is open longer. Bumping up the static compression will get back what you lose.
I know there is more to be said about this. I'm ready to learn.

[MadmanMark]

Re: Comp ratio vs cam duration

I think cubic inches has to get into this. More cubes = less duration per hp for a given comp ratio.

You could also probably correlate comp ratio, cubes, and max rpm (which could represent hp). The two curves put together would be interesting to see.

[JeffMcKC]

Re: Comp ratio vs cam duration

Longer intake duration means opening the intake earlier and closing the intake later using the same lobe center making less cylinder pressure, so you add static. The longer the stroke the more duration you need to fill the cylinder, but piston speed goes up, so the piston is farther down the cylinder at the same valve opening points making it harder to fill. Then you can start to fool things, with bigger .200 numbers at lesser .050 numbers it will keep you up nights if you let it.

Biggest mistake going right now is to much intake lobe. If you add it, you better be willing to spin it up to use it.

[TC]

Re: Comp ratio vs cam duration

How exactly does more squeeze benefit from longer duration?

More duration means more air/fuel that enters the combustion chamber. The more air/fuel you get into the cylinder the more power you'll make. Then there's the fact that more compression means more cylinder pressure. And more cylinder pressure equates to more Power. Now put the two together and you got a winning combination. ;D

The raise in compression that you would do when running a wide duration cam, is allowing you to harness more of the combustion's energy. Fact is by raising compression you are increasing the engines thermal efficiency. And thermal efficiency is the measure of how effectively an engine converts heat into mechanical power.

Then we can get into Expansion Ratio's and pumping losses/gains on port flow. ;)

[Barry Donovan]

Re: Comp ratio vs cam duration

that subject drives me nuts in race cars...bump up the compression, to lose some to the cam. Japanese is the crziest of all with that..its called vtec, or VVT.
trial and error would show, a high comp engine can go quite far with tight duration...it is very very stressful, but extremely freakin powerful. I have done it in my cheap newb days of learning.

Turbo engines actually need this duration increase, but they don't like it down low..
Whoever sets the rule like oem, is not exactly correct..strain the rules. they got racing parts for a reason.

[Eric68]

Re: Comp ratio vs cam duration

Cam duration and compression are VERY DEPENDENT on each other. They need to work together for best performance. Have you ever wondered why certain cars and combos seem to go faster than you think they should and some combos are much slower than you think they should be? It's all in the combo -- a good combo is better than the sum of its parts, a bad combo . . . well just plain sucks.

To understand the relationship between cam and compression you first need to understand certain basic engine parameters.

• Static compression ratio: the swept volume of the cylinder divided by the total volume of the chamber at TDC
  
• IVC or Intake Valve Close: When the intake valve closes, usually early in the compression stroke. This is an important event that is dependent on duration and the intake lobe center line.
  
• Dynamic compression: The swept volume after IVC divided by the chamber volume. In a race engine, the intake valve remains OPEN during the first part of the compression stroke. The longer the duration, the later the intake valve closes.
  
• Cylinder pressure: The same as dynamic compression ratio, except that instead of being expressed by a ratio it is measured in PSI. Generally speaking, the higher your cylinder pressure (in PSI) at a given RPM the higher your engines TQ output will be

Now with those basic terms being defined, you have to understand that since the intake valve remains open during the beginning of the compression stroke, at low RPM the engine cannot compress the intake charge in the cylinder UNTIL the intake valve closes. The longer the cams duration and the later the intake lobe center line, the later the intake closes and the less cylinder pressure the engine generates. UNLESS -- you use a smaller combustion chamber; this "props up" the cylinder pressure and allows the engine to make still make TQ.

You can have too much cylinder pressure which generates heat (Boyles Law) and the charge ignites spontaneously causing detonation which leads to preignition. On the other end, to little cylinder pressure results in a lazy engine with little TQ output. With BIG cams (like real big ones) and low compression the engine may never really get up on the cam and make good power at any RPM. You have to match the two together to make a combo work.

[yellomalibu]

Re: Comp ratio vs cam duration

Cam duration and compression are VERY DEPENDENT on each other. They need to work together for best performance. Have you ever wondered why certain cars and combos seem to go faster than you think they should and some combos are much slower than you think they should be? It's all in the combo -- a good combo is better than the sum of its parts, a bad combo . . . well just plain sucks.

I've seen this many times over the years. Good parts in a car, but not the right parts to work together, and the car doesn't run nearly as well as its owner expected.
On the other hand, someone has something that seems very simple and mild... but it hauls ass.


... but I've seen combos that shouldn't have run nearly as well as they did, too.

[Caveman]

Re: Comp ratio vs cam duration

Yup, yup, and yup.

Great points from all.

Now, I understand why a Pro Stock cam has such a wide LSA; because with such huge lift numbers in combo with big duration, you have to physically spread the cam lobes (Int & Exh) to avoid ridiculous amounts of overlap and intake charge reversion... which leads me to another thought thread.

For a street motor; Why couldn't you run really healthy duration (like barely-idling, dragstrip-cam type duration numbers) with a WIDER LSA, spreading those Int & Exh valves apart to the point where there's a normal (read 'street' or 'medium') amount of overlap... thus minimizing reversion, yet still giving a street motor tons of duration to use?

[TheSilverBuick]

Re: Comp ratio vs cam duration

For a street motor; Why couldn't you run really healthy duration (like barely-idling, dragstrip-cam type duration numbers) with a WIDER LSA, spreading those Int & Exh valves apart to the point where there's a normal (read 'street' or 'medium') amount of overlap... thus minimizing reversion, yet still giving a street motor tons of duration to use?

Like my cam?

VALVE LIFT

STOCK 1.55 RATIO:
IN: .478?
EXH: .475?

DURATION
AT .050
IN: 228
EXH: 247
ADVERTISED
IN: 276
EXH: 295
LOBE CENTER: 118


Spec's say there is a .5* of overlap.

[yellomalibu]

Re: Comp ratio vs cam duration

For a street motor; Why couldn't you run really healthy duration (like barely-idling, dragstrip-cam type duration numbers) with a WIDER LSA, spreading those Int & Exh valves apart to the point where there's a normal (read 'street' or 'medium') amount of overlap... thus minimizing reversion, yet still giving a street motor tons of duration to use?

Like my cam?

VALVE LIFT

STOCK 1.55 RATIO:
IN: .478?
EXH: .475?

DURATION
AT .050
IN: 228
EXH: 247
ADVERTISED
IN: 276
EXH: 295
LOBE CENTER: 118


Spec's say there is a .5* of overlap.

Nope, sorry. That's a baby cam.

A buddy of mine ran a solid flat tappet crane cam in his Chevelle... 355 small block, the cam had 270* @ .050". Under 11:1 compression. <-- Was it optimal? No way. ...shouldn't have run worth a damn, but it hauled ass.

[revolutionary]

Re: Comp ratio vs cam duration

I think Eric summed it up pretty well. You can't think of each item - compression ratio, valve timing events, or intake system- independantly. They're all dependant on each other. That's why you see so many cars that are just turds with stock heads/compression and a big cam.

Don't think of lobe separation or overlap as important units as much as the valve timing events themselves. IVO, IVC, EVO, EVC. The intake valve closing in relation to static compression is what determines when the engine really starts compressing the intake charge and is really a point that should be determined as a target. Intake opening can be moved around (with different duration) to vary the RPM where the engine makes peak power. Exhaust valve opening can be varied based on the length of burn of the fuel (alky vs gas) and piston speed (rod length) to make sure that it doesn't bleed off power by opening too early but still opens fast enough so that by the time the piston starts coming back up, the valve is well on it's way to max lift. killing a little low lift flow here (under .100") will allow you to open the valve earlier like with a not very aggressive lobe, and not lose too much pressure. This is why a lot of good head porters DO NOT back cut the exhaust valves. Exhaust valve closing point should be at a place where it allows as much as possible out without contaminating the intake charge. Tip there is to check low lift reverse flow on the intake to see if you can keep from contaminating the charge.

It all works together and you need to look at the whole system to see how it acts.

[TC]

Re: Comp ratio vs cam duration

I think Eric summed it up pretty well. You can't think of each item - compression ratio, valve timing events, or intake system- independantly. They're all dependant on each other. That's why you see so many cars that are just turds with stock heads/compression and a big cam.

Actually this is the reason most nitrous motors run crappy off the Juice. But saying that I've made many stock SBC's with a cam change, run great with a 250 shot. ;)

Don't think of lobe separation or overlap as important units as much as the valve timing events themselves. IVO, IVC, EVO, EVC. The intake valve closing in relation to static compression is what determines when the engine really starts compressing the intake charge and is really a point that should be determined as a target. Intake opening can be moved around (with different duration) to vary the RPM where the engine makes peak power. Exhaust valve opening can be varied based on the length of burn of the fuel (alky vs gas) and piston speed (rod length) to make sure that it doesn't bleed off power by opening too early but still opens fast enough so that by the time the piston starts coming back up, the valve is well on it's way to max lift. killing a little low lift flow here (under .100") will allow you to open the valve earlier like with a not very aggressive lobe, and not lose too much pressure. This is why a lot of good head porters DO NOT back cut the exhaust valves. Exhaust valve closing point should be at a place where it allows as much as possible out without contaminating the intake charge. Tip there is to check low lift reverse flow on the intake to see if you can keep from contaminating the charge.

Your kind of contradicting yourself here, if you move the intake and exhaust opening points you will change your LSA value. Well that is if all other cam values stay the same. ;) LSA is a very important value especially if your running a carb.

Also if your having a problem with reversion, then you have to much back pressure in the exhaust. When the exhaust valve opens there should be a vacuum that is pulled on the cylinder by the exhaust port and header tube, thus allowing the exhaust side to help pull the new intake charge into the cylinder before it closes and the piston starts its downward travel. I personally feel that Rod Ratios and LSA's go hand in hand. If you have a poor rod ratio a tight LSA can help get the intake flowing while the piston is "stalled" at the top of its stroke. Plus by running a tighter LSA the valves are closed for a longer period of time, thus creating more cylinder pressure.

Granted I do believe that all these things go together and must be matched up in order to take full advantage of the combination.

But saying this it is all relative, you can build a 454 with 8:1 compression put a 300 duration cam in it and still make 400+ hp. Sure it's going to be a lazy ass SOB on the bottom end, but once it gets going it's going to run. But then that's why they make stall converters, right ;)./ The fact is 8:1 isn't going to get you the full potential of what the motor can do. At 8:1 the rate of combustion expansion is very slow, so you basically can't burn as much fuel as you could with a higher compression. This also has to do with Piston speed as the 8:1 motor is going have a much slower expansion ratio compared to lets say a 15:1 motor where by the time the piston is about 1/4 way down the cylinder it has already doubled it's volume, thus pushing the piston down at a much more powerful rate. And since this more power rate is pushing the piston down faster it will actually pull more of a vacuum on the intake, so you will be able to get more of the intake charge into the cylinder. Basically this all gets into the effects of expansion ratio's and it's effects on pumping gains/loses. It's not that you need more compression to make it work, it's that you need to more compression to make it work efficiently.

So to answer the original question, of why you need more compression with a bigger cam, it's done so that you can maximize the full potential of the cam that you are installing.

[SpiderGearsMan]

Re: Comp ratio vs cam duration

duration means nothing on the compression stroke when both valves are closed
but the force created with the high compression is the means to burn more fuel and move more air=more power
you need the duration to prevent detonation
and burn more fuel !

[min301]

Re: Comp ratio vs cam duration

It (the cam) must match the airflow abilities of
the cylinder heads, compression needs to be
decided to match the cam needs.

It all must work together, or it will not run to potential.