Let's debate cam duration....
11-08-2001, 10:34 AM
#1
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Let's debate cam duration....
This was posted by Michael Delaney from the Endyn Message board and is a statement by Larry Widmer regarding the usefulness of ultra long cam duration e.g. Jun III's vs cams that have shorter durations. What do you guys think of this?
Just as in the Circle Track article of a year ago, where Jim Mcfarland asked me the questions, I still contend that the quality of mixture delivery to the cylinder is paramount to engine efficiency (including power production.) Obviously, if you deliver less mixture to the engine, it'll make less power.....assuming the mixtures are equally homogeneous. If the mixture quality takes a dive as the quantity is increased, you will certainly lose power.
You're right about some aspects of cam timing, and in fact, we've used reed-valves inside the intake tracts before to stop reversion, however, the primary reason that I'm not a fan of long cam timing is simple......the depth of the valve reliefs necessary to accomodate early intake valve opening destroy the combustion chamber dynamics we're attempting to create with the piston shape. Make those intake valve reliefs deep enough, and you've got no ability to bias the principal burn to the exhaust side of the chamber. Remember that the burn won't seek the best place in the chamber by itself, you have to force it to to the location you desire........and that's near the exhaust valves where heat causes ionization to begin first.
If the burn doesn't occur in the most efficient portion of the combustion space (chamber with piston as the floor), it won't burn as quickly, necessitating more innitial spark timing, which equates to more negative work for the engine.
I've been working on an article for seemingly months on the timing issues, but consider this........if the work produced after TDC doesn't exceed the work required to compress the expanding mixture before TDC, the engine won't run. The difference between the two work events (one positive and one negative) equals the power the engine will produce.
Optimizing combustion chamber dynamics is a tough situation. Take our pistons for example. Our high compression pistons have domes that interact with the chambers and quench areas so well that they are capable of running 1-2 points higher compression than any other pistons on the market. It tooks us six months to figure out how to make a lower dome (compression) piston that had the same detonation tolerance (mechanical octane), because when you just lower the dome you're losing significant squish ans while the CR is lower, the tendancy to detonate went up! We finally recontoured and moved the dome enough to make them work, but it was a lenthty ordeal.
I've stated before many, many times that the shape of the combustion space is what we work with and the static CR actually will simply fall where it may. We do not design pistons with specific compression ratios in mind. We design pistons to work with a wide variety of fuel qualities, providing the fastest, most complete (orchestrated) burn possible. If we go at it the other way, you'll end up with the same crap that other manufacturers are selling and you'll suffer attendant power losses, detonation, etc.
We received some JE road racing specials from a customer we're doing a head for last week. They look trick as hell with their high dome that's had a concave dish machined into it, but they have valve reliefs that are 1/8" deeper than ours, surface area that you'd be hard pressed to exceed, and they also weigh more than our pistons. Oh, and by the way........they have a net dome volume of only 2.2cc for "big" compression.
So Tuan, you have to think about the shapes that are going to influence where the mixture travels in the combustion space and what you have to compromise (possibly in cam timing) to create the ideal burn.
BTW Maximum piston velocity at 8200 rpm for the B18C occurs at 75 degrees past TDC and = 7730.33 ft/min.
Just as in the Circle Track article of a year ago, where Jim Mcfarland asked me the questions, I still contend that the quality of mixture delivery to the cylinder is paramount to engine efficiency (including power production.) Obviously, if you deliver less mixture to the engine, it'll make less power.....assuming the mixtures are equally homogeneous. If the mixture quality takes a dive as the quantity is increased, you will certainly lose power.
You're right about some aspects of cam timing, and in fact, we've used reed-valves inside the intake tracts before to stop reversion, however, the primary reason that I'm not a fan of long cam timing is simple......the depth of the valve reliefs necessary to accomodate early intake valve opening destroy the combustion chamber dynamics we're attempting to create with the piston shape. Make those intake valve reliefs deep enough, and you've got no ability to bias the principal burn to the exhaust side of the chamber. Remember that the burn won't seek the best place in the chamber by itself, you have to force it to to the location you desire........and that's near the exhaust valves where heat causes ionization to begin first.
If the burn doesn't occur in the most efficient portion of the combustion space (chamber with piston as the floor), it won't burn as quickly, necessitating more innitial spark timing, which equates to more negative work for the engine.
I've been working on an article for seemingly months on the timing issues, but consider this........if the work produced after TDC doesn't exceed the work required to compress the expanding mixture before TDC, the engine won't run. The difference between the two work events (one positive and one negative) equals the power the engine will produce.
Optimizing combustion chamber dynamics is a tough situation. Take our pistons for example. Our high compression pistons have domes that interact with the chambers and quench areas so well that they are capable of running 1-2 points higher compression than any other pistons on the market. It tooks us six months to figure out how to make a lower dome (compression) piston that had the same detonation tolerance (mechanical octane), because when you just lower the dome you're losing significant squish ans while the CR is lower, the tendancy to detonate went up! We finally recontoured and moved the dome enough to make them work, but it was a lenthty ordeal.
I've stated before many, many times that the shape of the combustion space is what we work with and the static CR actually will simply fall where it may. We do not design pistons with specific compression ratios in mind. We design pistons to work with a wide variety of fuel qualities, providing the fastest, most complete (orchestrated) burn possible. If we go at it the other way, you'll end up with the same crap that other manufacturers are selling and you'll suffer attendant power losses, detonation, etc.
We received some JE road racing specials from a customer we're doing a head for last week. They look trick as hell with their high dome that's had a concave dish machined into it, but they have valve reliefs that are 1/8" deeper than ours, surface area that you'd be hard pressed to exceed, and they also weigh more than our pistons. Oh, and by the way........they have a net dome volume of only 2.2cc for "big" compression.
So Tuan, you have to think about the shapes that are going to influence where the mixture travels in the combustion space and what you have to compromise (possibly in cam timing) to create the ideal burn.
BTW Maximum piston velocity at 8200 rpm for the B18C occurs at 75 degrees past TDC and = 7730.33 ft/min.
11-08-2001, 10:39 AM
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Re: Let's debate cam duration.... (Big Phat R)
Very interesting read.
I think duration is what makes power rather than lift, but ramp rate has a lot to do with it. For example, as you can see the Skunk2 cams versus the CTR cams..
I have NEVER seen a CTR cam make more power than a Skunk2.. and they have comparable dur/lift on paper. Ramp rate, and good duration and the cam is
I think duration is what makes power rather than lift, but ramp rate has a lot to do with it. For example, as you can see the Skunk2 cams versus the CTR cams..
I have NEVER seen a CTR cam make more power than a Skunk2.. and they have comparable dur/lift on paper. Ramp rate, and good duration and the cam is
11-08-2001, 12:17 PM
#3
Honda-Tech Member
Re: Let's debate cam duration.... (Mike K)
you guys need to stop posting this good information, don't you have any questions or polls about stickers? C'mon, geez, you'd think this place was a resource for information or something. 
..good post, thx!
..good post, thx!
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