Intake exh duration split??

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Steve.k
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Re: Intake exh duration split??

Post by Steve.k »

Thanks Bob. I really find this stuff interesting. Not many(grinders) like to explain why they are doing this or that (for obvious reasons i spose). But it is interesting no doubt. I guess I leave my grinder do his thing.
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Re: Intake exh duration split??

Post by Stan Weiss »

There are a number of ways to look at each or segment of each lobe. Lets take the exhaust and how Professor Blair looked at in different parts by using time area. If on blow down I can keep the same time area with a faster exhaust segment (EVO to BDC) I can have the cylinder pressure pushing on the piston for a few more degrees.

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Re: Intake exh duration split??

Post by hoffman900 »

Stan Weiss wrote: Sat Mar 17, 2018 12:08 pm There are a number of ways to look at each or segment of each lobe. Lets take the exhaust and how Professor Blair looked at in different parts by using time area. If on blow down I can keep the same time area with a faster exhaust segment (EVO to BDC) I can have the cylinder pressure pushing on the piston for a few more degrees.

Stan
Stan,

Absolutely. Thinking about this more over coffee before getting on with the day.

For the op, remember to think about this in these terms.

For a given camshaft design, time is the x-axis on a graph (so duration), displacement is the y a-axis (so lift). Integrate the displacement, and you get area. The area is contingent on time and displacement.

This is a good primer: www.profblairandassociates.com/pdfs/Back_to_basics.pdf

Note: Blair's Time Area is a little different as he's talking about a running engine. If you keep all the terms straight, you'll get it.

Just kind of ignore him peddling his software, which he often is doing in his articles, the underlying engineering behind everything is what you're looking at.

Harold, Mike, and Billy have all described instances of moving the valve too fast. Further studying of 1D (and 3D if you can afford it, and real data if you're lucky) you realize what they're talking about.

We've all theorized about camless / springless engines and opening and closing the valve infinitely fast, for a square like valve lift profile. I've realized this isn't the case.

What a camless engine could get you is an assymetrical valve lift profile, that can vary through the rpm range. You no longer would have to fit the shape of the curve to what the system is capable of supporting, but shape the lift curve to maximize trapped volume, use as much cylinder pressure to exert force, and expel it with the least amount of losses. Compared to a camshaft actuated valvetrain, the valve may need to move faster or it may need to move slower at any crankshaft degree at any rpm, depending.

Guys like Harvey, Harold, John Reed and other unnamed race engineers / top level builders were realizing this without the access of modern pressure sensor equipment and simulation software back in the '60s, '70s, and '80s. They had a natural ability to see this going on in their heads. I believe Billy used the term "pinball wizard" to describe Harvey about other things, but it applies here too. Mike , Billy, and the current generation of camshaft designers have also observed it, but they also are able to work with clients who can better measure this stuff.

Harold:
Symmetrical vs assymmetrical(or unsymmetrical) lobe designs......
I think the last symmetrical lobe designs I did were in 1976...... Many of them are still being sold today, and they work OK(not BEST, or I would still be designing them...).Assymmetric cam design merely means that the opening and closing sides are designed differently. You can open fast, shut slow, or you can open slow, shut fast.
I got my inspiration for unsymmetric cams from Racer Brown, who didn't do them,AFAIK. He wrote an article in the mid-60s about Toyota engines, in which he noted that cams that opened and shut the valve very fast made great power, but were brutal on parts. He also noted that cams that opened and shut the valve much slower were very stable in high RPM and didn't break parts, althought their low-end power was down.
I quickly identified shutting the valve hard and fast with breaking parts, and shutting the valve slower with less parts breakage and better high-RPM stability. Tying fast openings and slow closings seemed to me to be the best of both worlds, but it was at least 10 more years before I designed and made my 1st unsymmetrical cam lobe. This cam was the Comp Cams 324-4(I believe, I don't have a CC catalog available..), 324° at .020, 286° at .050, .4544 lobe lift---They still sell that lobe today. It turned 11,000 with ease back in 1977-1978.
My 1st unsymmetric hydraulic was the 268 High Energy cam, 268° at .006", 218° at .050", and .454" valve lift.
What I have said since 1977 is that by delaying the opening of the intake port, reversion(exhaust gas being pumped out by the rising piston on the exhaust stroke) is minimized, compared to a cam with an earlier opening. Because this reversion is minimized, when the piston starts down on the intake stroke, the lessened reversion cleans out of the intake port earlier, and starts airflow earlier. Would YOU let your opponent in a drag race have a .1th head start? What about a .25th?
Although I use very fat cams at high lift, this isn't actually a part of unsmmetrical cams. But because the properly-done unsymmetric cam has a higher port velocity, a fatter-duration cam in high lift area has MORE time to fill the cyclinder, and is filling it at a FASTER rate.
Shutting the valve gentler helps ensure aginst valve bounce, and the corresponding release of cylinder pressure back into the intake runner. It also helps parts life.....
I will try to answer more tomorrow/today. I have only Saturday and Sunday every week at home, The last 5 weeks have been spent in Bowling Green at Corporate HQS, learning how to fill out forms like a proper engineer.
There are also gains to be had with unsymmetric cams as exhaust lobes, and the two, intake and exhaust, work together very well.
I will see you all tomorrow....

UDHarold
The only typo above is that Racer Brown worked with OHC rocker arm Datsun engines. A symmetrical lobe design meant an assymetrical valve lift curve at the valve, and the seating velocities wouldn't be what you think they should be.
Last edited by hoffman900 on Sat Mar 17, 2018 12:44 pm, edited 5 times in total.
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Re: Intake exh duration split??

Post by groberts101 »

I miss Harold too but surely thank you(Bob) for keeping his wisdom alive and near the top of the threads for others to see and hopefully carry that knowledge forward. His willingness to share much of his hard earned lifetimes worth of knowledge always made him highly admirable to me and has taught me a lot over the years. =D>
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Re: Intake exh duration split??

Post by Steve.k »

Its amazing how those guys could figure that out. Especially when no computer sim software hardly available to them. Some very talented people out there to say the least. Thanks. Guys.
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