More exh duration

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More exh duration

Postby 2xmod » Fri Feb 08, 2008 11:27 pm

Why is it necessary to run more exhasust duration @.050 on a race engine with open exhaust? Im talkin IMCA modifieds runnin on ALKY. Almost every cam I see listed for this application has a 4-12 degree higher duration on ex @ 050 then the intake. :?: Is there a reason for this?

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exhaust duration

Postby bigjoe1 » Sat Feb 09, 2008 12:11 am

If you want the power curve to continue well beyond the HP peak, you need lots more Ex durationThis will let you rev the engine well up above the peak but still be making gook power. Dont tell everyone about this, it is a speed secret.

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Postby 2xmod » Sat Feb 09, 2008 12:20 am

THeres always a bad side to everything, what happens when you go too far with the ex duration. By the way, I feel privileged to have you answer my question. :D Thanks ,Brad

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Exhaust duration

Postby bigjoe1 » Sat Feb 09, 2008 12:30 am

When you go too long on the ex duration. you will hurt the low end and mid range power. It is kind of a balancing act, a little is good, but too much can be bad.


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Postby Metrobilly » Sat Feb 09, 2008 11:06 am

I have found it beneficial to use more exhaust duration on my 4 cyl road race engine that has a relatively poor exhaust port. It does as Joe says. Any thoughts on if it is more helpful to add the extra duration by opening the exhaust valve earlier, closing it later, or doing both equally?

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Re: More exh duration

Postby CamKing » Sat Feb 09, 2008 2:13 pm

2xmod wrote:Why is it necessary to run more exhasust duration @.050 on a race engine with open exhaust?

It's not.
The cams you see with the 8-12 degree longer exhaust are based on 1960's technology.
Back then, the average exhaust port only flowed about 64% of the intake port, so it took a lot of exhaust duration to empty the cyl. That's no longer the case with today's port designs. If your exhaust port is flowing 80% of your intake, you don't want a longer exhaust duration. On 4 valve engines where the exhaust flows above 80% of the intake, we run less duration on the exhaust.
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Postby wfolarry » Sat Feb 09, 2008 3:01 pm

If your exhaust is flowing 80% of your intake would you then use 8* less duration on the exhaust? What about lift. I see where they use less lift on the exhaust side as well.

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Re: More exh duration

Postby Windsor377 » Sat Feb 09, 2008 6:03 pm

bigjoe1 wrote:... It is kind of a balancing act, a little is good, but too much can be bad.


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CamKing wrote:
2xmod wrote:Why is it necessary to run more exhasust duration @.050 on a race engine with open exhaust?

It's not.
The cams you see with the 8-12 degree longer exhaust are based on 1960's technology.
Back then, the average exhaust port only flowed about 64% of the intake port, so it took a lot of exhaust duration to empty the cyl. That's no longer the case with today's port designs. If your exhaust port is flowing 80% of your intake, you don't want a longer exhaust duration. On 4 valve engines where the exhaust flows above 80% of the intake, we run less duration on the exhaust.


Those are 2 golden nuggets if there ever were any.

Thank you both.

Those two comments really tells us, in no uncertain terms, it all truely depends on the complete combination.

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exhaust duration

Postby bigjoe1 » Sat Feb 09, 2008 6:07 pm

That ratio of intake flow to exhaust flow doesent mean a thing A prostock engine has about a 50 percent ratio,. I never make a big deal on the ex to intake flow ratio stuff.

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Postby 2xmod » Sat Feb 09, 2008 6:23 pm

First of all, I hope I didnt start an argument. :D I just wanted to hear different opinions on the matter. Which different opinions there obviously is. I was dabbling around, I believe on Iskys web sight. I believe he aslo said something to the effect on the big split not needed. But yet his solid flat tappet had 4-10 degree splits. THAT confused me. So I came on here to hear the opinion from the pros. Thanks all of you for the input on the subject. The amount of information on this forum is almost overwhelming. :shock: Brad

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Postby RW TECH » Sat Feb 09, 2008 6:34 pm

Torque will suffer at low RPM with excessive exhaust duration, but it may not be as dramatic as you'd think, unless you're talking really low RPM.

I made a wholesale cam change on an engine, where the exhaust duration was increased 10 degrees and the lobe centers were spread 4 degrees. Intake lobe remained the same in both situations.

The first cam was a "typical" arrangement for the application & engine combination.

The engine lost only a few ft. lbs. below peak TQ, and was within normal year to year testing variation through the rest of the pull. Actual TQ peak moved to where I wanted it (7200 RPM) but the numbers were so close otherwise it really wasn't worth the change.

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Re: exhaust duration

Postby Windsor377 » Sat Feb 09, 2008 7:30 pm

bigjoe1 wrote:That ratio of intake flow to exhaust flow doesent mean a thing A prostock engine has about a 50 percent ratio,. I never make a big deal on the ex to intake flow ratio stuff.

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That's another good point. The SBF cylinder head designs that I'm involved with don't have as great an intake flow bias over the exhaust as compared to the BB PS heads we hear about. As a result, that's why I think these engines are running anywhere from 10 - 14* less exhaust side "advertised" duration than those BB enines, if I've got my numbers right on the BBs. Still the cams in these SBs run quite a bit more exh side duration than intake side.

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Postby Windsor377 » Sat Feb 09, 2008 7:32 pm

2xmod wrote:First of all, I hope I didnt start an argument. :D I just wanted to hear different opinions on the matter. Which different opinions there obviously is. I was dabbling around, I believe on Iskys web sight. I believe he aslo said something to the effect on the big split not needed. But yet his solid flat tappet had 4-10 degree splits. THAT confused me. So I came on here to hear the opinion from the pros. Thanks all of you for the input on the subject. The amount of information on this forum is almost overwhelming. :shock: Brad


I think this is a very good topic that gives folks a chance to explain how completely "opposite" solutions can each be successful.

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Postby ADR » Sat Feb 09, 2008 8:50 pm

2xmod
You have to be careful around here and pay attention to the fine print.

One "Pro" will say something based on a 50% flow ratio and another "Pro" will disagree with first pro but his info is based on an 80% flow ratio and they can pretty much argue all day about it but they are talking about two completely different things....its kinda fun really :D

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Postby MadBill » Sat Feb 09, 2008 9:19 pm

I was just looking at the Reher.Morrison site and noticed the cam for their 622 c.i. engine was 288°/316° @ 0.050" on a 117° LCA. They didn't list exhaust flow on the matching heads, but assuming it was similar to others with the same valve size (2.45"/1.85") it would be 540/290 or 53%...
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