More exh duration

General engine tech -- Drag Racing to Circle Track

Moderator: Team

2xmod
Pro
Pro
Posts: 261
Joined: Wed Nov 28, 2007 5:35 pm
Location: central Wisconsin

More exh duration

Post by 2xmod »

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?
bigjoe1
Show Guest
Show Guest
Posts: 6199
Joined: Thu Aug 24, 2006 8:16 pm
Location: santa ana calif-92703
Contact:

exhaust duration

Post by bigjoe1 »

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.

JOE SHERMAN RACING ENGINES
2xmod
Pro
Pro
Posts: 261
Joined: Wed Nov 28, 2007 5:35 pm
Location: central Wisconsin

Post by 2xmod »

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
bigjoe1
Show Guest
Show Guest
Posts: 6199
Joined: Thu Aug 24, 2006 8:16 pm
Location: santa ana calif-92703
Contact:

Exhaust duration

Post by bigjoe1 »

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.


JOE SHERMAN RACING ENGINES
Metrobilly
Member
Member
Posts: 118
Joined: Thu May 18, 2006 9:07 pm
Location: Maryland

Post by Metrobilly »

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?
User avatar
CamKing
Guru
Guru
Posts: 10709
Joined: Wed Aug 08, 2007 4:05 pm
Location: Denver, NC
Contact:

Re: More exh duration

Post by CamKing »

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.
Mike Jones
Jones Cam Designs

Denver, NC
jonescams@bellsouth.net
http://www.jonescams.com
Jones Cam Designs' HotPass Vendors Forum: viewforum.php?f=44
(704)489-2449
wfolarry
HotPass
HotPass
Posts: 165
Joined: Thu Aug 23, 2007 10:59 pm
Location: Oak Lawn,IL
Contact:

Post by wfolarry »

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.
User avatar
Windsor377
Expert
Expert
Posts: 810
Joined: Sat Apr 29, 2006 9:02 pm
Location: Maryland
Contact:

Re: More exh duration

Post by Windsor377 »

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


JOE SHERMAN RACING ENGINES
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.
bigjoe1
Show Guest
Show Guest
Posts: 6199
Joined: Thu Aug 24, 2006 8:16 pm
Location: santa ana calif-92703
Contact:

exhaust duration

Post by bigjoe1 »

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.

JOE SHERMAN RACING ENGINES
2xmod
Pro
Pro
Posts: 261
Joined: Wed Nov 28, 2007 5:35 pm
Location: central Wisconsin

Post by 2xmod »

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
RW TECH
Guru
Guru
Posts: 2387
Joined: Thu Nov 24, 2005 9:08 pm
Location: DETROIT, MI

Post by RW TECH »

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.
User avatar
Windsor377
Expert
Expert
Posts: 810
Joined: Sat Apr 29, 2006 9:02 pm
Location: Maryland
Contact:

Re: exhaust duration

Post by Windsor377 »

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.

JOE SHERMAN RACING ENGINES
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.
User avatar
Windsor377
Expert
Expert
Posts: 810
Joined: Sat Apr 29, 2006 9:02 pm
Location: Maryland
Contact:

Post by Windsor377 »

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.
ADR
Member
Member
Posts: 143
Joined: Thu Feb 17, 2005 10:37 pm
Location: Auburn, Wa

Post by ADR »

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
User avatar
MadBill
Guru
Guru
Posts: 15024
Joined: Tue Nov 15, 2005 10:41 am
Location: The Great White North

Post by MadBill »

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%...
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
Post Reply