straight vs. split pattern cams
#880733
12/15/10 01:04 PM
12/15/10 01:04 PM
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Joined: Jul 2006
Posts: 383 Essex, Ont., Canada
Fishmarket
OP
enthusiast
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OP
enthusiast
Joined: Jul 2006
Posts: 383
Essex, Ont., Canada
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Can someone explain when and why it would be advantaageous to use a split pattern cam with more exhaust duration, and when a greater exhaust duration would start to be detremental? I can see using a split on an engine breathing through exhaust manifolds but what if your heads are ported and you have a free flowing header/exhaust set up? Can too much exhaust duration hurt power output?
1972 Plymouth Cuda 340 4 speed
1971 Plymouth Road Runner 383 Auto
1970 Plymouth Duster /6 Auto
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Re: straight vs. split pattern cams
[Re: Fishmarket]
#880734
12/15/10 01:36 PM
12/15/10 01:36 PM
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Joined: Mar 2008
Posts: 5,746 Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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Testing has show for a given intake profile they will give a little more power with a longer exhaust duration. so lets say a standard 236/236 @ .050 110 LSA cam changing it to a 236/244 @ .050 cam will give a little more power and not effect the drive ability from where it is. the norm seems to be 6 to 10 more on the exhaust to get rid of that expanded hot exhaust charge.
Last edited by Dodgem; 12/15/10 01:37 PM.
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Re: straight vs. split pattern cams
[Re: Dodgem]
#880735
12/15/10 02:11 PM
12/15/10 02:11 PM
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Joined: Jul 2006
Posts: 4,319 Puyallup, WA
StealthWedge67
master
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master
Joined: Jul 2006
Posts: 4,319
Puyallup, WA
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The magic number, here, is 75%. You want your exhaust port to flow @ 75% of your intake port. Most stock heads (especially BB mopars) don't do this, so cam manufacturers figured they can compensate for the heads shortcomings by extending the duration on the exhaust side.
What has happened now, is that split pattern cams have just become the norm, and the general public percieves they are getting a better engineered product with varying #'s on the card (not really knowing why)
Most good aftermarket heads do support the 75% number, however.
The key when choosing a cam is knowing what your heads will flow, and complimenting that curve with the cam. With good heads that flow properly, there is a strong case to be made that you'll maximize the power available from a given duration and lift spec. with a square pattern cam. Your intake, carburetor, and header selection also plays a role here.
I conferred with Member, Jeff Patterson, when choosing my cam, and we both agreed that my combo seemed to call for a square pattern cam, so that's what we dialed up from Engle.
LemonWedge - Street heavy / Strip ready - 11.07 @ 120
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Re: straight vs. split pattern cams
[Re: StealthWedge67]
#880736
12/15/10 02:17 PM
12/15/10 02:17 PM
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Joined: Feb 2007
Posts: 5,080 organ
maximum entropy
master
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master
Joined: Feb 2007
Posts: 5,080
organ
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Quote:
The magic number, here, is 75%. You want your exhaust port to flow @ 75% of your intake port. Most stock heads (especially BB mopars) don't do this, so cam manufacturers figured they can compensate for the heads shortcomings by extending the duration on the exhaust side.
What has happened now, is that split pattern cams have just become the norm, and the general public percieves they are getting a better engineered product with varying #'s on the card (not really knowing why)
Most good aftermarket heads do support the 75% number, however.
The key when choosing a cam is knowing what your heads will flow, and complimenting that curve with the cam. With good heads that flow properly, there is a strong case to be made that you'll maximize the power available from a given duration and lift spec. with a square pattern cam. Your intake, carburetor, and header selection also plays a role here.
I conferred with Member, Jeff Patterson, when choosing my cam, and we both agreed that my combo seemed to call for a square pattern cam, so that's what we dialed up from Engle.
WHAT HE SAID.
for what is the good life if not doing things thoughtfully?
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Re: straight vs. split pattern cams
[Re: Dodgem]
#880737
12/15/10 02:32 PM
12/15/10 02:32 PM
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Joined: Dec 2008
Posts: 1,200 UK
602heavy
pro stock
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pro stock
Joined: Dec 2008
Posts: 1,200
UK
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Opinions are just that , here's mine FWIW. Running too much exhaust duration with low compression engines just kills torque , exhaust valve requires a later opening to take advantage of later peak pressure associated with low compression ratios , opening exhaust valve to early won't give the expanding gasses enough time/leverage to push on piston , using a faster lobe can help regards pumping lossess , sometimes a higher ratio rocker arm on exhaust can help when going shorter on exhaust duration................the other problem with long duration exhaust lobes is the overlap period , once the piston moves away from TDC on intake stroke it's drawing on both intake & exhaust port , any spent gasses left in the exhaust port will be sucked into the cylinder along with fuel'air from intake port , the longer the exhaust valve is held open on intake stroke the more contaminated the charge will be , this is the reason tighter lobe centres produce less vaccume & run pretty rough @ idle , the cleaner the intake charge the more power the engine will produce. Using restricted exhaust manifolds along with later closing exhaust valves (longer overlap period) makes matters worse on street engines that only spin to around 3500/4000 rpm , unless that exhaust can scavenge intake charge will be contaminated.......why is it advancing intake centreline makes more torque low down? , LESS CONTAMINATION......all you guys using split patterns (low comp) go loosen lash on exhaust valve & notice the motor pick up , will idle a bunch nicer too. Just my , feel free to bash the hell out of it.
Last edited by 602heavy; 12/15/10 02:44 PM.
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Re: straight vs. split pattern cams
[Re: Fishmarket]
#880738
12/15/10 03:17 PM
12/15/10 03:17 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
Testing has show for a given intake profile they will give a little more power with a longer exhaust duration.
Whose testing? I've seen some that show that the proper single-pattern cam will provide more power through the entire range. So, who do you believe?
Quote:
The magic number, here, is 75%.
Sorry, there is NO MAGIC NUMBER. That "75%" rule is outdated.
Quote:
Opinions are just that...
I won't argue that point.
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Re: straight vs. split pattern cams
[Re: 602heavy]
#880741
12/15/10 04:12 PM
12/15/10 04:12 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
Hey Budget , what does 'do not feed the troll' mean?
It means I'm in kind of a crappy mood and this is just the sort of thread that could cause me to post of bunch of smart@ss comments in my replies.
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Re: straight vs. split pattern cams
[Re: BradH]
#880744
12/15/10 04:35 PM
12/15/10 04:35 PM
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Joined: Dec 2008
Posts: 1,200 UK
602heavy
pro stock
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pro stock
Joined: Dec 2008
Posts: 1,200
UK
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Quote:
Quote:
Hey Budget , what does 'do not feed the troll' mean?
It means I'm in kind of a crappy mood and this is just the sort of thread that could cause me to post of bunch of [Email]smart@ss[/Email] comments in my replies.
I wanna know who's the TROLL.
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Re: straight vs. split pattern cams
[Re: 602heavy]
#880745
12/15/10 05:13 PM
12/15/10 05:13 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
I wanna know who's the TROLL.
That refers to ME. Although I tell my kids my parenting role model is Shrek and he's an ogre, not a troll.
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Re: straight vs. split pattern cams *DELETED*
[Re: Fishmarket]
#880747
12/15/10 09:39 PM
12/15/10 09:39 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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Post deleted by polyspheric
Boffin Emeritus
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Re: straight vs. split pattern cams
[Re: BradH]
#880748
12/15/10 11:46 PM
12/15/10 11:46 PM
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Joined: Jul 2006
Posts: 4,319 Puyallup, WA
StealthWedge67
master
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master
Joined: Jul 2006
Posts: 4,319
Puyallup, WA
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Quote:
Quote:
Testing has show for a given intake profile they will give a little more power with a longer exhaust duration.
Whose testing? I've seen some that show that the proper single-pattern cam will provide more power through the entire range. So, who do you believe?
Quote:
The magic number, here, is 75%.
Sorry, there is NO MAGIC NUMBER. That "75%" rule is outdated.
Quote:
Opinions are just that...
I won't argue that point.
Well, if opinions are "just that", then I'll consider yours to be the low side of the analogy. You've certainly provided plenty of facts to support your rant, there..... (oh, maybe not) Until you can show me actual numbers that say the 75% rule is untrue, I'm going to consider your opinion completely arbitrary. Perhaps I should have called it a "rule of thumb", not a "magic number", this I'll concede. But the facts remain. This number is based on the physics of volumetric transfer. I don't think these mathematical equations "expire" with time. Why does just about every internal combustion head ever manufactured feature an exhaust valve approx. 75% of the intake? You may really mean to point out that this rule of thumb isn't foolproof every time??. To which I'd still ask you to come up with a fact based equation that outweighs it. Bottom line, heated exhaust gas needs approx. 75% of the volumetric space to travel at the same velocity as the cooler, wetted intake charge. If a heads flow capability doesn't support that mathematical certainty, then adjusting the exhaust valve duration to compensate has become the accepted path that cam manufacturers have followed.
LemonWedge - Street heavy / Strip ready - 11.07 @ 120
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Re: straight vs. split pattern cams
[Re: Sport440]
#880750
12/15/10 11:59 PM
12/15/10 11:59 PM
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Joined: Jun 2003
Posts: 9,225 Charleston
sixpackgut
Drag Week Mod Champion
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Drag Week Mod Champion
Joined: Jun 2003
Posts: 9,225
Charleston
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what do the FAST class guys run?
Gen 3 power 6.22@110, 9.85@135 Follow @g3hemiswap on instagram
performance only racing, CRT, ultimate converter, superior design concepts, ThumperCarbs
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Re: straight vs. split pattern cams
[Re: sixpackgut]
#880751
12/16/10 12:08 AM
12/16/10 12:08 AM
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Joined: Jul 2006
Posts: 4,319 Puyallup, WA
StealthWedge67
master
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master
Joined: Jul 2006
Posts: 4,319
Puyallup, WA
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According to the writeup on Joel Nystrom's 67 GTX in MuscleCar Review last month, He reportedly runs heads that have been custom ported to flow 420 on the intake, and 315 on the exhaust.... (at this point I would ask you to do the math on that relationship).
The same writeup notes that he employs a camshaft that features 4% more duration on the exhaust side. This makes sense, since his hemi runs 1250 cfm worth of dual AFB on the intake side, but stock exhaust manifolds on the exhaust side.
Last edited by GEnsrud; 12/16/10 12:09 AM.
LemonWedge - Street heavy / Strip ready - 11.07 @ 120
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Re: straight vs. split pattern cams
[Re: StealthWedge67]
#880752
12/16/10 12:20 AM
12/16/10 12:20 AM
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Joined: Jun 2003
Posts: 9,225 Charleston
sixpackgut
Drag Week Mod Champion
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Drag Week Mod Champion
Joined: Jun 2003
Posts: 9,225
Charleston
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Quote:
According to the writeup on Joel Nystrom's 67 GTX in MuscleCar Review last month, He reportedly runs heads that have been custom ported to flow 420 on the intake, and 315 on the exhaust.... (at this point I would ask you to do the math on that relationship).
The same writeup notes that he employs a camshaft that features 4% more duration on the exhaust side. This makes sense, since his hemi runs 1250 cfm worth of dual AFB on the intake side, but stock exhaust manifolds on the exhaust side.
420/315 what does a head done for super stock flow?
Gen 3 power 6.22@110, 9.85@135 Follow @g3hemiswap on instagram
performance only racing, CRT, ultimate converter, superior design concepts, ThumperCarbs
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