FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: 427HISS on August 20, 2020, 06:35:26 PM
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About a week ago I had seen a comment on split duration on FE cams, but I can't find it. So does the FE's like them (exhaust I'm sure) why and depending on more factors, about how much ?
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I remember it too, vaguely.
My answer as I run some classic and new combos on the Gonkulator is,
It Depends!
I find the best split is anywhere from 3 degrees to 16 degrees, but in no case is a single pattern the best.
The best split depends not just on the tntake vs exhaust flow in the heads, but on the whole intake vs exhaust tract.
And of course it changes with power adders.
The Gonkulator thinks that both the 390GT and 428CJ like about a 4 degree split, ie 212-216 duration works better than the factory 207-221 duration with its 14 degree split.
But that's with open exhaust.
Thru mufflers, they all want more split. Maybe that's why Ford added some split for the 428CJ, and it ran so good on the street that way.
Its pretty obvious as an example that Fords last hipo big block, the 429SCJ, should have had a split pattern cam, with its sewer pipe intake ports, football stadium sized intake, and pinto-sized exhaust ports. It Gonkulates best with a 16 degree split, and would have probably smogged cleaner too. A 460SCJ with a 16-degree split cam would have been the true equal of the Chevy LS6 that year.
So it just depends!
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I've always try to split the exhaust based on the intake/exhaust flow. I recently did a dyno comparison of two identical engine combinations, except one had a 4° split and the other had an 8. Intake duration was kept the same. Overlap was kept the same. The engine with more split made 10 hp and 15 lb-ft more.
Now, with that being said, sometimes I can't split it as much based on overlap issues. If someone has a unique combination that requires a lot of vacuum or similar, then sometimes we have to compromise. But for the majority of the cylinder heads I run, I split them a lot.
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It would be a rare situation when a split duration camshaft "wouldn't" be more powerful. Why? Because you are flowing two "different " mediums. Fuel laden air on the intake side and hot , burning , expanding air on the exhaust. This is the same reason valves are not the same size. As Werby said the "flow balance" of the head is a factor and there are some that "can" use a single pattern , and some ( GM small block specific head) that wants SHORTER exhaust duration than intake duration. There isn't a general rule here. Guys that do allot of dyno testing on specific Ford engines have a clear advantage over ANY tech at a cam company. IMHO anyway. Why ask a GM guy what to run in a Ford?
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It would be a rare situation when a split duration camshaft "wouldn't" be more powerful. Why? Because you are flowing two "different " mediums. Fuel laden air on the intake side and hot , burning , expanding air on the exhaust. This is the same reason valves are not the same size. As Werby said the "flow balance" of the head is a factor and there are some that "can" use a single pattern , and some ( GM small block specific head) that wants SHORTER exhaust duration than intake duration. There isn't a general rule here. Guys that do allot of dyno testing on specific Ford engines have a clear advantage over ANY tech at a cam company. IMHO anyway. Why ask a GM guy what to run in a Ford?
Agreed.
I found a couple engines where the Gonkulator does NOT want a split pattern:
283/283 fuelie Chevy (in NHRA trim it actually likes a 4 degree NEGATIVE split, intake duration greater than exhaust duration)
396/325 L35 Rat in NHRA trim.
Even the big square-port Rats, with a decent exhaust port, still Gonkulate best with a 6 or 8 degree split.
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You will find some aftermarket heads that will probably want a single pattern camshaft or a very small split. They have very high exhaust flow per intake flow. I would imagine Blair's Pro Port heads would be in that category.
In the SBF world, the newer AFR and TFS-TW-R heads approach that, with almost an 80% aspect ratio. I typically only split them 2-4 degrees at most, depending on the application.
In the Pontiac world, the Kauffman High Port heads have an 80-85% ratio. I did a 434 once with Kauffman HP heads, Performer RPM intake, 10.5:1, with a 229/229 @ .050" hydraulic roller and squeaked out 600 hp @ 6000 rpm. Very streetable.
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Do we base the duration on just head the flow on the numbers we had on a flow bench, or is more information, like the fuel delivery, size of the exhaust ports, header tube size, back pressure etc ?
How is intake and exhaust duration calculated ?
If I gave you my intake and exhaust ports of my intake manifold and the same for my heads, what else would be needed to find out what the duration should be for a camshaft ?
Also, what would happen to the duration, both intake and exhaust, was not the engine needed and how it would react on both ?
it's say, that if you had .250 on the intake and .250 on exhaust you got the flow of .350. If you increased or decreased either by a small number of day, .2 or increased by , what would it do ? If you stepped it up by 5 more, then 5 more etc....(or by .2)
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You will find some aftermarket heads that will probably want a single pattern camshaft or a very small split. They have very high exhaust flow per intake flow. I would imagine Blair's Pro Port heads would be in that category.
In the SBF world, the newer AFR and TFS-TW-R heads approach that, with almost an 80% aspect ratio. I typically only split them 2-4 degrees at most, depending on the application.
In the Pontiac world, the Kauffman High Port heads have an 80-85% ratio. I did a 434 once with Kauffman HP heads, Performer RPM intake, 10.5:1, with a 229/229 @ .050" hydraulic roller and squeaked out 600 hp @ 6000 rpm. Very streetable.
Brent a good example is an alcohol injected BB Chevy where a single pattern cam "helps" by keeping a bit more heat in the chamber with the "choked exhaust. Another one is a small cubic inch SBC with an 18* chevy head.We ran 10* LESS on the exhaust because the exhaust port was TOO darn good. We also widened the lobe separation too . Overlap was our enemy on that head. Ford's ( for the most part) can use a bit more exhaust unless there is a restrictive ( stock) exhaust system. Then the back pressure negates the extra scavenging potential. You have a very good background/ data base/ experience since you dyno what you build.
Randy
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Do we base the duration on just head the flow on the numbers we had on a flow bench, or is more information, like the fuel delivery, size of the exhaust ports, header tube size, back pressure etc ?
How is intake and exhaust duration calculated ?
If I gave you my intake and exhaust ports of my intake manifold and the same for my heads, what else would be needed to find out what the duration should be for a camshaft ?
Also, what would happen to the duration, both intake and exhaust, was not the engine needed and how it would react on both ?
it's say, that if you had .250 on the intake and .250 on exhaust you got the flow of .350. If you increased or decreased either by a small number of day, .2 or increased by , what would it do ? If you stepped it up by 5 more, then 5 more etc....(or by .2)
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Brent a good example is an alcohol injected BB Chevy where a single pattern cam "helps" by keeping a bit more heat in the chamber with the "choked exhaust. Another one is a small cubic inch SBC with an 18* chevy head.We ran 10* LESS on the exhaust because the exhaust port was TOO darn good. We also widened the lobe separation too . Overlap was our enemy on that head. Ford's ( for the most part) can use a bit more exhaust unless there is a restrictive ( stock) exhaust system. Then the back pressure negates the extra scavenging potential. You have a very good background/ data base/ experience since you dyno what you build.
Randy
Don't forget the race Hemi stuff where they will sometimes murder the exhaust port to prevent/reduce cross flow from intake port right across to the exhaust without bothering with all that compression and combustion stuff.
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Absolutely! There isn't a "standard" when it comes to camshafts "in general" BUT specific heads DO favor certain designs. This is where an engine builder of the specific engine you have is the best person to ask. You don't want a Chevy person to tell you what to use. Same for "catalog descriptions"/"catalog cams". Often they are too generic and only occasionally "proper".My "day job" has me looking at customers camshaft information all day long and I see "trends" on every brand and type of engine based on use.