Intake exh duration split??

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Intake exh duration split??

Post by Steve.k »

You guys probably been asked this 10 million times but what do you like to see on the split. I know it's likely head specific so I'll say im using a a3 nascar head. On 436 inch motor. Current cam is 6* split. Flow was 378@800 & 256@800. Give your thoughts.
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Re: Intake exh duration split??

Post by andyf »

Split is probably the wrong thing to focus on. Figure out what the valve events need to be and the split will take care of itself.
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Re: Intake exh duration split??

Post by Steve.k »

Thanks Andy. I have used a grinder for a number of years and all is good. I just notice he likes a tighter split than others. So i was curious to why. We run primarily 1/8 mile so that maybe why. We typically run cleveland and bbf.
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Re: Intake exh duration split??

Post by hoffman900 »

The late Harold Brookshire:
This subject is chapters 3 and 4, and maybe 5, in the book I plan to write on the Theory and Application of Cam Design to Automotive Racing. It is too complex for a simple post or two.
Some quick answers:
Blown alcohol and turbo-charger engines normally use reverse-pattern cams, big intake, small exhaust. The reason is heat-production. Conventional cams with big exhaust profiles give cooler exhausts with lower back pressure. Blown Alcohol engines run very cold with these type of cam; a reverse-pattern cam retains heat in the cylinder, and also loses less charge out the exhaust port, keeping more in the cylinder.
Turbo-charger cams thrive on heat and backpressure, conventional cams produce low velocity, low pressure, cooler, exhausts, and lead to turbo-lag, bad throttle response. Reverse-pattern cams restores throttle response to them.
Conventional N/A cams are a blend of scieince and art. The science part is that exhaust to intake ratios produce a number that relates to how a cam designer designs cams. Different designers relate to different ratios.
I find that 72-76% ratios respond with the best overall power curve when the exhaust is about 8* bigger that the intake at .050". If the ratio gets above 85%, the engine does not repond to exhaust cams smaller than the intake at .050". Under 72% the engine needs a 10-12* larger exhaust.
All these numbers are for my type of design, and a perfect world.
Reality and what the engine builder wants to accomplish come into play.
Two-barrel engines are the prime example. These engines are generally limited by top-end airflow to the actual amount of horsepower they will produce. If that top-end is 450 BHP(just for example), putting an exhaust cam on that will get 600 BHP out of a 4 bbl engine will not help get out more horsepower; it will just blow a lot of bottom-end torque right out the exhaust pipe.
So we end up using an exhaust cam that will get out the actual power made by the engine. However, we find out that a little bigger intake lobe will help add mid-range power(still less than the maximum capability of the 2-bbl), and may help extend the flatness of power past peak power.
All this is more noticable in roller-cam engines than in flat tappet ones, I end up with a lot of wide LSA single pattern cams, or maybe just 4* bigger on exhaust. Top-of-the-line 2 bbl engines with headers and solid lifter cams might even go to 8* spreads.
Harvey Crane says "WEW", which stands for "What Ever Works!".

UDHarold
Mike Jones
No one said that.
Overlap is extremely important.
It is controlled by the Intake opening and exhaust closing and is not directly related to LSA, or the difference in duration between the intake and exhaust.
After I calculate the Intake opening and closing points, I calculate the exhaust closing point based on desired overlap, then the exhaust opening point.
If the exhaust duration ends up being 12 degrees longer then the Intake, or 12 degrees shorter, it's irrelevant.”
The long answer is, it's complicated. The short answer is, to quote Harold quotting Harvey, "whatever works". The flowbench has little baring of what the exhaust port is doing on a running engine. Both cam designers were/are very successful and typically ended up in similar places as far as camshafts go.
-Bob
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Re: Intake exh duration split??

Post by Steve.k »

Thats a very interesting read! Yes I wondered about that. Some guys say you need more split and some some say the other. I know I've had some cut by my guy and it was only 4* difference. I gave him our info and goals and thats what he came up with. It seems to work. I just had some guys asking about it so I thought I'd ask. Just like alot of things no specific rule applies.
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Re: Intake exh duration split??

Post by hoffman900 »

Steve.k wrote: Fri Mar 16, 2018 11:39 pm Thats a very interesting read! Yes I wondered about that. Some guys say you need more split and some some say the other. I know I've had some cut by my guy and it was only 4* difference. I gave him our info and goals and thats what he came up with. It seems to work. I just had some guys asking about it so I thought I'd ask. Just like alot of things no specific rule applies.
Everything I’ve learned, the more efficient the port, the less area it needs for a given rpm. As said above, treat them as their own, the splits will end up being what they are. You see everything from 15* less duration @ .050” to 15* more duration @ .050, depending, or .020” more or less lift. Just don’t reference it to anything. Or “whatever works”. :wink:
-Bob
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Re: Intake exh duration split??

Post by MadBill »

The vast majority of split duration catalog pushrod 2 valve cams have more exhaust lift as well as duration. The cynic in me ponders whether that's just because they're using the same generic IN/EX lobe family for both, so the longer duration comes with a side order of more lift.

How often has anyone found power with greater exhaust than intake lift? :-k
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Re: Intake exh duration split??

Post by KnightEngines »

The vast majority of split duration catalog pushrod 2 valve cams have more exhaust lift as well as duration. The cynic in me ponders whether that's just because they're using the same generic IN/EX lobe family for both, so the longer duration comes with a side order of more lift.
The cynic in you is well justified in being cynical, it seems to me that that is exactly the case.

I often spec custom cams for customers that are very similar in duration to a shelf cam, but I usually spec a lazier exhaust lobe & often change the lobe centres a degree or 2. I've been pulled up on it a number of times with the question being that the specs look pretty damn close to xx shelf cam, why do I need to spend the extra $ for a custom. Only answer I can really give is shelf cams are generic, pretty close to what you want, but what I'm getting made will make a bit better average HP, a slightly nice curve & a bit more peak, enough to justify the small extra spend.
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Re: Intake exh duration split??

Post by user-23911 »

If you reverse engineer a top of the line Japper DOHC cam, it's got more duration and lift in the intake than the exhaust.
That's an N/A.
Turbo is usually about the same int / ex duration..
A small turbine works best with a short duration cam but a big turbine works best with a much longer duration.
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Re: Intake exh duration split??

Post by Aukai »

Sorry for the interruption.
I miss Harold, that is all.....
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Re: Intake exh duration split??

Post by pdq67 »

Aukai wrote: Sat Mar 17, 2018 5:22 am Sorry for the interruption.
I miss Harold, that is all.....
Me too!

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Re: Intake exh duration split??

Post by hoffman900 »

joe 90 wrote: Sat Mar 17, 2018 5:13 am If you reverse engineer a top of the line Japper DOHC cam, it's got more duration and lift in the intake than the exhaust.
That's an N/A.
Turbo is usually about the same int / ex duration..
A small turbine works best with a short duration cam but a big turbine works best with a much longer duration.
It depends.

Mike Jones shared the Chevy IRL engine had exhaust ports that were more efficient than the intake at low lifts. That will effect things. If the exhaust port is less efficient, then it will need more lift area. A hemispherical chamber with good crossflow, will need something different.

Kenny Duttweiler on "turbo camshafts":https://youtu.be/dokOy3HM348?t=1h16m22s

Interview with Billy Godbold on camshafts: https://youtu.be/whmOxK4XDYQ?t=18m50s
Last edited by hoffman900 on Sat Mar 17, 2018 9:48 am, edited 3 times in total.
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Re: Intake exh duration split??

Post by Walter R. Malik »

A camshaft will usually be correct only in one instance.
Most people cam for a wide open throttle condition but, some engines will rarely, if ever, be run there and some will only be run there.; with a full exhaust system or open headers.
How an engine is to be used changes almost everything.
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Re: Intake exh duration split??

Post by Steve.k »

This is all very interesting. Most of my custom cams ended up between 4&5* splits. Which if you look back at the ol cleveland crane cams ended up close to 10. I just finished a 400 with a r-262 crane with that amount. Any how thanks guys. I guess if she pulls car all is good.
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Re: Intake exh duration split??

Post by hoffman900 »

The other thing that throws all of this into whack is asymmetry.

Harold's lobes have more aggressive opening sides than closings sides. The opening side acts like a small cam than the closing side. Harold said he designed lobes. Not intake or exhaust lobes, just lobes, and fit what he had in his arsenal to the application. Mike Jones has mentioned how Harold's cams ran best with more advance than his symmetrical lobes at a time (1980s). This was due to what Harold was doing with his lobe designs.

You can get Harold's lobes from Bullet (his older Ultradyne), Lunati (Voodoo, TL, TR families (the TL and TR being more recent designs)), and Howard's (Custom Camshaft Company / Arrington, which would be some of Harold's last commercial designs).

Comp has offset exhaust lobes (and intake), where the peak valve lift is biased to the opening side (positive offset). Billy told me some of their more popular positive offset exhaust lobes were for nitrous applications, but have become pretty popular with some naturally aspirated engine builders. He's tried up to 7* positive offset.

He describes offset as, Distance of Max Lift to the rated closing point - the Distance of Max lift to the rated opening point. Positive would bias towards opening side, a negative value would be the closing side.

Crane, Jones, etc. all also have asymmetrical (or as Harold called them "unsymmetrical" - saving "asymmetrical" for finger follower type lobes) lobes.

If you look at simulated and measured pressure traces on the exhaust side and calculated mass flow, especially on the exhaust side, you can see how positive offset, as Billy defines it, works.

The point? There are no hard and fast rules. Unsymmetrical cams will throw all of your rules of thumbs out the window. Harold's splits work for his style of lobes, which he mentions ("for my lobes"), and serve as a starting point - mentioning how it depends on what the engine builder wants or is doing. Do you keep the duration the same, but reduce lift? Do you reduce lift and duration? Do you increase any of this in relation to the intake? As Mike said, look at the exhaust and intake independently and don't look at the splits.

Make the exhaust side more efficient than it changes. You're not going to cam an engine with an Elston built headers like you will some $200 swap meet Hooker headers.

Again to quote Harold, quoting Harvey, "whatever works".
-Bob
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