Calculating cam I/E split from head flow numbers

[travis]

Found this a while back...posted from someone who does cams for a living.

Variables are CID and RPM but a good rule of thumb for most street engines is if the I/E ratio is 75% you need a single pattern cam at .050". If for every % below that you need about 1.5 degrees of split favoring the exhaust. For every % above that you need to add 1.5 Degrees to the intake.

Is this what you guys use? If not, then what?

This formula seems to be very much in the ballpark, based on some of the better running combos I have seen.

[Stan Weiss]

That sound like something that Chris Straub - Straub Technologies said.

Try this calculator - Estimate Cam Intake and Exhaust Duration Needed @ 0.050"
http://www.magneticlynx.com/carfor/calccame.php

Stan

[RDY4WAR]

Definitely sounds like Chris. I personally don't follow that rule. I don't put any correlation between the intake and exhaust lobes. Every engine is different.

[hoffman900]

Here are Harold’s thoughts:

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

From this thread: http://www.speedtalk.com/forum/viewtopi ... 7156cc0d92

Listening to a Kenny Duttweiler(spl) interview, people think differently on turbo cams now.

[groberts101]

Here are Harold’s thoughts:

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

From this thread: http://www.speedtalk.com/forum/viewtopi ... 7156cc0d92

Listening to a Kenny Duttweiler(spl) interview, people think differently on turbo cams now.

I wish Harold was around to further extrapolate on the bolded statement above. Makes absolutely no sense to me and most cam grinders will quickly change the exhaust lobes time/area to compensate for an overly strong flowing exhaust port. Over 90% E/I ratios can sometimes create a +20° reverse split design. And reduced lobe lift requirement too.

[FC-Pilot]

Definitely sounds like Chris. I personally don't follow that rule. I don't put any correlation between the intake and exhaust lobes. Every engine is different.

That may have been a rough statement for morons like me that are picking a cam out of a catalog, but the cam he cut for me did not follow that, and I am very happy with it.

Paul

[Scotthatch]

I have been fighting to find some way to know when to put the exhaust opening event for a long time .... I have moved the event as much as 10 degrees and seen no change at all on bigger cams ... on smaller cams it seems like even 4 more degrees can help a lot... I even took a straight pattern cam and just kept raising the out put with blower then nos and blower looking for something to show a restriction on the exhaust side and it just kept working .....

Part of the problem is intake and ex flow work differently because the pressure drop is different but there has to be some sweet spot like the cam above ... when it's at that spot more pressure from more HP moves more air and it works ... if you're below that point then there is a limit to what it will carry ... if above then no gain which is why moving it changes nothing .....

[Orr89rocz]

Pretty sure in a drag race pushrod engine most cams favor more exhaust side duration. From everything i have seen from BigJoe he always had a split. Its suppose to help extend hp curve beyond peak rpm so it doesnt fall off as fast.

[zums]

Its a complete useless rule of thumb, first of all, is it at .05- .200, .006, is it with full exhaust, bare port, intake radius entry only, intake and carb hooked up, with /without air cleaner, compression ratio dosent matter i guess?? either way just take a look where your piston is with your combo and see where you want the valve to open in relation to any work left
Tom

[Erland Cox]

Here are Harold’s thoughts:

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

From this thread: http://www.speedtalk.com/forum/viewtopi ... 7156cc0d92

Listening to a Kenny Duttweiler(spl) interview, people think differently on turbo cams now.

It is sad that Harold never got to write that book but you have done the next best thing to that by collecting his posts here.
Thank you for that.
I cam a turbo engine just like a N/A engine but with a little more lobe separation.
Usually not more than 3-4 degrees more.
Usually I use 8 degrees less on the exhaust to build more mid range.
I have speced a cam for a 2,8 liter Volvo 4 cylinder 8 valve engine that just made 1390 RWHP.
275 degrees intake and 267degrees exhaust at 0,050" at the valves. 16mm lift in and 15,3mm lift out, 109 degrees of lobe separation.



Erland

[hoffman900]

Here is the Kenny Duttweiler interview: https://www.youtube.com/watch?v=dokOy3HM348&t=5275s

He gets going on camshafts about 1:19 in, but listen to all of it as they come up several times. He mentions camshaft timing has changed over the years due to the efficiency and the drop in back pressure with modern turbo developments.