Intake to Exhaust Valve Size Ratios 2

[FischerMotorsports]

I’m currently cad designing, cnc machined, billet 4v heads for several ENDURANCE engines. I’m building them for both na and turbo applications utilizing a maximum rpm of 9,000 to 10,000 rpm. These are water cooled engines, utilizing thermal coatings (pistons, chambers, and ports) built for high output on pump gasoline.

I have been researching many different head combinations from 2 to 4 valves and analyzing their intake to exhaust valve size ratios. I understand that flow ratios are what you really look at but I also understand flow starts with and is limited to the valve size.

The ratios that I’m looking at are the Surface Area of the Intake to exhaust valve.

Everyone agrees that they would go to a smaller exhaust valve if they could utilize a larger intake valve. Everyone agrees that you give up energy through increased pumping loses but gain more from increased airflow and more power.

My question is: Do you also gain heat or longevity issues.

Out of everything that I’ve analyzed so far, drag racing has the smallest ratio (biggest intake valve and relatively small exhaust). I don’t see any endurance engines that have this small of ratio…. Is it because of heat and reliability?

I see pro stock running 2.6 in and 1.7 ex which is a 42.7%.
Alan Johnson sbc heads running 51.5%
Chevy LS2 and LS6 at 63.4%
ZO6 LS7 at 53.6%
Sport Bike Heads from Honda, Kaw, and Suzuki all at about 75 to 79% in the 1980’s…
They all now run between 68 and 69% currently.

My question is: How small on the exhaust is too small and what are you sacrificing in terms of reliability? Is it heat and is this why you only see this extreme trend in drag racing and not on endurance engines?

Thanks for any help.
682-558-9275

 

[thundair]

This has a lot to do with your head design and reversion tunning.

I have looked for empirical formulas and looking at everything from Ducati race bikes to Alfa Romeo

The secert lies some where between 1.22 and 1.3 or 1/X .

My guess is the pro stock boys with their huge intakes and 600# springs are sacrificing exhaust flow for the huge intake charge.

Also the scavanging of the long tube headers probably help with evacuation.

Regards

 

[ramblerman]

I think the measure you want to compare is "valve curtain area times duration". Valve curtain area is the circumferece of the valve times valve lift. This measures the wall of the cylinder that the valve moves through, which is the area of the opening provided by the opened valve. Using this measure of area instead of the area of the head of the valve eliminates the squaring effect. But you also have to consider the difference in the intake duration to the exhaust duration. As a former drag racer, I'll tell you that I found that most of the popular cams had longer exhaust duration compared to intake. This usually had to do with the fact that most american pushrod engines don't flow enough through the exhaust port to match the intake port flow.