Compression vs Airflow 1

[PFM]

Hello All,

I need some help on one of those what is more important in the production of horsepower, compression or airflow? At what point does a reduction in air flow, due to piston valve relief, overcome gains in compression due to piston shape?

OK the picture is this, the piston is a near flat top with a significant valve relief cut into it for valve clearance. We work hard to get good air flow numbers through the heads and then in the motor the valve is shrouded by the piston in the area of highest flow (the long side of the port / valve) as the valve goes through .25 lift.

I understand the piston and valve are moving away from each other and the shrouding effect is not long term but it must affect air flow. So do we re-shape the piston top to reduce the shrouding at the cost of compression or not?

The current compression is 16 to 1, on a guess it would drop to 14 to 1 to re-shape the piston to help the air flow. The effect on air flow is at the moment unknown.

Thanks for any input to this delema.

PFM

 

[rj426]

I'm going to revive this topic if I may, why is it that small block comp drag race motors seem to usually respond to 16-1 & 17-1 compression ratios yet some competitive 500 c.i. pro stock motors I know of don't seem to net any gains past 15-1 to 15.5-1?

 

[MaxRaceSoftware]

>>why is it that small block comp drag race motors seem to usually respond to 16-1 & 17-1 compression ratios yet some competitive 500 c.i. pro stock motors I know of don't seem to net any gains past 15-1 to 15.5-1?<<<


Quote from Darin Morgan =>
The Pro stock engines are only 15 to 1 static, but the dynamic compression ratio is about 17.5 to 18 to 1. The small blocks you are talking about probably have 16.5 to 1 but there dynamic is still 17.5 to 1. Why? because you fill the cylinder less so you have to compress it more. We fill the cylinder A LOT more so we need to compress it less in order to extract the same amount of energy. Some of our engines only have 14.8 to 1 because they are so efficient at trapping the air ( high VE ). When you say competitive comp engines what engines are you talking about? Single carb or restricted induction system engines of course need a couple points more compression to make up for the decreased VE.. The 358cid Pro Stock truck engines where only 14.5 to 1 and they made 968 HP . With the cam technology we have now there is no doubt in my mind that they could produce 985 to 990 HP now at 14 to 1. Its been a couple of years since I did one of those so I am speculating.

Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
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Arlington Texas 76001
817-467-7171
FAX-468-3147
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http://www.rehermorrison.com

Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
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