Technical > Matt Shaff's Engine Shop

Seat Pressure

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masracingtd1167:
Thinking about this today and wondering what your opinion's are . Do you think too much seat pressure can rob horsepower ? I run a lot of seat pressure 410 lbs but I also run the motor to 9400 rpm. Any thought's?

Frontenginedragsters:
Seat pressure is only part of the question.
Valve springs are rated in pounds per inch of travel.
When the valve is closed - seat pressure effects the valve only. You have lash in between the other parts.
But when that lifter rises and the rocker arm starts to open that valve, the pressure goes up at the rate of the spring.
Don't forget that open pressure is multiplied times the rocker arm ratio back to the lifter/pushrod side.
400 pounds seat is probably as much as needed. But do the math on the open pressure and look at your lifters often.
I assume you use titanium valves and small diameter valve stems like 7mm ?
9000 RPMs is what is needed to run big MPH and valve train stability means everything at that RPM.
There is good reading on Reher-Morrisons web site. David Reher spells it out in terms even an idiot like me can understand! ???
http://rehermorrison.com/category/tech-talk/
Thanks: Matt

masracingtd1167:
MATT Thank's for the advice . One thing I have never done is check the open pressure . I have to find someone who has a tester that goes that high!

KeithDyer:
FWIW - We have seen that these 400+ seat pressures will absolutely KILL the lifters if you are trying to run the stock .842 diameter. 

They have had to go to .903 or .937 to get the lifter bores to stay nice.

Take care, K

masracingtd1167:
Keith I am using the Jessel Keyway lifters in mine and I broke one last year . I have decided to back off the seat pressure to about 375 lbs . Thank you for the input Bill.

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