Seat Pressure

[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.

[JrFuel Hayden]

I've been running Jesel Keyways for 6 years, broke only one when it was too rich and deluted the oil.
Bill what dia Keyway and what roller dia are you running when you broke one ? I've been running 400+ seat pressure, with the .937, and .850 roller Keyways

[masracingtd1167]

Jon mine are .937 diameter and a .750 wheel . I don't turn mine quite as high as you guy's do about 9400 max . I was thinking it might free up a little horsepower . I still plan on using 375 on the seat . My biggest concern is how much open pressure they had . I don't have a spring tester that go's that high.

[hotrod316]

I CAN CHECK UP TO 1000 LDS IF YOU ARE IN THE DAYTON OHIO AREA SOMETIME, LET ME KNOW
STEVE M.

[coupemerc]

Jon mine are .937 diameter and a .750 wheel . I don't turn mine quite as high as you guy's do about 9400 max . I was thinking it might free up a little horsepower . I still plan on using 375 on the seat . My biggest concern is how much open pressure they had . I don't have a spring tester that go's that high.

Bill... My spring tester goes to 1000 lbs max. I am going to disassemble each spring (mine are doubles) and then measure the force of the inner at max lift and then the outer at max lift. Then add them both together to get the total open force.

[JrFuel Hayden]

We measure our closed spring pressure with the retainer, so we get the total spring pressure, just add the retainer thickness to your installed height.  Also your spring supplier should be able to give you open pressure once you give them your valve opening.
Bill, I don't know your motor combo, but we replace our 1 1/2" dia PAC springs when they test on the car at 329 lbs. 330 is min with my 10,000 rpm motor at .938 lift, on the RR 23° SBC , new they install at 400. One of the best changes we have made was when I switched to the 1 1/2" PAC springs, they are more expensive, but I don't have to change them nearly as often as the triple springs. Unless we have some other problem, I only replace 3-4 springs all season.
Bud's non raised runner 23° only ran 260 lb springs. Our Hemi needs 420-440 lbs new, bigger valves, and high 9,000 rpm.
Back in the day when I had a dual engine BBC Top Gas car, and we wanted to be one of the 1'st Top Gas cars to go 200 mph, we did run 202 when we installed weaker valve springs, no other change.
So, if you are not sure what springs to run, check with your cam grinder, and be sure to tell them 9400 max. 

Jon

[masracingtd1167]

Jon mine are .937 diameter and a .750 wheel . I don't turn mine quite as high as you guy's do about 9400 max . I was thinking it might free up a little horsepower . I still plan on using 375 on the seat . My biggest concern is how much open pressure they had . I don't have a spring tester that go's that high.

Bill... My spring tester goes to 1000 lbs max. I am going to disassemble each spring (mine are doubles) and then measure the force of the inner at max lift and then the outer at max lift. Then add them both together to get the total open force.

  Chuck that's a good idea I think I will try that  . Not making a big change in springs going from the 1249 's to the 1248 PSI springs .

[masracingtd1167]

We measure our closed spring pressure with the retainer, so we get the total spring pressure, just add the retainer thickness to your installed height.  Also your spring supplier should be able to give you open pressure once you give them your valve opening.
Bill, I don't know your motor combo, but we replace our 1 1/2" dia PAC springs when they test on the car at 329 lbs. 330 is min with my 10,000 rpm motor at .938 lift, on the RR 23° SBC , new they install at 400. One of the best changes we have made was when I switched to the 1 1/2" PAC springs, they are more expensive, but I don't have to change them nearly as often as the triple springs. Unless we have some other problem, I only replace 3-4 springs all season.
Bud's non raised runner 23° only ran 260 lb springs. Our Hemi needs 420-440 lbs new, bigger valves, and high 9,000 rpm.
Back in the day when I had a dual engine BBC Top Gas car, and we wanted to be one of the 1'st Top Gas cars to go 200 mph, we did run 202 when we installed weaker valve springs, no other change.
So, if you are not sure what springs to run, check with your cam grinder, and be sure to tell them 9400 max. 

Jon

  Jon I measure mine the same way.