NufNuffZ28

iTrader: (10)

I THINK I may be having valve float issues caused by boost in the upper RPM range.

I am looking for some guidelines regarding this.

My general setup is a 8.7.1 TPI SBC with a XR268R solid roller cam. I am currently at 21ishpsi and things start going weir at that point. I am upping to 25-26 to try for that 9 second time slip and we are going through everything before that to make sure we are good to go!


Cam: XR268R

http://www.compcams.com/Company/CC/c...?csid=304&sb=2


.552 lift intake and .564 exhaust

I am running 1.6 RRs.

Installed spring height intake 1.76
Installed spring height exhaust 1.78


I pulled a couple springs to check spring pressure.

They bind at .650, so I should be OK there.

As far as Spring pressures go we have 200 lbs on the seat and 420 lbs at .550 lift.

I THINK the 420 is a little low and causing the upper RPM issues.

After talking to comp and doing some research we are looking at the comp 917's

http://www.jegs.com/i/COMP+Cams/249/917-16/10002/-1

They have 200 on the seat as well, but 596@1.10, so it would surely be more then the 420 lbs I am seeing now.

They also look like they will slide right in with regards to installed height and size as well, so I'd like to swap to them, if the consensus is that I don't have enough spring pressure over the nose of the cam.

Thanks

Chris

Orr89RocZ

iTrader: (20)

Whats the current spring spec? Diameter? Double spring?

What intake valve size?

What spring retainer, steel or titanium?

What rockers and pushrod diameter? Are you positive the geometry is spot on? Stud girdle?
What rpms you turning?

NufNuffZ28

iTrader: (10)

Whats the current spring spec? Diameter? Double spring?

unknown custom specs on springs double spring with damper 1.55" diameter

What intake valve size?

2.02 1.60

What spring retainer, steel or titanium?

not sure on what brand. Steel retainers

What rockers and pushrod diameter? Are you positive the geometry is spot on? Stud girdle?

1.6 comp gold rocker arms.

cv7900 pushrods on intake 5/16th
cv7850 pusrods on exhaust 5/16th

no girdle

Yes, it is on point..... Sure of it!
What rpms you turning?
6000 so far. Want to spin it at least 6500-6600 so that when I shift I stay in big boost!

Orr89RocZ

iTrader: (20)

Seems odd to be having valve float at those rpms with that cam and valve size, if geometry is on with rocker/valve stem alignment, and those spring pressures.

Can you fit a 3/8 pushrod in there?

Sure its not ignition break up? Can you rev to 6500 on lower boost but not when in high boost?

420 lbs is alittle light on pressure for a big valve spring and solid roller. Its not that aggressive of a SR cam tho so thats why I am alittle surprised. Valve is small so its light and a 420 lb open load 200 seat would support hyd rollers to 7000 on that setup. I would like to see a smaller diameter spring if possible on that setup but more pressure would help if its a control issue and not ignition or geometry related. 1.55" spring retainers have some mass.

I went through a geometry related issue. WOuld pull 6500 on low boost but hit a limiter at 6000 on higher boost. Tried plug gap but no success. Went shaft rocker and set it up right, and problem went away. Its hyd roller but its a 2.1" valve with .640" lift and turns 7200 with 450 lbs open, 180 seat. Stability is important.

Those 917 springs would be 511 lbs open at .550". That should be more than enough IMO. Typical boosted apps do add some more pressure to the spring loads but its not like you need 20 psi more pressure because you run 20 psi boost.

86LG4Bird

iTrader: (1)

21 psi would push against the back of the intake valve with about 60 lbs of force. With the intake dynamics, I'm not saying that entire pressure is there when the valve is trying to close, but don't underestimate it. There's a hammer effect going on with the air column when the valve slams closed, so I wouldn't be entirely surprised if your seat pressure is too low for that amount of boost. It depends a lot on your cam's closing ramp profile

Orr89RocZ

iTrader: (20)

From what i read on the subject some were stating the differential pressures across the valve remain similar to na builds. So although there is 21 psi on valve there is also alot of pressure on backside especially if exhaust side had higher backpressure ratio. That was a huge concern of mine originally as i planned on 20 psi with my hyd roller and i was afraid the lifters would never handle the spring pressure. Turns out the pressures i run spec'd by my cam guy werent much different than what i'd run in a na build.

Harmonics however is a whole different issue. You may have enough pressure but not if those springs are in a bad frequency zone and become unstable, undamped. That will certainly cause float. New trend has been smaller diameter springs which have a higher frequency harmonic

NufNuffZ28

iTrader: (10)

I can't run 3/8 without swapping out other things as well.

I can turn more RPM with lower boost, rev, etc.


Thats another reason I think it is valvespring related.


with TPI and boost I am sure that the intake side of the valve is under much more pressure than the exhaust...

I am thinking that the boost, solid roller and relatively light 420 is causing the issue at upper RPMs.

Sounds like the 917s may be worth a try since they are so close to the size of what I'm running with just more pressure on the nose where the issue is at. 90 lbs would be a big difference.

Ideas?

Orr89RocZ

iTrader: (20)

Yeah if all other variables are non issue, then 90 lbs could go a long way to helping things out

NufNuffZ28

iTrader: (10)

At the price and relatively easy swap I may do that.


Just don't wanna keep throwing parts at it without research first...