antz01ta

iTrader: (3)

Just got the car back from the dyno, thanks to Mikey at Rapid for a great SD tune, the car drives and idles perfect.

2001 Trans-Am
421 iron Block
11.6:1 CR
ET 255 11` Cylinder heads
Jesel J2K rockers
Lunati 4.125” stroke crankshaft
Lunati Pro-Billet 6.125” Rods
Diamond Nitrous pistons
Moly coated skirts
Ceramic coated tops
Tool steel wrist pins
Rapid Motorsports Dominator Cam 250/252 .612 / .615 113LSA
Morel Hydraulic Roller Lifters
Ported Fast 90/90
Kooks 1 7/8 x 2 x 3 ½ headers
Kooks 3” y pipe
Mufflex 4” catback w/ dual outlet Spintech muffler
Rapid Motorsports SD Tune

Power coming through a steel flywheel, steel driveshaft, and 12 bolt 33
spline axles w/ 3.73 gears

No electric water pump, all belts on the car.

If you look at the graph you can see at 6300 it takes a nose dive, according to Mikey this cam should pull up to 6600 we are thinking valve float but with Comp 928 springs i cant see that being the problem.

chino_man279

iTrader: (7)

Does look like valve float...were the springs shimmed?

antz01ta

iTrader: (3)

we have 160lb on the seat and 380lb open, i cant see how it could be float, i am thinking more like lifter preload issue, i have to pull a valve cover to check it but they are installed with .030 preload as per Morel.

chino_man279

iTrader: (7)

That was going to be my next question...preload. Maybe go through and double check all the rockers. Pain in the ***, but relatively easy and quick to do.

What about your fuel system? Injectors? Pump? Pressure? Looks valvetrain related, but you never know.

This graph looks exactly like my friend's LT1 that had springs that should have handled his cam no problem. Turned out he had a bad batch of springs. We chased it for weeks. Swapped springs and it pulled cleanly to 7100 where as previously it dipped at 6200.

Slowhawk

iTrader: (12)

Thats valvetrain

Nate_Taufer

iTrader: (1)

That is definitely valve float. Try shimming the 928's to .050" or so within coil bind and see if that fixes the problem. Are you sure you have 928's and not 921's?

Nate

Patrick G

iTrader: (14)

That spring rate looks odd. My AFR 8019 springs are 158 seat and 440 open. No valve control issues even with my heavy 2.080" intake valve and LSK lobes.

Great power, but it definitely looks like valve control issues.

John02SS

iTrader: (47)

I don't think 380 is enough open pressure. I ran into float issues with my cam and I had about 380 open. I'm using a set of gold springs shimmed .050 from bind now. Car pulls all the way up now with no float issues at all.

antz01ta

iTrader: (3)

for sure they are 928's, i have to check the installed height of them, but it sounds like i need a different set of springs.

LASTLS1

iTrader: (7)

Springs seem light on the open pressure. Also putting a fast 90 on a set of ET heads is like putting dual quads on a 4 cylinder. Example... my 408 peaked at guess??? 6200 with a soild roller BIG cam. Sold the motor to my friend ran the same cam but with a gmpp intake and et heads and to spun over 8000!!!!

Big-DEN

There is something.

TQ is low.

408 thru manual will hit over 500rwtq, in the 510rwtq range.

your 421 IRON block should be greater than 520rwtq, when everyting is working in harmony. Of course the power number will be higer.

How the hell does the ET255 head work with a faast 90 intake? I'd figure the cross section would be too big on the head.

Cary@Perf-Induction

iTrader: (1)

that does look like a spring issue, possibly too much preload. I ran my preload around .010 with a comp r lifter and the PSI 1225 spring and it ran clean to 7500 rpm with a heavier valve. I think with that light of an open pressure, it is lofting the valve pretty far and the lifter will pump during loft and cause the valve to float when returning to the seat, thus causing the MAF to go nutts and the motor dropping off a cliff right where it is. The manifold area will cause a flat line when it is done working, this is not a manifold issue that your having.

mikey

iTrader: (6)

I could actually feel the motor lay down on the dyno. As I said, I suspect springs or pre-load.

Thanks for the kind words Glenn. Let me know if I can help with this.

Mikey

Beast96Z

iTrader: (21)

Even with the float, you making great power. Imagine if you can get it on up to 7k! Intake is also a major bottle neck, but dosen't seem to be the problem in the graph. After the float issue is fixed, start saving for a real intake, then you'll see some power.

antz01ta

iTrader: (3)

well i agree about the spring pressure and preload but it is a Speed Density tune so the maf is out of the picture. i am going to pull a valve cover and check it out.

antz01ta

iTrader: (3)

once i get a valve cover off i will be in touch. Andy told me he set the lifters right where you told us to so i am thinking springs.

redtail2426

iTrader: (10)

what about pushrod deflection??something to think about

LASTLS1

iTrader: (7)

Strange, mine peaked at 6200 and I was running 420 closed and over 800 open?

beardWS6

iTrader: (1)

Did you try it? Let us know the results. Thanks!~

SStrokerAce

iTrader: (2)

Well lets stop guessing on this and just give the lobe numbers on the cam out or tell me the .200" duration and then it could be a easy, ah HA it's not enoug nose pressure. Hell what's the installed height and coil bind, maybe it's a unrecoverable spring surge issue as well. If the spring can't control the valve when the lifter goes over the nose it's going to go into the toilet really fast even if it has crazy seated pressure.

I doubt you have a pushrod problem just because of the parts list, you are running some good pushrods correct?

Bret