Forcefed86

iTrader: (4)

Shims are on the way, I'll give it a go.

subeone

iTrader: (5)

Subd to see what the issue is. Had a similar issue on the dyno, fuel consumption went up, power stayed the same.

svslow

Did you figure out the issue?

subeone

iTrader: (5)

It was valve float, shimmed the springs and power went up. It was a 5.3 with an old school pt88, put down 600whp on 12 psi thru a th400 and 9 inch, put down 588 at 18 psi, hence i knew there was a problem, power flatlined after 6k rpm, it didnt dive it just looked like a straight horizontal line

Forcefed86

iTrader: (4)

What spring/cam combo? I was talking with Howards Camshaft tech this morning. They told me if it rev's in 1st to 7k with out issue it's not float. I don't think they are very familiar with turbo setups... but I wasn't going to argue with him.

subeone

iTrader: (5)

stage 4 btr cam (ljms), running pac 1206x

https://www.racingsprings.com/Dual-S...1206X/Item/762

that sounds like a bunch of horseshit.

One to take note is that the more aggressive the lobe is (ramp rates) the more susceptible it is to valve float. Lesson learned..

gsteele

At what point in first do you see full boost? I assume you maintain it from there till the end of the run.

Forcefed86

iTrader: (4)

I’ll have to look at the logs again. With the 30” tire I was making 17lbs at the top of 1st at 6800. Then creep to 19ish by 6200. I’m not sure with the 28” tires.

Gabbiani

iTrader: (20)

Curious. I'm chasing a similar issue. Boost goes down with rpm for me but I think I'm maxing out the little 7876. I'll spike @ 4k and tapers off 2-3psi or so by 6500.

tblentrprz

Curious where spring height is with valve on seat and open. Spring height at coil bind? Curious what seat and open pressure is with spring installed within .060' - .100" of bind?

General info for the masses - excellent vid to show dynamics and importance of valve spring and setup.

427

iTrader: (3)

It's not likely lofting that causes your problem if it's control related, your probably bouncing the exhaust valve off the seat. That will not cause traditional misfire, it causes weird overlap added at rpm. It will normally change the engines tone, making it sound like it gets muffled. O2 will sometimes drift lean although it's very likely it's not lean.

Kurt

Forcefed86

iTrader: (4)

In my experience that’s what happens when the turbo is done. Boost drops with RPM and IAT’s rise. I can make 30lbs no problem with reasonable IAT’s. Just doesn’t make power, boost never falls.

Gabbiani

iTrader: (20)

Sorry, I meant same boat as in not making the power it should. Of course very different setups but I'm also on dual spring installed at stock height. Added to the list of things to do.

Forcefed86

iTrader: (4)

If the overlap was doing odd things, would that show up as spikes in the back pressure readings?

My AFR is steady, but I’ve got my closed loop trim on. I’ll go see if it’s adding fuel at any particular RPM.

If these shims would ever get here, I’d have some answers. Going to an event Sunday either way. Then at least I'd know if Plug gap has an affect.

427

iTrader: (3)

Would not change BP as both sides of valve have near same pressure as it just closed. If you need to race it as is put the taller tires on or change rear gear as slowing the rpm will help with both turbo and spring problems, although it will make spark issues worse.

Kurt

Forcefed86

iTrader: (4)

Shim tracking indicates “out for delivery” so I should be able to shim them .030 at least by Sundays event.

I could put the larger tire back on, but I won’t know if my issue resolved itself then? Hoping to run the 275 tire and squeak some more RPM out of this combo. Feel like it’s leaving a lot on the table only pulling 6200 thru the traps with the 30” tire/3.25 gear. Before this issue the plan was to drop a 3.40ish gear in and be around 6800ish at 161-162ish. Shooting for 8.5x with a 1.4ish 60’.

If it does the same thing, I’ll stop for the day. I’ll only be doing TNT. I don’t have a road I feel comfortable on over 150 close by or I’d go test it now with only the plug gap change.

427

iTrader: (3)

150 on the street is bad, but I've done it a few times!
You need to measure it while it's apart so you can fix it right someday. Do you have calipers at your shop?

Kurt

Forcefed86

iTrader: (4)

Yes, I have digital calipers. I was planning to rotate the motor until the valve is fully open and stick some feeler gauges between the coils? Easier method?

Also thinking I could check the spring pressures on a digital scale I made with a bottle jack… or drill press maybe?

-thanks!

427

iTrader: (3)

If you can crush a spring cautiously in a vice and get the stack number, that will allow the math for shimming them right. take that number plus .050 plus lift and that will be set up number. So if stack is 1.050 it would be:
1.050+ stack
.050+ clearance
.612 lift=
1.712 set up.
Then cut off a piece of 1/4 inch tubing slightly longer and put it in drill and square the end on a sanding disc while both are spinning so it's square. Keep working it until the piece of tube is 1.712 long and square on both ends. After you pull the spring put the retainer back on with locks and pull up hard, it will lock on. Now put your tube in straight up and down and use feeler gauges for measuring clearance. Once you shim it up it will fit in tighter letting you know it's ready for assembly. If your using commercial shims you must work with .015 as the smallest shim, just get as close as possible.

Kurt

Forcefed86

iTrader: (4)

So the “stack number” is the same as the coil bind? If this info was provided by PAC would I even need to do this step? (I’ll check anyway)

Pac says the bind is 1.096 +.050 +.612 = 1.758 So .042 worth of shim would get me within .050 of bind assuming 1.8 installed height? So my .030 shim would be about as close as I should get?