Obviously with a turbo setup, playing with the boost is easy.

Given my blower, and relatively lowish boost, which isnt as easily toyed with, I think 9.35:1 CR should still be safe for me. Combined with the methanol injection, as long as it works, I think it should work well.

Boost really doesnt start to come in hard until about 4-4500rpm anyway, hence opting for higher CR than some.

With 8.7:1, I could run pump gas all day long with no problems with the same boost. So it was defo a safe number.

If the new build doesnt perform as I want, I'll look into additives then. But for now, I'll stick with what I know...and is cheap ( well as cheap as pump fuel can be, which is far from cheap !!! )

Although for all I get a chance to get over to England to race, I am starting to think throwing some race fuel in might be an option to consider, only when Im racing.
No sense wasting it on the road.

 

no problem mate.Just giving you a heads up on it...

 

bump on this thread.....

New 383 rebuilt, heads freshened up, all valves sealing fine. New Morel lifters, preload slightly high at 60-70thou, 1.7 YT rockers because the bloody Jesel's I bought will never fit ( ARRRGGGGGGGHHHHHH!! )
To get the correct preload, Id seem to need circa 7.300" which seems damn short ?? 224/230 0.581/0.591 on 115LSA, timed to full lift at 111 ATDC as per cam card.

And the damn boost still drops after 6000rpm.

Car performs fairly well up til this, but as soon as it passes 6000rpm, boost falls from say 17-18psi to 11-12psi and doesnt recover. The journey to 7000rpm is a difficult one.
AFR's stay consistent mid 11's

No belt dust visible at all.....so I dont think belt slip is an issue.

help....im pissed off.

 

just throwing this out there steve but did u ever think of trying to put stock ls1 rockers on the car just to see what would happen?im gonna be doing this myself as im getting alot of valvetrain noise atm

 

Have you checked valve springs????

While it may seem NOT to be he case, since problem happened when switching displacements, it sounds valvetrain related....

Good luck with it

 

Unfortunatley, I dont have any stock rockers to try
My valvetrain is quiet though.

Valve springs are 100% intact, and visually, all in good order. I did have the heads apart to grind in the valves during the rebuild. Although I havent checked spring pressure.

When I built the temp 346, I did sort the cam timing. The chain had stretched a fair bit during use in the 382 for over a year, but I just restored it to what it should be. I set this the same with the 383.
It had stretched some 3 degrees

SO it should be fine now too....

 

I you weren't so far away, I'd send you a set...

Seems like you checked mostly everything and I can't see how the rockers would have changed anything except for some uncertainty about push rod lengths????

I also can't see springs going bad overnight...But it sure sounds valve train related....

Did you have to advance the cam to get it back up to snuff?

Not questioning your ability, but too much advance can = loss of top end...but I don't see how boost would drop off so badly

Just thinking out loud

edit: here's a possible stupid idea...Can you run the car without the sc belt?

Might shed some light as to whether it is a sc or basic engine related problem, especially if the car still falls off badly at the higher r's

If the valves are floating, your boost would be going straight out the exhaust

 

did you do a boost leak test up to 15psi after you put everything back together? are you sure your BOV is staying shut and not bleeding any pressure off while in boost?

what kind of boost drop off are you experiencing? is it a sudden drop from 17-18psi to 11-12psi or is it kind of like a slow, bleed off, type of drop?

 

I'm following this one.............

 

If you grind the valves you lose some valvespring pressure, unless you put them on a scale and check them and install them at the proper height you will never know. They may look new but that doesnt mean anything. If you grind too much that may the reason you need such short pushrods.

Also use the .050 events when degreeing a cam to double check using the max lift method. Cam advance/retard shouldnt be your issue though. If you have the old style yellow terra rockers I would be a little suspect, I have personally swapped those out for stockers and eliminated valve float above 6800. Find a set of stockers and swap them out, 4.8, 5.3 and 6.0 trucks plus LSx cars all use the same ones. You can usually pickup a complete set on eBay for under 100.00.

Valve float with a centri looks/feels just like belt slip, it just gets to a certain point and stops pulling hard and the boost just levels off or starts dropping at high rpm. Its really hard to tell the difference.

 

I swapped out some old YT's and went back to stock rockers, my car pulls clean to 7000.

 

Getting access to stock parts is difficult in the UK.....but Id like to try some rockers in there !!..

yes. Boost pattern is exactly like belt slip, although possibly worse than any slip I have experienced in the past.
I assume I am right in thinking, that if the problem was ignition related, boost pattern should be as normal ?

Top graph AFR's ( still need work )
Bottom Graph, red = boost in kpa, and rpm in blue.
Boost starts to drop at 5900rpm, I close throttle at 6900rpm.

 

thats odd, really, you have verified the boost dropping off actually looking at gauge as well as the MAP sensor right? Thats pretty low rpm to be doing that.

You are 100% sure your bypass (or BOV) is getting manifold pressure to hold it closed?

Is it possible to change the pulley and slow the blower down to see if it does the same thing at lower boost levels?

Unless you have nasty missfire the boost should still be climbing with RPM.

 

Kevin....when I say grind the valves...I mean I ground them in by hand with valve grinding paste. I didnt get the seats re-cut or anything. ALthough I did have to replace 2 exhaust valves, as they were pitted a little ( possibly plug electrode damage from early July )
Installed heights would not have changed any noticable amount.


BOV's are fine. I tightened them up quite a bit, and still the same, except on closing the throttle, they dont vent enough.

Oddly...and maybe a little embarassingly...

I swapped my crank pulley for the one I had been using ( fractionally smaller, but still supposed to be 8.5" ) and a new belt.
Old belt whilst shiny on the back side, looks to be in good condition, no ribs damaged, marked or missing etc.

After a couple of tightens, boost seems to be much much better. But it still feels like a struggle to hit 7000rpm. Even trying, Ive only hit it once.

Ive checked lifter preload with the YT's and it takes about 520-540 degrees of bolt rotation, from first contact, to fully tight with the rocker bolts. This is with 7.325" rods.

I tried fitting the 1.75 Jesel SS rockers. Bolt on, without shimming the shaft mount, I would require 7.15" rods to give me close to 50 thou preload.

I tried lifting the mount up by circa 110 thou, and this would have taken PR requirement to a high 7.2x rod. I dont have either. But at this height, it looks like a PR could easily fall out.

I can only think the rocker platform on these heads are quite low ? compared to stock ? It would defo be interesting to see what length PR's stock lifters would require.

I'll get some short pushrods, and see if someone can get me a set of stock rockers as well. Then I can try both options
Im hoping to race this weekend, so I'll just run as it is for now. I dont think ET's will suffer greatly, but I defo wont hit 150+ if I cant easily pull 7000+rpm.

Thanks for any help....especially when some of this is down to me being stupid and not trying a new belt !!!

 

Are you running different heads now?

 

Same Edelbrock heads Ive been using since May 06

Same Rockers Ive been using since Ive had an LSx motor ( purchased used of another member )

Same camshaft since May 06.

I was on stock lifters until this 383 build thats been running since Saturday. Now on Morels.

I honestly havent a clue what preload I had before on my lifters with the 382, but I was on 7.4" rods with them. There defo was some preload though, and they had been working fine all last year. I could pull 7k, but not as easily as Id like, but much easier than it does now, and it felt smoother.

 

I honestly dont think its valve float after looking at the graph.

You may get some up top end float but I really think you are having belt issue or have something leaking causing the boost to level out.

FWIW here is my trap rpm @ 150mph with stock rockers, 8019 AFR springs, 7.350 pushrods, .050-.070 preload and the original 98 lifters that came in this car. It will pull hard until it hits the 7400 limiter. Generally you will see bad valve float more pronounced in lower gears when the acceleration is quicker then just loafing at high rpm in high gear.

 

When I ran 149.36mph last year, that was just shy of 7000rpm. So 7k+ would see me at 150+
It did a few 148+ runs that weekend.

But it is a hard pull. It was still accelerating, but not very hard ( relatively speaking of course )

For mine to hit 7400rpm, it feels like I would need to hold it forever, and it most definately would not be accelerating up there. Before it was ok, not great, just ok...but now it feels terrible.
It could do it in 3rd gear, but again....it doesnt want to.

So could my cam perhaps not be suitable for 7k+ ??

the only time I would ever use this sort of rpm, is in 4th over the line. I usually shift around 67-6800.
It just doesnt thrive on revs.

 

This cam pulls up top quite a bit harder then my old 222/228 115LSA XE. It was pretty well done after 7000 and that was with no advance at all. But naturally if your boost is falling off that would explain why it flattens out up there as well.

 

What spring pressure are you running?