Do you have access to a lab scope? I would try watching injector pulse width when it is happening does it drop off in that range for some reason. Granted adding more fuel with a prom change should in theroy change it but the computer itself may not be actually doing it because of some input issue. So after 5000 does it come back to life again? Just a thought

what about a govener ? you got one of them hooked on there?

<b>Yea, I actually started typing exactly that in one of my early responses and deleted it, I can't remember why I decided that wasn't the case. Was it someone on MAFB that had that problem, I can't remember where it was…. FWIW, it's happened before where the springs hit a resonant point and they just loose control of the valves for an rpm band, work fine above and below. </b>

That was my original thought, they put dampers into springs for that reason... do his have dampers?

I also think it could be the ECM too. my old ECM would do weird stuff, never that, but it would randomly throw 2-4* of extra timing into the mix without it ever showing up on the data outputs.

my new ECM sometimes opens the IAC when you first start the car but only shows 18-20 counts, then after a minute it suddenly pops up with 200 WiDE open and brings it back down.

Electronics are weird sometimes. what color are the plugs after this occurs? see any signs of detonation?

i PMed him the spring idea the next day after he started this thread..... athough he never returned my PM.

Valve springs would not make any sense. If you get into a float condition where the spring cannot keep up it stays that way till you bring rpm back down to let it catch up again. Looking at his dyno chart the motor picks up again after 5000 rpm?

if it was floating due to hormonics and not it being underpowered, thats entirely possible for the RPM band that the hormonics match in.

sounds wierd, but ive seen it happen... swap springs/dampeners/retainers and it went away.

It's not weird… everything has a harmonic/sympathetic frequency, like when you find the harmonic frequency of glass it shatters. It's also the reason that some driveshafts will "vibrate" even if they are perfectly balanced. There is a frequency that springs just will not work (leaving out scientific explanations). That's the reason for springs with different diameter/crossection dampers and secondary springs.

everyone who watched the dyno vid, you all heard the sound right before the bog happenes that sounds almost like a sword being sharpened with a diamond block?


listen for it... very odd...

i think there really might be something to this valve float issue with the spings- you can almost seemingly hear it in the video.... i suggest going with a beehive style spring if you upgrade, besides, i think those springs are a little light are they not eescpially with some boost trying to hold those valves open a little?

hey guys

FOr whatever reason the message board didn't alert me to any new post.. Thats wierd really usually its on top of that.

I don't understand how the harmonics would be any different NA than under a power adder. I've used these springs in great success on this car when it had a hotcam and 1.6's This new cam shoudl only spin up to 5400rpm and with 1.5's i'd figure it would be plenty of spring

The cam i'm using is basicaly a stock cam

Anthor thing.. I don't know what LT4 spring you use but these are single.. no inner dampner on these. its the same spring i've seen in quite a few kits and I bought them from GM directly.

I'll try putting more fuel in it.

one thing i did was put 8 more degrees of timing in the car and took away the retard (well put it down to 4 degrees) and the car loves it. It had a simular bog but as you can see in the following pictures its no where near as bad.

THis is a testiment to TTS Datamaster's dyno too.. One is on the same day as the dyno (look how close it is to original) and the other is with more timing and retard locked out.

I'll try more fuel for kicks and giggles but I can see black smoke now.. who knows though

OH.. that sound you heard is the Intake.. it sucks like crazy when the car is actually making power

By not supporting 330rwhp I know that you can get more out of them.. but my point was the car made 330 at 3800rpm which should mean it should make mid to high 400's and that is definately too much for a stock injector espeically on boost.

I still think it is the FMU. When I installed my Vortech Supercharger, the car would pull great till 4,000 and act like you just shut the key off. I took a fuel pressure gauge and taped it to the windshield. Sure enough, when the car hit around 4,000 rpms, I had 90+ pounds of fuel pressure at the fuel rail causing the injectors to not pulse. I sent it back to Vortech, they rebuilt it, and it has been flawless since than. Besides, the MAF code it throws!
give it a shot,
Mark

that was the day of the dyno

this is with a bit more timing and the spark retard locked out.

you're not going to be able to program this out.

my personal opinion is its the valve springs.


if you ever get around to swapping them, i'll be waiting for my $40

correction:


you COULD tune it to make **** for power there.... there by "improving" it from its current state.



but that wouldnt be fixing it... that'd be a band aid

i tried swapping valve springs on a LT1 with the heads on the car

never again.. i'll live with it if it comes down to that hehe

Serously.. took me probally 6 hours just to do springs just cause you can't get a decent spring compressor on the heads with them being under the cowling like that. as opposed to 20 minutes with the heads off the car.

i donno what GM was thinking with that over the motor....


anyhoo. 6 hours of your time would make all the diff driving it.... even if it took all weekend, looking at that dyno sheet, i wouldnt hesitate to do it.

Basically? What does that mean? The problem is the rate at which you're compressing the spring, not your seat pressure or "how big" the cam is.

Sorry, I wrote that after being up for something like 40 hours. The first part was right (that they are lighter then the stock LT1 springs, the LT4's supposedly had a lighter valvetrain… again, I'm not sure I believe it). The second part I was confusing them with the cheap, GMPP springs that IRS and a few others resell with the wrong retainers for like 3x what you can get them from at your dealer's parts counter, ignore that part.

Use the moroso valve spring compressor (or my choice, the half price summit copy). It will work under the cowl and if you're good you can do them in an hour or so on the car, hour and a half to 2 with some normal futzing around and if you count actually moving stuff out of the way to get to the valve covers… The actual job doesn't take any longer then off the head you just have to be more careful and less comfortable.

It's not a problem with the engine but the chassis. In most respects (definitly this one) the engine is the same or easier to work on as a traditional SBC

Hey - I posted first about the springs! I'll take $30, and let y'all split the rest He he he. Boost basically lowers the effective spring force, because it's pushing on the backside of the intake (and exhaust backpressure for turbos on the exhaust) valves.

Figure 2.08 dia intake valve - 3.4 sq. inches (minus the valve stem). At 10 psi, you're lowering the closing force of the springs on the valve by 34 lbs! Your marginal (wag!) 130 lb springs are now basically 100 lb springs, fit for a 1978 smog 350 that's good for 160 hp

Andris

lol thats exaclty what i said in my post except you have numbers, but u can keep the 40 bucks lol

OK, I went and looked it up. The stock LT1 springs are 86# on the seat and 376#/in, the LT4 Hot springs are 100 on the seat and 330#/in, so I guess the LT4's are a little stronger on the seat but weaker once opened. FWIW, in most cases LTx engines are happiest with springs in the 130# on the seat and under 350# open, personally, I've never had a problem with the crane 10308 springs, which drop right in.

i just put a fuel pressure gauge on the car and watch the fuel pressure.. It comes up to 60psi just like I expected it to. (it has a 4:1 FMU ) strangely enough it didn't surge at all all night.

The only thing i've done is unplug and remove the dial to my 6AL box and take the vaccum line off it. I dont' know if its a fluke or what.

The next time I put springs on the car i'll put some heafty springs. but it realy does take a long time.. even with the screw type compressors because the screw handle will hit the cowl.. theres just not much room there.

the Moroso/Summit compressor that I mentioned screws onto the stud and has a compoud lever that can be turned to face any angle (independent of the foot of the compressor), it's really pretty slick.

fair enough.. but using angles to clear that cowl.. the cross product would nett you with no advantage.. you would have to be a muscle man

Sorry a bit skeptical

I've used them on a number of LT1's, including my own '97 TA without any issues. It comes with a removable handle and even one of the smaller local guys was able to use it without the handle, it's cake with the handle on it unless you're using some killer springs.

It works similarly to how bolt cutters work, with multiple joints you end up with a mechanical advantage that allows you to exert a lot of force on the spring.

this spring compressor sounds pretty trick....

is it this one??
http://store.summitracing.com/partde...t=SUM%2D906784


i have to get one anyway.... currently, i can only do them with the head off the motor.

Yep, that's the one I have. Though reading the description I have no idea how the finish on it can be described as black. All the pieces are heavily chromed with the exception of the black rubber handle. It's IDENTICAL (well, the head is shaped slightly differently, but it's more cosmetic then anything else) to the Moroso one that costs 2x as much, and I'm pretty sure the Jesel and other attachments for the moroso will work with it.

OK..

I think i might have found the answer by accident.

I've always felt like the car had 2 differetn problems. With the crane or Stock ignition it felt like the car just couldn't fire under boost.

With the MSD box it felt like it could fire but just something else acted like a brake.

Well yesterday I was cleaning up under the hood and I decided to remove the MSD Dial (boost per PSI) because i'm retarding timing threw the PCM. That dial connects to the MSD box VIA a little weather proof connector. Its been on zero for months now. I also removed the vaccum line (which is shared by the boost gauge and FMU)

I took the car out and ran it twice last night and once today and got no surge what so ever. And the surge wasn't a come and go thing before.. its never not surged or misfired. So it seems like its not a fluke but it might be, it might have fixed it.

I'm thinking that the MSD dial has shorted and that its seeing maybe infinite resistance and retarding the full 3 or 3.5 degrees per PSI. That would bring timing down to almost after top dead center. That would certainly feel like a engine brake.

I'll have no way of checking htis until i get a multimeter. (I'm away from home right now)

OR- the on board map sensor is bad or is leaky and bleeding boost from the vaccum lines on that side and screwing with the FMU.

I think the first case is more reasonable.

Anyhow..

I hate to get excited just yet but serously the past 3 pulls on two different nights have been really smooth and the datalogs are showing wheel spin up into 45 mph which isnt' too bad when your starting in second at 30mph.

Well, I can't tell you that I'm that surprised if it's got something to do with the BTM, since from what I've heard typically they all stop working correctly fairly quickly.

not nessisarly the BTM but the lenght of wire to the knob.

It could partly be myfault for not running it into the cab better.

But yeah i just took it out again and no surge at all.