Ok, well i spend the entire day at the dyno basically. Did 8 pulls. with tuning and modification inbetween each one.

I'll post results and pictures from everything I did today a little later. I'm off to go get the vette registered in Texas before they close. (got it inspected LEGALLY today :D ).

Anyway, my best pull was 312rwhp and 291rwtq. A little disappointed, considering I was expecting 330rwhp and around 340rwtq. The first run i did i only pulled 291rwhp and 280rwtq! :eek: I was shocked. Fixed the A/F and pulled the same (almost exactly). Then the next one I added some timing and it jumped up 6hp 6tq. So i found what it was wanting. I ended up adding 9 degrees of timing before it showed any signs of spark retard... (up to 45degrees at 100kpa). It was incrimental; as i kept adding timing... more hp and tq showed up.

graph pics to come in a bit.. gotta run.

edit: Ok ... here are the dyno graphs:


First run stock LT4 programming. A/F is WHACK. I think the sensor fell out or something....


Better... A/F. I actually adjusted the programming to a 24.5 constant from 26.5 for this. Did that before finding out that the sensor probably fell out last time.


Raised 90-100kPa timing by 3 degrees between 2400 & 7000.


2 more degrees of spark in same areas


Bumped timing up to 45* (3 more from previous).


Brought injector constant back to 25.5 from 24.5 (and 26.5 stock).


Tried the stock LT4 computer.. only change to it is rev limiter and fans.


Added one more degree of timing... started getting some spark retard around 3500-4000 range, so pulled 1 degree back out from there.


[Modified by GlockLT4, 7:12 PM 2/17/2004]

 

What do stock LT4s put down?

 

I pulled 296 stock. So i gained 16hp with the cam. I still there there is more in it somewhere :yesnod: . I was really expecting about 330. I guess i need some exhaust work.

 

hmmm... that's way more timing than it should take. mine detonates on the 3rd dyno pull with stock timing. is it possible you put the cam in off a tooth? then the timing could be way retarded.

if you could temporarily replace the OBD-II ecm, it should tell you if there's a skew in the opti versus crank position. i think.

-michael

 

No, i'm sure that the cam was in position. I put the stock LT4 computer in there for one of the runs and only pulled 290/278. A guy in our club pulled 338 with hotcam and no cats... and stock OBDII computer.

There's something weird going on maybe :skep:

 

Glock - what type of dyno? Was it the same that you used before? Were atmospheric conditions similar? My LT4 when stock except for no mufflers pulled 306/309 on a DynoJet. My current 383 hasn't been tested yet, but the 350 TPI in the '89 pulled 285/371. So, I'm guessing your dyno is a tad on the conservative side. Paul :flag

 

Nice numbers Glock. Yeah there has to be more in an LT4 :smash: :smash: . Mine's a torque monster though (for an LT1). My torque is way higher than my HP. I'm thinking maybe some more tuning is still in it. But alas funds are low right now... so I'll have to hold off on the tuning equipment... for now. Good luck! :cheers:

 

Thats not bad! Some headwork would really improve things. :cheers:

 

You gained 16 hp but lost 15 lb-ft... I guess there must be some hp (and tq) lurking in there waiting to be let out, now you have to think about where it is hiding... :thumbs:

 

:yesnod: I know there is more in there. The only thing I can think that caused loss of torque would be the shorty headers.... but they are all sealed up now (no leaks).


[Modified by GlockLT4, 7:20 PM 2/17/2004]

 

It was on a DynoJet, but a different one than last time. I'll have to compare someone's numbers at the two (if they have the same setup for both runs) and see if there is a difference.

 

WOW!!! I never knew pulling the mufflers could yield 20hp at the flywheel over stock... :eek: I thought the cats could be a problem.. but I didnt think the mufflers where that bad... time to start looking at a new set of muff's..

 

i feel like a doomsayer, but something's definitely wrong. i just don't see these engines being able to take 9 degrees more than stock. can you try to verify the timing with a timing light? if you put your hub back on with the TDC mark in the right place, you should be able to set up a temporary pointer and use one of those timing lights where you turn the dial until you see 0 on the engine.

also, mine went from 296 to 325 with the HOT, no other changes, stock programming on timing and a/f.

how long of a pushrod did you use? how many turns of lifter plunge preload?

i forget the final outcome of the header saga, what did you end up using? because the regular SBC and L98 headers and gaskets do block the tops of the LT4 exhaust ports.

-michael

 

Yeah, i'm positive the cam is setup right. I hit a maximum of 50 total degrees of spark. The main/extended tables i have maxing out at 45 degrees. Did a quarter turn past lash on rockers. Used 12367777 gaskets. Headers are all sealed up.


[Modified by GlockLT4, 8:43 PM 2/18/2004]

 

That much spark lead says something is wrong. Look at the timing curves the LS motors use. I am using 31d total and putting out 430lbft on a Xfire. The more efficient the combustion chamber the less advance needed.

 

What else would cause that much more timing to be needed then? :confused:

 

GLock,

Is the cam timed properly? Did you degree it when it was put in? What about the pushrod length? Was it checked to assure that you are getting full lift? What is the static compression? Should be about 180psi? If its lower, too much duration, if its higher, not enough overlap.


[Modified by Dominic Sorresso, 8:36 PM 2/18/2004]

 

Haven't checked compression. pushrod is comp cams chromemoly 7.25" (stock length i think). Didn't degree it when i put it in. Was told i didn't need to worry about doing that. Haven't checked for full lift Guess i can get a caliper and measure that.

 

Compression test will tell you if you have too much or too little overlap/duration.
I use TFS heads and have 7.4" pushrods using a compCams HYDRLR. Take a black marker and paint the valve stem. Put rocker back on and see if the wear pattern is from upper third to lower thiird of stem. If it isn't below the halfway point, rod is too short. If it extends to the lower edge f the stem, its too long.
I would check cam timing to make sure it was put in correctly. Someone else put this in for you?

 

I do all my own work... i'm positive the cam/crank marks were lined up. I will double check the rod length like you said though.