96GS#007

Put the car on the dyno today. Power was obviously of interest as was taking a better look at my mysterious miss at high rpm. Previous best was 342, although most of the runs were 338-340. Since that time I had some mild work done to the heads, basically port matching, a little chamber cleanup, and some short side radius work. Since I have no plans to go to a cam larger than what I have (HOT Cam), there was no value in spending thousands on "Stage 3" work offered by the various venders. To wit, my total machine shop cost for the head work was $545. The results...

Max hp: 363
Max tq: 342
Avg hp: 269
Avg tq: 321

The runs were from 2000rpm - 6600rpm The curves are so smooth on all the runs it brings a tear to my eye :D...up until the point I see the dip due to the miss :( This was on the same dyno I've used the previous times. Initial run was just over 350, so I picked up 12hp or so via my tuning with LT1 Edit.

The miss is plainly visible on my charts. Fuel pressure was rock solid at 50psi during the runs so that pretty well vindicates the fuel pump. Looks more and more like I have a couple injectors fluttering at high rpm causing a temporary lean condition. I've been told the Multecs do have a resonance issue at combined high rpms / high fuel pressures. Duty cycles are now in the 90s, so they're maxed. Time for some FMS/Bosch 30# injectors. Hopefully they fix the miss, but also for peace of mind.

All in all, not bad. My goal was 360 and I accomplished that. :)

MaddMaxx

:thumbs:

superlund

Cool, for comparison I was at 363 w/ 358 torque using nearly the same motor combo. Had a little more compression & gapless rings which more than likely account for the torque difference. I was using the 28 LB/HR 2002 Z06 fuel injectors at 50 PSI; plan to swap them out for the FMS 30 LB/HR red tops.

Wish you luck in leaving your combo "as-is" I couldn't leave well enough alone @ 363, won't be happy until I hit 400 RWHP with endurance motor reliability...

I'll be watching to see if the FMS 30's help with the high RPM miss.

:cheers:
-Jim

STL94LT1

Great #'s Jim.

RichS

Pretty much what I had with mostly the same mods. Makes the car feel much stronger than stock! :cheers:

Mez

Jim,
Good job. You have proved the point regarding getting the most performance without spending more than necessary. Probably a lot of guys have spent more for less performance.

You realized that paying big $$$ for CNC heads are not cost justified with the Hot Cam. Guys, don't get me wrong here. CNC heads have their purpose but there is little or no benefit with the already good flowing LT4 heads with a Hot Cam. When I had my heads ported, since I was sticking with the OEM LT4 cam, the shops said don't spend the money.

GS057

Nice Gains. :cheers:

SouthernSon

Very impressive! I could use your input on tweaking Ed Wright's program using the MAF. I am going to the dyno tomorrow hopefully and need to flash a few more tables at my LT4. Any suggestions with MAF tables?

96GS#007

Mez,

You deserve a lot of the credit. I read some info on your webpage that got me thinking about what was really required. From there it was basically lots of research to determine if what I really needed for the HOT Cam. I should give credit to all the major shops. When armed with information and I asked the question of what I really needed, they were honest about not needing full up CNC porting

:cheers:

96GS#007

Hmmm...You must have an old Ed Wright flash. He scrambles his newer ones such that if you download them with LT1 Edit they don't make much sense.

Just briefly....With the MAF stuff, you're really attempting to tune idle and part throttle vs WOT. You want to have part throttle tuned such that when you do go WOT, the fuel trims go to 0%. If your part throttle fuel trim values are lean (adding fuel), you end up with inconsistent fueling at WOT. This is because the PCM will use the larger of the fuel trim values....0 or however much fuel is being added via the fuel trims. WOT is tuned by adjusting the PEvsRPM tables as well as the two timing tables.

For all the work and experimentation I've done for good idle quality and part throttle driveability, my WOT stuff is very close to the stock flash. Basically I played with the AFR until I wasn't seeing any more power, richened it just a little (I'm at 12.9:1) for safety, and then added timing until either no more power or I saw knock retard.

To get my fuel trims consistently at 0% or slightly below (subtracting fuel), I added about 10% to the MAF curve in the upper frequencies. In the lower regions it's about 5% greater.

In earlier cars, you tune using the VE tables. For whatever reason, I've never seen any affect from changing the VE tables except when I unplug the MAF sensor. It appears for '96, the VE tables are for backup only.

Good luck :cheers:

SouthernSon

Yes, supposedly these files are from an earlier time before scrambling. I appreciate the information greatly! It gives me good direction to fiddle with. I do have a question about the fans on/off though; Do the fans turn off at speed even though I have lowered the temperatures to around 187? Thanks again. :cheers:

96GS#007

Yes, as I recall the fans will turn off above 35mph. Again going from memory, above 35mph the fans are providing no additional airflow.

Shriker

Nice numbers , just about right.

What kind of "miss" are you having? I am having just such a problem myself with my combo , which is very similar to yours exept I am using 1.7 rr's and longtubes (and have dynoed ~13+ more rwhp than you ). My dyno looks great right up to 6100 , then takes a farely quick dive about 10-15 hp across maybe 200 rpm and then almost totally recovers before the redline at 6700. It is exactly the same every time......ALL ignition components are new (coil/wire, opti, wires, plugs) , fuel pressure and knock is fine , a/f appears to be fine. :confused:

96GS#007

YES!!!!!!! That's exactly what mine is doing!!!!! Until someone sees it or experiences it, it's hard for people to completely understand what's happening. You've described it to perfection.

Later today I'll post a pic of one of my dyno runs and the miss is plainly visible.

Like you, I've replaced everything on the ignition side.

What fuel pressure/injectors are you running? I've been told the stock Multecs have a resonance at high fuel pressures/duty cycles that causes them to flutter. The flutter causes a temporary lean misfire.

LT4POWR

:thumbs:

I also look forward to replacing my stock injectors

Mez

Jim,
My comments below are targeted at the LT4 heads mostly although some of what I say does apply to the LT1 heads too.

The shop that did my heads, Trevor Johnson Cylinder Heads in Rockwall, Texas was recommended to me by a local Dyno shop. I visited his shop and he seemed to know his stuff pretty well. He only does heads and does everything himself and has a good reputation in the Dallas area.

He pointed out that with the stock LT4 heads had bigger ports than the LT1/L98 heads. That unless you go with a much bigger cam in both in lift and duration, there is little to be gained from a full port job. When I walked in his shop, I pretty much figured out that with my stock cam, most of the flow gain (i.e. horsepower) needed to be generated at the lower valve lifts. From just off the seat to .400 inches of lift. He agreed. Remember, the valves are only at their peak lift for a very short time yet everybody seems to only look at the flow #s at peak valve lifts. Frankly, this is very misleading especially in light of all the dynamics involved with valves opening, closing, etc..

Also, it is important to understand that port velocity is critical to generating the maximum torque. Since valves open and close, the air in the ports is not smooth but starts and stops. The air has pulses. Smaller ports generate higher port velocity than big ports but run out of total flow capacity in the higher RPM range. Bigger engines need bigger ports (logical). Port velocity helps fill (or ram) more into the cylinders. I recall LT4 heads have about 195cc ports which is not the biggest on the market but much bigger than the stock LT1 or L98 heads. If you enlarge the ports too much, that can reduce the port velocity and could actually hurt torque/horsepower on smaller cam stock displacement engines. But since the flow benches measure constant airflow, you won't know it until you actual try the engine on a dyno.

The actual port shape has a huge impact to flow. It is widely understood that the port flow or short side radius needs to have a certain radius or the flow will separate, tumble, and actually cause turbulance before the valve seat and reduce flow. A skilled person on the grinder is what you want here.

The 3 angle valve job removes the rough edges in the port throat just below the valve seats. All stock heads need help here since the factory can not afford the time to smooth this area. This is not expensive and just about all machine shops offer this service.

The valves should have a 30 degree back cut to allow more air flow as the valves come off their seats. This also is not expensive to do. The area just past the valve seat in the combustion chamber can also be radius to allow better air flow at the lowr valve lifts. Almost a venturi effect.

If you look at heads as they come out of the CNC machine, they have these large grooves. Some shops leave them but others remove them by hand. According to Trevor Johnson, he would rather do a set of heads by hand than CNC because of this. He said all the best race engines still do their heads by hand not CNC. CNC machine just provide repeatability.

Lastly, with regards to valves and springs. The LT4 heads have good enough valves and springs from the factory. Remember, the LT4 intake valve is much lighter than the LT1. So unless the rev limit over 6400 rpm or go more than about .530" in valve lift, the stock LT4 springs and valves are fine and you would be wasting your money to change them. This is a major expense with the LT4 CNC heads out on the market.

I am dead set against porting your heads. First, you have to have the right tools. Second, unless you put them on a flow bench, you have no idea if you made them better or worse. Third, you will need to do a valve job anyway and a shop charges $100-150 for a three angle valve job. Forth, it does not cost that much. In Jim's and my case, around $600 at a local shop or about 1/4th of CNC heads.
Just my opinion.

sothpaw2

GS,
What size injectors are those Multecs?

Superlund, What was you injector cycle with the 28 lb Z06 injectors?

--My combo is similar to what you both have, just w/o the Meziere water pump. I still have my stock injectors and fuel pressure. I doubt it's adequate, that's why I haven't road raced yet. But drags should be ok.

96GS#007

The LT4 injectors are rated at 28# at 43.5psi. I'm running 50psi and adjusted the injector constant in my programming accordingly. I'm still seeing duty cycles over 90%.

96GS#007

Here's a pic of one of my dyno runs....



You can clearly see the miss and the recovery.

If the image isn't large enough to see, here's the link:
http://www.digitalcorvettes.com/phot...no_Run-med.jpg

sothpaw2

Duty cycle over 90%--was that for holding the throttle open at 6000 rpm in a particular gear (1st, 2nd, etc), or was it WOT accelerting in 1st gear from 2500 rpm to 6000 rpm??

My mechanic gave me numbers like the latter--75% for 3/4 throttle acceleration in 3rd gear from 2500 to 5000 rpm.