So I finally had the latest build of my engine tuned yesterday and the numbers were waaay lower than they should be. It was tuned by Mayhem Motorsports in Raleigh, NC and made 475/450. I was expecting hp to be in the 525-550 range.

Specs on the engine:

2000 Corvette Z51, M6 T56, 3.42 3 rib diff

402 LS2 (4.005" x 4.0")
- Callies Compstar crank/rods
- Wiseco pistons/rings, -8cc dish
- GMPP CNC LS3 heads with Comp Cams 921 springs (installed height verified), milled .036"
- Cometic 4.085" .045" thickness head gaskets
- 11.5:1 or so compression
- Comp Cams 875 lifters with (I believe- I can't find my notebook) .020" preload
- Comp Cams 7795-16 Hi-Tech pushrods, 7.375" measured for .020" preload
- Comp Cams 235/250 .621/.595 115+4 spec'd by Martin @ Tick
- LS3 intake/LS2 throttle body
- return style fuel system conversion
- Holley billet rails
- Aeromotive 13101 regulator set at 58psi plumbed after the fuel rails
- all Fragola PTFE braided stainless lines
- stock GM fuel supply line
- 60# Deka injectors
- KB BAP set to turn on at 4" of vacuum
- Melling 296 oil pump
- Edelbrock Victor water pump
- under drive pulley
- MSD plug wires
- NGK TR55 plugs
- LS7 MAF in a 4" housing mounted at the throttle body
- Vararam
- Comp Cams billet tensioner
- ARP rod bolts, head studs, and main studs
- AIR delete
- LG Motorsports Street Series 1 3/4 headers 3" collector
- Corsa dual 2.5" Tigershark exhaust
- no cats

Driveability was great from what little I drove it. After I got home from the tuner it sat until this morning when I blew a power steering hose- which will be here Thursday and will be installed on Friday morning.

I honestly do not know where to look. I was told by the tuner that I need larger primary tube headers and that they would net me probably most of my missing hp because LS3 port heads need a larger exhaust than cathedral port heads.

I want to be clear- I am NOT, in any way, bashing ANY of the companies whose parts are in it or have done work to it.

Here is they dyno sheet

 

What do the AFR's look like? How much timing are you running? It looks like it's peaking pretty early honestly.

 

I have no idea what the timing is at, I don't have HP Tuners to check it. Don't know the AFRs offhand, will have to get the power steering hose replaced and fire it up and check my wide band.

 

I think you would benefit from 1 7/8" headers. Also the 2.5" dual exhaust maybe borderline. You could probably use a dual 3" at the power level you are looking for.

Martin is pretty good with the camshafts but I don't know about the large split with the LS3 heads. The 235 part seems fine it is the 250 part I am wondering about. Then again he is a little wider on the LSA than most. Probably to make up for the 250 degrees and bring the valve events back in line. Also maybe he felt you needed a little more exhaust duration with the 1 3/4" headers. Just thinking out loud.

 

2.5" duals are not enough for the power you are wanting, I run 3" duals on mine and the car still gets faster with the cutouts open...and that's on a car that dynoed 390rwhp with the converter unlocked and cutouts closed.

Looks like things are getting fairly wavy and unstable up top

 

I would assume that it falls over early from a combination of relatively small:
i) primary pipe diameter
ii) exhaust diameter
iii) cam timing.


On a friends manual LS2 GTO I fitted CompCam 235 243 .620" .624" 113, 1 7/8" extractors and twin 3" into 2 1/2" system.

 

I would go to rpm trust me.i have a similar set up and mayhem was suppose to tune it charged me 450 for nothing couldn't even drive my vehicle.change nothing went to rpm Wednesday pick up 45rwhp and it drives,idle I'm happy with rpm

 

I had RPM do it with my last combo and was never happy with the tune. Also had a bad experience with the mechanical work they do- they lost several head stud washers and left them off, wires touching the exhaust, ect....

 

The tune I have now from Mayhem is very very well mannered. No surging or anything at low speed/throttle/rpm. Not saying the tune isn't the problem, I just know nothing about it lol.

 

I would drop in some stock length pushrods and see what happens. With .020" preload on a cold engine, you'd be lucky to have .005" once the engine gets hot. The aluminum block and heads expand with heat and you lose a lot of preload.

The torque is low for that displacement and compression, and it looks like it hits a ceiling right around 4500RPM, tries to hold it, and then crashes. In my experience, it looks like the valvetrain isn't happy and the lifter preload if the first thing that sticks out.

 

KCS, the recommended preload for the 875 lifters is 0.00 - .0125 so when it is at operating temp I should be right where I need to be. I think stock length will have the potential to bottom out the lifters. The heads have been milled .036 and I am running a .010 thinner head gasket.

 

Well, I would take the Comp recommendations with a grain of salt and at least check the preload since it seems like you're uncertain from your first post. If you have .00" preload when cold, you're going to have lash once the engine heats up. If it were me, I would want to know how much total travel there is in the lifter and try different preloads to see what effect it would have.

It's just a suggestion though, but its probably the easiest to eliminate and considering the issues people have had with those lifters, probably the most likely. Either way, I agree you're missing a lot of power/torque and I hope you find it.

 

Told ya so. Mayhem is not your problem, your combo is.

 

I never said Mayhem was the problem. I am also not running the cam I was planning on either. The causes I am looking at now is too restrictive exhaust or an issue with the lifters/pushrods.

 

That was the first thing that stuck out to me too was the lifters.

What issues have you seen with the 875 lifters?

 

tomorrow morning I am going to pull a valve cover and verify pushrod length with 0.00" preload so I will know for sure what preload I have. I don't have an issue with swapping out the lifters and pushrods again, just sucks having to get new head gaskets lol. Those Cometics don't even have 300 miles on them yet haha.

Also, when funds allow it after I rebuild my diff, I am going to upgrade to a dual 3" exhaust and see where that puts me.

 

I got to tear down an engine with those lifters and the valve train was really unhappy. A bunch of the retaining clips had broken and some of the pushrod cups from the lifters were in the trays, among other issues.

From the visual observations, it seemed like the lifter didn't have enough preload, maybe even some lash, and was getting beat up pretty bad when the lifter came up on the lobe.

The engine was rebuilt with a whole new valvetrain, including GM LS7 lifters, and no more issues.

 

Good idea Robert.

Sorry about the gaskets!

I will recommend Morel 5315's as a cost effective alternative. What push rods did you have before? A nice 11/32" Manton would be a good upgrade, or a thick wall 5/16".
Good information KCS.