Need ideas where to proceed from here.

Newly built, LS2 402,
L92 heads, Lunati single beehive springs, Manley Int /Rev Ex SS valves, Crane rollers, Harland_Sharp Adj Rockers 1.7.

L76 Intake, 42# Bosch injectors, 60 PSI fp, 90mm TB. Mild 220/.581 cam 10.5:1 ratio

Great Torque and HP, all the way til about 6000rpm, then BOOM just falls off the cliff, almost a vertical drop in both HP/TQ. Great AFR in the 12.3 range, throughout the entire rpm, even at the end. 423 RWHP at the top

Happens each time at exactly 6000-6100 rpm- like hitting the rev limit, although limit set at 6800. No funny noises heard.

What could be happening?

 

i would bet its valve float, with those heavy valves you really need to be running a patriot extreme spring shimmed to the right install height to get the most out it , in terms of RPM

 

I would agree. I had to go with Patriot Gold's on my set up to eliminate any concerns with valve float.

 

what kind of exhaust? just questioning as I had problems with flowmasters choking my GTO off.

It ain't right when your car is cammed decently and the HP still peaks @ 6k just like stock, went to flowmaster and the curves continued higher and stronger for another 500rpms.

 

What kind of car? 05 vettes have a acceleration based rev limiter that kicks in when the car revs too fast. Most tuners know about it and change the 500 in the Acc Thresh rev limit table to 0 which takes care of it. I can post up a screen shot of the table if you need it.

 

Well for one thing a 402 has no business above 6100rpm with a 220*cam!
That is tiny for that engine size, i agree valve size/weight should require more spring tension to keep things happy than standard Ls1 heads.
Im building 402 with hydraulic roller in the 250-260* range for street/strip use.

 

cam is way too small

 

if your power is dropping like a rock,i would suspect valve float..
how's the TQ with that small cam?

 

I'm going to heavier valve springs first.

Cam is too small, but with 420 peak torque in its present state, I want to learn to drive the car, before adding any more HP/TQ.

Thanks for all the input.

 

What? Choking and making it better?

 

If I remember, someone had posted some testing they had done on this intake and it stalls out about 6100. Just something to think about. We had the same problem with it falling over at about 6100-6200 on a FI set up as well.

 

is there an after market intake for the l92 heads that doesn't fall down at 6100rpm?

 

Yeah, welcome to the limitations of the L76 intake. When is FAST going to design a better one?

Check out my post on this:
https://ls1tech.com/forums/showpost....9&postcount=29

 

Carb intake> L76. Thats why it fell on its face.

Manifold GMp/n 25534416, $369.95 (when I bought mine) from scoggin-dickey
Gasket Fel-pro P/n 1222-3 (.060"), or 1222-2 (.040), $21.45, scoggin-dickey
edelbrock neck, carbplate-90mm TB front, pn 3850, $152, jegs

also need a vacuum fitting for the brake booster (pretty much anywhere, 90* NPT-barb) , and spacers for the fuel rails (thats on you) and after market rails.

But not having breathing problems is priceless.

 

Could you post a dyno of your carb version so we can compare the curve?

Here's the injected L76 (Bad tune) pull of mine.

 

you have a bad tune and still pulled 480rwhp?holy crap!let us know the numbers when you get a good one in there!

 

GMPP has a new victor type alum. sorry i dont have the Part no but its in the new GMPP cat. Its lower than victor jr so no cowl or hood probs.

Im currently doin this setup:

60lb, elbow, 300 shot direct port, 100 shot 2nd on a plate.

JT- get that cam into the .615s & ^ w/ springs. The l92 really needs it flowing out of the box @ 330cfms. The AF should be tweaked a little as well.

Stupid things we sometimes miss= TQ Mgt.

 

The car (Holden) L76 intake and the LS3 (6.2L) 'Vette intake are the same, I am pretty sure the 'Vette can turn over 6100 without any problems

 

I do not have Dyno numbers yet. Im still assembling the motor, and working on the flow of the intake and heads.

The carb intake flows over 1200 CFM. It maxed the flow bench.

My stock LS2 intake barely flows 750 CFM. even if the L76 flowed 800 CFM (which I doubt), the carb Intake is still 50% better flowing.

As far as the vette using the same intake, and not falling down, thats a OEM combo, with GM's full engineering dept behind it. Not exactly apples to apples. You cant ignore how the whole induction system functions. Its not just one part, and you can get a massive butterfly effect really easy by changing small things.

 

The stock LS-3 peaks at 6000 RPM's.

http://forums.corvetteforum.com/show....php?t=1819310