Manic Mechanic

iTrader: (3)

Well not much hoopla, just giving an account of what I've put on and it's benefit. Some searching here and on GMTrucks.com indicates that a stock 8.1/Ally will put down 240 RWHP. Horsepower Engineering here in Houston was doing a dyno day special so I paid $10 for two pulls. Both runs were in 4th gear, tow/haul on to lock the converter, and from 3700 RPM through 5250 (Chuck didn't know any better and it hit a rev limiter, these combos are done by 4800 lol) The 2nd pull was better as Chuck improved the technique a little after the first.

Peak #'s were 274 RWHP @ 4075 and 355 RWTQ @ 4075 on a Dynojet, 62*F, 30.2 in-Hg, 20% Humidity. SAE 0.95

That would indicate a 34 HP increase with my mods. Volant with ram-air tube in bumper, Ganetelli old school MAF, BBK 80mm TB w/port matched intake manifold, Champion 8.1 spec. copper plugs one heat range colder than stock, Delphi Marine 8.1 injectors at 39 PSI (flow is 10% richer than what the PCM expects), Hypertech programmer on 89 octane tune, and a Magnaflow x path muffler. I ran 93 octane Chevron during the test and engine and trans were at full operating temp 195 & 180.

HP stayed above 250 throughout the entire run and #'s actually started climbing steeply at 4800 where the Ally shifts but then fall just as fast as the fuel injection puts on the brakes by 5250.

TQ dropped below 300 at about 4650.

I was kinda bummed because I expected more based on how the truck performs but it's all torque so it's making good power just at a low engine speed. But at least I know it's up like I felt over stock. The best part of the experience was watching one of the techs setting up the runs open the hood. He didn't know anyone was watching and he still did one of those wide eyed WTH? stares at the big engine for a few seconds before it registered what he was looking at. They had never seen a 8.1/Ally truck there before.

Vernon

TieRod

That kinda seams low. I would of expected 10 hp and 40 tq higher.

Manic Mechanic

iTrader: (3)

It might have actually been higher if he had started the pull at a lower RPM. The peaks on my printout are actually where the pull started. They are used to working on high RPM engines and it was a quick in and out ordeal.

But it has been cutting out under WOT in 4th gear lately, I can't figure out why. It feels like a misfire but it's computer generated. Any advice?

Vernon

TieRod

I'll try to dig up one of my old dynos and see where my peaks are at.
I been having a similar problem with my Camaro. I haven't found whats causing the miss fire yet.

dirt track racer 81

iTrader: (45)

on our mustang dyno a stock 8.1 puts down like a round 180 i think it was..a stock 4.8 and 5.3 put that down..i wasnt impressed with them

TieRod

I've seen a 8.1 dyno 300whp with CAI, exhaust and tune through a 4l80e. I see allot of people say there not impressed. Not realizing that we have a Allison bus transmission and are done making power at 4500rpm.

BTW. Vernon if you take the standard Ally 38% power loss. Your making 378 at the flywheel.

dirt track racer 81

iTrader: (45)

and im sure that was on a dyno jet too

02_2500avalanche

I have yet to dyno my truck after I did all that I have done to it. It did lay down 240hp and 355tq with just exhaust...damn that was along time ago.

Alex

Kerr

doesent sound to bad to me considering where the rpm starter. Gm rates those motor at 330hp @4400 rpms and 450lbtq @3200 rpms... so as tie rod said take those numbers and minus the 38% true hp (if gm is not fibbing about power) should be 205 whp.. and 280 ish tq...

Also keep in mind that all dynoes are off. If they had the weight of the truck plugged in wrong etc etc etc... it will change the numbers a lot.