Any Ideas what is wrong with this LT4 Combo?
Any Ideas what is wrong with this LT4 Combo? Long story short, my Dad & brother (who knows his way around a BBC) built a LT4. It was a first time venture building a LT4. They had a mail order tune done and I talked the old man into getting it on a dyno to make sure everything was okay. So he went to Speed Inc and it pulled 272 RWHP and 319 RWTQ. That was after Speed Inc played around with the tune. 
Something has to be way off. I think it is a few things, number one being the cam. I know he is leaving some power on the table with his exhaust, but it can’t be a whole lot (tops 40?). Below are some of the specs, I am trying to get an unbiased answer to what the problem might be. Thanks in advance everyone!
391 Cubic Inch (4.01 bore x 3.875)
58 MM Throttle Body (Port Matched to intake)
Granetteli MAF Sensor
Racetronix Fuel Pump
42 lb/hr fuel injectors
LT4 Intake (Port matched to heads)
AFR Heads (#1057 210 cc LT4) (2.08/1.60)
Hydra Rev Kit
Comp Cam 229/245 .510 .540 112 LSA
1.6 Roller Rockers
6’” Rods
10.8 to 1 Compression
SLP shorty headers
Random Tech Y Pipe
Something has to be way off. I think it is a few things, number one being the cam. I know he is leaving some power on the table with his exhaust, but it can’t be a whole lot (tops 40?). Below are some of the specs, I am trying to get an unbiased answer to what the problem might be. Thanks in advance everyone!
391 Cubic Inch (4.01 bore x 3.875)
58 MM Throttle Body (Port Matched to intake)
Granetteli MAF Sensor
Racetronix Fuel Pump
42 lb/hr fuel injectors
LT4 Intake (Port matched to heads)
AFR Heads (#1057 210 cc LT4) (2.08/1.60)
Hydra Rev Kit
Comp Cam 229/245 .510 .540 112 LSA
1.6 Roller Rockers
6’” Rods
10.8 to 1 Compression
SLP shorty headers
Random Tech Y Pipe
TECH Fanatic
Joined: Jun 2002
Posts: 1,829
Likes: 0
From: Montgomery,tx
Re: Any Ideas what is wrong with this LT4 Combo? Cam shaft is way small, and check the flow of the heads....It should be able to make 500 at the crank no problem....There is a place in Louisianna that can get 600 at the crank on opump gass out of a LT4
Teching In
Joined: Aug 2003
Posts: 41
Likes: 0
From: College Station, Texas
Re: Any Ideas what is wrong with this LT4 Combo? Cam is probably a little small for that much displacement (391 c.i.). There's a lot of power being left behind because of that I bet. Research and see what cam would be good for that much displacement. I know stock LT4s put down 285ish rwhp and 300ish rwtq.
TECH Addict
Joined: Nov 2001
Posts: 2,207
Likes: 0
From: College Station, Tx
Re: Any Ideas what is wrong with this LT4 Combo? Cam is not that far off at all - I have run that same cam on 396's with LT4 heads and seen 400-440rwhp.
The SLp's are definitely hurting, but I agree - you aren't even ballpark.
#1 I would look for something large wrong. Get a compression and leakdown test done. Change all the plugs and inspect them to see if any look like they aren't firing. I would change the plugs and wires at a minimum, possibly the opti also.
How does the car feel/drive - is it smooth, or does it have issues? I am assuming the a/f looked normal, or did you check that? If not at least what do the o2's look like at wot? (if you are getting misfire/inconsistent spark you will see a huge "lean" condition from all the unused oxygen).
Are the heads out of the box AFR's? Were they independently flowed? If they were from AFR which porting level are they? What intake manifold gasket are you using - there is a GM LT4 part and a GM LT4 performance part (ends in -777) - if you use the former your gasket is hanging a least 1/2" down in the top of your intake ports.
The SLp's are definitely hurting, but I agree - you aren't even ballpark.
#1 I would look for something large wrong. Get a compression and leakdown test done. Change all the plugs and inspect them to see if any look like they aren't firing. I would change the plugs and wires at a minimum, possibly the opti also.
How does the car feel/drive - is it smooth, or does it have issues? I am assuming the a/f looked normal, or did you check that? If not at least what do the o2's look like at wot? (if you are getting misfire/inconsistent spark you will see a huge "lean" condition from all the unused oxygen).
Are the heads out of the box AFR's? Were they independently flowed? If they were from AFR which porting level are they? What intake manifold gasket are you using - there is a GM LT4 part and a GM LT4 performance part (ends in -777) - if you use the former your gasket is hanging a least 1/2" down in the top of your intake ports.
On The Tree
Joined: Feb 2003
Posts: 179
Likes: 0
From: Grand Rapids, MI
Re: Any Ideas what is wrong with this LT4 Combo? Check and see where the cam is actually installed at? Put a deg wheel on it and see where the cam is coming up at. This will kill power more than you know if the cam is off.
Re: Any Ideas what is wrong with this LT4 Combo? Something is WAY off on that car.
I recently built a 396 LT4 that uses less than ideal parts, but still puts down some decent numbers. The combo is as follows: 1996 C4, 396 LT4, Ported LT4 heads and intake, 52MM TB, GM Hot cam, stock exhaust manifolds, and stock exhaust system with Borla mufflers. I haven't had it dyno'd but the car has run an 11.55 @ 117.5 -- that equates to somewhere around 450 HP at the flywheel (based on 3500 lb race weight).
My point is that even with a small cam and through the restrictive factory exhaust manifolds, decent power can still be made.
I recently built a 396 LT4 that uses less than ideal parts, but still puts down some decent numbers. The combo is as follows: 1996 C4, 396 LT4, Ported LT4 heads and intake, 52MM TB, GM Hot cam, stock exhaust manifolds, and stock exhaust system with Borla mufflers. I haven't had it dyno'd but the car has run an 11.55 @ 117.5 -- that equates to somewhere around 450 HP at the flywheel (based on 3500 lb race weight).
My point is that even with a small cam and through the restrictive factory exhaust manifolds, decent power can still be made.
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Re: Any Ideas what is wrong with this LT4 Combo? Dump the Granetteli MAF Sensor. Get your money back. Those are like air foils (ie: pointless) They make the car hard to tune because the tuner has no idea what kind of air flow you're getting fuel for since it's not calibrated to the tables within your Flash file definition. All those things do is trick the PCM into thinking it's getting less air than it is (larger opening slows down the velocity of the air passing over the heated wires) and so leans out the fueling. They can cause knock retard by excessively leaning out your fueling. Though they (Mail order tuners) may have set the injector constant in the flash, you really need to see what the Block learns are so you can have the constant set to a more correct level if the block learns indicate it's off substantially.
Get some diagnostic tool on the car to see pulsewidths and Block learns as well as timing and presence of knock retard. Post what you find.
Get some diagnostic tool on the car to see pulsewidths and Block learns as well as timing and presence of knock retard. Post what you find.
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