LT4 Dyno question
Thread Starter
Racer
Joined: Sep 2005
Posts: 417
Likes: 0
LT4 Dyno question
I dynoed my LT4 yesterday and got bad results in my mind. Its a stock LT4 110k miles (which I know is a little higher number) with corsa exhaust and open air filter. I only pulled 289 rwhp. Air and Fuel were hitting great but when it hit around 270 rwhp the chart starts going up and down 5-10 rwhp like freaking crazy. I think it deals with the timing. It happens around 5200 rpms. And then the torque shoots straight down...no gradual drop or nothing just falls. I figure since air and fuel are perfect that it might be ignition. It goes straight up great until it hits that horrible up and down jump due to timing. Plug wires and plugs maybe? Would a chip help the timing? What all do I need to do?
Last edited by Achilleus07; Oct 20, 2005 at 10:30 AM.
Le Mans Master
Joined: Aug 2002
Posts: 6,080
Likes: 3
From: Clearwater FL
St. Jude Donor '06
I would start with a tune up. Have the plugs and wires been replaced yet? Might want to start there, could also be the opti if it is original and showing signs of aging (hard to know for sure, but the wires are a bear to replace and many do the opti at the same time). I'm sure others will chime in with other ideas. 
Le Mans Master
Joined: Oct 2000
Posts: 6,202
Likes: 9
From: St. louis, MO
I dyno'd at 299rwhp 2 years ago. Check plugs etc. like was mentioned. I just changed mine, opti went out 4 weeks ago, changed to Taylor wires and AC Delco plugs, and new opti, it feels like it is pulling harder.
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Did you check the fuel pressure during the run? It could be that the fuel filter is clogged or the fuel pump is getting weak. But then that would have showed up on the AFR readout as it would go lean at that point. However if you have ignition failure and a miss then that AFR at that point would also be affected.
Thread Starter
Racer
Joined: Sep 2005
Posts: 417
Likes: 0
Fuel and Air are perfect...not running too lean and not running too rich...its right where it should be. Just goes strong until 5000 rpm and then timing goes off the chart haywire and torque falls like a rock. I want Taylor wires for sure...so no desuading...Im going to get them from Summit...Should I get the Taylor ThunderVolt 8.2 wires or the Taylor Spiro-Pro. Is there really a difference?
Last edited by Achilleus07; Oct 20, 2005 at 11:04 AM.
Le Mans Master
Joined: Oct 2000
Posts: 6,202
Likes: 9
From: St. louis, MO
Originally Posted by Achilleus07
Fuel and Air are perfect...not running too lean and not running too rich...its right where it should be. Just goes strong until 5000 rpm and then timing goes off the chart haywire and torque falls like a rock. I want Taylor wires for sure...so no desuading...Im going to get them from Summit...Should I get the Taylor ThunderVolt 8.2 wires or the Taylor Spiro-Pro. Is there really a difference?
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Race Director
Joined: Feb 2003
Posts: 18,484
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From: Everyday you must choose between the pain of discipline and the pain of regret. Fredericktown, OH
Looks like all agree a basic tune up will help out.
As a reference, mine made 305 RWHP with Magnaflows and a Vortex rammer.
As a reference, mine made 305 RWHP with Magnaflows and a Vortex rammer.
Team Owner
Joined: Sep 2001
Posts: 62,155
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From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
Originally Posted by Achilleus07
Will a powertech LT4 chip help the timing problem or free up anything?
Instructor
Joined: Jan 2004
Posts: 171
Likes: 0
From: Fort Wayne IN
My 1994 had the same problem on the dyno. Plugs, wires, and a coil are not going to correct your problem. I wrote an article about it for VETTE magazine a couple of months ago. Replace your OPTI and save yourself some time.
Le Mans Master
Joined: Jun 2002
Posts: 6,097
Likes: 2
From: Cincinnati Oh
Originally Posted by vader86
No.
As stated, a "chip" will not help. You can get your pcm re-programmed if you believe gains are to be had or for other reasons (change the speedo, change the fan on/off temps, cags, etc). For the OBDII computers you do not replace a chip, but programming can change most things you would need.
Safety Car
Joined: Sep 2000
Posts: 3,524
Likes: 4
From: Hinckley Ohio
Originally Posted by Achilleus07
. ........... Just goes strong until 5000 rpm and then timing goes off the chart haywire and torque falls like a rock. .......
Racer
Joined: Jul 2001
Posts: 359
Likes: 0
From: Russellville TN
St. Jude Donor '06
LT4BUD and Vettef6 are correct. I've run many a vette on the dyno and you can see with scanner software running from the car's ECM on another computer the knock sensor interacting with the timing and the chasis dyno software.
Don't attempt to disconnect the knock sensor and run the car. This could leave your vette disabled on the dyno and needing a new engine!
Been there, done that.
Cheers,
Frank
Don't attempt to disconnect the knock sensor and run the car. This could leave your vette disabled on the dyno and needing a new engine!
Been there, done that.
Cheers,
Frank
Melting Slicks
Joined: Jun 2000
Posts: 3,215
Likes: 6
From: Tampa Florida
With your limited information...
Anytime one visits a dyno, you want the engine to be running in top condition. A fresh 'tune up' should always be part of the pre dyno visit checklist. Low mile Optispark, relatively new plug wires (I use the Taylor Spiro Pro's...nearly fit and look like the OEM wires-great luck with them), new plugs, fresh oil, new fuel filter, air filter, etc.
Anything that is 'normal maintenance' (and typically fairly inexpensive to replace) in other words. Why invest $75-150 for a few dyno runs when your 10 buck fuel filter is destroying the runs, etc. Another benefit is to insure the car is the 'same' for the next host of runs if you ever upgrade it-gives you good data if the mods helped or not.
Lastly, what dyno was it? Dyno numbers fluctuate from dyno model to dyno model and even between the same dyno models. I think we assume it was a Dynojet model...a decent running LT4 will return 300 WHP on that model.
Your timing comment leads one to believe it is either poor gas, engine is carboned up, optispark is failing, fuel pump is failing, and/or plugs and wires are old.
Don't try to modify the car to fix the 'problem'! An LT4 will pull strongly and smoothly to the fuel cut (6400 rpm)...if your's is not then something it wrong.
Anytime one visits a dyno, you want the engine to be running in top condition. A fresh 'tune up' should always be part of the pre dyno visit checklist. Low mile Optispark, relatively new plug wires (I use the Taylor Spiro Pro's...nearly fit and look like the OEM wires-great luck with them), new plugs, fresh oil, new fuel filter, air filter, etc.
Anything that is 'normal maintenance' (and typically fairly inexpensive to replace) in other words. Why invest $75-150 for a few dyno runs when your 10 buck fuel filter is destroying the runs, etc. Another benefit is to insure the car is the 'same' for the next host of runs if you ever upgrade it-gives you good data if the mods helped or not.
Lastly, what dyno was it? Dyno numbers fluctuate from dyno model to dyno model and even between the same dyno models. I think we assume it was a Dynojet model...a decent running LT4 will return 300 WHP on that model.
Your timing comment leads one to believe it is either poor gas, engine is carboned up, optispark is failing, fuel pump is failing, and/or plugs and wires are old.
Don't try to modify the car to fix the 'problem'! An LT4 will pull strongly and smoothly to the fuel cut (6400 rpm)...if your's is not then something it wrong.
Team Owner
Joined: Mar 2001
Posts: 30,856
Likes: 293
From: Boston, Dallas, Detroit, SoCal, back to Boston MA
Sounds like the opti needs a cap & rotor, but I'd put a new fuel filter in it first.
The dyno shop can hook a computer to see if the knock sensor is pulling out timing.
The dyno shop can hook a computer to see if the knock sensor is pulling out timing.