Lost HP/TQ with longtubes

TonyG · Mar 13, 2007

I have a 99 2v GT with a V1 S-Trim + FMIC pushing about 10-12 psi. Exhaust-wise, I had stock manifolds + bassani catted x-pipe. With this combo, my dyno numbers were 455 rwhp, 423 rwtq.

So I just had BBK longtubes installed, along with a catted X-pipe. I didn't see the charts yet, but from what I heard over the phone, I lost both HP and TQ. Most of the losses occurred at low rpms, but there are still some losses up high as well. I'll have more details tomorrow after I actually see the charts.

Maybe I should put the stock manifolds back on.

Any ideas what happened?

BCB00GT · Mar 13, 2007

I would be curious to see the graph. I haven't heard of many people losing HP/TQ on the low side with LTs.

mrvax · Mar 13, 2007

I would like to see the graph also. Especially a/f. Was there a long time between the last time you dynoed and now? I imagine after having long tubes placed on a S/C V8, you would need a tune to get the most from it. L/Ts are supposed to work very, very well with S/C cars.

CobraRed_96_GT · Mar 13, 2007

hmm, yeah - usually w/ FI car the less back pressure the better. This is an odd occurance. Was the weather the same? Same shop? Same type of dyno? Dyno numbers can be taken w/ a grain of salt, not to offend anyone, just my .02

BennyBlown2v · Mar 13, 2007

Might be an exhaust leak.

TonyG · Mar 13, 2007

Same shop, same dyno, same time of year. But the "before dyno" was done 2 years ago when it was dyno tuned. I know I didn't lose power in those two years, since I go to the drag strip regularly and keep careful records of my ET and MPH.

01BlkGT · Mar 14, 2007

Very unusual, b/c despite popular belief LT's actually give you can in both tq/hp down low as well as up high. The dyno's I have when I was n/s proved that. With LT's though, that is a big enough difference to where a new tune is necessary. Once it gets retuned, the numbers will show a gain (assuming something isn't wrong).

TonyG · Mar 14, 2007

Here is the dyno sheet

http://www.corral.net/photopost/showphoto.php?photo=28456&size=big&cat=681

I lost 20 rwhp and 20 rwtq in the mid range.

To be fair and objective, the "before" dyno was actually from April 2004. I should have done a pull right before the install. However I go to the track regularly, and my ET's and MPH have been very consistent over the past 3 years. I know that I haven't lost power from then to now. The only change I made since April 2004 was 4.10 gears.

I took the car for a ride, and noticed the peak boost was about 1-2 psi lower than before. My tuner says that if there is a boost drop due to a freer flowing exhaust, I shouldn't lose power -it doesn't work that way.

I think the best thing is to just take it to the track and see what happens. If I lose MPH, I'm going to flip out.

stangman · Mar 14, 2007

There is just no way that can happen with all things remaining the same. but you're on the right track, go to the drag strip & see how it does. dyno's can get messed up sometimes

Duke309 · Mar 14, 2007

you should definitely check for exhaust leaks before heading to the track.

stangman · Mar 14, 2007

ya know what, i'd check to see if they have all your s/c tubing etc.. tightened down

hotmustang331 · Mar 14, 2007

Dang, your new A/F is nearly 13.0-1 PLEASE get a retune before you start getting all over it....that lean condition isnt exactly safe.

And your previous dyno was all jacked up....the curve was all over the place and was missing sections.

So yea, check all of your clamps and get a retune. You may have to pulley down a bit too....once you get your boost back, your gains should go through the roof.

TonyG · Mar 14, 2007

hotmustang331 said:
Dang, your new A/F is nearly 13.0-1 PLEASE get a retune before you start getting all over it....that lean condition isnt exactly safe.

And your previous dyno was all jacked up....the curve was all over the place and was missing sections.

So yea, check all of your clamps and get a retune. You may have to pulley down a bit too....once you get your boost back, your gains should go through the roof.

Good call. I wonder why my tuner said it was safe to run.

BennyBlown2v · Mar 14, 2007

TonyG said:
Good call. I wonder why my tuner said it was safe to run.

Apparently your tuner is an asshat if he can't figure out why you really lost 1-2psi and 20hp lmao.

01BlkGT · Mar 14, 2007

BennyBlown2v said:
Apparently your tuner is an asshat if he can't figure out why you really lost 1-2psi and 20hp lmao.

You are close enough to one of the best tuners in the country. Would be well worth the trip down to ATL to have him tune the car (Tim Barth at Modular Powerhouse)

Stan Weiss · Mar 15, 2007

The holes in the original dyno run can be a big problem with the shape of the curve. Do you see in the top right hand corner where it says "smoothing". This means it will average out data to to remove the pikes and valleys and make a "smooth" curve. It looks like a large valley was remove which would raise the curve up.
Boost / blower pressure is another thing. With the same blower and pulley sizes a number of thing can change it. Different cam, or cylinder heads, or exhaust. If the new exhaust is scavenging better during overlap there will be less pressure in the cylinder and better intake flow N/A or FI.

jivepepper · Mar 15, 2007

Here's my last dyno from the same tuner! This 26th will be my third time there in as many years. I look forward to going. I have never had a problem. This was the break in tune.

After a few more back to back pulls

hotmustang331 said:
Dang, your new A/F is nearly 13.0-1

His last run was less than 12 flat by his dyno graph? There is just something wrong with the setup since. He has been driviing it for 3 years since his last tune.

TonyG · Mar 15, 2007

Maybe I'll get a retune and a smaller pulley.

I have a 3" Reichard ultragrip pulley with a 6-rib set-up, and it works great (no slipping). Anyone using a 2.8" ultragrip with a 6-rib set-up?

Lost HP/TQ with longtubes

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