So I bought an HKS true dual for my 07 based on hundreds of good reviews. Installed it, LOVE the sound, LOVE the looks (rolled me rear bumper too!). Only problem is my recent dyno......let me set this up for you.

I have had the car dynoed on 4 seperate occasions now (for different reasons) ALL ON THE SAME DYNO. The first three times were done stock and were consistent, the fourth time was three days ago with the exhaust and I was VERY disappointed with the results.
Dyno 1 - May 2007 - temp probably in the 60's - 266rwhp (stock)
Dyno 2 - November 2007 - temp in 40's - 266rwhp (stock)
Dyno 3 - April 2008 - temp high 50-s - 265rwhp (stock w/volks)
Dyno 4 - July 2008 - temp 100 - 233rwhp (volks/HKS)

Needless to say I was STUNNED. All these were done on the same dyno. My main question is could the very high temperature and bad air here in the summer be enough to make a 30+whp difference? Or is the HKS causing a power loss as well? Any input would be appreciated.

 

the heat didnt help you, but i doubt accounted for 30whp. ive always said this since the day i got here... large diameter dual exhaust is not good on a stock Z! it is just too much flow that the stock motor doesnt need, and in turn hurts power across the board.

that being said, its kinda wierd you lost that many hp. normally you might lose a little, or even go up a bit. but the major problem with big exhaust is loss of tq. what was your tq loss?

 

wow..thats weird as hell.

 

WOOOOOOOW..

 

Yeah I know! I am as shocked as you!!!

Torque loss was only 221 to 207. Maybe bad gas from the last fillup? I am just stunned.

 

Its not bad gas.. Its the NA factor and throwing a true dual.... For your application it was ftl.. Maybe if you get a tune youll be looking like before or better... The ecu could be doing that to you..

 

seriously wtf?!?! i guess i gotta go dyno mine and see whats up

 

finally someone that knows what theyre talking about.

and OP, allow me to elaborate on jay'z' statement. not bad gas. the tq loss is from the lack form in the exhaust flow, as the exhaust now has more room than its volume. this is no bueno in the exhaust world. since you spent so much money on this exhaust, going back is not an option. i would say that you could get the numbers back up N/A, but your gonna have to add more power for sure.

 

Oh I plan on adding more power! Thats why I am frustrated to see my first power mod send it the wrong direction!

 

what size are the pipes? 3"

 

I see COBB in your signature. Do you have the AccessPort, and any maps downloaded to your ECU??

 

it seems likely there is some other unknown variable affecting the dyno outcome.

 

Isn't this exhuast the HKS Ti TD? That's a 2.25" true dual exhaust iirc... that is certainly not 'oversized' for N/A application.

 

its def. not 3". I think it is like 2.25 or 2.3.

And I am the testbed car for cobb, hence all the baseline dynos

 

Please reframe from sayin DA comments like this. 2.25' exhaust is not to big by no means

 

HKS superior R&D =

JK- Its the weather. While a dyno does adjust for humidity/heat/elevation etc it has no idea how much timing your car is pulling due to extremly high ambient temp/ intake temps.

I have to look for my dyno plots but I dynoed several times during different dyno days (same place) and during a 95 deg day in the shade vs 78 degree day there was a 11 hp diff and about 5 lb/ft.

Do the math using my example and it adds up. Just go back on a cooler day or just laugh at your buddies results too!

 

a 50 degree difference can lower your HP. notice how the car always feels stronger in the winter........Cooler air is more dense and contains more 02. Your car runs leaner and makes more power and cools easier.

Your ECU probably reacts differently to those temps. At 100 degrees you would think the ecu would retard timing a bit to avoid detonation. Your AIT is pretty warm compared to 50 degrees.

 

for some reason, i was thinking the hks was 3". your right, 2.25 should not be too big. but still, being a dual exhaust it might still need more power. think of it this way.... with a dual 2.25 system, you have 4.5" of exhaust flow. if you put a single 4.5" exhaust on a stock Z, do you really think your not gonna lose power? not such a DA comment now is it?

 

so we should go true dual exhausts no greater than 1.5" per side?

 

still a very DA comment, especially considering you tried to redeem yourself and express that it was a good comment.

2.25 duals do not equal the flow or capacity of a 4.5 single.

remember the area of a circle from grade school.........pi*R*R

2.25*2.25*pi = 16 sq inches of flow
1.125*1.125*pi*2 = 8 sq inches of flow

not even getting into the fluid mechanics of a 4.5 pipe vs a 2.25.........leaving that to a ME