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Guest Message by DevFuse

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I need some SERIOUS thoughts.

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14 replies to this topic

#1 WJM



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Posted 13 February 2005 - 09:18 PM

We are about (we as in Pleaides and I it looks like) to take the RX over to a Mustang AWD dyno and calibrate it to Dynojet numbers....the only problem is that I am not supposed to have any changes to the car since the dynojet runs.....

I do have a change. The exhaust. Its still got the TWE header/DP and stock turbo....BUT...instead of having a 2.5" back with a glass pack, its now got a 2.5 inch 3 way cat, that after it reduces down to 2.25 and then down to 2.0 inches thru a 2.0 inch glass pack at the end.

And im using a different air filter.

Are these two changes going to seriously change the actual HP output? I didnt feel anytohng on the 'butt dyno'

Seriously...what does USMB think?

#2 4WDFrenzy


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Posted 13 February 2005 - 09:27 PM

I don't think that it would change the HP & torque numbers that dramatically. Maybe 1-3 on each, maybe. All in all, I think that you'll be just fine.

#3 Snowman


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Posted 13 February 2005 - 10:05 PM

Really shouldn't make too much of a difference.

#4 archemitis


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Posted 13 February 2005 - 10:12 PM

even a "sports" cat should give signifigant back pressure compared to a 2.5 inch glasspack.

when you dont have cams, or internal mods, your increased boost, headers, air cleaner, and exhaust system are the main mods, so... i think exhaust and air cleaner would have alot to do with dyno numbers, and more inprotantly, tuning. stoich!

#5 carfreak85


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Posted 14 February 2005 - 12:35 AM

Of course it WILL make a difference, the question is how large you think the difference will be. If it were me, I'd put the old exhaust on and test the new one to see the difference. But thats me! :grin:

#6 idosubaru


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Posted 14 February 2005 - 03:42 AM

all in all exhaust doesn't seem to make enormous differences in my (NA) cars so i wouldn't worry about it too much.

if you want to be picky, i think kevin might have some good dyno numbers and posts comparing 2", 2.25" and 2.5" exhaust diameters on an XT6. being NA maybe that's useless. going from 2.5" to 2" is a 36% decrease in cross sectional area. i think 2.5" or larger (and kevin i think agrees) is too big for an otherwise stock XT6, a noticeable difference from 2" and 2.25" exhaust. again this is NA, but we could notice the difference, not a very big difference but something. i'm a turbovirgin but that seems significant from what i've read about turbo's. in the end it probably moves the power band more than actually making the highest values that much different. that's what i don't like about the larger exhaust sizes on the XT6, loose too much low end for what i want.

#7 Flowmastered87GL


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Posted 14 February 2005 - 03:50 AM

My opinion...

If you have a cone air filter or a cut airbox, yes.

If you have a totally stock intake system, I belive the restriction of the intake will help to keep down any gains from the exhaust since its not fully modified (like a 3 inch turbo back or something...)

I found that glasspacking one of my turbowagons made no difference at all. (other than a bit of noise when I let off the gas)

A K&N Filter on my wagon back when it had a hitachi carb made a bigger difference! So I think these cars are more restricted on the intake side.

Like I said, its my opinion, not a proven fact.

#8 Scoobywagon


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Posted 14 February 2005 - 04:37 AM

Well, from a purely scientific point of view, the fact that you have made changes to your setup between dyno runs inherently invalidates at least one data set, if not both. That kind of depends on the purpose of making hte dyno runs in the first place. Having said that, I'm in agreement with the others. I really doubt you'd see a serious jump in power just based on your exhaust changes. I think the biggest change you're going to see is the fact that you are going from a 2wd chassis dyno to a 4wd dyno.

Using a chassis dyno at all is something of a crapshoot as far as getting usable results goes. Results are extremely dependent on operator skill as well as calibration of the dyno and, of course, the dyno's measurement method. I've actually seen a study where a lightly modded WRX only dyno'd out at about 98 whp. It turns out that a 4/AWD dyno loses something like 4 times as much power compared to a 2WD dyno do to heat and limited traction on the steel rollers (tire slippage). I suspect that you'll actually see a significant reduction in reported horsepower. I think your new exhaust will tend to offset that a small bit, but in the end, you'll see less power.

#9 WJM



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Posted 14 February 2005 - 06:43 AM

I have only used 4WD chassis dynos.

There was no slippage. The rollers are deisgned to not loose traction (aka they have stickey surfaces)

The Dyno Jet that I went to at first was brand new and freshly calibrated...it was 100% accurate on the numbers that other cars pulled in stock form.

This mustang experiment is to see just 'far off' the mustang is as compared to the dyno jet.

My intake remins almost the same...but I went from a mushroom-foam type filter to a regular cone-K&N style filter. ANd I cant put the bigger glass pack exhaust back on....i hacked it into peices to make the current one.

#10 4WDFrenzy


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Posted 14 February 2005 - 08:01 AM

Well, I know when using a Mustang Dyno to dyno a 2WD vehicle, you have to add like 20% to whatever the dyno shows as the WHP. I know this because my friend Roderick has a fully built and turboed engine in his Integra and it showed 395whp@15psi before the turbo blew the intercooler pipe off(faulty clamp). The gentleman who was running the dyno said to add 20% to the 395whp and it would be the actual number, which is 474whp@15psi. But since the RX is 4WD, I'm not sure what kind of change it will show.

#11 WJM



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Posted 14 February 2005 - 08:34 AM

30% loss ofr WRX and odl skool 5 spds 4wds.

#12 MorganM


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Posted 14 February 2005 - 09:56 AM

You have changed variables; the outcome is inevitably going to be different.

YOu could go to that same dyno you went to before now and come up with different values. Heck you could go back to that same dyno with the exact same setup and I bet your numbers would be different.

For it to have an ounce of validity you'd have to keep everything the same.

#13 WJM



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Posted 14 February 2005 - 10:28 AM

yeah...well...oh well.

#14 Hodaka Rider

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Posted 14 February 2005 - 01:18 PM

According to my research over the last few days, and some articles read previously, the most accurate dyno right now is the dynopack. They bolt directly to the hubs, eliminating the tire/roller friction loss and loss to the rotating mass of the wheel/tire. These two losses are different for every different wheel/tire combination. So two cars that make identicle power at the hubs would get different numbers on a normal chassis dyno if one had bigger wheels/stickier tires. Just a thought.
Also, why the reducing diameter? You will be losing power through most of the RPM range, I would estimate. Hot gasses tend to want to expand. If you reduce the diameter, you are reducing the volume available for expansion (creating backpressure). I would say the losses would be higher at the top end, though. most likely under 2000 you will not lose much, but over 4500 or so you would see a drop.

#15 WJM



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Posted 14 February 2005 - 01:34 PM

I have noticed no loss in top end. Goes to 6500 with no issues. The reason to decrese the dia of the exhaust was to equip the car with a quiet and streetable exhaust that doesnt drag the ground. :o Since its got edecent sound, it relatively quiet...and seeming doesnt reduce power that I can tell.

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