Tried my different setup, and got a lesson on the "myth of backpressure"

[MustangEater8251]

Basically I ran the car last night, maybe 50 degrees out.

I was not looking for any new numbers, infact did alot better then I thought.

I have on my car at the moment, a stock manifolds, y-pipe, cutout, catco Cat, 2.5 in.s-pipe, 2.75in. I-pipe, stock z28 muffler and tips.this is a different(and much more legal setup) then when I ran my 15.4

I know having the cutout open was going to have to much free flow(did something similar before but never had hard datat to back it up)

1st run Cutout open

ran a 15.98@like 84.XX
2.18 60', and I dead hooked the launch
I lost abotu 2 mph, over my average runs, and abotu .4-.5 of my average runs.

2nd run Same setup
2.16 60" another good hook
ran a 15.917@ like 84.8
little bit better but probable due ot tempratur drop, sun was down for longer(big differnece in temp in FL)

3rd run

I was planning on runnng with cap on to see how the stock z28 system does.

got creative and botled a colelctor I had laying aroun in my car up, then wedged 2 washers, between the collector and cutout. This made the diameter slightly smaller then and a littel more backpressure.

ran a 15.78@85mph
2.11 60" dead hook launch

4th run

Added a few more washers, car was feeling peppier last run but still too much flow... ran a 15.60@85... still too much flow, acording to my A$$ Dyno.
2.08 60'

5th run
Ran it agian same setup, but was a little too focused on a race with another f-body, redlighted and spun the launch, and ran like a 16 flat I think...

2.3 It hink the 60' was didn'te really pay much attention.

his time I think wa a 15.9, earlier he hit a 15.4 I think was a 98+ 5-speed camro with a few mods.


My r/ts were high .5s, and low .6s, and one .4XX.
Main reason I went tonight was to work on my r/ts, only got to the track 3-4 times all last semester, getting rusty with my r/ts and I was thinking about running eliminator rounds at SLP Appreciatrion day in March... we will see.


Either way I know there is room for improvement, and my plans for cutting down on the backpressure will be a little less ghetto and probable more efficient then some $0.03 washers wedged in my car.

But I did very my cars tiem by about .4, and 2mph with some washers... thats like a 20-30 hp difference?

BTW, the different 60's, all my launches except the last one were really good, I think they were more related to power then my launch.

[CamaroL32]

So having the cut-out half open seemed the best?

[MustangEater8251]

depending on the location of oyur cutout...

your exhasut setup is prebable better then mine... I got my CATback free so I ain't complaining.

mine is more like 1.4 the way covered up nad not ideal, I was hitting better mph with a different setup.

[CamaroL32]

Mine is on the I pipe. I seem to get my best times with it open. I have a 2.5" S pipe now with crappy bends and hope to change that over to 3" when the heads and headers go on here in a month or so.

[ January 30, 2003: Message edited by: CamaroL32 ]</p>

[MustangEater8251]

yeah you probable will... my car ran pretty strong back when it had stock cat, s-pipe and I-pipe, and no muffler.

[Supersix]

My best is with an open cutout

[12secondv6]

When I had rk sport headers, 2.5 inch y pipe, gutted random tech 2.5 inch cat, 3 inch cut out OPEN and edlebrock cat back I put down 282 rwhp and 335 tq
Same set up 15 min later I capped the cut out CLOSED and put down 291 rwhp and 362 rwtq

I have not done a back to back on my new nitrous set up.

My cut out stays closed.
Our cars DO NOT need such free flowing exhaust systems, back pressure is a good thing

[SLOWBRAEATER]

My guess would be evey car reacts differently because magnus runs open y pipe and he picked up .2 and 2 mph and there is like no back pressure at all.

[12secondv6]

Maybe if he add's a cat back or something with back pressure he might up the mph?
I don't think he has ever made passes with it on.
We would have to speak with him concerning that

[Magnus]

High stall verter + gear:

Catback on + cat: 14.8@87
Catback on + 2.5" orp: 14.7@88

Open y-pipe: 14.5@90

all on the same day

I pick up so much with NO backpressure because my engine is always in the higher rpm's. No way in hell am I ever racing with backpressure again. I like making it easier for the exhaust gasses that already made the power to escape. [img]smile.gif[/img]

A 3.4 might not do so well because they are more of a lower RPM engine.

[ January 31, 2003: Message edited by: Magnus ]</p>

[Oryan02]

I'm with Magnus on this. How do you guys figure backpressure is good? You can't go by track numbers for your conclusion that making more backpressure is good just because you shaved of a couple hundredths of a second. You could have just been driving better as the night went on and temperature dropped.

12secondv6 I don't see how your HP and TRQ went up when you capped the cutout. When I dynoed mine I hit 183HP and 214TRQ with the cutout open and got 178.7HP and 228TRQ with the cutout closed.

When racing it would be ideal for me to get that 228TRQ in the begining of the race then open the cutout after I got past the 4200-4500 rpms when tourque drops and HP picks up. This is just my opinion of course.

Edit: deleted an image that didn't show up. It was the dyno chart from Dominics web site under 3800-II specs.

[ February 01, 2003: Message edited by: Oryan02 ]</p>

[Roy]

ive seen LS1s dyno BETTER with their cutouts closed than open. when i had a borla (on the v6) i played around with the plates. i found when i ran no plates that it seems slower off the line, but had better top end, and closed was the opposite. the only way to tell if something works for your car is to try it on your car. my car dynoed a little higher with the cutout open, but ETs seem to be about the same, but mph is down a little making me think that i am losing power through the muffler. every car is different and reacts so.

[StockV6Cam]

You could get one of those Q-tec cutouts that open electronically. Then you could launch with it closed and open it up at the 1/8th or something.

[Oryan02]

A Qtec is in the future. :D

[12secondv6]

<blockquote>quote:</font><hr>Originally posted by Oryan02:
12secondv6 I don't see how your HP and TRQ went up when you capped the cutout. When I dynoed mine I hit 183HP and 214TRQ with the cutout open and got 178.7HP and 228TRQ with the cutout closed.
[ February 01, 2003: Message edited by: Oryan02 ]<hr></blockquote>
Nitrous likes it
I will redyno w/ the new engine and direct port shortly