i have heard people saying for years that loss of back presure will cause loss of low end tq. is there any dyno proof for this? what rpm are we talking about? here are my charts, i will probably have one more set of charts to look at before the turbo goes on too but for now this is all i have. for this test at 2500rpm i was allready seeing an increase in tq and hp, the lower chart apears to be flatlineing at the lower rpms wich would indicate to me that sure the diference would be less at lower rpms but it would still be an increse in hp + tq way down in the rpm's.

anyone have any dyno evidence to help prove this "loss of tq" theory because i have my doubt. on the gtech i typicaly see .2-.3 reduction in ET's by pulling the exhaust at the y.

here is a closed vs open chart from my truck



and here is a chart that shows the actual hp diference of the chart above

 

I can't verify either way, but I know on the street I can get better mph out of it in a certain strip of road. My track times are to incosistant to measure changes like that. I need better tires and a locker for that.

 

better with open or closed?

 

open
It isn't much either ... just a mph or 2.

 

on a basically stock truck you will loose some tq down low but depending on the motor and mods. my lt1 camaro with cai, 4.10's, maf, ypipe, no cat, dynomax exhaust, and a few other things runs great. when i had the 3" flowmaster exhaust it was a pig up to about 4500-4800rpm then it came alive. i put on the dynomax and regained that low end that netted me a .4 reduction in 1/4 mile times while there was only 2mph diff with the more restrictive dynomax 2.5" exhaust. the ls1 based motors though love to be wide open and you will gain some up higher in the powerband but it also raises your powerband. where you once made tq at 2800 it may now peak at 3500. for racing this is good because it allows you to launch at a higher rpm. for everyday street driving it gets annoying like having to big of a stall, takes more effort to start off in normal driving. there is a good balance of both but it requires trying different setups. i have been through5 diofferent mufflers on my car and finally went back to the dynomax. good luck!!

 

I have 3" cutouts in place of my cats. I had my truck running very well before I wnet to the track, but that was with the cutout closed. When it was time to run, I opened up the cutouts - sounded great. Taking off N/A, it felt dead until I got it in the upper rpms. It wasn't pulling as hard either. I then ran a nirous run, and broke out my laptop and logged my last N/A run. It was very rich and my MAF was at least 6 pounds below where it normally runs.

Now if I had a bigger stall, and had tuned the truck to run with the cutouts open, I would probably have run a much better time. I need to tune for the cutouts and then I'll try it again.

At this point, I'm mixed on whether open exhaust is a benefit. It hurt me, but had I been tuned for it, it may have netted me a better ET.

 

I also read that on single patten cams there is no need for backpressure, I don't really believe that too much. I have a cutout on my F150 and aside from making a shitload of noise, it's considerably slower from the hole shot.

BUT when you go FI, back pressure is bad. Turbo, blower, and nitrous are basically all FI. Hell my WRX has a 3" exhaust on it now and that's huge for a 2.0L motor.

When you were shootin that 150hp shot through that 6.0 that was essentially going forced induction. Besides that, the stock exhaust on there is severly inadeuquate for what you were trying to do let alone a N/A 6.0

In conclusion, I believe that on a street vehicle a properly setup exhaust system will yield best results. Full length headers, big cats (optional), and high flow muffs.

 

You DO realise that dynomax mufflers have some of the highest flow numbers of anything on the market, and that you probably were getting better flow out of the 2.5" Dynomax mufflers than you were the 3" Flowmasters, right??

Flowmasters are sort of like the import crowd... noise and marketing are more important than actual performance gains. Yes, they are an improvement over most stock mufflers (or more specifically exhaust systems), but a true high performance muffler they are NOT.

 

Tex, do you still have that video around?

 

whenever i have my cutouts open it seems like the low end is dead. it's louder which creates the illusion of more speed, but it just feels like it's not pulling near as hard as it does with them closed running through the bullets. mine has great low end when running through the exhaust.