JohnEd

iTrader: (1)

Guys,

I hope I am not letting some of you down with this. I had intended an extensive series of measurements of the back pressure that exists in my 93 SC400. I ended up doing a severly limited test that still gets to the bottom line. IMHO

I could not get a reading at the O2 sensor port located between the exhaust manifold and the pre cat. The O2 sensor was extremely tight and I needed a claw foot hydraulic fitting six point. I couldn't find one and I am sure that it is a special Toy tool. The back pressure there is no doubt higher than at any point that I managed to measure. That might be moot as the cat is a high flow item and hard to improve upon. My first measurement was at the point between the first/pre cat output and the first Y.

The pressure at the input to the first Y was 2.5 psi. I couldn't access the point just after the first Y and had to measure the point after the second/ center cat and that is in front of the second Y. That measured 1 psi. I concluded that the first Y and Cat gave 1.5 psi of back presure or 3/5ths of the total. The single psi in the rear is distributed across the rear Y, resonators, mufflers and those pesky bends and partly colapsed 2 inch SS tubing. You may get more noise with a different muf or by taking out a resonator but you can't get much better than 1 psi and no matter what you install, that replacement item must have at least some back pressure creating flow resistance. It looks to me that the rear is as good as it gets for a stock engine making less that 300HP.

The 1.5psi is created by the resistance in the center cat and front Y. I think the front Y is as good as the rear Y and that most of the back pressure is caused by the center cat alone. The good news here is that the center cat is only required in California. I have read posts by Californians that they removed the center cat and passed smog with flying colors. The fly in that ointment is that the law says you can't modify the smog system. They don't care if your new system is cleaner....it is still illegal. So either don't do that or don't talk about it. I promise not to do that and will tell you how it worked out in terms of back pressure and net HP change in the near future.

The conclusion, if you believe my crude test results, is that for a modest cost you can improve the ex sys by reducing the ex back pressure by 3/5ths. My spendy local muffler shop said $100 to replace the center cat with 2.5 inch pipe.

I don't know what to tell all of you with center resonators. It looks like a choke point but who can say what it would measure? Maybe one of you will measure that for the forum and post results.

I recently listened to a post by redpheonix. He has replaced his entire sys. That is the greatest sounding V8 ever. I hope the recording of the ex note wasn't taken from inside the car. In that case...Holy Bat farx Batman.

I hope this helps some of you decide what to do with your cash.

Good luck

John

MJHSC400

It seems that the total of the rear section will be effected by the drop of the cat/first y also-- and should be remeasured after the center restritction is replaced by something that flows better.

I'm thinking that the two pressures are relative, and when the center is opened up the rear reading may go up--

Also, it seems that 1 psi is quite alot considering it's an open section of pipe -- As in it's very different from pressure on a closed vessel. I'd be interested to see what pressures in a zero loss system look like..

I'll bet the engineers designed this system for quiet/power-- and allowed some losses for silence --


Nice work ole' fella-- It's definitely part of a big picture of important data.

It's always a task reverse engineering anything made in Japan.

Good luck!

JohnEd

iTrader: (1)

Matt's comment about things changing after the change is valid. I will remeasure the rear end after the new center pipe. I can also split hairs back there as i have a measure port between the resonator and muf.

Good critque.

thanks

John

MJHSC400

sweet dude