What difference did you guys notice when you put on the BHAF? Truck is stock except what's in my sig.Thanks.

It's a BHAF thread, but there is a little discussion on exhaust. ... ..... There truly are very few applications where a 3" exhaust is acceptable on a ISB over 250 hp. The 3" exhaust was barely enough on a stock 02 HO, with any sort of mods a 4" is needed. Guys will put 4" intake tubes on their air filters, and leave a 3" exhaust. The exhaust flow is roughly 2.5x that of the intake at 1200°.

Interesting. Whats the impact of running the 3" though - just higher back pressure and therefore drive pressure ? Most guys wouldnt know if that was happening without measuring DP. For example mike has a 400+hp truck and he has a 3" exhaust and his EGT are in check and his efficiency is good with over 22mpg. So only downside I can see is higher DP ? I went to 4" and never saw anything noticeable. But I was under 400hp at that point.

The impact is EGT's, back-pressure, and flow. IIRC Mike drives off of EGT's, as they approach 1200° (I think that's what he said) he will downshift, so with his driving style he may not know if he has too much resistance. 3" of Hg is supposed to be the max backpressure on a turbocharged diesel. The 3" exhaust works fine at low-mid power levels and low-mid rpms, its when your towing heavy, or using additional rpms that the back-pressure can be too great. The fact is that a 4" wont hurt anything down low, but a 3" can hurt performance up top.

If I may butt in real quick, what is drive pressure?

If I may butt in real quick, what is drive pressure?

The pressure the engine puts out to drive the turbo. Think of putting a boost gauge where the EGT probe is.. From what you read in the post before your's, the back pressure is created by the turbo. After all, with no turbo and a free flowing exhaust, the engine has no real back pressure. Put a turbo there for the engine to drive and then the engine has "pressure" against it, so it has to actually work to push the exhaust gases out and this is seen as pressure because of the restriction of the turbo and having to drive the turbo. Known as back pressure on the engine or drive pressure for the turbo.

Cool. That makes perfect sense to me. Thank you for clearing me up on it

The exhaust back pressure and drive pressure are not the same. As it was stated the drive pressure is the pressure in the exhaust manifold. Ideal is 1:1 or better with boost. So at 40 psi of boost you would want 40 psi or less of exhaust pressure, some turbo's run more (like the stock HE351CW on my 2005) and some run less (like the GT3782R I run now). Back pressure is the pressure in the exhaust between the turbo and the tail pipe, it's not supposed to go over 3" HG. There is a big difference in pre-tubo and post-turbo pressures. Flow is created by pressure drop, so a truck running 30 psi of drive and a 3" exhaust with 5" of HG will have less flow than the same truck with a 4" exhaust and 3" of HG.

i have a bhaf on my 02. when i first put it on i cleaned the turbo real good. i just got an 01 with the $400 afe , took it off and the turbo was caked. i then pulled the bhaf of the 02 and turbo is still as clean as the day i cleaned it! i do run the pre filter cover though. its a long dusty road up here. i clean cover every oil change with air. filter has 20 k on it and is white as rice. oh afe is for sale, although i cant justify the price.

That's very interesting information. Another one for the simplest way being the best.

then trying to protect them from rubbing holes in them another pain in the butt.

Simple fix. Take a 5" piece of 1X6 board and lay it on top of the studs that held the stock air box on. Use a hammer to tap on the board the make impressions of the three studs. Pull the board up and drill the indentations with a 3/16" bit and put it back over the studs. You may need to tap it down with your hammer, but it will stop the wearing of the filter on the studs plus the filter wearing into the A/C line it sits on too.

I mounted a piece of 2" aluminum angle over the two back studs. Then I zip tied a piece of heater hose to the aluminum angle. The BHAF shows no wear. Also clears the AC line. I then either use Ag-Loc, or a nylon strap to hold the filter down so it is not bouncing around.