CAT-Back Exhaust System ...

This is an excerpt from an email discussion regarding exhaust and intake modifications to a Montero Sport. Drawbacks an benefits are discussed. A lot of basic information on the operation of an intake and exhaust are also talked about.

The names have been changed to protect the innocent.:-)

********** wrote:

... But I do have more questions for you if you don't mind... Your page on increased gas mileage is really interesting. I would love to do the 3000gt conversation that you did. However my question is this, I once had a VW GTI that i raced, 2000 model. And i did the BIG intake and BIG cat back ready for a super charger or turbo. But i ran in to problems with the computer, ran too rich, ran to lean and the same print out??? Go figure... But here's the thing, i understand how you killed the battery, and rest the comp. But i really don't want to go that far just yet. Do you think that by doing the gt kit and a k&n it will give me problems? Even better, do you think if i put a snorkel on it would mess up the comp? I know you don't get much mud, but i'm sure you've crossed some water...All and all i'd really like to improve my gas mileage. I do have an auto. And i'm getting around 15hwy. So the intake and hubs idea sounds good to me. I'm trying to gear up for the ad on's. The ARB, rack and i think i will get some sliders. ....
From: ES_97Sport
3:43 PM
Subject: Re: 2000 Montero
SportTo: *********

... As for the exhaust, I believe I know why you were having that problem. Typical turbo/supercharged exhausts are built for the capacity of a forced induction system. They do NOT work on naturally aspirated vehicles. WAY too big a diameter pipe, muffler(s), etc.

Forced induction relies more on forcing the exhaust OUT of the system. If you put a really large diameter exhaust on the vehicle you end up with the same problem that people with naturally aspirated vehicles have when the exhaust is detached from the headers. Effectively, if you have a large enough capacity exhaust it'll behave the same way as an open exhaust on a naturally aspirated vehicle. Unfortunately, it'll change depending on the RPM. So, yes, half the time it'll be too rich and half the time it'll be too lean. Pretty messed up. And being computer controlled doesn't help much because the computer can't "learn" fast enough to adjust. I can reproduce the exact problem on my '68 Firebird - more consistently because its carburated. With the exhaust capped make all the adjustments. Uncapping or even an exhaust leak throws everything out of wack at various RPM.

On a Montero Sport it takes about 6-15 miles for the computer to "relearn" to a base adjustment. The system then goes from there to do continous fuel tuning, but even that doesn't happen very quickly. Basic stuff like knock adjustments happen right away, of course, but the other things take time. Especially the fuel curve.

BTW, resetting the computer actually can be done with some of the diagnostic scanners. I have one from Checkers that'll reset it to default without unpluging the battery.:-) VERY nice piece of equipment. 'Course they cost $350, but it was well worth it. Saves me a BUNCH of money at the dealership being able to read all the codes and OBDII codes at home or anywhere else I go. Readouts are actually in English, too!:-)

The one thing people usually run into that causes problems is not adding the air cleaner. Its the exhaust. WAY WAY too big. On a 3/3.5L engine you do NOT need a 2.5-3.5" exhaust. One of the main functions of a naturally aspirated exhaust is to assist in the removal of exhaust from the cylinders. Backpressure is a "wive's tale". Never EVER buy an exhaust for a naturally aspirated vehicle based on how much back pressure it creates. You don't need "back pressure" for the engine to run as some people say. You have it whether you want it or not and its not a bad thing unless its excessive.

What you want to focus on is the ability of the exhaust to scavenge the cylinders. That ability is enhanced by increasing or decreasing - yes, decreasing - the diameter of the exhaust tube, mufflers and CAT. A properly tuned exhaust will litterly suck the exhaust out of the cylinder when the exhaust valve opens increasing the vacume in the cylinder. That increase in the cylinder vacume will actually draw more intake charge into the cylinder thereby increasing HP and torque. More fuel/air in the cylinder, more power. Its the oposite of forced induction. You're using vacume to pack the cylinder instead of pressure. All this works off of the velocity of the exhaust flowing through the exhaust system. The faster it flows the more vacume is created at the head - exhaust valve - of the system. A common misconception is that exhaust flow stops when the valve closes. It doesn't. Common high school psychics test question. A body in motion tends to say in motion. That body is the exhaust fumes and they're going to keep moving in the direction they are started in creating a vacume at the head where the exhaust trys to "pull away" from its starting location.

So, you want a small exhaust tube diameter to keep velocity as high as possible without it being so small that the exhaust can't get through it. Kind of a pain since the optimum size at WOT is obvioulsy not the optimum size at idle. Of course, on a street car, you compromise.

We drive trucks. I don't care what anyone says. Anthing that weights 5000lbs is a truck. And to move that kind of weight requires torque. That's another thing. HP does NOT move your vehicle. Torque moves your vehicle, HP keeps it moving. So, it get a heavy object moving you need torque. Big exhausts optimize flow at WOT, but destroy exhaust velocity at lower RPM ranges. Unfortunately, the lower RPM range is where you need the torque to get your vehicle moving - ie. normal street driving, stop and go, and highway. And, the velocity in the exhaust is one of the major things that improves torque in the low RPM ranges. Montero Sports are not race cars so there is no point in putting an induction or exhaust system on that only works at 5000 RPM.

Actually, a small diameter exhaust is one of TWO major things that improves that. The intake works the exact same way only backwards, so to speak. A naturally aspirated vehicle needs an intake that will use intake velocity to "pack" the cylinders - JUST like a turbo or supercharger system does with a piece of equipment. The higher the velocity the more air/fuel you'll pack into the cylinders when the valve opens. Of course, just like exhaust this is a compromise. Too big of runners/induction and you have no velocity thereby only partially filling the cylinders - less then 100% volumetric efficiency - at low RPMs. Too small and you "choke" the engine and again only partially fill the cylinders. Another common misconception is that you can NOT reach or exceed 100% volumetric efficiency - filling the cylinders 100% with the air/fuel charge - in a naturally aspriated system. That's a load of "hooey" since Indy and other race cars have been doing it for 20 years.

We're not changing the intake manifold so the issue of velocity really doesn't come into it. It would if you changed the tube INTO the intake from the sensor/air cleaner assembly. What we have to deal with is the "chokeing" effect of the air cleaner box, and smooting the air flow into the sensor. Of course, accomplishing these two points we ARE actually increasing velocity a tad but mostly its flow QUALITY and to a lesser extent QUANTITY. What we're doing is decreasing the amount of vacume required in the cylinder to draw an equivelent amount of air/fuel in. However by DECREASING the amount of force required we DO actually draw more air/fuel - again, increasing HP and torque.

BUT - Too big of diameter intake runners and air intake WILL DECREASE VELOCITY. And this WILL kill you're low end torque and HP. Up to about 3K-3.5K RPM. Bad since we need it from 0-3.5K RPM.

Now the good thing about the FIPK is it doesn't decrease the velocity much but increased quality and quantiy a lot. More then makes up for the minute decrease in runner velocity. So the improvement is accross the board. STILL more so at the higher "highway" RPM levels of course due to the much larger intake and relief from the air box "chokeing" off the engine but very significant at the lower 1000-3000 RPM leves, too. More then enough to justify the cost of the unit.

The good thing about the exhaust is that @ 2 1/4" it eliminates the stock muffler - deficient for a couple of reasons - removes a bend or two and INCREASES the exhaust velocity at low RPMs - 1000-3000+ - without being a restriction at the higher 4-5000 RPM levels. You could significantly increase upper RPM performance with a 2 1/2" exhaust but low RPM levels would suffer significantly. BAD for a 4-5000lb vehicle since what would suffer would be the torque output.

Now, my system is a compromise. Normally, you see big exhaust, big intake or small exhaust, small intake. But for a VERY good reason I can't do that. My Sport is a daily driver. So it needs to be decent - 0-5500 RPM - on the highway, in town and off road. I could optimize any one of these but the others would suffer a LOT. The FIPK - for good reason - operates best at higher RPM leves. No, operates isn't the right word. The benifits are most apparent at the higher RPM levels because its dealing with something slightly different then the problems the exhaust address - HP. The exhaust is designed to address problems in the low to mid-range RPM levels - building torque. Like I said, its a compromise, but a very good one even if I do say so myself.:-)

Now we get to you question.:-) The computer. Neither of these affect the computer because there are no drastic shifts for the computer to try to adjust to. You read my piece on the exhaust and unpluging the computer.THAT was a drastic shift and that's why I had to reset it. I could have just driven it for two days, and let it do it itself but that would be silly. Like I said above, I monitor the fuel, ignition curve, etc. and the system has responded very very well without any hic-cups.

One of the major problems with computer controled vehicles is making changes that go outside the abilities of the stock software of the stock software to deal with. These two changes are not. I'm sure if I put on a huge exhaust and throttle body and air cleaner and tube and etc. I'd have the exact same problems you have. But, I've been through this repeatedly - from injectors, TB, regulator, etc. Everything on the vehicle is well able to deal with what in reality are rather minor changes.

The snorkel - on the other hand - is slightly different. There, if set up this way, you could actually create a sort of forced induction with it. Whether that would create a problem I don't know because it depends on how MUCH of a forced induction you create. I could very well see problems at high speeds where it'll create the most effect because I can assure you the comptuer is NOT going to adjust the fuel curve fast enough if you're doing short high speed highway jaunts. So, yea, its going to go rich on the highway and then be lean on the street until it readjusts. I KNOW this will happen, just not to what extent. I've delt with muscle cars that have "ram air" induction - which is what a snokel set up that way - would be. And doing air/fuel adjustments on a ram air equipted vehicle is a pain in the butt. From a functionallity standpoint, though, you're right. I've done deep water crossings up to that mottled gold pin striping right below the windows. 3-4.5 ft of water for 30 yards. A snorkel would be nice insurance, but our Sport's air elements - even with the FIPK - sit high enough that if you DO get water to the element you're either doing the crossing wrong or you shouldn't be in there in the first place. And, of course, now mine sits so high that the only way I could drown it is by doing something REALLY stupid. Deep enough water to drown my air filter would actually float the vehicle first - a situation you NEVER EVERY want to get into!!!!!!!! Most probably, I won't be putting a snorkel on any time soon. 'Course, I WANT to do an air box for the FIPK that will suck cold air from outside the engine in the summer so a snorkel at that point would be a trivial addition.

The hubs will probably be the BIGEST improvement you can make. 15 MPG is appaling! Although, I guess I shouldn't be surprised. I just can't for the life of me explain WHY the newer models are so BAD on gas! MINE didn't do that bad the day I bought it! I think it was 21 Hwy and 17-18 town rated. It exceeded that without any mods. Sigh. Hmmm. Well, you can expect between 2-4 MPG from the hubs. That's the range from everyone that's put them on - Monteros and Montero Sports. Seat of the pants power jumps, accordingly, by a significant amount to say the least.:-)

That would be my first mod. Next would be the air filter kit and the exhaust. I should have the exhaust kit ready here shortly. It's a CAT-back system and uses 2 1/4" tube with a Flowmaster muffler. I like the Flowmaster bacause its probably the most indestructable muffler on the planet. Its small compared to the stock 55 gallon drum monstrosity we have. Flows better by far. Its quiet! Not as quiet as stock, but unless you're outside you can't really hear it at all. It's distinctive - AND it doesn't sound like a souped up sewing machine. AND its flat! Which means you get tons of clearance under and around the muffler when its installed. VERY nice off road and also when you're under the vehicle. Oh, yea, and it get rid of that dangly dang tip hanging off the back. Stupid stock design catches on EVERYTHING.

I'd also buget in cost of a stabilizer for the steering. Most people don't like the "loose" feel when on the highway with the hubs unlocked. I didn't. I doesn't cause any problems, but I like a tight steering feel with some resistance in it. It was too much like driving a 2-wheel drive Courier pickup for me. One thing I like about my new cross-over steering. Excellent road feel without being "sloppy" feeling.

Tips on the other stuff. Don't put a rack on if you're going after gas milliage. SUCKS gas. Aerodynamics play a big part in our vehicles - and THAT is a big part. The ARB bumper weighs in about 100lbs. If you lift you're vehicle by turning up the bars you won't be able to put a winch in. The XD9000 comes in at about 88lbs. That's more then the torsion bars will support if they're already being used to lift the vehicle. You can go with the teflon winch rope instead of the steel cable - which I recommend - at 4lbs vs. 25lbs. It'll help but that's still not enough. That was why I mentioned the aluminum bumpers @ about 40lbs. 40lbs + 4lbs + 55-60lbs is considerably less then it would be with the ARB and steel cable. ....