Performance improvement 101 Breathing

[David Lang]

One of the simplest ways to improve the performance of your Diesel Scout is to improve the flow in and out of the engine. In addition any other performance improvements you make will be limited unless the existing bottlenecks in the intake and exhaust aren't addressed. Diesel engines are even more sensitive then their gasoline counterparts to limitations in this area (not being an engineer I am not sure why, but I suspect that the high compression ratios are a large part of the reason)

In addition to improve the airflow are some of the cheapest modifications to make (in moderation) and as such are good things to consider for new owners.

Starting at the intake there are two cheap and simple changes that make a very noticable improvement to the truck.

First look at the air filter housing, you will notice that it has a rather small inlet pipe, actually smaller then the tube exiting the filter housing. This drasticly limits the amount of air that can get to the filter. The good news is that this is very simple to fix. remove the cover and filter, plug the intake with a rag, and attack the sides of the housing with a hole saw (1 1/2" to 2" is a good size). Leave enough of the housing to hold itself togeather and hold the top on firmly (remember the air filter is sandwiched between the cover and the bottom of the housing, you don't want dust, etc to get past the filter). Make sure you clean out all shavings prior to reinstalling the air filter and starting the engine.

Second, drive down to your local parts store and order a high flow air filter (K&N or equivalent, ~$50, the part numbers are in the sticky thread). you will already notice a difference in the truck and when the filter arrives (it's a scout so of course they won't have one in stock) put it in and you will notice another significant impovement.

Now that air can get into the engine we can turn our attention to getting the exhaust gasses out of the engine.

For this section I will specificly refer to the turbocharged engine because that's what I have, the same basic ideas will apply to the normally aspirated engines, but I can't cover the specifics of them.

The SD33T has a cast iron fitting attaching to the turbocharger that is one of the worst designed pieces of exhaust ever fielded. It forces the exhaust of the turbo to make two 90 degree turns within a few inches, and in the process reduces from the 2 1/2" opening on the turbocharger to a 2" pipe (see attached photo)


In addition the standard exhaust pipe has a crimp in it after crossing the axle (I guess to make it fit the clamp more effectivly)



These seriously affect the exhaust flow and anything that you can do to improve this will definitely help.

Sammy is working on an adapter that will connect to the turbo and allow you to hook a 3" pipe to it and make any bends in the pipe, I couldn't wait for his testing to finish and lucked out in finding an alternative (not quite as good, but still a significant improvement over stock)


This is a 2 1/2" pipe with ~6" radius that bolts to the turbo and then has a standard three bolt header flange on the other end with a 2 1/2" pipe running from that. compare it to stock.


The tricky part in fabricating a new larger exhaust is getting past the starter. The pipe shown above angles out and then goes straight down beside the starter with ~1/2" to spare before heading to the rear. I understand that this is the area Sammy is having the most trouble with as well with his 3" exhaust.

After clearing the starter you have a fundamental decision to make. Do you put in a muffler or not?

The turbocharger breaks up the impulse noise from the engine so you can get away without any muffler at all, the truck will be significantly louder then it was to begin with, but still not as loud as many V8 powered rigs you hear. The most common comment I have heard about my rig since putting in the new 2 1/2" exhaust without a muffler is that 'it sounds like a real truck', including the ability to hear the turbo whine. If you do decide to put a muffler on make sure you get a high flow version.

I was able to pick up the first few feet of the exhaust system for ~$50 (the part shown in the picture above) and then went down to Pep Boys and spent ~$50 for various bits and pieces to complete the exhaust (I welded the pieces togeather myself, put in a new hanger to hold it in the rear, and clamped to the piece coming down from the turbo so that it would be easy to take out in the future) for a total cost of ~$100.

If I was doing it again (and knowing what pieces I would need rather then designing on the fly) I would have spent a few more bucks and ordered mandrel bent pieces online to improve the airflow even more. From what I was seeing from the links provided on this board (and I hope people can provide those links again in this thread) it would have cost me ~$120 to go with mandrel bent 3" tubing (with an adapter from the 2 1/2" piece that clears the starter) instead of the $50 I spent for the parts. Someday I'll probably redo this job and do it right (when I have some spare time, right )

When I asked local muffler shops to price the job I was given several estimates of ~$300 (southern California, you can probably do FAR better elsewhere)

This finishes the simple/cheap modifications to the intake/exhaust system, but is nowhere near the end if you have the time, money, skill, and desire to go further.

Other modifications that have been discussed here on the board include

- Adding an air-air intercooler between the turbo compressor and the intake manifold to cool the intake allowing the engine pull in more oxygen (cooler air is denser, so more oxygen at the same preasure). Some people have discussed using other intake manifolds to make this easier (IIRC a ford 6 cyl manifold of some sort will bolt up but the intake pipes cross over the top of the engine so you can pipe from the turbo to the intercooler to the intake without having to cross back over the engine with your intercooler plumbing)

- Moving the air intake to pull air from the cowl area (Pressure builds up there so you have a double advantage that you get denser air into the engine and by pulling air from the high pressure area you improve the aerodynamics of the truck as well)

- Get more turbo boost (either a larger turbocharger or tricking your existing one into producing more)

- Add propane to the intake air (Ok, this isn't strictly an airflow modification, but that's where it gets hooked up). Depending on who you listen to adding propane to a diesel improves power and milage as well as reducing soot and wear on the engine becouse it acts as a catalyst causing the diesel to burn more completely, or because it burns slower then the diesel, keeping the combustion process going in the cylinder longer and burning more of the diesel in the process.

- Port your head/manifolds. This consists of CAREFULLY grinding in the ports on your head/manifold to clean up anything that would limit airflow and open them up to allow more volume of air to go through. If you aren't careful you can easily ruin the head by grinding away to much and weakening it (or grind through into the water jacket)

As you consider these more advanced modifications think through them carefully.

The intercooler should be safe to do, it adds air to the engine without raising the preasure. The problem is running the 2" (or larger) air lines around the engine compartment. If they pick up to much heat from the engine you have defeated your purpose.

Any of the ideas that boost your intake pressure need to be considered carefully. If you increase the pressure too much you will damage your engine. Remember that with a 21:1 compression ration a 1 psi boost of the intake air translates to a 21 psi boost just prior to ignition (and if you arrange to get more fuel into the engine as well to take advantage of the extra oxygen this means your combustion will be significantly stronger as well). I don't know what the upper limits of these engines are, but remember they are 20 year old engines so while they were designed with a significant saftey margin, some of it has been worn off over time.

Propane seems to be a win-win (but expensive) option. It promises improved milage (20-40% although some of that will be countered by the cost of the propane), improved horsepower (30-60%), improved torque (30-60%), cleaner oil, cleaner exhaust, without increased risk to the engine. The added horsepower and torque are gained by the combustion lasting longer and being more complete so the peak pressures in the engine don't go up (and some claim may even go down slightly). The biggest drawbacks to propane are the cost (~$1000 with tank for commercial kits) and the difficulty in finding a safe place to install the tank (I'm planning to rig mine up to be able to operate off the propane bottles designed for portable grills as well as the large tank. Use the large tank for normal driving, then remove it and go to the small bottles when I'm planning to go somewhere that would risk the large tank getting punctured)

One thing you can (and should) do to protect yourself as you modify your truck is to get a pyrometer (exhaust gas temperature gauge). They cost ~$200-$250 but are invaluble in learning how hard your engine is really working. If you get one installed before you start your modifications (recognising that you will have to reinstall the sensor if you replace your exhaust) you will be able to get familiar with how your engine operates normally and keep it from getting too far out of this range when you have modified it (driving by the pryo will probably also improve your mileage as well)

[Eddie]

Awesome summation, Lang! Too bad we're basically the only ones on this planet who appreciate it!


1. SBC guys have had the heat-soaked starter problem for years, and I gather it's a problem when eliminating that stoopid double-elbow reducer thingy. Developed with the SBCw/header crowd is this piece: http: //www.summitracing.com P/N DEI-010402. This is a strap-on starter-wrap radiant-heat blanket that runs around $30.00 and actually works

2. Without getting deeply into it, small-diameter pipe produces higher gas velocities but lowers the volume of gas that can pass through it; the reverse is true, also.

Truck engines normally have smaller diameter intake and exhaust tracts, as the higher velocities promote better cylinder-filling at low RPMs, i.e., better torque low down but obviously limits high RPM performance.

Notice I said "intake and exhaust tract". I am defining "Tract" as as total length, valve seat-to-plenum on the intake side, valve seat-to-collector on the exhaust side.

Because of the torque-producing nature of the constant-pressure Diesel cycle, we want to enhance the off-idle low-end efficiency as much as possible, which means maximum cylinder filling at low RPM/low valve lift. Therefore, any porting work done has to be oriented toward minimal enlargement and maximum velocity, which will come through flow enhancement and intake ram tuning, and long-tube tri-Y design headers for N/A 33s.

Dr. Phillip H. Smith, Ph. D., was the seminal thinker in the field of "Scientific Design of Exhaust and Intake Systems" and another important thinker was Sir Harry Ricardo in "The High-Speed Internal Combustion Engine". Everyone should read those two books (if you can stay awake though 'em!)

(Ricardo designed and licensed the swirl-chamber pre-combustion design we're running.)

Smith basically states that long, small diameter tracts promote better cylinder-filling and emptying, and the longer the tract, the lower the RPM at which the tract is most effective. Modern engine designers have taken this to heart on the intake side--look at any Tuned Port Injection inline-cylinder naturally-aspirated EFI engine, gas or diesel, and you will see some ramtune runners of all lengths.

My present pickup has an EFI gasoline engine with an intake of this type (pretty nice factory cast-iron/dual down-tube header, too), and makes lots of torque just off idle and runs out of breath promptly at 4,000 RPM--gee, sound like another engine we all like?

The intake on this pickup is as you describe, looping over the valve cover to the opposite side of the motor--obviously, the only way they could sandwich in a 16-inch intake manifold runner length!

I plan on scaling this design down and making the manifold from mandrel-bent header components, probably 1.250" or 1.375" diameter tubing, 3.00" pipe for the plenum.

The fact that the plenum is now placed in an optimum position for a crossflow intercooler is a lucky bonus; the intercooler could just as easily have been a U-flow unit.

I've asked myself for years why a ram manifold of this type wouldn't be EXTREMELY effective in a turbocharged system--the ram effect would be invaluable in off-boost and free-wheel conditions, and mitigate turbo lag. Obviously, Mercedes-Benz feels the same way 'cause they're using such a system in their current production diesel cars.

Turbocharger turbines are primarily driven by heat, by the expansion of exhaust gas. That said, the stock cast-iron manifold should be just fine with some port-matching and flow cleanup.

Same for the cylinder head ports-put a nice, smooth finish on 'em, don't touch the short turn, lose a bunch of guide boss, and match the bowl to the valve seat.

I'm hoping for some big numbers (relatively speaking) from the heat retention abilities of heat-barrier ceramic coating.

Notice that all of my propositions are for increasing efficiency, not RPM. Notice that a HUGE amount of million-mile tractor-trailer rigs use many of these same principles.

Notice that I have too much time on my hands.

Notice that I'm outta here.