Thread: Exhaust Options - 300 Tdi

Originally Posted by marko66

Hi All

Rovercare asked me to post again so I will

Rovercare said you don't lean out a diesel but you can over fuel it.
Bush 65 said that your diesel engine will love more the leaner you run it

I will agree that for small simple upgrades the chances are that the original fuel parameters and boost compensator will cope - probably wont be ideal but will cope.

So if you can't lean out a diesel we get a diesel engine say a 300 tdi and compound boost it to 65psi and leave the pump standard and while money is no object and flights of fantasy are catered for we make a new dual 3 inch intake manifold and dual four inch exhaust and because you cant lean out a diesel all cylinder head temps and exhaust gas temps will be perfect I believe not.

Regards Mark

Via PM

Originally Posted by rovercare

Originally Posted by marko66

Hi Rovercare

Didn't want to rebutt you publicly and i agree that overfueling will do all the things that you said and would also say that people not checking their injectors is one of the biggest problems.

But the way that your post could be construed is that someone could seriously boost a non turbo motor or seriously uprate the power of an already turbo motor and no tuning is required?

Regards Mark

But putting forward your rebuttle on there is what a public forum is all about

Correct, they can, all that is required is an Exhaust gas temperature gauge and some common sense, followed closely by a boost gauge helps along way, worlds apart from a petrol engine

The fact is, the temps will likely be cooler as you'll be forcing more air in, without any extra fuel to burn, how does a fire work without fuel? can;t get more heat without more fuel..............Oh, how do you propose to obtain 65psi, from the standard pump settings?, impossible

Ditto, no fuel, no boost.

Keeping the EGT's at an acceptable level is the most important thing, and this is infinitely variable due to the complete engine package you are working with IE some engines are more efficient than others in their initial design, these will make for good upgrades BUT they may not have the % gains of say a less efficient standard engine brought up to a decent output.

Leaning out a diesel engine will just cause it to run cooler,(Good for it) and use less fuel(Good for you) but it'll be a LOT slower on hills.

JC

Originally Posted by marko66

...

If fitting bigger turbo and exhaust then it might be a good idea to get the injection pump tuned to the new mechanicals as it may be possible to lean the motor out.

Regards Mark

Mark, your earlier post (above quote) reads to me as though you are advising Jock to have the IP tuned on the basis that by fitting a larger turbo and exhaust system that he is in danger of the air/fuel ratio becoming lean.

I responded with a post, close enough (but exactly) to what you quoted in your 2nd post (quoted below).

To explain my post:
Many people make similar remarks/warnings about lean air/fuel ratio with diesels. This comes from a misconception from trying to apply petrol engine experience to diesel engines.

A petrol engine must have air and fuel very close to the stoichiometric ratio to run, and can be damaged if too lean. the stoichiometric ratio is approximately 14.7: air to fuel (the figure after the decimal mark may be slightly out - just my memory).

With a petrol engine, the accelerator pedal operates a throttle that increases or reduces the amount of air that the engine can get (to go faster or slower). The carburation or fuel injection system then closely meter the fuel to maintain the acceptable air/fuel ratio.

A diesel engine however runs much leaner than than stoichiometric ratio. The air fuel ratio that causes black smoke (incomplete combustion) is approximately 18:1 (air to fuel), and it is best that it be not less than 20 or 22:1. This is at full load, when maximum fuel is injected.

A diesel engine does not have a throttle - except newer computer controlled diesels which can have a throttle, which is part of the emissions control/egr system. Effectively it is as though the engine always has a wide open throttle - no different at idle through to maximum speed and load.

The accelerator pedal controls, via the governor, the amount of fuel injected by the fuel injection pump (or injection system). When the pedal is depressed more fuel will be injected to make more power. The older mechanical fuel injection pump doesn't know how much air the engine is getting.

At idle, the air/fuel ratio is very high - it can exceed 120:1

The higher the air/fuel ratio, lean the motor out (to use your expression), the cooler the exhaust gas temperature will be. When the ratio gets closer to the smoke ratio (18:1) the exhaust gas temperature starts to get high, very quickly. A diesel engine can not endure exhaust gas temperature anywhere near as high as a petrol engine.

The excess air from higher air/fuel ratios, also removes a considerable amount of heat from the engine.

If a diesel is working hard and blowing black smoke for much time, it can be close to the limits of the materials used in the pistons and turbine. Changing to a lower gear will quickly bring the exhaust gas temperatures down, because the engine will get more air, due to higher rpm, but at the same time the injection system will reduce the amount of fuel injected, because the governor will sense the reduction in load - in other words, lean the motor out (your expression).

Originally Posted by marko66

Hi All

Rovercare asked me to post again so I will

Rovercare said you don't lean out a diesel but you can over fuel it.
Bush 65 said that your diesel engine will love more the leaner you run it

I will agree that for small simple upgrades the chances are that the original fuel parameters and boost compensator will cope - probably wont be ideal but will cope.

So if you can't lean out a diesel we get a diesel engine say a 300 tdi and compound boost it to 65psi and leave the pump standard and while money is no object and flights of fantasy are catered for we make a new dual 3 inch intake manifold and dual four inch exhaust and because you cant lean out a diesel all cylinder head temps and exhaust gas temps will be perfect I believe not.

Regards Mark

There is no doubt whatsoever that the original fuel parameters and boost compensator will cope.

A diesel engine does not require a boost compensator on the fuel injection pump. The boost compensator is there for emission reasons.

The function of the boost compensator is to reduce the amount of fuel that is injected if the boost pressure is low. If the boost pressure is low, the engine will not be getting as much air flow and the boost compensator will then reduce the amount of fuel (to lean the motor out) reducing emission of particulate matter (black smoke).

The higher the boost pressure providing the compressor (or compressors) are operating at a reasonable efficiency value, the more air that is provided to the engine and the lower the exhaust gas temperature.

Unnecessarily high boost pressure is a waste of energy. The engine pumping losses increase, and the engine will be operating away from peak efficiency. The engine efficiency increases with temperature and similarly for the turbo with exhaust gas temperature. These temps need to be high, but not exceed safe limits for the component materials.

The necessity to meet reduced emissions requires improvements in efficiencies. The latest crop of diesel engines are using staged (compound) turbos for increased boost pressure. Turbo manufactures are using better materials and investing in research to allow the turbo to operate with higher exhaust gas temperatures.

Originally Posted by C H T

Just a quick note - don't be tempted to for too large an exhaust - if the diameter is too large for the engine total gas velocity will be reduced and therefore negatively affect engine performance. i have been advised that optimum for a 300Tdi is 2.5 inch mandrel bent. (if you use an exhaust with ordinary bends that crimp the pipe you might want to try 2.75 inch diameter pipe. When Beaudesert exhaust built the system on my 3.9 Isuzu turbo Defender they specified 2.75 inch, which has worked brilliantly. (Beaudesert explained that if the pipe is too big for the engine the effect is akin to passing the gas through a 44 gallon drum gas in = gas out but without any scavenge effect and at recduced velocity)

C H T

There is no such thing as a "scavenge" effect on a turbo diesel. I think too many exhaust manufacturers and tuners come from a petrol background and think the same principles apply. (however - as an aside - have a look at the exhaust size on a 2.0L turbo WRC car...)

I agree with John's(Bush65) posts above that bigger is better wrt turbo diesel exhausts. Ignoring legalities, just the dump pipe alone and no exhaust would be optimal in terms of maximising engine output. Like this:

At the same time outright power isn't what we need on a road vehicle,if you had that drag car engine in a road car it would be undriveable,imagine trying to park.There isn't a right and wrong with road cars as they must be useable under all conditions so the exhaust that gives the best off-idle and midrange is the best pick.On a few of the brit sites they go with the 2 1/2'' mandrel on a stock Tdi but Jocks has a VNT with bigger outlet so needs a bigger pipe to suit. Pat

Originally Posted by PAT303

At the same time outright power isn't what we need on a road vehicle,if you had that drag car engine in a road car it would be undriveable,imagine trying to park.There isn't a right and wrong with road cars as they must be useable under all conditions so the exhaust that gives the best off-idle and midrange is the best pick.On a few of the brit sites they go with the 2 1/2'' mandrel on a stock Tdi but Jocks has a VNT with bigger outlet so needs a bigger pipe to suit. Pat

Where is the evidence that exhaust size has any affect on low speed driveability??? I think you are also confusing petrol engines with diesels.

I am sure that the difference between a 2" and a 3" exhaust would make no noticeable in low speed driveability.

When I drove my 110 to the exhaust shop with no dump pipe or exhaust connected to the turbo it was perfectly driveable across the whole rev range.

Yeh the exhaust is just for decoration,it doesn't do anything usefull like having a bearing on engine performance. Pat

I have a straight through system on my 300tdi . No mufflers at all ! bit noisy but spools up quicker