Best air fuel ratio for economy & power

[Jaymz]

Guys ive done some searching around and i'm struggling to find the right answers.
I have spent alot of time around performance petrol engined cars and have also had my hand in tuning a few on a dyno. Getting the best performance & economy out of an engine normally will see AFR's around 12:1 to 12.2:1

I know you can lean on a TDi a little harder and can run leaner mixtures but who has had some experience tuning TDi's and what is the best AFR's to aim for?

[gldgti]

Guys ive done some searching around and i'm struggling to find the right answers.
I have spent alot of time around performance petrol engined cars and have also had my hand in tuning a few on a dyno. Getting the best performance & economy out of an engine normally will see AFR's around 12:1 to 12.2:1

I know you can lean on a TDi a little harder and can run leaner mixtures but who has had some experience tuning TDi's and what is the best AFR's to aim for?

hmmm.

AFR in a diesel is a somewhat arbitrary thing...

a diesel is unthrottled... you run as much air as possible at all times. you add what fuel you need to keep the engine running as fast as you require. when you run out of air for the fuel to consume, you get black smoke being produced.

the only thing that AFR really affects with diesel is the exhaust gas temperature.

maximum torque in a diesel is reached just past the black smoke limit - when it is producing a little smoke. the torque will not drop off considerably if you are producing more smoke than a little bit, and unless you are producing a little, you are not at max torque.

so the basic equation is air+fuel=power. too much fuel = melted pistons. too much air = wasted energy getting it in there.

to illustrate - you can add a supercharger to a diesel and it will slow the idle right down, because you dont get any more power even though theres more air going in. likewise, if you have that supercharger there, you can drive it around all day every day with no extra fuel, and the engine wont mind at all. nor will you get any more power.

my AAZ mk3 1.9td will make 4-5 psi boost going downhill, off throttle, in any gear once your above 3000rpm. it makes no difference at all.

its not really accurate to use "mixture" in the same sense as one does with a petrol engine. in the case of diesel, your accelerator pedal is the mixture adjustement, and thats it. there is no throttle. so when you are idling, or cruising, the "mixture" is lean.... very lean. when you accelerate, it is rich.... very rich!

[gregozedobe]

.....so the basic equation is air+fuel=power. too much fuel = melted pistons.

in the case of diesel, your accelerator pedal is the mixture adjustement, and thats it. there is no throttle. so when you are idling, or cruising, the "mixture" is lean.... very lean. when you accelerate, it is rich.... very rich!

Good post, GG.

When I added a turbo to my Landcruiser I was told to fit an exhaust gas temperature guage (sometimes called a pyrometer or EGT gauge), and to watch it when working hard (eg towing a heavy trailer up a long hill). If it started to get over 1100°F then I either had to back off the accelerator (reducing the fuel, thus reducing the temperature), or change down a gear. This allowed me to run a higher state of tune (ie more torque and power), in return for me being responsible for not cooking my engine.

Many of the turbo diesel tuners like to run at least a boost gauge and an EGT gauge so they can monitor what is going on in their engine.

Popular power boosts seem to start with injection timing, duration and fuel quantity (typically with larger injectors), increased boost (sometimes via higher capacity turbos), and better intercoolers (lower temp air going in to the motor). On earlier TDIs this was mainly mechanical mods, but on the latest TDIs most people start out with a remap of the engine's ECU.

One thing about modding TDIs that is quite different to petrol motors is that TDIs don't respond very well to getting more power via increased revs. This tends to lead to the more heavily modded TDIs having a somewhat limited power bands (eg have to wait for the turbo to get onto full boost at say 2000rpm, but hitting the power peak by say 4500rpm).

If anyone wants to spend several 100 hours reading about performance mods for TDIs, this link will keep you amused for a while

http://forums.tdiclub.com/forumdisplay.php?f=13

[Jaymz]

Thanks guys, now we're getting somewhere. Great posts.

So tell me then, with the latest generation diesels we have here (the PD series) can you control the fuel and the timing separately?

Also with the latest generation cars with a DPF, correct me if i'm wrong but producing black smoke to find the max torque may be a little difficult? So what is another way to optimise the tuning and making sure i don't have a melt down?

Cheers,
Jamie

[Jaymz]

If anyone wants to spend several 100 hours reading about performance mods for TDIs, this link will keep you amused for a while
http://forums.tdiclub.com/forumdisplay.php?f=13

Have done this, excellent web! Very few on there with a PD170 though.

Cheers

[Mischa]

Also with the latest generation cars with a DPF, correct me if i'm wrong but producing black smoke to find the max torque may be a little difficult? So what is another way to optimise the tuning and making sure i don't have a melt down?

Cheers,
Jamie

cut it out, thats what im gonna do eventually if i ever start modding my daily tdi.

[gldgti]

if your modding for serious power over stock, then the DPF will have to go anyway as a matter of course - it provides too much restriction as far as EGT's are concerned.

maybe you could run "high flow DPF's"???? hahahahahhahaha

straight pipes all the way!