Diesel Transmissions

Several diesel threads have got me wondering. Since they rev so low are they somehow geared much higher than a diesel engined car?

It seems as if I read a while back that the VW/Audi cars had the same gearing whether they were gas or diesel.

Regardless of how many torques they are claimed to have, everyone I've driven has seemed so gutless.

They're generally geared about the same, give or take. The RPM range is similar.

Mind you, this isn't a rule, more of a generality. The shortest VW 020 final drive comes from a diesel. 1.6l of nonturbo diesel "goodness" needs all the help it can get for gathering momentum, and it's not like the top speed is very high so who cares if it redlines out at 90 or so? It won't ever actually go that fast anyway.

The idea that diesels make a lot of torque only occurs if you throw a turbo and a bunch of boost at it, but that's true for gasoline engines too. Mainly, they just don't rev very high because of the combustion characteristics of the fuel, so your options are larger engines or heavy boost. (A 1.9l TDI needs 15-17psi boost to make the same power as a naturally aspirated 2-liter gasoline. Of course one is going to have more torque than the other! Now, how much power and torque will the 2-liter make with the same amount of boost?)

Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser. So you don't get the surge like a gas engine. Gearing usually isn't much different either, the diesel just doesn't rev as high so top end is limited. Turbos help with the power, but go drive a big NA diesel (like a Ford 7.3L IDI/pre-Powerstroke) and they have a good amount of torque, especially right off idle. Of course a turbo helps make even more power as well, as the late model diesels prove.

81cpcamaro wrote: Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser.

You mean directly, right? And you are assuming a non-turbocharged application, right?

My turbo diesel most definitely gets a LOT more air when I press the "go pedal". Granted, it's indirect, but it most certainly does get more air.

SVreX wrote:

81cpcamaro wrote: Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser.

You mean directly, right? And you are assuming a non-turbocharged application, right? My turbo diesel most definitely gets a LOT more air when I press the "go pedal". Granted, it's indirect, but it most certainly does get more air.

I think he means you are injecting more fuel, but know opening a valve that allows more air in, and the air pulls fuel in (either directly, or via sensors and computers)? I think I see where he's coming from.

DrBoost wrote:

SVreX wrote:

81cpcamaro wrote: Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser.

You mean directly, right? And you are assuming a non-turbocharged application, right? My turbo diesel most definitely gets a LOT more air when I press the "go pedal". Granted, it's indirect, but it most certainly does get more air.

I think he means you are injecting more fuel, but know opening a valve that allows more air in, and the air pulls fuel in (either directly, or via sensors and computers)? I think I see where he's coming from.

On a gasoline motor, you push the pedal, which opens the throttle. More air goes into the engine, which requires more fuel.

On a turbodiesel motor, you push the pedal, which adds more fuel to the engine, which causes it to rev higher, producing more exhaust gasses that spool the turbo and push more air into the engine.

In other words, gasoline motors are sort of the reverse of diesel motors. With the former, you add air and then fuel. With the latter, you add fuel and then air.

At least, that's how it was explained to me. All that black smoke you get from rednecks hopping up their diesel trucks is because they're dumping a metric crap-ton of more fuel into the motors without an upsided turbo to supply the needed air, spewing unburnt diesel fuel in the form of soot. A properly-tuned diesel does NOT produce a lot of black smoke.

Sky_Render wrote: A properly-tuned diesel does NOT produce a lot of visible black smoke.

FYP. Even the best ever tuned diesel still requires a particulate filter to pass any current standard.

Sky_Render wrote:

DrBoost wrote:

SVreX wrote:

81cpcamaro wrote: Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser.

You mean directly, right? And you are assuming a non-turbocharged application, right? My turbo diesel most definitely gets a LOT more air when I press the "go pedal". Granted, it's indirect, but it most certainly does get more air.

I think he means you are injecting more fuel, but know opening a valve that allows more air in, and the air pulls fuel in (either directly, or via sensors and computers)? I think I see where he's coming from.

On a gasoline motor, you push the pedal, which opens the throttle. More air goes into the engine, which requires more fuel. On a turbodiesel motor, you push the pedal, which adds more fuel to the engine, which causes it to rev higher, producing more exhaust gasses that spool the turbo and push more air into the engine. In other words, gasoline motors are sort of the reverse of diesel motors. With the former, you add air and then fuel. With the latter, you add fuel and then air. At least, that's how it was explained to me. All that black smoke you get from rednecks hopping up their diesel trucks is because they're dumping a metric crap-ton of more fuel into the motors without an upsided turbo to supply the needed air, spewing unburnt diesel fuel in the form of soot. A properly-tuned diesel does NOT produce a lot of black smoke.

even a gas turbo engine will produce a small puff of "Smoke" when going to full boost. At least my Saabs have done it and most Subys and Mitsus seem to

"Even the best ever tuned diesel still requires a particulate filter to pass any current standard."

Yes, that statement is true, but until a "few" years ago, there were next to no emissions standards for diesels.

BTW, in the last two areas I lived in, there were no exhaust ("sniffer-type") tests done on diesel vehicles of any kind...even cars. Gasoline powered cars and trucks? Yes, but not diesel powered cars and trucks.

As far as torque goes, the 2 old Rabbit diesels I drove that belonged to friends and relatives didn't seem all that slow, just that changing gears didn't seem to have any effect. In a gasoline powered car you change up gears because the engine sounds like it's going to fly apart...yet oddly, the diesel with it's much higher compression doesn't feel like it's bothered if you want to go 5 MPH or 55 MPH in first gear. You feel like you are gathering momentum, not speed in a diesel powered car. At least the older ones feel that way.

The other interesting thing I learned about Diesels, when I had an n/a Volvo 245 Diesel, is that (at least IDI) Diesels have an efficiency peak that differs from gasoline engines.

With a gasoline engine, peak fuel efficiency occurs by being in the highest gear that won't lug the engine. Not so in a Diesel. Even though my 245 could be shifted into overdrive at 40 mph (and would feel happy doing so) it actually got better mpg by leaving it in 4th, and revving higher.

alfadriver wrote:

Sky_Render wrote: A properly-tuned diesel does NOT produce a lot of visible black smoke.

FYP. Even the best ever tuned diesel still requires a particulate filter to pass any current standard.

Yes, this is true. But my point is that all the mouth-breathers spewing huge amounts of black smoke out their "stacks" are imbeciles that have improperly-tuned motors. Once you start dumping enough fuel in for 100 additional horsepower, you need to install a larger turbo as well.

Sky_Render wrote:

alfadriver wrote:

Sky_Render wrote: A properly-tuned diesel does NOT produce a lot of visible black smoke.

FYP. Even the best ever tuned diesel still requires a particulate filter to pass any current standard.

Yes, this is true. But my point is that all the mouth-breathers spewing huge amounts of black smoke out their "stacks" are imbeciles that have improperly-tuned motors. Once you start dumping enough fuel in for 100 additional horsepower, you need to install a larger turbo as well.

Certainly can't argue with that.

integraguy wrote: "Even the best ever tuned diesel still requires a particulate filter to pass any current standard." Yes, that statement is true, but until a "few" years ago, there were next to no emissions standards for diesels. BTW, in the last two areas I lived in, there were no exhaust ("sniffer-type") tests done on diesel vehicles of any kind...even cars. Gasoline powered cars and trucks? Yes, but not diesel powered cars and trucks.

Actually, there have been- gas and diesel's share the same emissions standards in the US. It has only been recently that the PM standard went from 50mg/mi to 10mg/mi, and that is scheduled to drop to 3mg/mi in about a decade. Even 50mg/mi is tough without treatment, 10 requires it. Even Europe has finally figured out that PM and gas emissions are very bad- to the point that Euro VI and VII will likely force diesels out of cars that cost less than $30k.

And don't mistake the lack of local testing with the lack of standards- most of the US do not have sniffers nor spot checks on individual cars, yet the entire country is covered some kind of emissions rules.

Sorry to get off topic.

My F-250 is spinning at around 2,000 rpm at 70 mph, so I don't think it's geared much differently from a gas-powered rig.

Sky_Render wrote:

DrBoost wrote:

SVreX wrote:

81cpcamaro wrote: Another thing with diesels is that you are only adding fuel as you push down the go pedal, instead of more air and fuel in a gasser.

You mean directly, right? And you are assuming a non-turbocharged application, right? My turbo diesel most definitely gets a LOT more air when I press the "go pedal". Granted, it's indirect, but it most certainly does get more air.

I think he means you are injecting more fuel, but know opening a valve that allows more air in, and the air pulls fuel in (either directly, or via sensors and computers)? I think I see where he's coming from.

On a gasoline motor, you push the pedal, which opens the throttle. More air goes into the engine, which requires more fuel. On a turbodiesel motor, you push the pedal, which adds more fuel to the engine, which causes it to rev higher, producing more exhaust gasses that spool the turbo and push more air into the engine. In other words, gasoline motors are sort of the reverse of diesel motors. With the former, you add air and then fuel. With the latter, you add fuel and then air. At least, that's how it was explained to me. All that black smoke you get from rednecks hopping up their diesel trucks is because they're dumping a metric crap-ton of more fuel into the motors without an upsided turbo to supply the needed air, spewing unburnt diesel fuel in the form of soot. A properly-tuned diesel does NOT produce a lot of black smoke.

Yep, you all pretty much covered it. On diesels you don't have direct control over airflow (with a throttle valve), just fuel, if that makes sense. It is more pronounced in a NA diesel than a turbo one, the feelling that pushing the pedal down doesn't have as much effect as a gas engine does.

Well all that certainly explains the lack of guts feel of the diesel and why you can use the same geared transmission & just limit top speed along with keeping things simple for the manufacturer. It just seems counterintuitive tho.

I don't know about feeling a lack of guts. Nobody has ever accused my truck of being slow, even when towing a 30' enclosed trailer.

I wouldn't want anything that weighs 6,000 pounds to accelerate any faster thats for damn sure.

Every diesel I've driven has had a lackluster quality to the acceleration. There's been no drama and they definitely haven't felt fast. There was an initial neck movement and then just the same ol, same ol after that.

In other words there was no excitement, more of a ho hum quality to the powerband.

The only diesel truck I've driven while towing was nice, but not really any nicer than my gas powered pick up, but then again it wasn't a huge load and like you said, you don't want to be accelerating too fast while pulling a trailer, things will fly off.

81cpcamaro wrote: Yep, you all pretty much covered it. On diesels you don't have direct control over airflow (with a throttle valve), just fuel, if that makes sense. It is more pronounced in a NA diesel than a turbo one, the feelling that pushing the pedal down doesn't have as much effect as a gas engine does.

This is why you "feed" fuel to a diesel (gradually increase how much you push down the pedal) not mash the pedal and wait for the engine to catch up.

carguy123 wrote: Every diesel I've driven has had a lackluster quality to the acceleration. There's been no drama and they definitely haven't felt fast. There was an initial neck movement and then just the same ol, same ol after that. In other words there was no excitement, more of a ho hum quality to the powerband.

I got a ride in a modified 6.6 F350 that scared me. It probably could accelerate more quickly than it could brake back down, especially after the third 0-90 blast in a row.

This is not indicative of the breed. And the only smoke from it was roiling off of all four rear tires...\

I've driven 6.9 and 7.3 nonturbo Fords that were positively appalling. That is to say, every single one of them. Just no response at all. They're a different kind of engine, they're made to be loaded down like a workhorse, not accelerated rapidly.

They're a different kind of engine, they're made to be loaded down like a workhorse, not accelerated rapidly.

But for simplicity's sake they sure look attractive in a Locost - until you drive a diesel anything and then ask yourself "Is this the sporty performance I'm seeking?"

I was seriously impressed with the mini cooper diesel I rented in the uk last month, I'd bet that there's not a significant difference in acceleration performance between it and the gas turbo cooper s. at least to around 80-90 mph, then it kinda started to fall over a bit, probably more due to the limited rev range (redline was 5k iirc)

Back on topic.

In most traditional RWD applications with separate transmissions and final drives, the transmissions were identical to the gas counterparts with a different torque converter made to stall at a lower RPM in automatic applications. The final drives were changed out, to a lower numerical ratio for cruising or a higher ratio for towing.

My Dodge had 3.55 final drives, not many of those used in the 2500 gassers I would imagine.

Most modern diesel trucks (2005 or so and up) have a special transmission made to handle the power of the diesel engine. Duramax/Allison, Ford has their 5R100 or whatever they get nowadays, and Dodge has the 48RE or again, whatever its called. Usually these transmissions are only offered with the diesel or the big block gas engines (V10s, 8.1s)

carguy123 wrote: Every diesel I've driven has had a lackluster quality to the acceleration. There's been no drama and they definitely haven't felt fast. There was an initial neck movement and then just the same ol, same ol after that.

which diesels have you driven? anything recent?