fast vs slow acceleration

[jakobnev]

Originally Posted by Diesel_Dave

Also, when you're in town, fast acceleration generally means you're going to be going faster.

Not if you have the discipline to stop accelerating sooner.

[t vago]

Originally Posted by Diesel_Dave

Right on!

It's been debated back and forth as fas as what rate of acceleration is best. Personaly, I've found it hard to notice a difference either way. What really kills your mileage is unecessary acceleration. Hard acceleration is generally thought of as bad becuase, in many cases, people accelerate hard only to have to brake hard at the stop that's right up ahead. It's really the braking that is wasting all the energy.

Besides that, hard acceleration is bad in some OBDII vehicles because those vehicles' engine computers will go into open loop and perform enrichment of the fuel/air mix to the point where you'll see AFRs of 12:1 or even 11.5:1 (like my truck, for instance). Fuel economy goes right out the window, even if you wouldn't have to brake hard.

[Diesel_Dave]

Originally Posted by jakobnev

Not if you have the discipline to stop accelerating sooner.

Agreed. That's why I said "generally"

[hackish]

Originally Posted by t vago

Besides that, hard acceleration is bad in some OBDII vehicles because those vehicles' engine computers will go into open loop and perform enrichment of the fuel/air mix to the point where you'll see AFRs of 12:1 or even 11.5:1 (like my truck, for instance). Fuel economy goes right out the window, even if you wouldn't have to brake hard.

This is true, although many newer vehicles are staying in closed loop much longer. Some of them will stay closed loop for everything but WOT.

I just did some testing on a 1.9l TDI and actually found that the engine was more efficient under higher load with more fuel than factory. Surprised me but I don't tend to tune a lot of Diesels. Sadly, adding fuel sure didn't help the emissions. The HC and particulate readings were sky high

[Diesel_Dave]

Originally Posted by hackish

This is true, although many newer vehicles are staying in closed loop much longer. Some of them will stay closed loop for everything but WOT.

I just did some testing on a 1.9l TDI and actually found that the engine was more efficient under higher load with more fuel than factory. Surprised me but I don't tend to tune a lot of Diesels. Sadly, adding fuel sure didn't help the emissions. The HC and particulate readings were sky high

That's odd. I've tuned diesels and it's pretty odd for efficiency and PM/HC to both increase (ususally you'll see efficiency & NOx go together). It does happen on occasion, however, if the conditions are just right. Since it's a turbo remember that adding more fuel will alter where you are on the turbo maps. That will change your efficiency as well (maybe good or bad).

[hackish]

Originally Posted by Diesel_Dave

That's odd. I've tuned diesels and it's pretty odd for efficiency and PM/HC to both increase (ususally you'll see efficiency & NOx go together). It does happen on occasion, however, if the conditions are just right. Since it's a turbo remember that adding more fuel will alter where you are on the turbo maps. That will change your efficiency as well (maybe good or bad).

I measured efficiency in amount of fuel consumed per horsepower generated (BSFC really). I usually do this in vehicle simulation mode since you can use an OBD2 scanner to deliver the fuel consumption values on some vehicles. Proper fuel flow meters are expensive and injector pulse is hard to get into the dyno. Emissions results are usually done on a separate 5 gas analyzer that the dyno can't datalog

Adding more fuel should not change the location in the turbo efficiency maps unless there is a corresponding increase in boost pressure or RPM since RPM will increase the number of pounds of air. Increasing the boost will increase the pressure ratio (and subsequently Lb/h of flow). Adding or subtracting fuel will have no impact on this.

[Diesel_Dave]

I guess I was assuming that you were not necessariy holding boost pressure constant. Regardless, if the boost remains constant the mass flow will remain constant, however, the exhaust temperature can change significantly. A change in exhaust temperature means a change in exhaust pressure, therefore the pressure ratio accross the turbine changes, therefore where you are on the turbine map changes (even if where you are on the compressor map hasn't changed).

[Floordford]

Originally Posted by HydroJim

thanks for all the replies! I'll definitely be integrating these driving techniques into my driving style.

Any ideas what kind of car I should get?

• I'm looking at $3000 and under
• 4 cylinder for sure(or 3 if I can find one)
• the transmission doesn't matter but manual would be nice.
• I definitely want cruise control because I'm going to be testing different fuel delivery modifications and the cruise control will allow for consistent testing without human input. I wish I had a dyno in my garage
• the car has to be 96 or newer due to OBD-II. I'm looking to play around with the ECM so OBD-II is a must.

I've looked at the chevy/geo metros/suzuki swifts, but here in cleveland, they're covered in rust. Plus people have seemed to catch on that they get great fuel economy so they are pretty pricey. I don't really want to pay $3000 for a go-kart if you know what i mean.

I've also looked at the Saturn S-series. Probably my number 1 choice at the moment. Good fuel economy, cheap, the body is generally rust free(plastic body panels ) and my dad used to be a mechanic for Saturn so he knows everything about them.

The third car I looked at was the Chevy Prizm. They're basically Toyota Corolla's without the price tag. Same engine and everything! Unfortunately, there are not a whole lot of them around.

I also have looked at Honda Civics or a Toyota Corollas. But, they somehow manage to hold their value forever so I've had a hard time finding one for $3000 that doesn't have 300,000 miles on it

I know I basically covered my choices, but does anyone have any other ideas? Or maybe they know someone who knows someone who is just looking to hook me up with a deal

I hate the way clean bone stock CRX HF blue book for under 3k but people sell them for 5k. But if people pay it I guess thats what the value is. I do recommend you take a look at what others here are driving that get awesome mileage here.

[Sven7]

Good discussion here; I was wondering the same thing!

Jim, have you considered a Mk3 or Mk4 Golf/Jetta TDI? I've heard of them getting 50mpg easily but not sure if they run OBD-II.

[Diesel_Dave]

My understanding is that every (US) vehicle after 1996 is OBDII. I'm not sure about other countries.