Rev-hang revisited - fix may be available soon

[stevenj]

Hi, I am a new member, automotive engineer by profession.
I read a lot about rev-hang when engine speed stays high for seconds after throttle pedal was returned to idle after acceleration. This makes smooth gear shifting difficult/impossible. I also read various advice ranging from rare good technical insights to frequently wrong conclusions and stupid explanations (e.g. heavy flywheel. limiting pressure in intake/crankcase, or wrong driving style)
I test cars for living and am very familiar with the rev-hang in EFI cars with FBW throttle (before those, dashpots were used on carby cars for the same reason - exhaust pollution control). The dashpots could be easily removed but these days the rev-hang is programmed in engine control computers. Car manufacturers are forced to do it by the Clean Air regulations even when they know very well that drivability of their cars is sometimes severely affected. In short, when throttle is closed abruptly, engines produce increased amounts of NOx (oxides of nitrogen) above legislated limits (I have seen test result proving this). Car makers’ universal solution is to force closing the throttle slowly by ECU signals, regardless of the throttle pedal being at idle position, logically demanding no fuel.
There are 2 possible fixes. For normal car owner, like you and me, the first one is neat, expensive and unrealistic - reprogram the ECU. The second one is more readily achievable, definitely cheaper but not for everyone as it will call for some technical expertise, as I will explain shortly.
After years of being annoyed by the rev-hang in manual transmission cars (if fact I avoided buying some models, like 2009 Subaru Forester, just because of this " fault" and kept buying automatics instead) and encouraged by the general dislike of it expressed in Internet discussions, I started building a small microprocessor controlled box. It will connect to a number of existing sensors and control fuel injectors in a suitable manner to kill the rev-hang. My proposed solution will be universal, suitable for all makes and models of petrol cars and readily transferable from one car to another. This universal approach has a significant drawback in that owners of various model cars will be left to figure out their own connections. I have no hope of knowing where various sensor wires are located in hundreds of makes and models and can only provide instructions as to what type of sensor will need to be connected, like RPM, Speed sensor and others. At present I am at the "proof-of-concept" stage and once the box is finished, my target selling price will be about $300, if everything works OK.
Any interest?
In order to avoid making future long posts like this one, I intend to put a more technical document on the Web and wonder if I would be allowed to include a link to it on this board?

[ClutchFork]

Some thoughts: I guess my first question is, if the rev hang is an unavoidable offshoot of anti-pollution regulations, then will a device that eliminates rev hang, result in increased pollution? If so, then the device might be considered in the same legal category as straight pipes installed instead of a catalytic converter. Now, if you can eliminate or at least tone down rev hang without affecting pollution control, then you would have something quite worth while. Many folks have to get emissions tests and will need to pass. So if your device is worthwhile, it would ultimately be best if sold direct to the automobile industry so that we get the cars off the assembly line with no rev hang. But selling to the auto companies is going to be extremely difficult. If you sell to the consumer, it seems that you really need a wiring connector that an existing connector on the car plugs into so that the existing connector sandwiches your added connector. Then it would be plug and go. For example, I had an electonic tranny connector like that once for my motorhome (think it was Banks' TransCommand). I did have to pull one pin out and replace it with some other wire on their connector and mount their electonic box, but it was easy.

[theholycow]

Hello and welcome.

Can you explain further about rev hang existing to reduce NOx emissions? If we accept the premise that closing the throttle slowly helps, which is easy enough to believe, why would they instead control RPM to stay exactly the same, instead of closing the throttle slowly (but, perhaps, steadily)? Instead, it seems that with rev hang the throttle immediately snaps closed to the point necessary to hold the exact RPM, then stays right there for seconds at a time (or even longer) rather than slowly closing. Then when rev hang ends, the throttle snaps fully closed and RPM drops at a natural rate.

As for the product, it's an interesting idea but why would someone spend $300 on it instead of the safer, easier, and more advantageous programming? I think it was $250 for the VW I had (whose rev hang was absolutely horrific) to be done by a reputable, well-reviewed company. It's the same or less for other vehicles whose prices I've seen.

[ClutchFork]

Also, isn't it the last bit of throttle closing that counts for the pollution reduction, so that you could have it close about 85 to 90 percent, then go slowly. I think that was what the old dashpots did. So if that were the case, rev hang would not be a nuissance as it would close to near idle right away.

(I was once told the dashpot was to prevent the throttle from snapping entirely shut so suddenly that the engine would stall--presumably total misinformation anyway.)

[tankinbeans]

The only problem I've ever really had was throttle delay. This product sounds like it could be interesting though, even if not necessary for my car.

[Shadow]

There are 2 possible fixes. For normal car owner, like you and me, the first one is neat, expensive and unrealistic - reprogram the ECU.

Lots of people buy "canned" tunes to rid their manual transmission vehicles of rev hang. This is a cheap and easy alternative to "custom" tunes. Of course either tune usually includes other benefits, such as sharper throttle response, removal of the speed governor, and usually a modest gain in HP (unless of course it's a forced induction car, where the HP gain will be very significant). For that reason, I doubt there will be much of a market for a little "black box" that can only remove rev hang. Another drawback is the necessity to wire it in or piggyback it to the PCM/ECU.

[Shadow]

The only problem I've ever really had was throttle delay. This product sounds like it could be interesting though, even if not necessary for my car.

They already make a little black box to adjust your throttle response. Look here:

http://www.sprintbooster.com

[Shadow]

Some thoughts: I guess my first question is, if the rev hang is an unavoidable offshoot of anti-pollution regulations....

It's not. If that was the case, then every new car with a manual transmission would have rev hang. But that begs the question: If some cars can meet or exceed modern pollution standards without exhibiting rev hang, then why can't all cars? I don't think that's a question that anyone can answer. Maybe some companies just spend more time calibrating the factory tune. I dunno.....

[stevenj]

Thanks for the interest in this topic. Engine speed is controlled by the supply if air/fuel mixture to it. The throttle valve controls the air and fuel injector pulse controls the petrol. ECU has tables to keep the two in sync to maintain the right mixture for various engine operating regimes (full load, partial load, no load, overrun, etc.). Suddenly leaning the mixture too much makes the NOx go high. Extending the duration of injector pulse avoids leaning the mixture too much to cause problems at the cost of keeping the rpm higher than they should be.

The concern about maintaining emission standards is a valid one. I live and work in Australia and we have so called Australian Design Rules (ADR) for new cars. The full ADR test procedure is complex and expensive (like $2000) and I had it done twice at my own expense for some other inventions. In-service testing is nowhere as demanding. In fact, we do not have any compulsory emission in-service testing as yet. I doubt very much that any in-service test anywhere in the world would approach the stringency of the manufacturers’ testing when proving compliance with emission standards. Comparison with removal of catalytic converter is incorrect as it is very obvious how that would change the exhaust pollution. Any adverse effect of my proposed solution would be minimal and undetectable by the current in-service testing methods. In addition, the box will be removable, if need be, with minimal changes to the car original setup. I located a supplier of injector connectors for all major makes, so my connection to injectors will definitely be by means of a 3 inch long extension male/female harness between the injector and original harness (true plug-in). Unfortunately, engine sensors cannot be so treated, as I explained earlier.
Correctly changing ECU programming is a complicated business because car manufacturers keep the program listings to themselves. As far as I understand, ECU program modifications are sort of black art unless done with the manufacturers’ help. Personally, I would avoid them out of concern about unwanted side-effects, but this is not to say that people providing this service (I know nothing about US situation) are professionals who know exactly what they are doing.
My cost estimate is approximate as I have not finished building the box yet. It will not be just a few resistors but a proper micro-controller. Electronic parts are relatively cheap but the programming will cost money and I will have it done by professionals. The box will be transferable, unlike ECU re-flash, so when you change a car, you keep the box. In future I cannot see this unwanted “feature” of rev-hang being easily eliminated by car manufacturers when their products have to maintain ever more stringent emission standards.

I did not know about the SprintBooster, looked at their Website and did not find any English comments after 2009. Whist it may sharpen throttle response, by the way it is connected I cannot imagine it would have any effect on the rev-hang.

[Shadow]

I did not know about the SprintBooster, looked at their Website and did not find any English comments after 2009. Whist it may sharpen throttle response, by the way it is connected I cannot imagine it would have any effect on the rev-hang.

The Sprint Booster does not eliminate rev hang. It only adjusts throttle response.

[ClutchFork]

At the expense of appearing ignorant on the subject (I am), the prospective red neck fix would be to install a real throttle cable, a retro-retrofit. But I suppose with all the electronic baggage on cars these days that would not be a simple project.

I am thankful that I don't deal with rev hang as my Ranger has a throttle cable.

[Rope-Pusher]

Revs can hang even if you don't have electronic throttle control. The idle speed hasn't been controlled by the throttle plate position for a number of beers now.

[ClutchFork]

Revs can hang even if you don't have electronic throttle control. The idle speed hasn't been controlled by the throttle plate position for a number of beers now.

That is true. When I got my 1995 F150 (used in 1999) it idled at 1000 rpm. I discovered there was no way to mechanically adjust the idle speed. Still, that is all in the low/fast idle range I would think, so should not be a nuisance. But if it hangs at say 3000 rpm on an up shift, that would be more of a problem.

[theholycow]

Also, isn't it the last bit of throttle closing that counts for the pollution reduction, so that you could have it close about 85 to 90 percent, then go slowly. I think that was what the old dashpots did. So if that were the case, rev hang would not be a nuissance as it would close to near idle right away.

It doesn't take much throttle to keep RPM high when declutched/neutral. Closing in 90% would leave plenty of throttle opening to keep it revved way up.

At the expense of appearing ignorant on the subject (I am), the prospective red neck fix would be to install a real throttle cable, a retro-retrofit. But I suppose with all the electronic baggage on cars these days that would not be a simple project.

That would be a major pain in the butt project. You'd have to reprogram the computer much more than what a professional tune does, to start with.

If you weren't bothered by a constant Check Engine Light you could add a small bypass circuit that cuts power to the throttle actuators, or gives them the appropriate PWM signal for idle, when the accelerator pedal reaches 0%. In fact, the consequences would be less than the proposed fuel injector control box (which would wreak havoc almost as bad as the throttle cable idea). The only major problem I see (besides the CEL which is annoying and would make a car fail inspection in places with inspections) would be fail-safe mode that the computer chooses when the throttle doesn't react as expected. For that, you could also fake the throttle position sensor.

[AHTOXA]

In my experience ecu flashes are not expensive nor complicated. Many companies offer it and in case of VW rev hang was mostly eliminated after ecu flashes.