Rev hang - a curse or blessing

[aquasurf]

About 7 years ago car owners, including Honda fans, started noticing that revs tended to drop very slowly when upshifting gears, making smooth gear changes painfully slow. Numerous comments appeared on various Forums devoted to different makes and models of cars with manual transmissions. Some were critical, others indifferent and few even praised this "feature" for supposedly making gear shifting and "rev matching " easier.

Range of possible explanations were offered, most commonly asserting that rev hang was somehow meant to get rid of unburnt fuel from intake manifold or cylinders. Some argued that heavy flywheel was to blame, while others insisted the rev hang was intentional to make rev matching easier for people not used to driving stick.

I spent the last 2 years examining the rev hang and experimenting with possible ways of its removal from different car models. I came to the conclusion that its introduction was indeed intentional and inevitable but not for the purpose of assisting the driver. It came about because of the need to meet the ever-tightening exhaust emission standards imposed on car manufacturers by Government clean-air agencies all over the world. I explained the intricacies of rev hang, including videos and charts in my Website devoted to the rev hang.

Car owners minding, disliking or even hating rev hang and wishing to fix their cars are being offered various "tunes" predominantly designed to increase power and improve throttle response, torque delivery/spread, drivability and provide other advertised benefits. Judging by Web responses some tuners were completely successful in eliminating the rev hang whilst others were able to reduce it somewhat. The apparent downside of "tunes" involves, aside from the cost, question marks about their impact on continued warranty of new cars modified in such a way. Regardless of what people may say, the fact remains that current OBD (on-board-diagnostics) protocols include provision for car manufacturers to detect when ECU was last flashed. It remains to be seen if and how they decide to use this facility in assessment of claims for a major engine failure, if it were to happen.

There are car owners not seeking power increases and hence not interested in "tunes", who are still annoyed by the rev hang in their cars. They have by now become almost resigned to the situation that "rev hang is a common feature in modern cars". This seems to be the prevailing attitude, judging by the diminishing number of complaints about the rev hang found online in past 2 years or so. As young people buy newer cars, they become accepting of the new way they drive, perhaps because they never experienced the delights of changing gears in a car not afflicted by the rev hang. Drivers are forced to adjust their driving style to suit their cars. I turned 60 and can say that it was never like that before. Until recently car manufacturers built cars to accommodate the needs and desires of their owners. The tables have turned, it seems.

I wonder what other Honda drivers think of the rev hang. If it bothers you, do you believe that market exists for a cheap stand-alone module designed to do nothing less and nothing more than completely remove rev hang from any car? Such module exists and is easily removable, unlocked and freely transferable between different cars.

[RenzokukenJ]

It is quite the bother. I believe its a "feature" introduced from the DBW (drive by wire) throttle bodys. I had this same issue on my Polo GTI which was annoying. Especially with a lightened flywheel. I believe the only way would be to get a tune, as you have already mentioned. Unfortunately there is not much else you can really do with getting rid of the pain that is DBW

[curtis265]

Rev hang is annoying but there for emissions sake as you say... I learned to drive with it, even then I could tell it was awful. Had to granny shift and couldn't double clutch like I should.

Have you considered one of these thingys? http://www.ozhonda.com/forum/showthr...tle-Controller

[aquasurf]

... I believe its a "feature" introduced from the DBW (drive by wire) throttle bodys. ...

The DBW throttle is not causing the rev hang, its introduction only made it easier to implement the rev hang. Even before DBW in mid-70's car designers made the carburetor butterfly close slowly by adding a vacuum operated dash pots.
The pain is not in the DBW throttle system, it is in the ECU program instructions using the DBW to perform unnatural acts, like the rev hang.

....... Unfortunately there is not much else you can really do with getting rid of the pain that is DBW

I wonder why you said that when I wrote yesterday that an electronic module has been invented that kills the throttle hang. I have one in my '12 Accord Euro and it works a charm (demo is in this YouTube video).

…….Have you considered one of these thingys? http://www.ozhonda.com/forum/showthr...tle-Controller

Yes, I bought one in January 2014 and thoroughly tested it when connected to a throttle pedal in test rig. Its function is to change the relationship between the position of throttle pedal and the sensor voltage. It simply creates additional maps (9 for power and 7 for economy modes) and these have no influence on rev hang in any conceivable way. The device just makes the driver feel that his car is performing better and this is essentially true in the sense that the engine revs higher for any given throttle pedal input when in power mode against the standard setting. Attachment 28594

[curtis265]

Oh, I was under the impression rev hang was a function of the pedal input.

a signal amplifier is a bit lame..

well, you can always use the haltech elite system )

[RenzokukenJ]

The DBW throttle is not causing the rev hang, its introduction only made it easier to implement the rev hang. Even before DBW in mid-70's car designers made the carburetor butterfly close slowly by adding a vacuum operated dash pots.
The pain is not in the DBW throttle system, it is in the ECU program instructions using the DBW to perform unnatural acts, like the rev hang.


I wonder why you said that when I wrote yesterday that an electronic module has been invented that kills the throttle hang. I have one in my '12 Accord Euro and it works a charm (demo is in this YouTube video).


Yes, I bought one in January 2014 and thoroughly tested it when connected to a throttle pedal in test rig. Its function is to change the relationship between the position of throttle pedal and the sensor voltage. It simply creates additional maps (9 for power and 7 for economy modes) and these have no influence on rev hang in any conceivable way. The device just makes the driver feel that his car is performing better and this is essentially true in the sense that the engine revs higher for any given throttle pedal input when in power mode against the standard setting. Attachment 28594

Out of curiosity what cars did they introduce that with?

And yeah, I forgot about that module lol.

[amant02]

Its a Curse of DBW in my opinion.

It happens in my STi, if your shifting gears like race car on Intelligent Mode.

Im just curious on your reasoning behind it being an emissions thing?

Unburnt fuel gets sorted by PVC System. Plumbed back into the intake track for a second shot at redemption.

This is my 1st DBW Car and its the only car that I've owned does this.

I also know its cos Im shifting like race car when it happens.

If just driven Civil, granny shifting in traffic is all good.

I also don't think its an emissions thing. On S & S# there is no hang in my car, behaves like a cable operated throttle & more sensitive pedal & more power.

Little bit better then module as on I - limited to 60% throttle at max pedal depression. IE throttle plate wont open past 60% even when given the full blow.

I also don't see how it would help upshift don't you want the rev's to drop when upshifting instead of hanging around?

Its the downshift where the effect would be sort of desirable to newer drivers.

[aquasurf]

Out of curiosity what cars did they introduce that with? ....

I do not know when rev hang first appeared. My first experience was in February 2003 in Spain where I hired a new Fiat (I think it was a Punto model) and in 7 days I drove it I could not figure out why I could not change gears smoothly.

…Im just curious on your reasoning behind it being an emissions thing?

I am positive about that. The problem car designers try to solve by rev hang is the sudden leaning of mixture when throttle is closed abruptly as it generates a spike in NOx (oxides of nitrogen) emissions. I had car emission lab test results from about 1985 (the voluntary test costing me over $1000 at the time), proving that beyond doubt.

… Unburnt fuel gets sorted by PVC System. Plumbed back into the intake track for a second shot at redemption.

I agree the rev hang is not about removing unburnt gasses. In fact, it works to the opposite effect because it intentionally feeds additional fuel to the engine when throttle pedal is off. This causes the engine (and car) decelerate much slower that would otherwise be the case, bringing on all the nastiness of hanging revs.

… This is my 1st DBW Car and its the only car that I've owned does this. I also know its cos Im shifting like race car when it happens. If just driven Civil, granny shifting in traffic is all good.

People describe their experience with hanging revs in different situations. Some write about revs hanging at fairly low engine speeds, say 3000 rpm, others cannot get idle down to the normal 700-800 rpm without revs first hovering around 1500. My concern and solution deals with the problem of revving my engine high and the revs not dropping fast enough when upshifting. Generally, the rev hang gets worse with increasing revs. I can upshift at low rpm without difficulty but I did not buy my Honda VTEC to "shortshift" when I wish to use all of its 140 kW at 7000 rpm.

… On S & S# there is no hang in my car, behaves like a cable operated throttle & more sensitive pedal & more power.

I understand some cars, like yours, have several programs in their ECU and engine control can be varied by the push of a button.

… Little bit better then module as on I - limited to 60% throttle at max pedal depression. IE throttle plate wont open past 60% even when given the full blow.

I find it interesting that people often refer to the opening of the throttle plate as if it was the determining factor in setting engine revs. Would you believe that in EFI cars it is possible to have the throttle plate fully open (by the ECU, of course, taking cue from the DBW throttle pedal) and yet go nowhere? This happens when the ECU cuts signal to injectors, stopping supply of fuel to the engine.

… I also don't see how it would help upshift don't you want the rev's to drop when upshifting instead of hanging around?

Of course, I want the revs drop fast when upshifting. This was the reason behind my efforts to delete rev hang. If you want to read more about my rev hang investigations, look at my rev hang webpage.

[curtis265]

I thought rev hang was to allow a smoother throttle cutoff, so that you don't dump unburned fuel into the ca, supposedly prolonging it's life.

Unburnt fuel gets sorted by PVC System. Plumbed back into the intake track for a second shot at redemption.

How do you suppose unburnt fuel goes into the PCV system? I think your motor may be on the way out if that's the case. Perhaps you mean EGR? Even still this does not loop 100% of the exhaust back into the intake, so this is why they introduce rev hang, to burn the last of it.

Little bit better then module as on I - limited to 60% throttle at max pedal depression. IE throttle plate wont open past 60% even when given the full blow.

How do you know that the power is not being controlled by the wastegate? or valve trickery?

I also don't see how it would help upshift don't you want the rev's to drop when upshifting instead of hanging around?

I would rather my revs drop to the right level rather than hang and sacrifice smoothness

[Hondarally]

I think that rev hang makes the car easier to drive for the average commuter. I've been driving modern cars for so long now I'd forgotten what it was like before.

I recently put my EK4 on the road and it has a good, old school cable throttle, and revs almost instantaneously return to idle when you lift off. It takes me a couple of gear changes to get used to it (in fact, the faster you shift, the smoother it is to drive - within reason of course). My L-plate son, on the other hand struggles with the immediate drop in revs, and it takes him much longer to get the hang of driving the Civic. Once he hops back into the Focus, he's immediately smoother on the controls. He's still at the stage where he takes his time shifting gears, and generally does everything fairly slowly when he's driving. This works with modern DBW systems, but isn't so smooth on old school un-assisted everything cars.

[aquasurf]

I thought rev hang was to allow a smoother throttle cutoff, so that you don't dump unburned fuel into the cat, supposedly prolonging it's life….

I am struggling to understand this supposed prevention of "dumping of unburnt fuel" as the reason for rev hang. Let's say we have an engine without rev hang to perform the stated function. Backing off the throttle shuts both the air (via throttle plate) and fuel (via injectors) immediately. My question is how do you explain this "dumping of unburnt fuel" immediately afterwards? It just does not make sense, especially as I know for sure (I have a lot of test data, videos etc.) that the rev hang in fact keeps feeding fuel to the engine after the throttle pedal was returned to idle position. If there was to be any problem with dumping of fuel, the rev hang would certainly make it worse and not better.

You are right that (small degree of) rev hang allows for smoother throttle cut off. With zero rev hang drivability of manual cars would suffer through drive-line snatch. This is the accumulation of clearances and small free-plays in the gearbox and differential that could otherwise be felt as an unpleasant harshness, jolt or thud/shock when engine power was suddenly applied or reduced.

How do you suppose unburnt fuel goes into the PCV system? I think your motor may be on the way out if that's the case. Perhaps you mean EGR? Even still this does not loop 100% of the exhaust back into the intake, so this is why they introduce rev hang, to burn the last of it.

This article should answer your question about the PCV. Based on my findings I would say that PCV and EGR recirculation systems have nothing to do with the rev hang.

[aquasurf]

I think that rev hang makes the car easier to drive for the average commuter. I've been driving modern cars for so long now I'd forgotten what it was like before....

Exactly right. Thanks for confirming my opening words that as young people buy newer cars, they become accepting of the new way they drive, perhaps because they never experienced the delights of changing gears in a car not afflicted by the rev hang. My two daughters learnt to drive in a vintage 1991 carbureted Suzuki Swift with cable throttle and are now able to recognize the rev hang when they come across it in their newer cars. They do not mind it much, though, as they are usually in no hurry to change gears.