Rev hang - a curse or blessing

[curtis265]

Theoretically speaking, would you not say that releasing the pedal suddenly will cut off ignition? So any fuel that hasn't finished the compression cycle will be dumped out the exhaust. It will ignite in the cat creating that crackling and popping we all know and love.

I do not know if a dbw equipped car has no popping.

[amant02]

I 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. 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.

Wouldn't that lean out the fuel? More air without increase in fuel?

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.

So using this logic curt, your engine must be on its way too. Wait wait 100% cars on the road, their engines must be on its way out, cos of unburnt fuel in the PVC System. All engines will make a little bit of unburnt fuel. Specially Factory tune turbo cars as they are tuned to run rich from factory.

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

I guess to some extent these are used as well, isn't the throttle basically a valve for air? so it is valve trickery. Since there are changes to amount of air intook by the engine so corresponding adjustments must be taking place in the wastegate, since there is less spool.

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

This I agree with

[aquasurf]

Wouldn't that lean out the fuel? More air without increase in fuel?

I do not think so. You can possibly lean out a fuel mixture, that is to change proportion of air and fuel in it, but if you have no fuel, then you do not have a mixture to lean out. You have just air and it ain't no pollutant.

Edit: Initially, the injected mixture would suddenly lean out as fuel supply was being shut off and emissions would spike. This is the very reason why car designers try to avoid it by adding the rev hang.

[JohnL]

I’ve been contemplating replacing the ancient CB7 (24 years and 380,000km) with a somewhat newer Euro. I haven’t yet driven one, but am disheartened to discover that they seem to suffer the dreaded rev hang quite badly, possibly steering me toward a different car (Mazda 6 maybe, but that may be no better...?).

The CB7 (and probably many other Hondas of the era...?) also suffered excessively from rev hang ex factory, but since it predates FBW Honda ‘achieved’ it by means of the IACV (idle air control valve). On the overrun the ECU would momentarily hold the IACV open, creating the highly irritating 'rev hang'. I ‘fixed’ this by blanking off the ports in the IACV, simply by inserting a thin aluminium plate between the IACV body and plenum bodies. Of course this disabled the idle speed control function, so parasitic loads on the engine resulted in a lower idle speed (I had to increase base idle speed to 900rpm prevent excessive vibration at sub 500rpm with AC etc on).

Initially I tried simply disconnecting the loom from the IACV, which did stop the rev hang, but caused other issues. Of course it triggered an irritating CEL, but worse it seemed to cause the ECU to cut the ignition and / or the injection on the overun, which created unacceptably harsh off / on throttle behaviour. Simply blanking off the IACV ports didn’t do this (the ECU doesn’t ‘know’ that the IACV isn’t working, so doesn’t seem to cut overrun ignition and / or injection), only stopped the rev hang with no other issues (other than disabling idle speed control, which I could live with).

I agree this is all to do with reduction of Nox emissions , this was what I gleaned from my related research some years ago. Simply holding the IACV open (or with FBW cracking the throttle plate) won’t cause the rev hang on its own. For the rev hang to occur fuel needs to continue to be injected (and ignited) while the throttle plate or IACV allows air to pass into the plenum and cylinders. If only the unwanted air flow is permitted (with no fuel and / or ignition), then if anything the rpm would drop even faster, somewhat like an unrestricted diesel engine on the overrun.

This is because unignited air entering the cylinders will act to slow the engine, because cylinder compression is still occurring (less air = less compression, requiring less ‘work’, but more air = more compression etc). The effect is likely small as the compressed air will ‘give back’ some of its ‘stored’ energy to the crank past TDC, but when the air is initially compressed it will get hotter, so rotational energy is ‘lost’ as heat to the cooling system and down the exhaust, which in theory would manifest as a somewhat faster rev drop (with a partially open throttle or IACV allowing more air in, but no fuel burning).

I can’t see any easy way around this problem with a FBW system. Only some sort of piggy back device on the ECU, or reprogram some parameters of the ECU itself (possible with the Honda ECU??). Maybe I need an old (but nice, enough...) Integra or Prelude instead, something with an IACV system I can bypass...?

Regards,
John.

[Fredoops]

There's a much cheaper ecu reflash that gets rid of the worst of revhang

[aquasurf]

….I can’t see any easy way around this problem with a FBW system....

John, I agree with every word you wrote except the above. If you are in or get around to Sydney, I would love to offer you a test drive in my 2012 Accord Euro (CU2) 6-speed M/T I managed to cure of the rev hang completely and without ECU re-flash.

I encourage you not to give up on the late model Accord. When mended, it is a beautiful car to drive. I never had a Honda before and picked up my first one a month ago (mainly as a challenge to fix its rev hang) at auction for less than $16K with GPS and 50K km (also with hardly any thread on tyres and 2 days of rego left). I can imagine that previous owner got tired of the rev hang and decided to dispose of the car. His loss was my gain .

I love the car and revving the VTEC to 7 grand for a snappy upchange. I made a minor mod to the traction control (or VSA - Vehicle Stability Assist) to disable it automatically on each take off and re-engage at 30 km/hr. to avoid the horrible bogging-down feeling at speedy take offs. The VSA was far too aggressive, cutting down power at first sign of traction problems, well before any wheel spin.

[RenzokukenJ]

John, I agree with every word you wrote except the above. If you are in or get around to Sydney, I would love to offer you a test drive in my 2012 Accord Euro (CU2) 6-speed M/T I managed to cure of the rev hang completely and without ECU re-flash.

I encourage you not to give up on the late model Accord. When mended, it is a beautiful car to drive. I never had a Honda before and picked up my first one a month ago (mainly as a challenge to fix its rev hang) at auction for less than $16K with GPS and 50K km (also with hardly any thread on tyres and 2 days of rego left). I can imagine that previous owner got tired of the rev hang and decided to dispose of the car. His loss was my gain .

I love the car and revving the VTEC to 7 grand for a snappy upchange. I made a minor mod to the traction control (or VSA - Vehicle Stability Assist) to disable it automatically on each take off and re-engage at 30 km/hr. to avoid the horrible bogging-down feeling at speedy take offs. The VSA was far too aggressive, cutting down power at first sign of traction problems, well before any wheel spin.

How exactly did you fix the issue?

[aquasurf]

How exactly did you fix the issue?

My story of the rev hang fix is here.
The VSA fix was simpler but different.

[RenzokukenJ]

My story of the rev hang fix is here.
The VSA fix was simpler but different.

Thanks, I might look into it for the fn2...

[JohnL]

Aquasurf,
Thanks for the kind offer. I’d take you up on it but it’s a long way to come from the Mid North Coast, and at the moment I have no other excuse to visit the Sydney rat race...

I’m fully in your camp re the horror of rev hanging, it really spoils the gear shifting experience. My old CB7 became a much nicer car after it had been eliminated (and the light flywheel, and my modified short throw shifter, and the Konis, and the stiffer springs, and the stiffer rear ARB, and deleting the front ARB, stiffer bushes, braces etc etc).

The VSA mod sounds interesting, but is it this that has addressed the rev hang issue? If so how? If not, then what did you do??

When I bite the bullet on a new car, it will be a much older ‘new’ car than yours. Older higher K Euros are only now starting to drift into the reach of my bank balance (besides, spend too much $ on a car and there’s less left over to feed the racing kart...). I’m starting to see what at least appear to be reasonable early MY Euros on the market for sub $6000, which would be around where I’ll be looking (when the time comes).

Experience with Hondas suggests higher ks aren’t necessarily a problem if the car has been looked after. I’d be happy with a mechanically sound car that might be a bit ratty on the outside (I live on a dirt road, and with a dirt road you shouldn’t get too worried about your cars being cosmetically pristine, that way lies madness...). At any rate, the CB7 is getting just a bit too tatty even for me, time to think about retiring her. Our other car is a Saab 9-5, which is nice but nowhere near as reliable as a Honda and too thirsty (head said Honda, heart said something less rational...). So, it will either be a Euro, or an Integra vti-r, or a late model Prelude (though these seem a bit thirsty?). The Euro does seem a more ‘grown up’ car, and less likely to have been thrashed (though Hondas do seem almost immune to ‘thrashing’, so long as they’re otherwise looked after...).

Regards,
John.

[JohnL]

My story of the rev hang fix is here.
The VSA fix was simpler but different.

Aquasurf,
That's good, but rather generic and beyond the scope of most (certainly myself...).
If I had a rev hanging Euro, what could be done tomorrow to address it...?
What hardware would be needed? What other changes? What would it cost, and who would I give the $ to??

Regards,
John.

[aquasurf]

….The VSA mod sounds interesting, but is it this that has addressed the rev hang issue? If so how? If not, then what did you do??...

The VSA mod is separate from the rev hang delete I implemented by the connection of my QREVS module. I did use one function of that module to trigger my VSA activation speed threshold (30 km/hr.) to re-engage the VSA after reaching it but that was just for convenience, it could have been handled independently by the gearbox speed sensor signal. Otherwise the VSA mod is just a collection of 2 timers and some relays - in fact it is very primitive but it does the job. How it works is that after ignition on and engine running the timer waits for 1.1 second to switch on contacts of the VSA dash switch for 0.5 sec. (basically the same action I used to do by hand before) to deactivate VSA before launch. When speed is reached, different timer switches on the same contacts for another 0.5 seconds to activate the VSA. When speed drops below the speed threshold, the VSA switches off again in readiness for another launch.

From your description of mods you did I see that you are REALLY interested in improving your cars. I wish you luck in finding the right next "new" car. If you had a chance to read my rev hang website you would see that the rev hang delete module I built is universal and suitable for many different cars. The hardware remains the same, only the software differs. I proved it by fitting it to 2 different cars (2013 Yaris 1.5 and 2012 Accord Euro 2.4). I did not expect the general skepticism of car owners complaining about the rev hang in their cars that I have encountered online in different Forums when I wrote that the rev hang can be fixed. So it was a pleasure to come across you who understands what needs to be done to stop this dreadful malaise of modern motor cars. Perhaps we will meet one day. Cheers.

Oh sorry, I missed your other post. To fit my module you would need to identify pin assignments of your ECU (revs, speed, clutch switch). I know these for the Yaris and Accord Euro but not for other cars. Then I would build one module for you and provide instructions how to connect it. About $200-300 should cover the cost. I might even come to visit you to help fitting it.