My own engine is a good example. Opel 3.0i 24V (C30SE)

204 HP with

223 HP without

Maybe some HP added because of the replaced exhaust system.

BTW I was just kidding tc. Catalyst saves nature (a bit more). But my humour is still WIP juno?

So you had a dynomometer/rolling road session. First with the catalyst, then (on the SAME day) took of the catalyst and changed NOTHING else, and retested it? If not, your results are meaningless. If you did, then you must be made of money to afford wasting dyno time on a decat pipe, and should be able to afford something better than a Vauxhall.

Quote from tristancliffe :

If you did, then you must be made of money to afford wasting dyno time on a decat pipe, and should be able to afford something better than a Vauxhall.

I made just one test. Why testing factory engines? And it´s a question of taste what car you like and what don´t. I don´t like girls roadsters like Miata f.e.
I like big, old Rüsselsheim Iron.


Edit: Till I own my 68 Charger =)

Why testing factory engines? Because they are all different - each one will vary in power output. If you think that the tolerences on a mass produced engine are such that they are all the same you are mistaken. Even F1 engines, with near zero tolerences, produce noticably different power on the dyno, and the best ones are reserved for competition whilst the worse ones are used for testing and destructive analysis.

Whatever. All I wanted to say is that carburator cars (which I prever) and old efi cars don´t have a cutoff.

Quote from evilgeek :

of course it would be simple to program the ecu to tell the injectors to do whatever you want, but unless you have an actual example of a motor that cuts them off, then you are speculating just as much as anyone else. i don;t doubt that some motors reduce the amount of fuel injected at high revs and low throttle, but i am still highly doubtful that any engine would cut the injection completely off, because of the potential damage to the motor. that was my only point, and i don't see why you gotta be so sarcastic.

I posted a few links in the reply to Tristan. My own car, built way back in 1988, has this feature, and it is one of the most dangerous around to run on a lean condition - turbo rotary and all that. Wasn't trying to be particularly sarcastic, I just find it interesting how many people post answers in threads like this without having the answer - but I guess I wasn't expecting deceleration fuel cut to come as such a surprise to people.

Quote from Crazy Harry :

With a carburator engine you better never lift complete in downhill because this will close the throttle valve (damn language barriere) completly, no air is taken and your engine uses a huge amount of fuel.

I use engine braking all the time on my bike (250cc single-cylinder, DOHC, air-cooled, single 34mm Mikuni carb) and while I get a little bit of light popping from the exhaust, I somehow doubt I have fuel pouring into the engine.

Blowtus, in reference to those links, they make no mention of lean mixture conditions. At some point after the fuel is cut off there is no fuel, so leanness doesn't exist (and thus no change of detonation or pre-ignition or anything like that), but for a few moments the mixture will go lean. Is this just ignored, as it's for a brief time (probably), or do they combat it in a different way.

Catalysts are very sensitive, as I'm sure you're aware, to mixtures far from stoich, quite apart from the detonation issues (which could be resolved by retarding the ignition), and I'd have thought that even a few seconds of leanness a day would eventually do nasty things to it.

To be perfectly honest I'm more of a carbs man myself, and have little understanding of the systems control and feedback routines in a modern (ish) ECU, so you can teach me

OK, I can confirm that a medium-sized modern diesel engine does cut the fuel completely on deceleration.
This is based on data taken directly from the ECU of an engine running simulated drivecycles on a transient dynamometer.

I don't know exactly what conditions cause the ECU to cut the fuel injection...I could find out, but all the engine software manuals are strictly confidential so I couldn't share the information.

My understanding is that, if there is no throttle demand from the driver, the ECU will provide just enough fuel to maintain idle speed. I therefore assume that if the engine is being motored above idle speed the ECU decides that fuel injection is not needed.

As for the effect of catalytic converters...there will be a very small pressure drop across them but I don't think it would be significant. They're generally quite open-celled and don't present much of a restriction. We've never done any testing with and without cat because it would be a complete waste of time.

Quote from tristancliffe :

Catalysts are very sensitive, as I'm sure you're aware, to mixtures far from stoich, quite apart from the detonation issues (which could be resolved by retarding the ignition), and I'd have thought that even a few seconds of leanness a day would eventually do nasty things to it.

A quick search has a few people talking about pulsing the injectors on occasionally during DFCO (automotive acronym for deceleration fuel cut off, useful for further searches if you're keen) and I know my car kills one bank of injectors before the other - whether that's enough to mitigate any damage, or even whether the damage is worth worrying about, I have no idea.

Virtually ALL modern-ish (like late 80's and newer) fuel-injected engines cut the fuel COMPLETELY on overrun (deceleration/foot off the throttle). In fact, if you happen to have a car with throttle body injection, you can even verify this yourself. Simply pull the intake ducting/air filter, rev it up to like 4k rpm and close the throttle. The injector(s) will not inject any fuel until the engine speed drops below a predefined threshold (somewhere between 1500-2500rpm).

On engines with sequential port injection you can even wait for each cylinder to complete its power cycle and avoid incomplete combustion/a lean mixture.
Engines with batch fire injection will have half of the cylinders running lean for a split second, but if you're running batch fire, you're not terribly concerned about emissions in the first place, and the lean condition lasts WAY too short to damage the cat, exhaust valves or anything else.


In addition to "Deceleration Fuel Cut Off" virtually all ECU's feature "Deceleration Enleanment" to anticipate throttle movements and reduce the fuel amount in advance. This is necessary because the pressure behind the throttle plate basically equalizes with the speed of sound when you close the throttle, but the fuel travels a good bit slower. In essence you have to approximate the throttle position a few ms down the road, or else you end up with excess fuel in the cylinder and a bucking engine.

The only fuel injected engines that don't shut off the fuel during overrun are (forced induction) high performance engines that need excess fuel for the cooling it provides, and ones with very very basic injection systems.