How to make RPM drop faster?
Discussion
My Westfield has a Vauxhall 2.0 16v engine. This is fitted with QED's Q450 cams, higher compression pistons and a lightened steel flywheel. It has direct to head throttle bodies and a 4-2-1 exhaust manifold. The ECU is a very old (1996) Emerald M3D that senses crank speed, throttle position and air/water temp. At the moment, if the throttle is "blipped", the revs rise quickly but die away quite slowly. This is more noticeable at lower revs, but even at higher revs, the reluctance of the crank to decelerate means gear changing has to be a relatively leisurely affair.
Given that I only have a fuel map and an ignition map to adjust, what strategy can I use to make the rpm reduce more quickly?
Given that I only have a fuel map and an ignition map to adjust, what strategy can I use to make the rpm reduce more quickly?
The speed at which revs drop is primarily a function of friction and pumping losses working against component inertia. The XE does have a heavy crank so it's never going to behave like a motorbike engine if that's what you were expecting. They have tiny multiplate clutches and very little flywheel inertia effect.
Make sure the throttle butterflies are shutting completely and instantly and no air leaks anywhere. Also make sure that idle ignition and fueling are optimised to get an acceptable idle speed (lowest possible for preference) with the least airflow so the butterflies close as much as possible. Experiment with ignition advance to get the highest idle speed and then adjust the throttle stop to bring it down again. An ecu that cuts fuel completely when the throttle closes above a certain rpm should make a small difference too but I don't know if your one has that option.
Do a compression test to make sure nothing is amiss. After that it's down to component weight and that's an expensive avenue for small changes in how the revs drop.
Make sure the throttle butterflies are shutting completely and instantly and no air leaks anywhere. Also make sure that idle ignition and fueling are optimised to get an acceptable idle speed (lowest possible for preference) with the least airflow so the butterflies close as much as possible. Experiment with ignition advance to get the highest idle speed and then adjust the throttle stop to bring it down again. An ecu that cuts fuel completely when the throttle closes above a certain rpm should make a small difference too but I don't know if your one has that option.
Do a compression test to make sure nothing is amiss. After that it's down to component weight and that's an expensive avenue for small changes in how the revs drop.
Suggest you rev the engine to around your change-up RPM in neutral, switch off and see how quickly the revs drop. If that's similar to the behaviour when changing gears, that's as good as you're going to get without changing the hardware.
If the revs drop more slowly with the ignition on, if you have an idle control valve, make sure it isn't opening during overrun. I expect you have full fuel cut during overrun but you ought to confirm that.
If the revs drop more slowly with the ignition on, if you have an idle control valve, make sure it isn't opening during overrun. I expect you have full fuel cut during overrun but you ought to confirm that.
The ECU doesn't do fuel cutoff on overrrun. I've done it manually in the fuel map.
The suggestions around air leaks and ensuring the butterflies are shutting has made me wonder if the air bypasses in the throttle bodies are letting excessive air through - I haven't touched those since the last RR session.
The iidea of seeing how it decelerates with the ignition off is a good one. As soon as the temperature in the garage rises above freezing, I'll have a tinker with that to give me an idea of what I should expect.
The comment about the weight of the rotating/reciprocating parts in this engine is a good one, but my old Sunbeam Lotus used to lose revs quickly on a closed throttle, and that engine didn't exactly have light internals; also its 45DHLAs wouldn't have had fuel shut off.
ISTR, in an old post, Max_Torque giving a long description of how OEMs used various fuel and ignition settings to promote crank deceleration; but I can't find the thread, so may have dreamt it!
The suggestions around air leaks and ensuring the butterflies are shutting has made me wonder if the air bypasses in the throttle bodies are letting excessive air through - I haven't touched those since the last RR session.
The iidea of seeing how it decelerates with the ignition off is a good one. As soon as the temperature in the garage rises above freezing, I'll have a tinker with that to give me an idea of what I should expect.
The comment about the weight of the rotating/reciprocating parts in this engine is a good one, but my old Sunbeam Lotus used to lose revs quickly on a closed throttle, and that engine didn't exactly have light internals; also its 45DHLAs wouldn't have had fuel shut off.
ISTR, in an old post, Max_Torque giving a long description of how OEMs used various fuel and ignition settings to promote crank deceleration; but I can't find the thread, so may have dreamt it!
cmsapms said:
The ECU doesn't do fuel cutoff on overrrun. I've done it manually in the fuel map.
Have you checked that you're running in the part of the map that you intend to?I've read elsewhere on PH about somebody with a throttle butterfly that stuck partially open when lifting off the throttle at high revs, which I think was attributed to air pressure on the butterfly increasing stiction. In your case that might also keep you out of the part of the map you configured to provide overrun fuel cut-off. Watching what the ECU is doing in real time would give you a much clearer picture about this type of thing.
for my rover K running a more modern Emerald K3, I found that having too much idle air can cause the revs to hang in a little at something like 1500 rpm before it idles. The drop from whatever to 1500 was rather quick.
Reducing idle air, adapt idle fueling as a consequence and advancing the ignition a tad for the idle to remain low fixed this.
If you need the fuel cut off, any more modern version, M3DK, K3 or K6 have fuel cut on overrun. I have never looked into the oldest version of the Emerald, I admit.
Reducing idle air, adapt idle fueling as a consequence and advancing the ignition a tad for the idle to remain low fixed this.
If you need the fuel cut off, any more modern version, M3DK, K3 or K6 have fuel cut on overrun. I have never looked into the oldest version of the Emerald, I admit.
cmsapms said:
The ECU doesn't do fuel cutoff on overrrun. I've done it manually in the fuel map.
It is worth noting that the value in the fuel main table will be referenced to inj open time. Added to the inj open time value in the map is the dead time. If the dead time value is wrong, then you could still be getting fuel.cmsapms said:
The suggestions around air leaks and ensuring the butterflies are shutting has made me wonder if the air bypasses in the throttle bodies are letting excessive air through - I haven't touched those since the last RR session.
Air leaks are immaterial if there is no fuel to burn. You could have a wide open throttle and a fuel cut. No fuel, no power.cmsapms said:
The iidea of seeing how it decelerates with the ignition off is a good one. As soon as the temperature in the garage rises above freezing, I'll have a tinker with that to give me an idea of what I should expect.
This will give you the best case rate of deceleration achievable.Do you have a Lambda/WBO2 gauge? It should be reading off the scale lean if there is no combustion taking place.
stevesingo said:
cmsapms said:
The suggestions around air leaks and ensuring the butterflies are shutting has made me wonder if the air bypasses in the throttle bodies are letting excessive air through - I haven't touched those since the last RR session.
Air leaks are immaterial if there is no fuel to burn. You could have a wide open throttle and a fuel cut. No fuel, no power.Mignon said:
That is not the issue. The majority of engine braking (rpm drop speed) in a petrol vehicle is caused by the closed throttle which the engine has to pump against. The closer to completely shut that throttle is the greater the pumping losses.
But if the throttle is closed, there is little or no air being pulled in to pump ?And certainly when driving, having a fuel cut off on overrun vs not having it, the fuel cut is always more abrupt in feel when it occurs on closing the throttle ?
Is it that the old Emerald doesnt have an overun cut off...or just that it is not enabled or active on your setup ? Does it have any closing throttle fuel adjustments you could use ?
DVandrews said:
I know the Emerald pretty well.. how much air are you drawing in each barrel at idle? If you reduce this and compensate by adding a little more advance at idle to maintain idle speed this may have an effect.
This is what I'm going to try next. The Jenvey TBs have air bypasses to (I presume) allow balance adjustment between individual throttle barrels. I'm going to make sure these are letting the minimum air through consistent with maintaining balance across each butterfly and then shut down the throttle as far as possible at idle and re-establish the idle speed with igition/fuel adjustments.To answer the earlier question, this version of Emerald doesn't have fuel cutoff. I have the fuel map set to zero on closed throttle (load site=0) above about 2000rpm. The point made above about injector "dead" time means I might still be getting fuel injected even with a fuel value of zero. I'll check for this with the WBO2 display I've written to work with the M3D. I wonder if I might also be seeing an interpolation effect if the raw value of the TPS is not reducing to its minimum, although as I'm seeing load site 0 when the throttle is closed, I think the raw value must be getting to its minimum value.
If fuel is still being injected despite the fuel value being set to zero, would drastically retarding the ignition at closed throttle (load site 0) above 2000rpm help? I'll give that a try too. I wonder what effect I'll see with such a big step from load site 1 to load site 0?
All this will have to wait until the weather up here on top of the Pennines improves!
As an aside, as a bicycle pump's air inlet is not the hole you can put your finger over, it's not really analagous with an ICE?
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