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Gear shift with ignition cut

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hi ,i just wanted clarify if ignition cut during gear shifts is meant that we cut ignition for a certain millisecond or do we retard ignition timing to drop the engine torque down ? thank you

It can be done either way depending on the amount of torque reduction you require but generally we would be using an actual ignition cut. This is often followed by ignition retard to soften the torque as it's reintroduced.

Something I noticed is modern factory race cars doesn't sound any more like they have an ignition cut during shifts. Sombody told me last week, that rally cross cars use only fuel cut for any limiting, Launch control, Antilag and shift cut. Can anybody confirm that or isn't fuel shift cut something that is going to work well?

I recently calibrated a shift cut on a Evo 4G63 with a sequential gearbox and Motec M400. I tried different settings, but there are always big bangs during shifts. So I was thinking about switching to Fuel shift cut. The shift bang can be heard good in this video


I doubt there is an across the board rule that fuel cut is the only limiting option that is used, however to generalise, fuel cutting is a safer option for the engine when compared to ignition cutting.

There's a lot more to the shift cut that will affect the popping that's audible than whether you're using a fuel or ignition cut. Normally if you're getting a lot of popping it's indicative that the cut period is too long (of course this may be a limitation of the gearbox design and shift speed). On the pikes peak hillclimb car I configured recently we ended up using a 60% ignition cut which was just enough to allow the dogs to disengage, followed by an aggressive ignition retard to allow the shift to complete and reintroduce the torque smoothly. When you get this right it's almost seamless and there's little to no popping present.

Antilag is a special situation where you need to use ignition cut/retard as it relies on uburnt fuel/air making its way into the exhaust system.

Hi Andre

Thank you for the feedback. To be honest i have not gone below 80% ignition cut and retarded only 5deg. I use closed loop gearcut, so I can't imfluence cut time to much, but I use to instruct the Driver to shift and release the gear lever fast. Can you number how much retard you've used and how many milliseconds you ramped the torque back in? I know every engine-gearbox combination is different, but it will help to get an idea of the ballpark.

I going to try it again. The testing time was quite limited until today.

The percent cut is interesting. I always used to set up gear cut with a 100% cut but if you drive a sequential box that has no gear cut control, you generally only need a slight lift on the throttle to allow the dogs to release. That is where I started reducing the cut. The amount of cut you'll require depends on the gearbox design though so you need to test and see what works for your box. If you reduce the cut too much then the gear won't release so you know you've gone too far.

My approach depends on the ability of the ECU. With the MoTeC M1 you can target the % rpm match that the cut will be active for. In other words the ECU will calculate what the rpm should be in the next higher gear (let's say it's a 1000 rpm drop from 4th to 5th gear). If you target a 100% rpm match then the cut will remain active until the rpm has dropped by 1000 rpm. This works but is slow. I've found it works well to target a drop of around 30-40% (300-400 rpm) and following that I use around 20-30 degrees retard to allow the shift to complete and soften the reintroduction of torque. The retard might ramp out over 50-100 ms.

Ok that makes sense.

Doesn't lead the high retard to overboost diring shifting? Especially when the driver shifts very slow.

The retard is very momentary and is also off the back of an ignition (or fuel) cut where there has been no combustion which normally results in the boost actually dropping. What I found with the Pikes Peak car on the M1 was we saw a very slight over boost after the gearshift but this was nothing to do with the retard. instead it was the reaction of the closed loop boost control system to the momentary drop in boost during the shift that resulted in a slight increase in duty cycle in order to correct the boost error. When the engine power was reintroduced the boost naturally exceeded target momentarily before it caught up. I actually suggested to MoTeC that the boost control system reverted to open loop for a fixed time during the shift to prevent this.

I see, usually, the retard is too momentary to have any ill effect. Now I'm wondering how is it, during a miss shift? In worst case gear cut could be applied for 500-1000ms with 60% cut. This means 40% of combustion events are burned very late (retarded). Do you think the 40% with a lot of retard is not enough to lead to boost spikes?

Okay, I just found the same problem with over boost during shifts on an Evo 4G63 with OEM transmission and LINK G4+. I'm not sure how Motec handles the 3 boost stages. The problem I found on LINK is that the system goes into stage 1 which is usually 90-95% WGDC. This is usually not too much of a problem on small frame turbo where you have high WGDC's. But on big turbos and external wastegate which need only 20-30% WGDC to reach target boost, the short fall back to stage 1 (=90%WGDC) during shifts (sometimes transient too) is enough to spool the turbo up above the target. I guess you experienced already the same problem...

I think this scenario is going to depend a lot on exactly how the ECU deals with the cut and retard. For example is the cut and retard active simultaneously or is the retard used post cut? If you miss a shift the cut length is going to be extended initially which results in no/limited combustion if it's solely a cut at this point. It is possible that you may see some affect on the boost but I'm not sure that I'd personally be too concerned on this scenario just for the very minimal chance of a miss shift taking place.

I personally don't find the Link closed loop boost control to be effective and there are several known problems with it. The way boost is handled in the MoTeC is completely different and I've had very good results with it. I've done some testing for Link and shown them the problems I see with their boost control. these issues have been accepted but as yet I haven't seen a solution and will continue to use open loop control.

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