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Engine stutter

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G'day. I've got an engine swapped MX5 NC, 2.5L with Rotrex C30-84 (upgraded pump and injectors), intercooled, 2.5" ss exhaust head to tail running on E85. It's also got custom torque converter on about a 2500 stall. (I know, it's an automatic but when I bought my 'sports car' the wife's stipulation was auto so if needed she could drive it) I've had the car since new and done all the work myself, except tuning. I've had to rely on a guy in the US and, as much as the car can go I have some issues. I've mentioned them a couple of times but he doesn't appear to read much of what I write, just looks at my logs. This is the primary reason for my being here, learning to EFI Tune, but hoping someone will be able to point me in the right direction. Also issues I was unable to replicate on dyno, not that I could afford much time to investigate.

First issue is take off, even if I try to stage up a bit on the line, as soon as I go WOT she just bogs. It 'feels' like too much fuel but not confident that's all and I think it may also tie in with my second issue. Once again, under WOT, usually second to third but sometimes first to second, once it shifts, revs drop a bit and she just stutters, almost like hitting rev limiter but it's easily 1k rpm under limit and it's more a surging stutter and it doesn't stop until I let off. Thing is, because it's so transient my EcuTek logs don't appear to have the granularity to pick it up. Having said that, would I be wrong to be suspicious of the stock VLIM? Is it possible the butterflys could start oscillating due to the pressure 'blip' on gear change? Also, only a modest ~5psi@6000 and redline set at 7200.

Any pointers or general observations most welcome, it's all learning :)

Hi Chris, that's an interesting issue you've got there. Ultimately it could be a transient enrichment issue that you're facing when you try to take off. The other part though on the gear shift is unlikely since I'll assume the throttle remains open on the shift. I'd be very surprised if the VLIM is your issue here since under WOT conditions the manifold pressure should remain pretty consistent.

In order to really analyse what's going on it'd be helpful to have a wideband in the car and gather some data of the issues if possible.

Thanks for the feedback! Will have to start saving my pennies for a wideband kit... considering I'm running a stock ECU I'll probably need something that will pass narrowband as well as something separate to log the wideband? At least that's my presumption from my limited knowledge... Need to get back out on the road to replicate the issues but with with someone else on the laptop to mark the logs, trying to do it all myself isn't really the safest.