Duratec 2.5 - Torque / VE Dip

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shifty35
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Duratec 2.5 - Torque / VE Dip

Post by shifty35 »

Greetings all! I've put together a motor in my 2006 MX5 street toy, and have an odd torque / VE dip right around 5k rpm. Here's the vitals on the motor.

2.5L Duratec 89x100
~12:1 compression
Crower single valve springs + Ti retainers
Esslinger Stage 4 cams - 245* @ .050" intake, 235* @ .050" exhaust
Ecoboost Mustang intake manifold - single 9" runner, ~2L plenum volume
68mm throttle body, full 3" intake / MAF housing
1.6" diameter shorty 4-1 header (~22" primaries), full 2.5" exhaust with catalytic converter

I was street tuning today doing VVT sweeps to nail down the VVT map, so I've got a pretty graph with those results, but this motor has been running since November, and has always had a pronounced dip at 5k. The graph is actually measured load from the MAF sensor, so it's a VE dip, not related to either AFR or timing. As you can see, no change in intake cam angle resolves it either. It was there with the original OEM dual-runner intake manifold as well.

I really suspect it is some kind of exhaust resonance, but the other candidates are some kind of valvespring harmonic or intake harmonic? I know a few others with very similar setups and it seems somewhat common. I am hoping going to a longtube header down the road will resolve it, but I also plan to put on an adjustable exhaust gear to play with in the near future as well.

I appreciate any feedback!
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ptuomov
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Re: Duratec 2.5 - Torque / VE Dip

Post by ptuomov »

Not pretending to be an expert, but that sort of big VE dip would likely come from intake manifold plenum resonance going the wrong way or exhaust blowdown interference during the overlap period.

If you buy a EngMod4t license and program the engine into that simulator, you can likely identify from simulated pressure traces whether it's an intake or exhaust issue.
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shifty35
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Re: Duratec 2.5 - Torque / VE Dip

Post by shifty35 »

I suspect you are right that it's an exhaust interference phenomenon. Plenty of people with this setup with shorter duration cams and no issues, but once the duration is long enough, there is a problem. Kind of surprising that moving the intake cam doesn't change things since that could potentially reduce the overlap in the problem zone, but if it has more to do with the exhaust cam timing, then that obviously wouldn't work.

I do have an adjustable exhaust cam gear that I may play with, and I suspect it will probably just shift that dip around in the powerband rather than eliminate it - I suspect I'll need a real longtube to solve that completely.

Thanks!
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ptuomov
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Re: Duratec 2.5 - Torque / VE Dip

Post by ptuomov »

The fact that the intake cam timing doesn’t change the torque hole size or location does indeed suggest it’s more likely a plenum resonance issue than an exhaust blowdown interference issue.
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