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Exhaust Changes After A Dyno Tune

EFI Tuning Fundamentals

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Discussion and questions related to the course EFI Tuning Fundamentals

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Hello HPA friends,

I had my 97 Miata tuned on a dyno with a CAI running on MS2 and stock internals/exhaust.

Of course, like most you, I can’t stop modding and found a full header back exhaust system that I added (cat deleted) post tune. My poor wallet….

Before returning to another very expensive dyno session, I’m wondering how much the change truly impacts ignition timing? From the HPA course, I learned any change to the engine will require a retune but I’m thinking the exhaust side would mostly have impact on VE and therefore could potentially be adjusted by a simple road Tune or VE analyze off the existing fuel table. Am I on the right track? Or should I take this back to the dyno fully have the MBT redone?

I’m a bit nervous to try steady state on the road as I’m mostly concerned with screwing up and missing out on valuable track time…

One would expect the engine's VE to improve, but that's not always the case as the OEM was already quite well tuned for the vehicle.

In some cases there can be a loss of bottom end.

So then the ignition timing should be set as is? My logic is if the previous tuner found the appropriate timing without knock, then the exhaust change might leave HP on the table but not necessarily create an unsafe operating condition?

Personally, I'd leave it as is, while keeping an ear open for knock and checking the plug readings.

What sort of logging options does the MS3 offer for lambda and knock?

Gotchya, I’m running an MS2 and it logs knock and Lambda. The only thing is that I’m still waiting for my knock sensor to arrive and install as the 1997 Miata’s don’t come with them.

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