I currently am tuned and am running a Walbro 255 fuel pump. I'm geting ready for an e85 flex fuel tune. I want to get redy by replacing my Walbro 255 with a Walbro 400 e85 pump. Will this affect the drivability of my car or the current tune in any way?

 

It shouldn't effect it enough to notice a difference. Assuming you're not going wot the whole way there

 

It might. The additional flow from the pump may increase fuel pressure at the rail if your FPR can't keep up. If you have an adjustable FPR then measure fuel pressure at the rail before the swap, and after the swap adjust your FPR so that rail pressure matches what you had before.

Increasing fuel pressure at the rail will certainly affect the tune and driveability.

 

You'll be fine if it's just for a few days and you stay out of WOT
+1 on checking if the fuel pressure matches before and after

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I may not be geting tuned anytime soon. I just want to know if I swap to a bigger pump that the tune will not be affected and that I can still drive my car and go wide open throttle everyday. If it will affect my tune then I won't installed it until just before I go dyno. I also do have an aem adjustable fpr.

Before I was turboed I had a Walbro 255 pump for like half a year and the car drove normal just like as it did with the stock pump. My friend also has a 340lph pump in his stock s2000 and it drives 100% normal.

 

Well you should still be fine since you are going bigger, not smaller. Worst case scenario is that you will run richer. Might hurt performance, but won't harm the engine.

If you can't get the fuel to match by lowering the FPR, you can just edit the map yourself to cut back some fuel. Shouldn't need a dyno for that if your wideband is accurate.

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Low load will be fine given you have an aftermarket FPR and fuel trims will be able to compensate.

You will run richer at WOT/open loop due to additional flow provided by that pump at any given pressure compared to a 255. You can run a car too rich; you can damage a motor with excessively rich conditions.



Get a tune.