Will be having motor dynoed shortly but will not have the Stainless Marine manifolds nor extended riders strapped on for dyno session. How much will it affect the tune once motor is back in boat? I have contemplated buying and using a set of midlength 1 5/8ths primary x 3" collector headers as the shorter primary length to collector distances seems reasonable to what the manifold/riser will be. Any suggestions on the best way of going about this would be greatly appreciated!

Outside of your numbers not being a 100% accurate I wouldn't concern myself with it. Every engine I've done has been with dyno headers for the ease of EGT's. Once jetted to satisfaction on dyno I typically end up jetting down a little after a trip or two out with plug checking. The climate where I boat etc jetting has always been close. Make sure they use the same octane fuel you'll be running on the water.

Running open loop EFI setup and at sea level

75-90 degree days. Will be using same fuel. I will be bringing it

Bump

What your reasoning for open loop?

Jeff

Wanting to get away from a permanent o2 sensor in riser

Just for some background,
A friend of mine is running a GM ECU with the o2 in the SM manifold just under the V band clamp. Tapped a hole there. It has been working for four years now.
The fuel economy is nice with the closed loop setup.

I would dyno with 1 3/4 headers. The SM manifolds are big breathing.
Jeff

It is tapped directly above V band clamp currently from where previous o2 was. this software has the capability to run closed loop if i choose to at a later date. Will look into the 1 3/4 headers for dyno. the overall length was my main concern so that they were similar length in total to my manifold/riser setup.

I think it will run a little richer in the boat vs on the dyno with dyno headers. That's been my experience at least.