AFR table for NA Marine engine??

Mr Gadgets · Post by **Mr Gadgets** » Tue Jun 20, 2006 11:22 am

I did a search but didnt find much.. did I miss it?

Has anyone setup their squirt for a large NA marine engine used in an offshore boat. I am about to make the switch from ______ brand to MS. I am wondering what values of AFR to use at idle, coming on plane and afterwards. I know that at some point I want to see 12-12.5 (peak torque?). Just not sure where to start.

Boat is 28' Checkmate, 5700#'s, 565" BBC, sheet metal intake. 1000cfm x2 throttle bodies. Decent size solid roller cam. 6300 rpm max.

engine load will be light at idle, but will increase as boat comes on plane, then level off when on plane. As aceleration increases load does too.. So do I start with AFR of 12 across the board or what..

Anyone been there yet, can give me some pointers.. I would surely appreciate it.. Hate to burn down a piston in the new motor..

Thanks
Dick

boost junkie · Post by **boost junkie** » Tue Jun 20, 2006 1:27 pm

For light load you can get away much more lean. Shoot for 14-15:1. Save some fuel.

Medium/Heavy load will need to be richer, 12-13:1 range. Usually you'd want to tune it on a dyno to see what the motor wants. Without the dyno I like to run it on the rich side to keep it safe.

The real fun is in tuning the ignition timing!

fscott · Post by **fscott** » Tue Jun 20, 2006 7:20 pm

I'm curious about how you'd dyno a marine engine. Are there actually dynos that hook up to the prop shaft? How effective is this compared to simply mooring the boat to a really sturdy dock and getting on the gas?

Fred

Mr Gadgets · Post by **Mr Gadgets** » Wed Jun 21, 2006 7:07 am

I understand that concept of light load/lean. At what point is it considered light? How many inches of vaccum or KPA does the transistion from light to moderate to heavy.. Can you put a number on it or is it relative to each engine setup??

With my old setup I have not seen maximum vaccum at idle because it wont clean up.. but if I remember .. somewhere around 6" of vaccum...

There are prop dynos available, but they usually are for smaller motors.. and outboards... I just didnt want to spend a stack of money on dyno tuning with the new setup... The boat acts like a dyno with a load on it all the time...

Thanks
Dick

PSIG · Post by **PSIG** » Thu Jun 22, 2006 5:35 am

I built a few offshore race engines like 20 years ago that were carbrueted. The mixture was tuned on-the-fly in the cockpit similar to an aircraft. The top wrench of each boat did the actual tune setup. In general, the mixtures were 13-13.5 for launch accel at full throttle, which was preset as the Full Rich mixture setting for max torque at Cold temp (140Â° outlet temp IIRC) . Once stable and positioned (race position), it was back to roughly 85-90% throttle and full lean. Full lean was adjusted based on peak EGT and rpm response, but I'd guess in the 16:1 range for typical engines at an adjusted outlet temp of about 180ishÂ°. Lots of cool water out there

HTH,
David

EDIT: Oh, forgot you wanted idle, too. You can get that by setting mixture for the lowest MAP at idle and maybe a touch on the rich side from there for idle stability. AAMOF, if this is a pleasure boat and not on the ragged edge, you can set your fuel and spark maps the same way since it's loaded all the time. Set your throttle smack on an rpm bin and adjust fuel for lowest MAP, then set timing for best clean rpm's, then reset throttle back and recheck fuel. Move on to next bin and repeat. Chase the MAP bins with different loadings to cover all the bins you can. Easy squeezy. Eric's Auto Tune may get most of the VE's roughed for you and then tweak it from there for reliable efficiency.