just as the topic says, I am planning to install a TPI onto a stock 260HP carbureted Mercruiser 5.7l engine. I think this will be a great combo since the max recommended RPM on these motors is 4500-4800 rpm anyway. Took me a while to find the cam specs: "269/280 adv and .394/.403 lift. What surprised me is when I plugged all of the stock motor parameters into dyno 2003 and it was very close to factory HP specs: 255hp@4500 and 334tq@3500 with 304tq@2000. I changed just the induction to Tuned Port and the new numbers are 252HP@4500 and 340tq@3000 with 318tq starting at 2000. Not really any measurable gains but I bet it will save some gas in the longrun and run very smooth. I will be using a 727 ecm mounted under the dash to keep it dry. Here's the questions: how in the world will I fine tune the ecm? It will of course be running in open loop since there are no O2 provisions on the manifolds.. not so bad but I will need some kind of feedback to know what the engine is operating. Then I thought of a method they used in the old days.. ua vacuum gauge. Could this be an (ok) tuning aid? Should I start with a stock 350 $8D prom? I would also think only a limited number of VE cells would need to be tweaked since the load will be pretty much linear for a boat. What about AFR and timing? Any thoughts, comments or suggestions would be greatly appreciated. I will be sure to post back pictures if anyone is interested. Thanks

 

The cam in that Mercruiser engine is very similar to a stock TPI cam. The head flow and compression are likewise very similar. My only assumption is that this is NOT a Vortec-headed Merc engine (my brother knows a lot more about the Merc motors than me, but I know a bit about them).

That being said... Job 1 is you need to make sure your lower intake manifold on the TPI will work with the bolt pattern on your heads. Whether they be old style 85-down bolt pattern (auto application- Merc used the old style heads for many years after GM abandoned them), 87-up bolt pattern or Vortec bolt pattern (TPI Vortec lower intakes are available from Scoggin Dickey- SDPC2000.com).

You have an O2 issue, that you already know about. If your motor has Vortec heads you have an EGR issue as well (Vortec heads do not have an exhaust crossover passage in the heads to feed the EGR system on an F-body TPI intake).

If it was me here's how I'd proceed: Get a custom chip based on the factory proghramming but one that deletes closed loop (O2) and the EGR function (if necessary) but leaves the rest of the fuel maps and such stock. It'll probably run pretty good right there. The next step up would be a "best guess" custom chip from one of the usual sources like Fastchips, etc. Beyond that, there's not much more you can do without pulling the motor, running it on the dyno with a wideband O2 and having the fuel and spark maps custom tuned for the application.

 

^Whoa there; you sure know you're stuff! And btw I have recomended you to a few people regarding tuning Q-Jets.. so just incase.. err, well, you know

 

it is an 88 motor with centerbolt heads.. straight plugs, not sure of the casting #s yet. the 86 tpi base I will be getting has the older style angle on the center bolts but I can buy the small wedges to correct the angle there. I have all of the hardware to modify my own prom "Tunercat, memcal flash adapter, and eeprom programmer. I will start with a factory AUJP mask BIN "90-92 SD 350 TPI auto" and can start by removing the O2, VSS, VATS, and EGR diagnostics. What else can you think of?

 

Paul, I think you got it about covered. If you have the older style TPI intake don't forget about the 9th (cold start) injector. I think you can just leave it unplugged and run it with the later year programming that didn't use one.

Custom chips aren't really my bag. I know a little, but honestly you probably already know a lot more than me.