Ideal Fuel Rating for 351M?

[dentside79]

I have always been running 93oct. fuel in my 351M ever since I rebuilt it about 30,000 mi. ago. I have tried 87 and 89 and the truck wants to spark knock. It has never spark knocked on 93oct. but almost every time I try a grade lower, it does. This engine is bone stock other than being stroked. It has all new parts inside and out. I am running Autolite double platinum spark plugs w/ Motorcraft plug wires. It has a new distributor, vacuum advance, cap, rotor button, coil, everything. The timing is set at 6 deg. BTDC, I have even tried 8 deg. and 10 deg. BTDC to see if it made a difference seeing as how some older 351M's ran at this degree. Plugs are gapped at 0.040 (at least I think that is what I remember gapping them at? I cannot remember off the top of my head). The fuel pump and lines are new, as well as the filter, diaphragm in the carb. and power valve in the carb. The engine runs absolutely perfect! It doesn't skip a beat and runs like new. I just find it strange that it will not run on 87 or 89 octane fuel? What were the original recommendations for this engine from Ford? Why would you think that I am unable to run a lower grade fuel?

 

[surewhynot]

You mentioned the engine is stroked. It is possible that the compression got bumped up in the process. These were originally low compression engines designed for 87 octane fuel. That would be my guess.

Welcome to FTF!

 

[TexasNomad]

Can't turn down the timing some to stop it from pinging Tony?

 

[FirstOnRaceDayAg]

Are you sure the engine was stroked instead of bored? My 351M was bored .040 when it was rebuilt ~ 3 years ago.

Plug gap on these engines is .042-.046 but that shouldn't really have any effect.

Do you still have EGR on the engine? If so, and it's operational, check the EGR plate under the carb. The runners in the plate on my engine will get filled with carbon and the engine will ping like there is no tomorrow.

If you no longer have EGR on the engine, the distributor advance must be recurved to compensate for the missing EGR.

Stock timing on my 77 351M (non-catalyst, no a/c, manual trans) is 12*BTDC. If you advance yours to that, it will really ping in your current situation.

Just some things to think about.

 

[hollenjoe]

By stroking you probably mean you had it turned into a 400, only pistons and crankshaft are different....

 

[JBradley500]

maybe someone put some flat tops in it and made it a 400. that will get you up there in the compression

 

[TexasNomad]

Hey J didn't I trade Carbs with you awhile back?

 

[JBradley500]

2 and a half months later... yeah we traded carbs.