I know the 1977 351m was given a horsepower rating of 161h.p.@3400r.p.m. but I was wondering what you guys think my engine has. The engine runs fine and is in good shape. I recently gutted my catalytic convertor and put on a cherry bomb muffler and I was wondering what you people think my horsepower might be at now since the mods? I can definetly notice an increase in acceleration.

By the way, my info on the bottom says 79 Bronco but it has a 77 engine in it. Also, disregard the carb and intake info because I put the stock equipment back on.

 

I thought the 351 was rated at 135 hp. The 400 at 165.

If all you changed was the cat and muffler I'd guess 10 or so hp.

If you really want it to run, you have to put in a matched cam and timing chain.

 

I wouldn't put too much faith in factory HP ratings, they were typically under-rated at that time- measured at the back wheels, some with all accessories running, i.e. if the vehicle had air conditioning, they rated it with the A/C on.

I believe any rebuilt 351M with a stock-level replacement cam will pump out around 250 HP at the flywheel. With a 4-barrel and bigger cam and headers, 300 HP would be no problem. This is assuming no cat converter, EGR, or other power-robbing emission controls.

 

We've had this discussion before.

Post 1971 factory HP ratings were not taken at the rear wheels. They were taken at the flywheel but they did include a 'standard' set of accessories and full exhaust system. The results were called 'SAE net horsepower'.

Pre 1972 ratings were done on a stripped down, bare essentials only motor and were known as 'SAE gross horsepower' ratings, primarily for inter-manufacturer bragging rights.

 

Bill,

I have to disagree there, SAE net means, rear wheels.

No offense but, where ya been ??

 

SAE Standards J1349 and J1995 which outline the test procedures and correction factors for determining gross and net horsepower of an engine. Neither mentions testing vehicles at the rear wheels, only at the flywheel. The only difference is the accessories, emissions equipment and exhaust system. In general SAE Net HP is about 80% of SAE Gross HP.

 

OK I'll give a little on this one, as SAE standards TODAY for most vehicles use flywheel with accessories on, but that is/was not always the case.

During the 1970's, each mfgr. used their own method. Some used different temperatures, altitudes, rpm levels, and flywheel or rear wheel ratings- before it became "standardized". But now, let's look at this logically from our own standpoint, for the sake of discussion.

Gross pay- what you make before taxes and deductions.
Net pay- what you actually get to take home and spend.

The same goes for HP. IMO, we can't say the flywheel HP is net HP and be 100% correct, as the power must still go through the trans, driveshaft, and rear axle of a truck, before it reaches the rear drive wheels. (I use rear drive cuz this is a truck forum, and we all have RWD as primary drive mode) All these drivetrain pieces absorb power. We've found about a 50-70 HP loss on race cars, from the actual dyno sheet for the engine, to the track times in mph. Been there, done that. 50HP or more simply "vanishes" in the drivetrain, that was there at the flywheel when the engine was dynoed.

The typical 350 HP vintage 1970 engine, made only about 275 HP to the rear wheels after parasitic losses.

So, "IMO", net at the flywheel, isn't really net. To be truthful to ourselves in the discussion, we have to mention and recognize ALL the drivetrain HP loss. It would be illogical to ignore what the trans, driveshaft, and rear axle use up. It's a big number, typically around 15% total engine power lost from those components, in a street engine !

Bottom line, I don't count money that I never get to spend- and I don't count HP that never gets to the rear wheels. Why ?? That's what turns and makes the truck go !

Good discussion...

 

No, prior to 1972 manufacturers used SAE Gross HP ratings and while they may or may not have "fudged" them a little by using specially prepped engines depending on what the competition was doing they still used a specific SAE standard measured at the flywheel.

In 1972 they decided to use a different standard for whatever reason but it was still measured at the flywheel. It's just that simple. I've never seen factory published numbers measured at the rear wheels regardless of year.

 

o.k. guys. I just wanted to know how much h.p. my truck has now. Still not sure.

 

One of the 4 wheel drive magazines dyno'ed a 78 or 79 bronco with 351. It was a tire compare between the hp to the ground with 33" and 35" tires if I remember right. Their bronco with bone stock worn out 351 only managed to put around 90 or so ponies to the ground. Not very impressive at all, but reasonable given the hp rating of 135 or so that most of my books read.

If you want to check it out, I think it was 4 wheel and off road, but I'm not sure, and it was about tires, so you would have to start there for your search.

Many people seem to forget that now is the best time ever to live in regards to hp/cu in available from the factory. In 1985 a 4bbl 350 chev in 3/4 ton trucks was rated at 175-180 hp depending on gvw and smog equipment. Nowadays any v-6 will approach or exceed that while giving about 50 % more fuel economy.

The only difference is that you could take that same 350 and work it over to get 300 hp relatively easily. Things aren't nearly so simple these days. Not only that, but torque in many of the newer engines is dismal, and peaks somewhere near 4000 rpm, where it is of little use in a heavy truck.

 

Thank you very much Willd,
Do you know what the h.p. ratings are for a 77 351m? cause my 79 Bronco has a 77 in it.

 

You can find the factory specs for your engine here:

Bubba's M block page

 

thank you

 

1big79, you had 161 and then came carbon, lost compression, worn bearings etc... and you added like 10 hp as mentioned before but realistically you back to stock if anything.

 

Well, even though my engine is old, it still keeps an oil pressure of 75 p.s.i. when started and it hasn't dropped below 50 p.s.i. when driving. So I think by completley gutting out my catalytic convertor and adding a free flowing muffler, I have added more h.p. to it than stock. Also, I am guessing that the guy who owned the Bronco before me wasn't an idiot and the 1977 351m he put in it was better than the stock engine. Why else would anyone do that?