I have a 1979 2wd with a 351M, 3 speed automatic, it runs and drives ok up to 60 mph then it start to get a bit hairy, seems like if I push it will blow. I want to be able to take it on the freeway but I have no idea what to change or why. The engine was just rebuilt and seems solid and I hear the 51M is used for all sorts of hot rods and racing etc (I'm sure with work done).

I dont need super performance, just need the increase in something to get and stay on the freeway.

Any ideas?

P.S. I wont be doing the work, I'll have the garage do it but I want to be able to understand the lingo before I go in.Thx

 

What do you mean by 'hairy'? We need more to give you better advice.

 

What rear gears do you have?

 

Install a tachometer. It'll provide better info on cruising speed vs RPM.

Generally, aim for 2000 to 2200 RPM at whatever speed you prefer.

Calculators abound on the web... use 'em. At a minimum you'll need to know your tire height. For example: http://www.summitracing.com/expertad...tio-calculator

65 MPH at 2200 RPM with a 30-inch tire results in a desired rear gear ratio of about 3.00.

 

No idea, sorry

 

It means that it seems it's at its limit and would need another gear or two

 

Taller tires will help...like in the 33in range. Unless your already running 33's.

 

As an example my 78 f150, 300/6, 4 speed np435. With i THINK 3:55's runs about 3k at 70 with 235/75/15's

 

Before OD transmissions were available/common trucks ran higher revs on the highway. Personally I think the low 3k RPM range is OK on a healthy engine. It shouldn't hurt it... nor will it likely go 200k+ miles. New vehicles typically cruise in the low 2k RPM range with OD.

To help guide your effort, you really need some more info. Rear axle ratio and rear tire diameter being most important. Tire diameter you can look up on the manufacturers website for your size. Original rear ratio is specified in the door plate (if not changed).