1939 Plymouth sedan 4 door 82 hp

[Jamesjames]

I have a 1939 Plymouth with a flat head six. My question and my curiosity is wondering how I can keep the original engine in but add some horsepower to speed up the ride a little bit. It has 82 hp right now but if I can enhance that it would be great. Any ideas on how to add horsepower without changing the engine?

[Rusty_OToole]

Start by doing a compression test and checking the oil pressure. This will give you an idea what kind of shape the engine is in. They are foolers, and can continue to run smooth and quiet in an advanced state of wear. The only symptoms are, they become hard to start, burn more gas and oil and are down on power.

 

If the engine is in tip top shape you will not notice any lack of power in normal driving. I know 82HP does not sound like much but your long stroke engine produces so much torque at low to medium speeds, that it feels a lot more powerful than it really is, and gear shifting is almost redundant. On level ground it will pull away from 10 MPH in high gear without strain.

 

I know people with similar cars, one was a 1941 Plymouth. The owner, like you, thought of changing the engine or transmission for better performance. But first he had the stock engine rebuilt. This brought back all the missing horses, and took away all desire for a different engine or transmission.

 

Another owned a 1947 Dodge. It ran smoothly enough but had very little power, and compression was practically non existent in some cylinders. She drove it to the garage for a rebuild. They were astonished it would even move under its own power as it had more than one broken piston and was in an advanced state of wear. With the engine rebuilt it ran as smooth and quiet as a new car, started instantly, and had all the power anyone would want for normal driving. Notice I said normal driving, as in keeping up with normal traffic, climbing hills etc. not racing or burning rubber.

 

Normal engine life 50,000 to 80,000 miles with one or two overhauls. By that I mean, ring and valve jobs. If your engine needs to be rebuilt all parts are available cheap and they are not a hard engine to work on. Vintage Power Wagons has a lot of NOS parts. They have NOS pistons for $75 a set of six, how good do you want it?

https://www.vintagepowerwagons.com/_files/ugd/a95fd2_06771433e51b4d40ba239f331ee71bf7.pdf?index=true

 

It is possible to hop up your engine but be sure it is in top shape first. The easiest way to blow up a worn engine is to try to soup it up. Be sure the bearings, pistons, valves etc are all like new before you do anything else.

 

If you put the engine in like new shape and still feel a lack of power come back and I will give you a few tips.

Edited September 8, 2022 by Rusty_OToole (see edit history)

[Rusty_OToole]

It could be a sharp tuneup or a ring and valve job is all you need. A compression test and oil pressure check will tell you a lot.

 

PS you do not say if you are in the US or Canada. Canadian made Plymouths and Dodges used a different engine from US made models but they look the same. It makes a difference if you need parts.

Edited September 8, 2022 by Rusty_OToole (see edit history)

[Jamesjames]

Thank you that is very helpful. I am in the US. And the car starts great I actually did a 6 V to 12 V conversion on it as soon as I bought it and got everything running great and it runs fine and starts immediately. We live in a mountain area so there’s a lot of uphill and downhill. But TopSpeed is only about 38 miles an hour. I figured if I can add a few parts to it to just game a little bit of horsepower it would be all I need to get around a little bit more conveniently.  I will do the oil pressure test and see where I stand.

21 minutes ago, Rusty_OToole said:

Start by doing a compression test and checking the oil pressure. This will give you an idea what kind of shape the engine is in. They are foolers, and can continue to run smooth and quiet in an advanced state of wear. The only symptoms are, they become hard to start, burn more gas and oil and are down on power.

 

If the engine is in tip top shape you will not notice any lack of power in normal driving. I know 82HP does not sound like much but your long stroke engine produces so much torque at low to medium speeds, that it feels a lot more powerful than it really is, and gear shifting is almost redundant. On level ground it will pull away from 10 MPH in high gear without strain.

 

I know people with similar cars, one was a 1941 Plymouth. The owner, like you, thought of changing the engine or transmission for better performance. But first he had the stock engine rebuilt. This brought back all the missing horses, and took away all desire for a different engine or transmission.

 

Another owned a 1947 Dodge. It ran smoothly enough but had very little power, and compression was practically non existent in some cylinders. She drove it to the garage for a rebuild. They were astonished it would even move under its own power as it had more than one broken piston and was in an advanced state of wear. With the engine rebuilt it ran as smooth and quiet as a new car, started instantly, and had all the power anyone would want for normal driving. Notice I said normal driving, as in keeping up with normal traffic, climbing hills etc. not racing or burning rubber.

 

Normal engine life 50,000 to 80,000 miles with one or two overhauls. By that I mean, ring and valve jobs. If your engine needs to be rebuilt all parts are available cheap and they are not a hard engine to work on. Vintage Power Wagons has a lot of NOS parts. They have NOS pistons for $75 a set of six, how good do you want it?

https://www.vintagepowerwagons.com/_files/ugd/a95fd2_06771433e51b4d40ba239f331ee71bf7.pdf?index=true

 

It is possible to hop up your engine but be sure it is in top shape first. The easiest way to blow up a worn engine is to try to soup it up. Be sure the bearings, pistons, valves etc are all like new before you do anything else.

 

If you put the engine in like new shape and still feel a lack of power come back and I will give you a few tips.

Once everything is checked out. I would love to invest in some aftermarket products that could help boost the horsepower as well. Any ideas

[28 Chrysler]

Spinning on 12V will mask a lot, a good Mopar 6 will do much more than 38 MPH.

Do a compression test, if it is low in 1,2 or 3 cylinders do a leak down test, check the timing, plugs and wires.

 

[Rusty_OToole]

When new top speed was in the low 80s and it would cruise at 50 - 55 without strain.

If you really want to go big there are dual carb intake manifolds ($200 - $300 plus carburetor) high compression, finned aluminum cylinder head ($685) dual exhaust manifolds ($250).

I don't recommend any of those things because of cost. If you can find one, a stock head off a newer engine will give higher compression. 1957 -59 Plymouth and Dodge I believe had the highest compression. Or you can have your old head milled. This works especially well at high altitudes where the air is thin which lowers compression pressure.

 

The oil pressure gives you an idea of how good the bearings are, compression test tells how good the rings and valves are. If compression is over 100 in all cylinders your motor is in good shape. If it is all over the place like, 60 50 90 80 etc you know the engine is badly worn. In that case you will need to take off the head and inspect for worn cylinders and worn or burned valves.

[Jamesjames]

Great useful information. 
exactly what I needed. Thank you so much from a rookie exploring I appreciate it. I will go through all of these suggestions and get it all fixed up. 

[Rusty_OToole]

For your amusement, see these back yard hacks revive a flathead Dodge that has been decaying in a field for 50 years. Those old Dodges and Plymouths are hard to kill. By the way in spite of appearances they are good mechanics. They like to have fun messing around with old cars.

 

 

[Rusty_OToole]

One other tip about doing a compression test. Write down the pressure for each cylinder. If it is low squirt some oil in and test again. If compression comes up you have bad rings, if not you have bad valves.

[Rusty_OToole]

If you really want to go crazy a Chrysler or DeSoto six will bolt in your car, although it is 2" longer and requires a few mods like moving the motor mounts and radiator forward. You can get up to 265 cu in and 135HP stock, more with a mild hop up.

I DO NOT recommend this, if your motor was running like new I am sure you would have all the power you need.

[Rusty_OToole]

230 cu in six from a later model Plymouth or Dodge bolts right in and gives up to 132HP stock. If your engine is shot and you can find one reasonable it might make sense to rebuild it and put it in your car. Depending how original the car is and how original you want to keep it.

[Jamesjames]

That’s great to know. Right now the current engine runs good that maybe just getting peak performance out of it might be what it needs without the move up . But if the engine was shot definitely would be my new direction and great to know they can actually fit if I needed more HP

[1939_Buick]

9 hours ago, Jamesjames said:

 But TopSpeed is only about 38 miles an hour.

 

My uneducated guess is something is seriously wrong with the engine.

On level ground should do more than that, as above.

[wayne sheldon]

Yeah that speed for a top speed is absurdly low for a car like that! Every model T Ford I ever had would go faster than that! And that includes the TT truck I had forty years ago. It did have an overdrive, and in overdrive, it would do 42 mph on level roads.

[Ben Bruce aka First Born]

Yeah, should do more than 38 mph in first gear!    This could be an interesting thread. Please keep us up to date. 

 

  Ben 

[nickelroadster]

Pay attention to rRusty.  He is giving very good advice.

[bryankazmer]

I used a 41 Plymouth (same basic engine but I think 85-87 hp by then) as a daily driver.  While not a fast car in its day, as others have said 50-55 mph was a practical speed, and in town there were no issues of driveability.  Rusty's advice is spot on.

[Rusty_OToole]

If your engine needs to be rebuilt here is some info that may come in handy. Your engine is a 201cu in with 3 1/8 bore cylinders. A few years later they made the same motor with 217 cu in and 3 1/4 bore cylinders. Your engine can be bored 1/8 oversize and use stock size 217 pistons.

Here is why this is good to know. Stock size pistons are always cheaper than oversize pistons. I suspect the low priced pistons from Vintage Power Wagons are stock size. Now you know, stock size 217 pistons will work in your motor if you bore the cylinders.

[vermontboy]

Have you checked the timing, both at idle and at higher rpm to make sure the advance is working. As others have stated - top speed of 38 mph doesn't make sense unless something is drastically wrong.......

[Jamesjames]

I definitely will check that thank you so much

[Grimy]

Suggest you also test for a clogged exhaust system.  It's easy, cheap, and non-invasive.  On a warmed-up engine, plug a vacuum gauge into a source of vacuum (such as for vacuum wiper motors).  Start engine and have an assistant hold the throttle/accelerator pedal to maintain about 1,500 rpm *continuously* and *steadily* for TWO full minutes while you monitor the vacuum gauge.  The gauge reading will stabilize within about 15 seconds.  Note that initial, stabilized reading and watch to see if the vacuum reading diminishes over the balance of the two minutes.  If so, that's an indication of a clogged exhaust system, which could be collapsed or crudded up muffler internals, or critter nests in the tailpipe, or even a stuck manifold heat control valve.

[cpwalter]

One thing I would do is look for a 1939 Chrysler overdrive.  It will allow the car as is to cruise at 60 plus on the highway.

[Jamesjames]

I just found the pull knob that says overdrive on a kit. It’s good to know . Thank you I will definitely be getting that going.

[32Buickman]

Jamesjames,

Where did you find the over drive kit?  My friend has a 1939 Plymouth.