Technical Powerband / Peak Torque Chrysler Flathead 6

Hey all, have a souped up Chrysler inline 6 cylinder in a 55 Fargo pickup.
These engines are long stroke and typically reach peak torque at around 1200 RPM.
Now what would be a minimum RPM on the highway, with windage, and running an overdrive trans.
For example, would 1600 RPM at 65 MPH be okay, or would 2200 RPM at 65 mph be better..

 

The closer to the peak torque RPM the better. That's where your engine is most efficient.

 

1200 RPM torque peak seems low to me. Especially if it's souped up a little.
Did you dyno it to know for sure?

 

Yes its a lot lower than peak HP which is near 3600 RPM

Chrysler Flathead 6s have peak torque from the factory from 1200 RPM to 1400 RPM, long stroke engines undersquare and peak torque just off idle.
My current engine is a 228 long block, dual carbs and exhaust, other wise stock, cam is stock, cam profiles change torque to the needs.
The Chrysler flathead 6s have a very low RPM peak torque band, much lower in RPM than a Furd Flathead V8 or Stovebolt 6.
My current old tired engine has a bore of 3 3/8 and stroke is 4 1/4, my donor 265 engine is 3 7/16 bore and 4 3/4 stroke.
The 265 has peak torque of approx 228 ft lbs at 1200 RPM stock...

 

Yes, I'm well aware of the relationship of the cam to the torque and power curves. I'm also aware of bore to stroke relationships, but I wasn't aware that this motor has a 4 3/4" stroke! I can't say that it is a torque monster at 228 ft lbs., but it is definitely going in the right direction. If it pulls it at 1600 rpm @ 65 mph, go for it.

What kind of rpm does it pull now with the 228 motor @ 65mph?

Does somebody offer a hop-up cam for that motor, or would it be better to change the gearing and leave the cam alone?

 

With the 228 and my current rear gears 3.23 and stock 4 speed it is turning 2600 rpm at 65 MPH. Although this engine has good oil pressure and does not smoke, it's tired and leaks in 4 out of 6 cylinders. Its compression is down across the board too. Its runs well but is tired, its a real dog from 40 mph to about 55 mph, the tired engine, and the 3.23 most likely the cause...
Once my new 265 engine goes in I am using a Chrysler A 833 overdrive (.73) and 3.55 posi diff but would keep the 3.23 gears if it would work out okay with the overdrive reduction..
Yes these engines, a 251 has a 4 1/2 inch stroke and the 265s a 4 3/4 stroke, mine is already bored .040 over, my cam profile will be changing, but not sure to what just yet.. hope to achieve near 175 hp or so and rev to 4500-5000 RPM capabilities.

 

Are there mountains where you drive?

 

Hi Jim, the immediate area is flat like the Great Bonneville flats. Hilly areas not far but rarely would I go there.
I live near a city close to 1 million people, but traffic is slow on highways/freeways in this area, average speeds 65 mph for the most part..

 

I think you'll like driving better with a bit more RPM. 2200 at 65 would be fun to try....let us know how it works out.

 

You seem to be talking at cross purposes here, you want to make your peak HP at higher than stock (3600) with the cam but then gear your vehicle to cruise at low rpm. If the OD gearing is apx 70% then your 3.55 will be be around 2.5. Your rpm at 60 is around 2000 depending on tire size. I ran the calc. using 28.3 Which approximates a 225/235 75 radial. Torque gets you going HP keeps you going. When the factory put ODs in cars, the picked higher numerical diff ratios, 3.80 to 4.13 to give you good acceleration and hill climbing, and then used the
OD to calm down cruising rpm. I had a friend who had a slant 6 volare with the A883 OD as I recall it wasn't really good at any thing because the rear end was geared in the low 3's. Had no zip in 2 and and 3rd and you couldn't use top gear on even the slightest up grade or into a head wind. We went from Syracuse to Buffalo going into a 20 mph breeze and couldn't sustain 60 mph without downshifting to third. I would think you would want tires and gears that in OD would yield about three to 1 in crank to diff turns. Maybe 2200 at 60. That would give you good driving response in traffic, and easy highway cruise with a lot of pedal left for grades and breezes. So maybe no lower than 3.7' s or so. Do you plan to use the truck to haul or tow, or just to cruise with maybe just a passenger?

 

Hey Greg, yes a slant 6 powered pickup or van would or certainly could be a real dog with an A833 Overdrive, with 3.08 or 3.23 gears. I would think 3.55 or or ? don't think the Mopar 7 1/4 or 8 1/4 came from the factory with 3.73s.
I would venture to guess a stock 265 would have a whole bunch more torque than a stock 225 slant 6.
With my intention on the 265, obviously it will not be stock, and most likely will be going with 3.55 limited slip gears in a Mopar 8 1/4 diff, plenty of power for my needs and area.
My tired old 228 does not bad with the 3.23 gears in direct drive, this might not be the case with a .73 overdrive unless doing 70+ MPH.
My thread was more aimed at being these long stroke engines, with a peak torque band at 1200 RPM, what might be the best highway RPM for all around cruising, don't think it will cruise at 1500 RPM on the highway like my 2015 Hemi Ram.

 

There's an interesting thing about large bore vs long stroke, regarding torque. There really isn't much difference at all. Consider two engines of identical displacement, with different bore/stroke ratios. If you have a larger bore, and a shorter stroke, the piston has less leverage on the crankshaft..but at the same time, it has more force pushing the piston down, since the piston has more surface area, and combustion pressure is about the same.

I doubt the flathead six has more torque down low...I think it just has less, up top. since it can't breathe worth a darn.

 

Interesting thing is this too Jim.
The 230 L head, in the early 1950s had like a 102 hp, and by the end of it's reign it was near 138 hp.
The biggest change was the compression and head, and to a lesser degree some had a 2bbl carb and intake.
Now take the slant 6, basically not much difference in the l head 6 and slants bottom end, but the overhead valve head makes a big difference.
The nice thing about the Chrysler L head 6s is this, balanced forged cranks, full pressure oil, bearing inserts, 6 exhaust ports.
Limiting factors 3 Siamese intake ports, where a single carb is the least efficient and a triple carb intake can be the best.
I do know going with my 265 is going to push my little Fargo around a lot better than my old tired 228...thanks for your Time...

 

Be prepared to try some different rear gear ratios, let us know how they work out.

 

You don't need or want overdrive.
I had my M37 to over 60mph on 37" tires on flat ground. It had 5.83 axle gears. This equals about 3100rpm. That was a hard as I could push it. But it would run all day and be very happy at 50mph. Which was about 2600rpm.
It had a mil spec 251 with dual carbs and header, flex fan and even power steering.

My friend and dodge mentor has a 40 sedan with a 251, four speed and 4.10 rear gears. And 8.00-15 rear tires. It has 23mpg and scoots along quite well. Easily well enough for 100kmh traffic.

Your engine will actually be way under its sweet spot at 1400. It will lug really bad, and not have any throttle response either.

 

yes with my current rear gears (3.23), and 28.5 inch tires spinning about 2600 RPM at 65 mph, I had her up near 80 mph the other day, interesting ride at that speed but thats another story.
Yes the sweet spot is much higher than 1400 RPM and more like 2000 RPM and over.
Here is the situation, with this old engine, and its a bit tired, when I hit 4th at say 40 mph, and the speedo drops to 1600 RPM, it is a dog until it hits 2000 rpm around 50 mph, then she goes again.
The fresh 265 should improve things a lot, over my tired 228...

 

might just want to shift a little bit later

my wife has a late model sporty car that turns close to 4000 at freeway speeds. In 6th gear.

 

I've got a '56 Dodge with a 230 and overdrive. With 3.50's in the rear it was okay on the flat but dogged pretty quickly on a grade. I switched to a 4.10 gear set which improved drivability but it it's a little too low geared and noisy at 60+. I've got a set of 3.73 's waiting to go in which I think will be ideal with the OD. You having a few more cubes I would think the 3.55's should be about perfect. My tires are about 28-29 inches tall if you want to do the RPM calculations.

 

Yes and I do when necessary, the trans in 2nd is 1.68 to 1, so once in 4th 1 to 1 , the RPM drops quite a bit.
Interesting this old engine likes to rev to about 3000 RPM, but when I was doing up my dual exhaust, this interesting thing happened, with 1 muffler on and none on the other pipe, this engine would spin up higher to 4000 RPM, but with 2 mufflers on it would not. I am running 2 inch pipes and glasspacks. To this day I have no idea why this would be, even with straight pipes it would not rev up as high...

 

Ya gotta remember tees things are not light. Torque and gears get it done, horsepower numbers are irrelevant.
And also remember that the crank weights over 100 lbs. spin it up and keep it spinning. Don't let it drop under the torque peak because it won't pull worth a dam under it.

Or put it in a 1200lb car and spin the tires till they explode.

 

let me guess you have old school glass packs.? Back pressure on these engines costs a lot of power. Put a pair of hushpower mufflers on it.

And if you have split stock manifolds throw them in the scrap and put headers on it. It probably has so much reversion it is trying to run in exhaust not air.

 

Do you have an R10 overdrive from a car in your truck.
230 is a good little engine, right now I am still running the 228, but this 265 will be going in at some point...

 

Got the red 2inch Thrush, the kind you can get at crappy tire.

 

Don't know about the truck but my coupe goes about 2870 pounds add 425 for people, dog and trunk junk so 3300 going down the road. Engine is 56 Plymouth 230, dual carbs, single exhaust, .030 over, .050 milling between block and head so right around 8 to 1 cr. Stock cam, 127 HP at 3280 rpm at the rear wheels. I haven't seen the sheet in a few years but I think 221 lb ft at 1470 rpm. on Mustang Chassis dyno. 4.11 rear, 28.3 inch tires.

Sunday we went on a tour with the local chapter of the AACA, reaffirming the reason I hate follow the leader parades. The lead car chose to drive between 30 and 45 mph. About 10 of us had old cars, most driving their dd cuz it was sprinkling. Most of the folks driving older cars, were complaining about all the shifting they were doing, trying to match pace on hills and curvy sections. I just left mine in high and lugged down to 25 and pulled curves and hills back up to 45/50 with no drama. We went 112 miles on 6.3 gallons of gas.

We visited three car collections of basically over 150 wasted cars and motorcycles. Cars from the 30's through the early 70's just sitting packed like sardines in pole barns garages and barn basements. Some with registration stickers last valid in the 90's and early 2000's. Some in progress on a lift getting exhaust installed for the past 10 years. Some stuck up in the rafters wrapped in what looked like Saran Wrap. Nice to see them, sad to realize they probably will never see the light till somebody dies. Oh well still a good day driving our old Mopar. Enjoying the torque.

 

Thrush are alright, I had the longest ones they make on the dodge but in 2.25" single. Some of the cheap ones are a weird bypass design and don't flow at all.

You didn't say weather or not you have a headers or split manifolds?

 

They are stock manifolds, but a true split not just some outlet welded on

 

Get rid of them. As indicated by the weird rpm limits with and without them they are not working.

 

In your words why?

 

You wrote the engine will rev to 4000 without the one pipe attached, but not both. That tells me in one easy step you have a lot of backpressure. Which means the exhaust can't get out the exhaust fast enough. If you can get the exhaust out you can't get fresh fuel and air in.

The stock dodge Plymouth manifolds are a horrible design. A set of Fenton headers or some tube headers is going to free up a bunch of breathing, throttle repose and gain you a bunch of driveability.

 

That's not what happened , with 1 muffler off, it would spin up, both exhaust manifold outlets had pipes.
I have cylinders leaking really bad, so I attribute more RPM in this situation to that fact alone when 1 bank of 3 cylinders had had more flow, compression is down a lot. 85-90 psi and more than 50% leakage on a few cylinders in a leak down test.
I have no doubt this will not be an issue with the 265 with higher life longer duration cam, spinning to 5000 RPM should be attainable. Stock lift is usually .375 lift, and mine will be 400 lift 250 degrees, spinning north of 5 grand...duration
These custom made manifolds are well made, have been used by many, many, flathead owners, a friend of mine has them on a 265 powered 50 Plymouth, with 3 carb intake, 3 ring Venoila lighter weight pistons, Edmunds head, this thing spins to 5500 rpm with ease.
There are some good header designs out there, just don't think the Fentons built for a Chrysler flathead are one of those, they look nice, but not sure they flow well at all..