Help choosing suitable stall ratio for converter

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Headintheclouds

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Upgrade from 727 to 42RH is nearly there. I have found out that the stall of the converter currently fitted to my 42RH is 2300.
The car is geared for 130MPH at 5000 RPM, so 26MPH per 1000, 2000RPM at 52MPH. If overdrive comes in at 55MPH and lockup around 65MPH, then driving on main roads at 60MPH in overdrive it will be ticking over at 5000/130 x 60 = 2300 x0.69 for overdrive = 1590RPM. Way below stall speed.
My main reason for changing to overdrive is for comfort and to save fuel but I want to retain as much performance as I can. Is a 2300 converter going to cost me serious miles to the gallon on main roads? Obviously it will be ok on the freeway when locked.
The car weighs about 3500 pounds, 185/15 tires (26.5in diameter) with rear axle ratio of 3.09:1
Thanks for any help
 
They're not designed to be that way even if you're cruising under the converter it's going to be locked up that rating is Max torque
 
That's a relief! I have just discovered that the original 727 converter was 2:1 and original engine delivered max torque at 2800RPM. I have read that stall should be about 500 to 700RPM below max torque so I seem to be on the money.
The engine now has a very slightly hotter cam than original so peak torque may be slightly higher than it was new.
 
Headintheclouds said:
The car weighs about 3500 pounds, 185/15 tires (26.5in diameter) with rear axle ratio of 3.09:1
Click to expand...
185s? on the back? what's up with that?
and a 3.09 rear gear?
with a 2.45 low gear?
and a 2300 stall?
in a 3500# chassis?
That means, your starter gear is gonna be
2.45 x 3.09=7.57, which, at 5 liters is gonna be pretty sluggish out of the gate. 6 is better, 7 is just fine......
With 26.5 tall tires, my math says 65=1760 rpm(see below). Even with a stock cam no SBM, with a stock-type distributor, will make any decent fuel economy at this rpm/roadspeed; because that distributor cannot deliver adequate cruise timing, under these conditions. In fact, I'd be willing to bet that it will do better at 2200.
Now throw a cam into the mix, and it will only be worse. For fuel economy, this ratio demands an aftermarket stand-alone timing computer.
Or
just gear it up to 2200 and call it done. The engine will want about 56 degrees of cruise-timing, and the factory type D cannot be modded to even deliver that, but it gets close, maybe 45/50 degrees.
For 65=2200, you'd need a rear gear of ~ 3.55s and your starter-gear is now; 3.55 x 2.45=8.70, a very significant 14.9% upgrade to performance, from 7.57..
As a sidenote, I have been cruising at 65=2240 for decades, and I find it decent.

But if yur running say 6 or more liters, with a decent amount of cylinder pressure, well, that will help pull the 3.09 x 2.45 starter gear, pretty good.
But good luck getting 6 or 7 liters to be fuel efficient at 1760 rpm.
I've done really good with 6 liters, tho, but forget camming it up much.
The lowest rpm that I have run, with my 6liter, is 65=1590. It did pretty good with just 44* of timing coming from the stock-type modified distributor. But I installed a dash-mounted dial-back timing device with a range of 15 degrees. Thus I would have been able to bring that 44* up to 59*. I don't remember anymore (that was 20 years ago) what that combo liked, other than; at 1590, that 59* would have been too much. I didn't run that combo for very long tho, perhaps one summer.
My current combo runs 65= 2240rpm@52 degrees cruise timing. It likes more, but this is all she gets.

Note-1
(65x1056x3.09x.69)/(26.5 x 3.1416) = 1758rpm
 
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Not sure what your question is but this is a European size. It's an 80% profile
 
It looks as though you would know this size as 27 x 7 15". In the land of UK we are still caught between imperial and metric. Timber would be bought as 4.2 metres of 4 x 2. Like your 2 x 4 but lying down...
 
Headintheclouds said:
Not sure what your question is but this is a European size. It's an 80% profile
Click to expand...

If 185 is the width of the tire in millimeters that's a very very narrow tire that will easily be overpowered unless you have a small engine (as in well under 3 liters) that does not produce much torque/HP. I don't think he misunderstood the size, in North America tire sizes are rated in "mm width/aspect ratio-wheel diam in inches". For example the rear tires on my 1970 Duster are 275/40-18.

Anyway to answer your original question, like @MOPAROFFICIAL said torque converter stall speeds are rated at the (expected) maximum input torque. Below that torque, say cruising or taking off at part throttle the stall speed (more correctly, difference in input vs. output RPM) will be lower. Also since your 42RH has a lockup converter, in most cases when cruising the torque converter will be locked so there will be no slippage or difference in input and output RPM from the converter at all.
 
There is almost zero information here for anyone to give a converter recommendation. You may as well have asked, "I need a converter for my car, it's blue"
 
RustyRatRod said:
There is almost zero information here for anyone to give a converter recommendation. You may as well have asked, "I need a converter for my car, it's blue"
Click to expand...

The title suggests a converter recommendation but I think he's actually just asking if the converter he already has in his 42RH will slip too much and affect gas mileage during street driving... Otherwise I agree lol
 
Thank you for taking the time to give such full replies. I really appreciate the constructive answers. I wish I understood enough to ask the right questions. MopaR&D is bang on. That's exactly what I'm asking.
In stock form with the original 727 with 2000 stall the car was returning 20 MPG (imperial) on the freeway, 15 MPG city driving and 18 average. Converting to US gallons, I think this converts to 16.6, 12.5 and 15, which seems pretty good to me. 0-60 time around 8 seconds, which is ok by me. It's pretty quick to 120MPH but takes a while to get to 130MPH. All a bit academic as there are few places where it can be driven above 80MPH.

A friend who has put a 46RH into an otherwise identical car is getting 28MPG on long freeway journeys. I think that's 23.3 in your money. These are not theoretical figures. They are repeatable and have been tested over a couple of years. His car even has a 2.88 final drive - and the 46RH has a longer first gear. His choice of final drive has slowed it off the line a bit. A dyno test gave maximum torque at 2140RPM.

By the way, the car is red and I intend to respray it green. Does that affect choice of converter?
 
Headintheclouds said:
Thank you for taking the time to give such full replies. I really appreciate the constructive answers. I wish I understood enough to ask the right questions. MopaR&D is bang on. That's exactly what I'm asking.
In stock form with the original 727 with 2000 stall the car was returning 20 MPG (imperial) on the freeway, 15 MPG city driving and 18 average. Converting to US gallons, I think this converts to 16.6, 12.5 and 15, which seems pretty good to me. 0-60 time around 8 seconds, which is ok by me. It's pretty quick to 120MPH but takes a while to get to 130MPH. All a bit academic as there are few places where it can be driven above 80MPH.

A friend who has put a 46RH into an otherwise identical car is getting 28MPG on long freeway journeys. I think that's 23.3 in your money. These are not theoretical figures. They are repeatable and have been tested over a couple of years. His car even has a 2.88 final drive - and the 46RH has a longer first gear. His choice of final drive has slowed it off the line a bit. A dyno test gave maximum torque at 2140RPM.

By the way, the car is red and I intend to respray it green. Does that affect choice of converter?
Click to expand...

Sounds about right regarding the MPG improvements. What kind of car are we talking about here? 2.88 and 3.09 are not common final drive ratios for Chrysler rear axles...

On that note your average fuel economy (and acceleration) would improve significantly if you paired the 42RH swap with a final drive ratio in the 3.5x range.
 
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