Let's have some fun!
Let's start with a base line, the stock 455 3.07 gear package.
One thing we don't talk about much is torque converters. And what I want to talk about the effectiveness of balancing your power train system.
This will not be about mileage. I think most agree, it's about 125 HP to run them down the at 60 mph. That is going to take X amount of Gas. To get
more MPG we need to burn the fuel more efficiently or use a more powerful fuel(diesel). If you know of a way to get fuel to burn more efficiently in
any motor that will fit in a GMC let me know. I remember reading that in the fuel injection age, the top engineers have only got about 5% more
internally efficient motors.
So for our project today,let's create a baseline. Let's call it torque for acceleration. TFA.
For the discussion we need to establish a few things. First, that we don't spin our tires from a WOT start. I know most can. 2nd, our stall speed on
our stock torque converter is 1800 rpm. If someone knows for sure that was not the origanal new number let me know. How do I think that is the correct
number. Go back to 1973, we were in a 55 mph speed limit. with the stock 3.07 gear 55 cruise would have been 2090 rpm. I have been told since 1975 on
a non lockup converter, stay 200-300 below your cruise rpm so the trans does not slip excessively and over heat. Higher gear more stall. Remember
this, it's important later. Also the torque multiplication of average converter 2.5.
Let's make our baseline.
torque at stall X first gear ratio X final drive ratio = TFA
1. Stock 455. 350(stock 455 at 1800 from Jim's dyno chart) X 2.48 X 3.07 = 2665 TFA
2. Stock Caddy. 400(from what I can tell form Cadco site Caddys make about 50 more across the range) X 2.48 X 3.07 = 3045 About a 15% increase. That
power you feel in the seat of your pants. No bolt-ons (headers intake)on the 455 will give you that much. Maybe 35-40 at that rpm range. So why didn't
GM just put in the Caddy. My guess, the peak torque of the Caddy at 500+ way more than they though would live in front of the 425 under the heavy load
of the GMC.
3. Add a gear to Stock 455. 350 X 2.48 X 3.50( a lot of people like this gear)= 3038
About the same as the Caddy. And according to sources the gear they wanted to put in but didn't. And the same lesser load on the transmission.
4. It's 2017 and we know we cruise a little fast let's use 65 mph. New cruise rpm 2470, so new stall speed is 2100 rpm. About 400lbs from Jim's sheet.
So with just a torque converter change it would feel like the Caddy. 3045 TFA
5. Lets add a Gear. 400 X 2.48 X 3.50 = 3472 That's a 30% increase! That's more than seat of the pants!
6. Let's get it all. I know I like to run at 70. Right at 3000 rpm with 3.50 gear. Lets put in a 2500 stall converter.
450(from jim's chart and right around max} X 2.48 X 3.50 = 3906 TFA That's a 47% increase and we never touched the motor!
FYI the switch pitch convertor is about 1800 low and 2400 high from what I find. And as side note, my switch pitch will not normally spin the tires in
low, but under the same conditions will spin in high.
So what is this all about?
Balancing your system. Most agree that GM didn't even do it right in the beginning. We got a 47% increase in feel, and didn't touch the motor. No
added load to the transmission. The only concern would be running in the slip range of the converter. and if you don't monitor your trans temp now,
you should be. And even cruising in the slip range won't be that bad. Remember stall is a function of load input.
Or as I like to look at it, it takes 47% less to move the same load.
Somebody said something about luging the HT383 that I am proposing. We are already luging our 455, not to mention the poor 403s.
Reducing the load on your engine, I don't care which one, is going to make it last longer.
Rock away!
Let's start with a base line, the stock 455 3.07 gear package.
One thing we don't talk about much is torque converters. And what I want to talk about the effectiveness of balancing your power train system.
This will not be about mileage. I think most agree, it's about 125 HP to run them down the at 60 mph. That is going to take X amount of Gas. To get
more MPG we need to burn the fuel more efficiently or use a more powerful fuel(diesel). If you know of a way to get fuel to burn more efficiently in
any motor that will fit in a GMC let me know. I remember reading that in the fuel injection age, the top engineers have only got about 5% more
internally efficient motors.
So for our project today,let's create a baseline. Let's call it torque for acceleration. TFA.
For the discussion we need to establish a few things. First, that we don't spin our tires from a WOT start. I know most can. 2nd, our stall speed on
our stock torque converter is 1800 rpm. If someone knows for sure that was not the origanal new number let me know. How do I think that is the correct
number. Go back to 1973, we were in a 55 mph speed limit. with the stock 3.07 gear 55 cruise would have been 2090 rpm. I have been told since 1975 on
a non lockup converter, stay 200-300 below your cruise rpm so the trans does not slip excessively and over heat. Higher gear more stall. Remember
this, it's important later. Also the torque multiplication of average converter 2.5.
Let's make our baseline.
torque at stall X first gear ratio X final drive ratio = TFA
1. Stock 455. 350(stock 455 at 1800 from Jim's dyno chart) X 2.48 X 3.07 = 2665 TFA
2. Stock Caddy. 400(from what I can tell form Cadco site Caddys make about 50 more across the range) X 2.48 X 3.07 = 3045 About a 15% increase. That
power you feel in the seat of your pants. No bolt-ons (headers intake)on the 455 will give you that much. Maybe 35-40 at that rpm range. So why didn't
GM just put in the Caddy. My guess, the peak torque of the Caddy at 500+ way more than they though would live in front of the 425 under the heavy load
of the GMC.
3. Add a gear to Stock 455. 350 X 2.48 X 3.50( a lot of people like this gear)= 3038
About the same as the Caddy. And according to sources the gear they wanted to put in but didn't. And the same lesser load on the transmission.
4. It's 2017 and we know we cruise a little fast let's use 65 mph. New cruise rpm 2470, so new stall speed is 2100 rpm. About 400lbs from Jim's sheet.
So with just a torque converter change it would feel like the Caddy. 3045 TFA
5. Lets add a Gear. 400 X 2.48 X 3.50 = 3472 That's a 30% increase! That's more than seat of the pants!
6. Let's get it all. I know I like to run at 70. Right at 3000 rpm with 3.50 gear. Lets put in a 2500 stall converter.
450(from jim's chart and right around max} X 2.48 X 3.50 = 3906 TFA That's a 47% increase and we never touched the motor!
FYI the switch pitch convertor is about 1800 low and 2400 high from what I find. And as side note, my switch pitch will not normally spin the tires in
low, but under the same conditions will spin in high.
So what is this all about?
Balancing your system. Most agree that GM didn't even do it right in the beginning. We got a 47% increase in feel, and didn't touch the motor. No
added load to the transmission. The only concern would be running in the slip range of the converter. and if you don't monitor your trans temp now,
you should be. And even cruising in the slip range won't be that bad. Remember stall is a function of load input.
Or as I like to look at it, it takes 47% less to move the same load.
Somebody said something about luging the HT383 that I am proposing. We are already luging our 455, not to mention the poor 403s.
Reducing the load on your engine, I don't care which one, is going to make it last longer.
Rock away!
