relationship between boost and hp/torque

[TStClair]

Hello folks,

I would like to know the relationship between boost, hp, and torque. E. g. If I am running 22 -23 lbs of boost max, what does that translate to in hp and torque? Part of the answer depends on rpm, but it seems that similar hp ratings in Ram trucks and motorhomes have different torque ratings. I have a 1991, 30ft motorhome with a non-intercooled 5. 9 cummins. It is stock except for a 14cm exhaust housing, and a 'tighter' torque converter for the allison 542 trans. I have a digital SPA gauge for the exhaust temp and boost.



If I understand correctly, 22lbs of boost very roughly translates to 220hp (probably a bit less), but what kind of torque does this translate to? Torque matters more to a motorhome than hp. If I recall the numbers correctly, an early 210 hp motorhome has 605 ft lbs of torque, while a Dodge of similar hp has much less torque (to keep the driveline alive). What is the difference in 'tuning'? The intercooler is larger in the MH, but what else is different? The stock settings on my rig were 160hp/400ft lb, the same as the truck. I want to bias most future modifications to torque increases, but I'm not sure how to. I plan to have a 3. 5" exhaust (currently its 2. 75) to lower egt's, installed at the same time as an exhaust brake (w/ 60 lb springs). I will adjust the pump marginally, but I don't want a lot of smoke. I'm not sure I want to change injectors if just a pump adjustment will suffice.



It is amazing to me how well a 5. 9 liter engine can push around a 30ft brick, but I always want more power (sound familiar?).



Tom

 

[KEENO]

Welcome Tom!

Hey Tom!



Welcome to the First Gen Club!



I'm don't have any hard answers for you on this one... I'm just part of Welcoming Committee today!



I've heard the 12 valve "P Pump" guys mentioning that 1 lbs of boost = 10 hp as a rule of thumb, but I feel this is too general to estimate actual HP. I do know that "Power Wagon" has a very involved formula for calculating actual HP/Torque.



You can find him lurking around the "Cummins Coversions" & "General Diesel" forums.



Tim "KEENO" Keen

 

[Cummins Cowboy]

one thought is that max hp on our engines is measured at 2600 rpm add more fuel around there and i am sure you will get more hp, now torque is measured at what somewhere around 1800rpm add more fuel here and yes you will get more hp but this will pump up the torque at where it is maximum. so i think the differences in hp and torque with the same engine all depend on where and at what point the fuel is added and the rpms the readings are being measured

 

[BushWakr - RIP]

Personally I think TStClair is on the right track. First if you've received your latest TDR mag. you'll find a reprint of HP/Torque numbers and their relationship in it. It's a little hard to read in places but still good to go over. When you apply fueling and how much will directly affect your HP/Torque. No matter when you do it though you cannot avoid the corresponding increase in both. I think the MH application uses different injectors than our Dodge trucks do,hence the different torque numbers. You can 'pick things up' a bit with pump adjustments (free) but the VE44 pump will only take so much of that. Pump timing is important.

It is also my understanding that these engines produce the torque at or around 1700-1900 RPM.

Bill K. has said many times that. . "when you uprate your power you effectively raise your converter stall speed. That is a major factor in my view. If the convertor stalls at 1800 RPM now, raising the HP will also raise the stall above the peak in the torque curve. (I think)

So, you definitely need a converter tuned to the application/modifications/HP. From the article in last TDR mag read that the HP/Torque relationship is different in the 12 valve engines than in the newer entines. One is a curve the other being more "linear" in nature. (again thats just my read on it)

The boost/HP rule of thumb was my understanding as well. Also you can (rule of thumb) add 10*F for every 1lb of boost to your pyro reading if your probe is post-turbo.



Sorry if this was long guys.

Regards, Bush'

 

[TStClair]

Thanks guys,

I took Keeno's suggestion and sent a message to Power Wagon. He didn't have the formula anymore due to a computer crash, but was able to provide some useful information.



Apparently modifications change hp and torque in a linear fashion on the early models with the rotary pump. Every hp increase adds 2. 5 ft lb of torque. So, every lb of added boost adds 25 ft lbs of torque, and 10hp. He suggested that my pump timing was probably not retarded like the pickups, and that I should investigate injectors rather than changes in the pump adjustment. Minimize the stress on the pump and get added flow out of injectors. Makes sense.



I'll post the results of these changes. I'll also have to be careful with the transmission, even though it can handle a lot of torque, I'm already over its official limit (and reducing the stall speed, closer to the torque peak didn't help).



Again, thanks for the input.



Tom