Figuring Horsepower on 2 Stroke Detroits

[TomC]

Without having the exact horsepower and torque in front of you that Detroit published, it is easy to figure your horsepower compared to your injector size on mechanically injected engines.  I just remember one engine size, the 8V-71TA, which is the last version Detroit made of this engine-hence the most modern version of it.  It is rated at 400hp with 1200lb/ft torque using 80 injectors.  From that you can figure out just about any horsepower version and be pretty close to the output. 
For instance, if you want to find out what a 6V-92TA will be with 90 injectors, first take 400 divide it by 8 then multiply that by 6 (cylinder change).  Then divide that by 80 and multiply it by 90 (injector change).  You come up with 337.5- which is very close to the advertised 335hp.  Same with torque.  It's fun to play with.  Good Luck, TomC

[bevans6]

Another way to look at it is 5/8th of a HP per injector unit per cylinder.  My 8V71N is 5/8 X 55 X 8 cylinders is 275 hp.  what the formula shows is that it's mostly the amount of fuel you can stuff into a cylinder and burn effectively that counts in making power in these engines.  things like turbo or not, cylinder capacity, etc, are all functions of that.  Bigger cylinder or more turbo boost or intercooler just allows more injector size.  As you say, kind of fun!

Brian

[JohnEd]

Ah, yes.... Grasshopper.  That is the story, properly told by you, where the pencil meets the paper.  Or keenest interest is where the "rubber meets the road".  Normally, a wheel dyno would get us close "enuf" but there is a better way that will include ALL VARIABLES in total.  I use my G Tek PRO  SS meter.  The way this goes is you weigh the vehicle carefully, tell the meter what ya got and then floor the throttle wide open.  How long it takes you to reach 30 or 60 MPH is a physics problem that has a unique answer.  Only Sean could argue this point so we can be sure it is a reliable theory.  The G Tek is a accelerometer and measures G force.  It sits on the dash or any horizontal surface and "talks back". If you take your rear wheel HP number and subtract the G number you will have, in HP, your loss to air friction, bearing loss, alignment loss and brake drag and anything I haven't imagined.  Get a baseline reading and compare after replacing fronts for an idea of the comparative rolling efficiency. air pressures, or break job or bearing replacement.  Just hold the pedal down from a stop till a given speed and WALA.  Or eureka or whatever you ethnic persuasion.

For those that want to try it I will have it with me at Bus'n USA in Rickreal, Oreygun in June. 

John

PS:  While I have done this often on gas engined cars. I haven't tried it yet on a D.

[fe2_o3]

  JohnEd..HUH?  Can you translate that into English by Rickreall? Hope to see you there.
  Tom and Brian I under stood. With a 671 and 60 injectors I can have up to 225hp.
  Sure don't feel like it goin' up the passes...Cable

[lostagain]

What is more meaningfull in terms of every day driveability is the power to weight ratio. For example, the 180 to 200 HP I get out of my 4-71T is pushing 22000lbs. A 8-71N (318HP) in a 40000lbs coach is not much faster than mine up a hill, if at all. I should hold an annual bus race up a long hill just for fun. Lots of that around here.

JC

[bevans6]

The G-tech is a neat device that just measures work done over time and derives "road horsepower" by measuring how long it takes to  accelerate from one speed to the next.  As debated endlessly in those HP vs Torque threads on other chat groups, HP is a measure of work done over time.  If you tell the G-tech how much you weigh, and then accelerate from one speed to another, it can calculate how much power would have been needed to accomplish that task, and tell you how much horsepower your power-train managed to put down to the road via the tire.  That's an interesting number, but of relatively  little use until you make a change to something.  For example, blow up all your tires to the sidewall maximum, you should get a better number since the drag due to tire friction is lower.  Disable your alternator, you can figure out how much HP your alternator takes.  Turn on the OTR AC to full, and you should see the result in a drop of HP to the road since you are turning the AC compressor and running about 150 amps worth of fan motors.  One that I think would be interesting on an MCI is take the blower fan belt off and see how much HP the blowers take - I bet they take about 20 HP or more.

It's just a way to measure work done, without annoying people doing 0 - 60 runs, or timing things and all of that.  The hot rod car guys use them to see how much the couple of grand they dropped on a new turbo got them.

"Road HP" is what I call the power left over after everything that saps power has done it's sapping.  The G-tech adds in wind speed and road grade to that, since it basically just measures acceleration.  What's interesting is that the drop box and ring and pinion on my MC-5C probably takes a full 10% of engine power all by itself.  Hypoid gears are not very efficient.

Sorry for the ramble and the loss of on-topic-ness...

Brian

[white-eagle]

so if i calced right and read right, an 8v71 and an 8v92 have the same horsepower?   

8v71 with 80's = 400   8v92 with 80's = 400 also?

in the example, the 6v92 was calced using (400/8)*6 which seems to indicate a 71 and a 92 cylinder develop the same horsepower and only the injector size makes a difference? 

what did DD get from the larger displacement?  Am i confused.  somebody please tell me what i missed or what dd did? 

[JohnEd]

Brian,

I agree with everything you said.  It also gives you1/4 mile and 1/8 mile and other stuff but you have to give your engine rpm to get it and without and ignition signal you get less info.  I got almost 20hp from a LEGAL ECU modification.  They, ECU guys, said to expect 15 to 17 hp gain so they underrated their product.  When has that happened recently?

Cable,

Did that clear things up?  Need more?


John

[TomC]

Detroit took the 4.25" bore with dry cylinder liner series 71 and punched out the bore to 4.84" to create the 92 series.  What DD was trying to do was to keep the interior cylinder pressure the same.  So a 8V-71TA using 80 injectors with 400hp at 1200lb/ft torque will have the same cylinder pressure as a 8V-92TA at 510hp at 1550lb/ft torque (100 injectors).  All in the name of having higher horsepower with over stressing the piston rings-which on a 2 stroke is critical-since a 2 stroke doesn't have time to cool down between strokes like a 4 stroker does.  Good Luck, TomC

[fe2_o3]

So...Your G gizmo measures the change in inertia. Factor in the weight and it gives road horsepower required to make that change?
Also..What I'm hearing here is if I put in big'nuff injectors, my 671 can develop the same horsepower as a 692 with the same injectors. However it will be for a very short time. Heat is my enemy...Cable

[luvrbus]

Keep your eyes open for DDEC inline 671 330 + hp and it will last


good luck

[cody]

huh?

[fe2_o3]

                       What?

[luvrbus]

Guys a buddy of mine has a DDEC 671 in his 4104 set at 330 hp great little engine and he averages around 11 mpg with it. 
I saw one in a boat set for over 500 hp as Ripley says believe it or not   


good luck

[fe2_o3]

    What alterations did he do to keep it cool? My 210 horse 671 warms up on long hills. And at 28000# I only get about 7.5mpg...Cable