Quote:
Originally Posted by Booyah45828
Good question. Since most fail to understand the different operational characteristics between an inline 6 and a v8 I'll lay them out. For some reason, oem's decided that v8 engines are more capable of rpm. Its not my guestimation, its a fact. 6.9 idi has rated Hp at 3300 rpm, while the 6bt is 2600 rpm. Why? I'm not an engineer, but my guess is crank length and torsional stress. V8 has a shorter crank and doesn't have to worry about it as much as an inline 6.
Moat people buying buses on here aren't speccing them out. They're looking for something affordable with minimal problems. Just because you have a 6.9 doesn't mean you can get away with no gear options. Famousinternetjesus has what, a five speed with a 4 speed brownie? All I'm saying is if my redline is 3500 and yours is 2500, and we both have the same gearing/tires, I will be able to go faster then you. Its note bragging, its simple math.
I just wish some of you guys would understand that a v8 isn't a 6bt, a dt466, a dt360, or any inline 6 cylinder. You guys are never comparing apples to apples when you share your experiences because its simply not the same scenario.
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not all V8 diesels spin that fast... the max safe RPM on a T-444E is 2700... in fact my DT-360 *CAN* be spun up to 3200... though navistar says it will shorten its life.. it wont blow up....
a lot of RPM ranging is as you mention stroke length but also the weight of the pistons.. heavier pistons on a long stroke run the risk of stretching the rod at high rpm and slapping a valve or the head with the piston..
less pistons + same displacement = heavier pistons..
the pther piece is cam profile in relation to getting good low-end torque.. while a V-8 diesel mayu be able to spin fast, im guessing the dyno curve is it reaches peak TORQUE at lower than the 3300 RPM...
if its profile is such that it reaches peak torque well into its RPM band.. then theres a good reason right there as to why it as short gears..
in the consumer world the stall speed of torque converters is a big part of getting rated power out of an engine.. seems not to be such the case in the allison world...
while my T-444E runs up to 2700 (2600 in my case I limited the computer purposely).. its max HP is at 2300.. I like to think the max TQ is a little lower.. cant remember.. that engine is going to "LIKE" to run at 2300 RPM.. and at 55 it does just that... it will GO 65 because I run it up above that sweet spot of 2300..
at some point there was a DESIGN speed for these busses.. and what was ordered... many suburban districts never run a bus above 45 except for a field trip.. . many large city districts have routes that require short amounts of freeway time...
rural districts are in busses that spend majority of their route at 55 or above...
as those of you who work in commercial vehicles know.. ordering a bus isnt like ordering a car... there are FAR MORE options on a bus.. page after page of just drivetrain options.. (well not as much anymore.. like when many of our busses were built)....
im extremely cautious on any older engine before "turning up the smoke".. you take a bus thats never spun above 2400 RPM and suddenly you turn it up so it can soin at 3500... there WILL be some rod stretch and further piston reach.. whats that mean? that nice ridge in the cylinder that the rings have created over the past ??? miles and hours will now be a stressing point for that top ring as it either gets stopped by or flexes to overcome said ridge at high speed... far fetched?? maybe .. but breaking up the rings in a cylinder aint a fun day... trust me ive done it.... yeah on an oldsmobile gasser that I "turned up the smoke" on so to say... big cam, big carb, and blooeey went the top ring in 2 cylinders....
-Christopher
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