Tom, you do realize the Detroit has ZERO bottom end, right? They are designed to SCREAM to be in their powerband, just like every 2 stoke is
You don't think I was bragging about dragging that boat off the mud at idle do you?? However, your assumption is, Detroits don't produce power until they're screaming. Well.... that is just not correct!
Their peak torque is actually at the low end. On a 2100 RPM Detroit, max torque is actually at around 1500 RPM.... not at 17/18/1900 or even 2100 rpm, but 1500 rpm!! At 1200 RPM it is almost at peak!!!! Only a 20ftlb difference in a peak torque of 610 or something like it.
However, at 1200 rpm I took a decent strain and dead headed at about 1450 rpm's. My 6-71 is a 2100 max r'd vessel, this puts it at less torque than at 1200 rpm's or so. The big torque starts around 1150/1200 rpms @ around 550ftlbs and ends somewhere around 1500 rpms @ appx 610ftlbs.... which I cannot reach when dead headed with my current prop and trans configuration in that scenario.
So when I am able to reach 1450 rpm's, I'm almost at the top of my torque curve!! Capiche'?
A Cummins 450 on the other hand.......AAAAHAHAHAHAHAHAaaaaaa..... oh! Sorry to the Cumminheads.... well... I'm not the Cummins expert, but... their torque doesn't add up until they hit 1200rpm's.... guess what? Turbo's gotta be spinning at that R to get the torque... when dead headed.... a Cummins won't spool..... unless you can drop trowel on a tranny/clutch.
BTW- the article that gatorman put up for us.... read this part.... I think it's what I've been stating all along:
"Detroit Diesel The two cycle diesel has been the mainstay of Detroit Diesel for nearly 60 years now. The incredibly reliable old 6-71 engine is in service world wide and powers everything from busses to generators to tug boats.
Its service record is unparalleled.
Two cycle and four cycle engines each have their pros and cons. The operating system of the two cycle is radically different than the four cycle, which operates more like an ordinary gas engine without spark plugs. The Detroit Diesel engine is unique and is not offered by any other manufacturer. This engine utilizes a moderately heavy cylinder block that is quite complex in its casting. Compared to a MTU or Volvo, it is light; compared with Yanmar or Cummins, it is heavy.
It is a long stroke, smaller bore engine than most four cycles. The Cat 3208, for example, has a fairly short stroke, as do the Cummins V-8’s. This gives the Detroit engine considerably higher torque at lower speed ranges, but limits its top RPM range: the longer the stroke, the slower the engine has to run. Thus, higher power at lower speeds. The push rods of these engines are also about 33% longer than most 4 stroke engines, creating more rotational mass and therefore more rotational torque. Altogether, the engine delivers more power from less displacement, lower speed, but higher casting weights, while fuel consumptions remain approximately the same.
And here's the coupe' de grassie:
So the DD engine is left with certain advantages such as more immediate power delivery and higher power at lower speeds. This is one of the things that has made these engines more popular with the commercial boat industry.
The four cycle engine has to throttle up to a higher RPM before a similar amount of power is delivered to the propeller.
Knock/knock!! Hello!?
Uuuuum.... yeeaaaaah!
Thaaaaaaank yeeeeeeeew! Very much!
Not trying to be a brat here... but not only do I know my engine.... I know what the other fellas boats-n-bragging can't do.
To be fair, I can't come close to their speeds either. I think top R's on a Cummins is around 1800, but their torque is huge at 1400, over 1100ftlbs- twice what a 6-71 N/A gets at any RPM....but.... ya gotta get the turbo to spool or Ya! Ain't! Got! Schnitz!
No matter how much fuel you dump into that cylinder, if the crank cannot turn exponentially...or increase in rpm, the turbo will not spool, and you will get nothing in power in return...PERIOD! Might as well be a starving N/A at that point.
Remember this about Detroit... it won't make much more power the more R's you pump out of her.... this is why some ignant truckers wind those Beetches up and over.... shift points are waay too high and are just wasting fuel. Keeping the engine within torque is what matters when hauling weight.... not jacking up rpm's. Gotsta know where the power bands are.
A good truck hauler will know exactly when to shift..... just before the throttle feels empty and R's go up real fast.
I was reminded of this by Towcat and others in their tranny threads.... low shift points = high torque all the way through and shifting earlier than later.
I screwed up my own tranny rebuild by forgetting to tell the builder to keep low shift points. In the end, it was still a decent shift point... 1700 v 1500..... but it isn't optimum for towing.
Same rules apply to the IDI as the DD. Gotta know where the torque begins and ends, to go to the next step. Small HP means nothing when it comes to power.... it's all about the torque.