671 detroit swap??
- Thread starter pennsylvaniabo
- Start date
What turns the prop.... ain't rubber bands!
Single engine, single prop made for general purposes isn't like tuning tires, adding turbos or NOS injection. It isn't every day the boat yanks at full BHP, it has to be tuned for daily use...slipping props are not good for travel, loose too much bite and forward travel....baaad for fuel economy.
4 stroke.... 2 stroke.... huh?!!
Which creates more torque at low end?
Which creates more torque throughout the power curve...until turbos kick in?
Small wheels.... funny pushing little props to let turbos squeal won't pull dead weight off the mud... props will cavitate and may as well not have either turbos or props. Too much pitch, props won't turn... too much low end load, less pitch, bigger diameter, same scenario.
Gotta have a prop designed to not only pull, but to get you where you're going efficiently.
How many ponies you think a twin 450hp engine set up was producing at Bollard HP in the above scenario? I'd guess less than 150 total. Decrease prop size to pull, loose your HP when you lose your grip.
Besides, torque is the real power number we need to see, not hp. Torque is better made in longer, more frequent power strokes. The Cummins set up relies on the magic of extra fuel and air stuffed into a short stroke.
Detroit uses a longer stroke with twice as many of them to create a more torquie environment.
Their problem isn't wheels, their problem is torque. They can't get their high speed equipment to turn fast enough to produce anything.
The advantage a vehicle has is, an adjustable clutch...whether manual or automatic, it can be used to regulate rpm... no such critter in a marine transmission...one gear all the time for all performance variances, all weights all weathers.
The Cummins has no low end torque.... can't dig itself out of a hole to save its ****!
gatorman21218
Registered User
haha wheres freight train when you need him???
anyway Brad, this ones for you. you say the old jimmys are slow on hills huh? well what about on the flats?
http://www.youtube.com/watch?v=J0bD2pZdTWQ&feature=watch_response_rev
a 16v92 setting a land speed record
anyway Brad, this ones for you. you say the old jimmys are slow on hills huh? well what about on the flats?
http://www.youtube.com/watch?v=J0bD2pZdTWQ&feature=watch_response_rev
a 16v92 setting a land speed record
mabc926
Mojo
I've rode in a 97 Dodge with a Cummins and a 5 speed, and that **** was loud for being stock, so I know I'd go crazy if I drove a truck with a Detroit in it.
I'm sure the noise could be fixed with a few layers of sound deadening............And some thick carpet......
I'm sure the noise could be fixed with a few layers of sound deadening............And some thick carpet......
Had to turn that up real loud...that's freeking awesome..reminds me of when dad took me out as a young boy 4-5 and would do the twin stick...SOO cool
Thanks...it woulda been his 72 his birthday yesterday and made me grin ear to ear..Thanks!
This is my favorite
http://www.youtube.com/watch?v=dVjXwS8HzFw
The drive-by that starts at the 1:30 mark reminds me of sitting at home, we were about that far from a grain elevator. Heard that all year.
http://www.youtube.com/watch?v=dVjXwS8HzFw
The drive-by that starts at the 1:30 mark reminds me of sitting at home, we were about that far from a grain elevator. Heard that all year.
I wouldn't want to listen too one all day long, and Brad is right about the downshifting before you even hit the hill... But man those things sound AWESOME!! I rebuilt a 3-53 in a course I took during high school. What a fun project especially hearing it scream on the dyno 
I witnessed this one first hand
Good times! http://www.youtube.com/watch?v=AbfI7CEWnL4
I witnessed this one first hand
Good times! http://www.youtube.com/watch?v=AbfI7CEWnL4
Diesel_brad
Dunce
I wouldn't want to listen too one all day long, and Brad is right about the downshifting before you even hit the hill... But man those things sound AWESOME!! I rebuilt a 3-53 in a course I took during high school. What a fun project especially hearing it scream on the dyno
I witnessed this one first hand
I love how that cummins kicked both their the detroit ***** w HAVLF the engine displacement
I wouldn't want to listen too one all day long, and Brad is right about the downshifting before you even hit the hill... But man those things sound AWESOME!! I rebuilt a 3-53 in a course I took during high school. What a fun project especially hearing it scream on the dyno
I witnessed this one first hand
Both those vids are great... Ye Ole Dragon sounded sweet and smooth as silk!!
Take note of the Cummins v Detroits..... the lil short stroker will always have advantage when you can up the R's. He could probably put that thing at 7,000 rpm compared to the Big D's at maybe 4000 R's.... I'm just guessing here, I know nothing about tractor pulling. I'm judging by estimated RPM sound only, based on my experience of many different engines at speed.
That said... Their clutches are not "on" until they juice up the R's on either truck configuration... you can see that in the exhaust as they spool up and drop trowel to get off the line.
Now, take my scenario and force the competitors to dead head their tranny's from the git go, and see who pulls further.... 2 stroke or 4. That Cummins couldn't pull 2" before quitting....even with NOS.
The tranny I have behind my engine is also a 1.91:1.... this is considered very small for what I do on occasion. If I had a 3 or 4:1, imagine the schnitz I could yank around getting up into the torque range peak! I'd actually tear my tow bit right off the stringers...I'd be lucky not to rip the bottom out of the boat.... and my 9 knot top speed would result in about 3.5 knots.... top speed.... not pretty. As it is, my top speed is slow enough, however, my tow speed is almost the same.... that is, I only lose about 1-2 knots of speed when towing.... it's the torque doing that....not the HP.
Diesel_brad
Dunce
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
And only produces what 550 ft lb.
And guzzles fuel (land version).
In a boat, they're fine, constant load/RPM (or close to it) but on land for anything that is geared they suck because the power band goes from almost nothing, to hey look at me, back to almost nothing about that fast. I have operated a Loader with a 6-71TA, hated it nutless load and drank about 15-16 gallons an hour, Because you had to work the hell out of the machine to get anything done. The Hugh with it's N/A IH I6 on the other hand I could play around, get more work done, and still only drink 5 or 6 gallons an hour. And yes I have driven a, IH daycab with a N/A 6-71, I think I would rather have a 6.9...
Only on start up... Vroom bap bap bap vroom bap bap bap.
gatorman21218
Registered User
2 cycle diesels sound like a 4 cycle diesel about to blow!I remember every transit bus in Baltimore must have had a detroit of some kind in it when I was growing up.
yep they had the old pancake or slant 6-71s or 6L71s. my neighbor down here used to work on them for the balto trans back in the day. said they were the dirtiest things he'd ever seen
gatorman21218
Registered User
http://www.yachtsurvey.com/comparing_diesel_types.htm
here is a good site comparing the 3 main marine engines. Cat, Cummins and Detroit
here is a good site comparing the 3 main marine engines. Cat, Cummins and Detroit
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.
