Living with Detroit Diesel 12V71bs

Can anyone help me answer this

Can anyone help me answer this question.
I have twin 1976 12V 71 turbos and I am looking for information on the bore and stroke of the engines.
If anyone can help it will be greatly appreciated
Thank you

The bore of all the 71 series

The bore of all the 71 series (71 cubic inches per cylinder) is 4.25 inch. The stroke is 5.00 inch. This demension won't change with overhaul as each cylinder is repaired with "kits" that come as liner and piston sets and the liner is the selective part for replacement.

"Thank you so much Jimmy, I ve

"Thank you so much Jimmy, I very much appreciate you taking the time to help me
kind regards"

"I have another general questi

"I have another general question to ask. My engines are 12V 71's x 2 turbo in a 57 foot boat, given that it should be more powerful than i could need but sadly its not, 15 knots maybe 17. The engines have not been used in two years is it the consensus that the turbos are in need of replacement or have I got other more serious problems. I am led to belive that each engine has about 650HP so something is not quite right. Any advice or thoughts are gratefully appreciated."

"Hi Kamara,

I was waiting t

"Hi Kamara,

I was waiting to see if anyone else with more expertise would reply to your question. I really don't have much to offer as to why your vessel is apparently so slow. There are just too many variables to consider and any assessment would need someone familiar with boats of this size and horsepower "on the scene", so to speak, in order to perform a qualitative evaluation.

Some things, in the form of questions, come to mind and I will share them with you.

Maximum hull speed is effected and affected by many factors:
How clean/dirty is the bottom? In what condition are the propellers? Are they properly sized? In what conditon are the shaft bearings? Are the transmissions delivering the engine's torque efficiently and not slipping? What was the "designed" hull speed when new? ie;A tugboat will travel much slower than a "planing" hull such as a sport fisher.How is the vessel loaded? And...on and on.

As far as comdemning the engines as the culprit, only a detailed, SCIENCE BASED, examination of both would be required to point out any deficiencies.

For instance:
It is my experience that two turbochargers don't usually "go bad" simultaneously and they can easily be evaluated for specific regulated boost pressures with the proper knowledge and test equipment. An engine that may be down on power could have more than one problem causing that condition and it would take a technician much superior to myself to stand in the engine room and point to the problem without some precise tune and operational checks. And, that is where you start. You find a QUALIFIED and well equipped mechanic (not cheap!) to perform a comprehensive operational test. This might (should, actually) start with a compression test of both engines and, at the very least, injector spray pattern and fuel pressure/volume checks.

You might be terribly disappointed if you just bolt on a couple of new or rebuilt turbochargers only to get the same results you are getting now.

It would be a shame if you spent upwards of $5,000(us) replacing the turbos only to find out later that the engines were starving for fuel due to clogged delivery lines or some such other simple problem.

I don't think I would want to take that boat out of the harbor until I was sure about the power package. It is truely a matter of life and death.
Good luck to you."

"Hi Jimmy
Again I must thank

"Hi Jimmy
Again I must thank you for your very sensible and direct suggestions to what is I agree a complex issue.
I will have a mechanic look at the fuel delivery first and start ticking boxes after that. I suppose from my perspective having such an (potentially) overpowered boat its a shame not to at least stretch her legs so to speak.
There are two very good 700mm props and recently cleaned hull so I think I can start ticking some boxes already.
Again I must thank you for your time and advice, Its very much appreciated."

"You're very welcome Kamar

"You're very welcome Kamara! I think what most owners and, sadly, many mechanics don't understand about turbocharged engines is that the turbo needs the engine to burn the proper amount of fuel primarily to deliver the right quantity of exhaust gas to spin the turbine. A poorly tuned engine will act as if the turbo is not working correctly simply because it can't power the turbo as it should.

The engine burns fuel then spins the turbo. The turbo delivers more air to the engine so that it can burn more fuel. The engine, in turn, burns more fuel and spins the turbo faster, etc...

So, you always have to start with the tune and fuel delivery to make an intelligent decision as to what needs to be done. You are doing the right thing starting there. I hope you get a good mechanic to work with you."