Engine Dilemma ?

[DCJ]

In process of complete resto of 31SF into FBC best I can tell hull is 66 model. Years back when I joined this site, greatest site ever, I was doing a minor upgrade on my first 31 which had 454 gassers. Unfortunately she went to the bottom of Mississippi River during hurricane Issac. During the time I owned her/him/it I came across what I thought was a smokin hot deal from a local marine shop on two 6bta 300 hp remans with new 220 ZF 1.5 trans. 10k each. Doing my due diligence I found the machine shop that did the reman and after talking to the owner discovered the remans had actually been balanced and blueprinted. So now I’m pumped and on my way to a diesel repower but during the interim I lost my boat. Let me tell you, losing your prized 31 is very emotional. So okay here we are have the motors sandblasted, epoxy primer, awlgrip painted and they are going in the boat I am currently doing a complete restoration on. Here’s my dilemma, after extensive research on this site I am reading that the motors I have, which are the 1990 models, were not the best Cummins design and they should be treated as 250hp vs 300hp. With that said will changing my shaft angle from the stock 15 degree to the recommended 12 for better ride, fuel eco, etc. be a mistake? If I have to repower after these alleged crappy motors TAS the 12 degree angle is in place and good to go along with 6” exhaust for a more modern repower. We fish in GOM out of Port Mansfield where our wave sets are 3’-4’ - 3 seconds on average. So, this ride thing, motors back, more weight behind main bulkhead is big. So here’s my question, will my motors operated as 250hp power develope enough torque to make the boat ride for the smoothest, driest, most economical ride. And even bigger question, is there anyone out there with the same year model, same hp engines that would share their hours and problems?

[Tony Meola]

It is always beneficial if you can change the shaft angle. Reducing the shaft angle increases efficiency. Be careful how you do it. You need to adjust the shaft angle without reducing the prop tip distance to the hull.

That means lowering the engine so it is going to be maybe an inch or so off the hull. It's tight.

Now think of the current strut as a fulcrum. The new strut will be the same size as the current one but the angle of the barrell will change.

Can't tell you anything about that engine. If the machine shop blue printed the engine, I would think they will know the horsepower.

If the turbos are sea water cooled, then you should see if changing the injectors and fuel pump will bring the horse power up. Again the machine shop might be able to help.

[Ironworker]

My 69 B31 had a couple of Volvo TAMD 60B diesels which was rated at 220HP. They were heavy engines at 1500#s each. My boat with those old engines topped out at 30 kts and cruised at 22. The old gal rode pretty well.

[Carl]

My 02…


Build the boat for 300hp, use it like it’s 250 hp.

Under prop the wheel some so you’re not pushing it at the edge.

If you feel the need to run a 300hp motor you can swap the motors as everything will be setup for it…just repitch props.

[DCJ]

Thanks for the prompt response guys. The motors are indeed 300hp. As far as anymore correspondence with the machine shop the owner passed away and shop shut down. Mechanic is scheduled to be here with crane truck to help with engine install Monday morning, FINALLY!. Big day. Had them in before for motor Mount mock up and yes they are going to set with tranny 1/2” above hull, however it appears that the fiberglass shaft logs that I cut to length as per Cpt. Pats specs are going to be too short due to new shaft angle. Has anyone else run into this problem? Will try to post pics when motors are in.

[Tony Meola]

The only other one who I know that changed his shaft angle idea Bob Lilco. Sounds like you need to replace the tubes.

Could you find a tube who's inside diameter was big enough to slip over the installed tube. Slide it over and glass it in place

[Snipe]

David I think you will be just fine with those engines and if in the future you want to change them the newer ones will bolt right up as far as I know they are the same layout. Friend of mine is running older Cummins and they run great. Glad you are close to getting them in the boat it is going to feel so good. The shaft logs I made to Capt pats specs almost didn’t work for me either I had to work some magic to get them to work. Longer ones would be much easier. I bought the material from fisheries supply.

[Carl]

Jason, from what I recall the pre storm blocks were ok at 300hp, not bulletproof. The idea was to underrate these motors a bit to keep them banging away for years with no worry. Once the new n improved storm block came out you could really lean on the motors.

[Snipe]

Got ya Carl yea my buddy does not beat on the boat either he runs it kinda easy and is always up on maintenance.

[DCJ]

Thanks guys

[DanielM]

If your 1990 Cummins are CPL 970 there is a lot of info on this site and boat diesel dot com about them. Search for 970 if they are that CPL. The main problematic issues I think were the larger injectors and the sealed aftercoolers.

I picked up a pair of CPL 970 running take outs that had been setting in a fellow’s shop for ~20years a couple of years ago. I haven’t put them to use yet, but I think if you address the known issues and run them as 250hp they should last. At least I hope so.

I gave Harry Babb a call when I bought them and asked his thoughts as he had rebuilt a couple and put them in De Nada. He said he was comfortable using them. Given that Harry owns and operates a machine ship I put a fair amount of stock in his opinion.

Anyway I bought my 970s at the time knowing the issues, not Cummins best engines, but the concerns are known and can be addressed. Hope they work out for you.

[DCJ]

Thanks Daniel

[DCJ]

So back to the “larger injector “ issue, would it be wise to replace with smaller?

[DanielM]

I don’t have any personal experience, but from what I can glean from the internet, there are bushings to allow using 7mm injectors in the 9mm heads.

I ‘think’ that is just to allow putting new 7mm injectors in if they need to be replaced, as 9mm injectors may no longer be available. I can’t see where just changing the injector would help with the head cracking issues. It seems like the deficiency was in the head design. But I could be wrong there.

The better fix was to upgrade to the later designed head. Or just run them easier (the run them as 250hp comment I see a lot)

In regard to the sealed aftercooler it looks from the pictures in the your other engine thread, the sealed/welded aftercoolers have been changed out with an aftercooler that can be serviced. So it looks like somewhere along the way someone addressed that issue.

Given that they were rebuilt, it might be entirely possible the head was upgraded to the 7mm head at that time. I’m sure there are casting numbers on the head to research which head you have, but I don’ know where you would find them. It might be worth a call to your local Cummins distributor. I’ve found Cummins office in Houston helpful over the years.

[DCJ]

Daniel, you are spot on regarding the upgrades and I was able to find an old post by Bob LIco where he had included the casting numbers on the upgrades. Found numbers and starboard engine has the upgraded head. I suppose at sometime in its life the old one cratered and was replaced. Leaning towards going ahead and replacing port now while I’m in rebuild process. Peace of mind thing. Thanks

[DCJ]

BTW, what part of Texas coast are you?

[DanielM]

Freeport south of Houston. If you ever pass through, stop in and say hi. I can usually be persuaded to BS for way too long.