Mercruiser 7.3 litre power curves

[AndyL]

Would anyone have any information on the Mercruiser 7.3 ltr Diesel power curves. Can't find it anywhere. About to re-engine from the Mercruiser and am trying to compare similar engines performance at cruising rpm ranges. Any direction or help appreciated.<br />Thanks in advance<br />Andy

 

[Richard Petersen]

Re: Mercruiser 7.3 litre power curves

Punch up GM marine engines . Then your model, they do have the curves available.

 

[uaw9fan]

Re: Mercruiser 7.3 litre power curves

The 7.3 diesel isn't a GM engine.It's an International/Navistar Engine.

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Thanks for the input. Would I be right in assuming that the power curves for that particular engine would be the same in whatever the application it was fitted in. eg a marine application or a truck?<br /><br />I am trying to compare between the Mercruiser that is currently installed but getting tired to a Cummins, Volvo, John Deere, and Iveco.<br />The othres are easy to find power curves for but the mercruiser doesn't give up the data easy.<br /><br />Any advice or input much gladly received.<br />Thanks<br /><br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

Originally posted by AndyL:<br /> Thanks for the input. Would I be right in assuming that the power curves for that particular engine would be the same in whatever the application it was fitted in. eg a marine application or a truck?

Simple answer is no. Be especially careful interpreting truck curves. They show along the full lug curve which despite many, many opinions and posts is almost meaningless to a marine application. Probably the most important numbers are max bhp and the RPM that it makes it. Go to any major manufacturer's site for marine diesels (or gasoline) and you will find the prop curves and note that they NEVER reach peak torque when compared to an on-road counterpart. With all of this said, I am sure there is data somewhere. I found this link to a guy who discusses a 160 bhp naturally aspirated 7.3 repower after about 2 minutes on Google with lots of different search word combos.<br /><br /> http://www.gulffishing.com/ol001.html <br /><br />If you can determine the max bhp and the RPM, you should be able to determine a) whether you will need a gear ratio change for anything you consider and b) what engines are in the right output range. I would not get too hung up over the "curve". Nice to have, but IMHO not absolutely necessary.

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Thank You Quietcat, Your information is warmly received as is your opinion. I had a good hunt around Google before posting but kept winding up in Trucks and other non marine applications. As you can appreciate I am looking for an engine that can cruise comfortably at say 70%mcr thus saving fuel and wear on the motor. I don't want to have a motor that has to be run at high rpm to achieve optimum cruising speed / fuel consumption.<br /><br />Thanks for taking the time to reply.<br /><br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

AndyL,<br /><br />What RPM and speed and boat size do you have now? Cruise and WOT RPM. What drive? What gear ratio? What prop setup? Maybe we can zero in on some options.<br /><br />BTW, I hate writing WOT as there is no throttle on a diesel, but I'll get over it . . .

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

That was quick,<br />The application is a semi displacement 26 foot hull approx weight 4.5 tonnes. The prop I believe is 22 X 24 inch. Gearbox is 2.1 :1 ratio.<br />With this Mercruiser as is fitted I am getting appx 24 knots at full rack (3450rpm) and normally cruise at 2800rpm giving me 16 knots and a more comfortable ride. Fuel consumption at cruising rpm is 10 gall / hr.<br />The motors that I hope to chose between is the Cummins or the Iveco both of which are straight 6 configuration and appx 330 hp. The current Mercruiser is 275 hp.<br /><br />Hope this gives you a lead.<br />Thank you.<br /><br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

Why do you want to pay for the 330 if you don't want any more speed?

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Its the best deal basically. Its not for the extra performance that may be there.

 

[QC]

Re: Mercruiser 7.3 litre power curves

Here's what I would do after a quick look.<br /><br /> http://www.gce.cummins.com/mce/mce_6b/assets/fr90209.pdf?SMSESSION=NO <br /><br />I'd change from your 2.1:1 ratio to around 1.7:1 This would give the same shaft output speed (1642 RPM at the shaft today vs. 1647 with the Cummins at the 1.7:1 ratio and 2800 RPM). The new cruise would be 2272 RPM for a 1336 shaft output speed, today you are turning 1333 RPM at the shaft at your cruise RPM of 2800. My .02

 

[QC]

Re: Mercruiser 7.3 litre power curves

Yeah, I just looked up the 330. The only diff IMHO is the rating. Check this out:<br /><br /> http://marine.cummins.com/public_cummins/content.jhtml?tlaId=6&anchorId=39&marketId=13&menuId=1 <br /><br />It is important, but based on your posted info you qualify for high output ratings. Soooooo, depending on price I would even look at the 305 HO. I would bet my house that ALL three ratings (305 HO, 315 commercial and 330 HO) would give you the same fuel consumption at the new cruise RPM of 2272 . . . I can even help you understand why if you want.

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Good on you Quietcat,<br />Will muse over the data and come back again if the "grey mist" descends over the figures.<br /><br />Thank you for your interest and help.<br />Will let you know how the decision goes.<br /><br />All the very best<br /><br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

Are you Aussie? I don't recall ever hearing "Good on ya" in the UK . . .

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Spent a bit of time in NZ and Tasmania with work.<br /> <br />What a place!!

 

[QC]

Re: Mercruiser 7.3 litre power curves

Roger that. Never actually been to Tazzie or NZ (Airport once) but have been spending a bit of time in Oz.<br /><br />How do you know the bhp output of your 7.3? Is it turbocharged?

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

Good day again Quietcat,<br /><br />The current Mercruiser is indeed turbo charged and that 275hp is what is stamped on the engine plate plus the manual confirms same. With the links you gave to the Cummins engines if I could have got the same data for the Merc it would have given me an idea how the two would compare.<br />I know what rpm and speed actual I get with the Merc so with the power curve I thought I could then discover what HP I am using to achieve that speed. I could then enter the Cummins curves at that horsepower and see what RPM I would need and to some extent the fuel consumption possibly.<br /><br />I take on board your points regarding the gearbox as I can order the box with the new motor. Just have to decide whether to go for the Twin Disc commercial box or the more leisurely orientated ZF220 box but that is another story.<br /><br />Oh the joy of boating!!<br /><br />Were you working in Oz or just cruising around?<br />Would have loved to have settled out that way as the pace of life and attitude really is the way to live.<br />All the best<br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

I travel to Oz 2 - 3 times a year for business. I also lived/worked two weeks a month in the UK for about 18 months ended a year ago, but I have been back twice for short visits. We have a little factory (we would call it a shop) in Leyland.<br /><br />Propeller load curves are fairly linear. If you use some existing curve and find a way to mathematically convert it to your RPM etc, I think you could get close. In fact with the Cummins data and the other gear ratio I think I can tell you within around 10 bhp . . .<br /><br />Yup, I think it is probably slightly less than 150 bhp at cruise. The Cummins I used was the 315 soooooo, the bhp on that propeller curve would be higher than what you have now from the 275. I played with the 280 @ 2600 rating which would do a great job for you and it shows 141 bhp at 2100 RPM. This is the RPM that would correspond to your cruise now with an even lower numbered gear ratio.

 

[AndyL]

Re: Mercruiser 7.3 litre power curves

That is a great response Quietcat,<br /><br />You have been a great help on this one and I thank you.<br /><br />Will post an update when the deal is done and the motor fitted. Its difficult to get through the sales pitch and get down to the facts, but when the outlay is so much you need as much input and impartial advice as you can muster.<br /><br />All the very best and hope we can discuss again in the future.<br /><br />Andy

 

[QC]

Re: Mercruiser 7.3 litre power curves

Yeah, I am real curious about how this all works out. Have those guys verify my figures as they do it all of the time I suppose. There are many a salesmen that are confused about efficiency though. Between diesels of the same displacement there is really no truth to the concept of paying for more bhp than you need to get better fuel efficiency. That sometimes works in an on-highway vehicle but not a boat. The only other concern I can see is being too fast at idle (tick over) with a low numeric gear ratio i.e. too fast for wakeless speed etc. Good luck!!