1992 Mercrusier 7.4L MIE 340hp Gen IV

[machined]

Hello,
Have a pair of these in a 31' Uniflite. Would someone supply a torqu/hp curve for this engine.
Thanks

 

[alldodge]

Unable to find a power curve. I'm finding 330 hp around 3800 rpm and torque around the same area.

 

[Scott Danforth]

If you can find a GM Powertrain PDF from 1992, you may find a torque curve. However Mercruiser did not publish it. The manual has peak power as AD mentioned

 

[machined]

Ok
I'll go that route
Thanks

 

[machined]

Actually...what I'm really wanting to do is compare these engines to my yanmar 4lha-ste 230's and see if they match up well at my cruising rpm of 3100. Any input on this would be very helpful

 

[alldodge]

Didn't do in depth investigation, just skimmed through the docs.
First the simple stuff Diesel-D Gas-G

D or G weight about the same

G-more HP and higher rpm
G-will out perform D-but D-will last longer

Hole shot is faster with D over G
Top end better G over D

 

[machined]

Thanks....
Current BBCs run 2800rpm at 24.5kts. Not looking for better performance but don't want to loose any for sure. Looking to increase fuel economy. Don't care about a hole shot but need to be able to drop rpm/speed for rough sees with feeling doggy

 

[HT32BSX115]

Actually...what I'm really wanting to do is compare these engines to my yanmar 4lha-ste 230's and see if they match up well at my cruising rpm of 3100. Any input on this would be very helpful

Howdy,

Don't know where you would cruise the Yanmars but the max RPM for that model is 3300, so I would guess cruise would be someplace around 2500-2800.

According to Yanmar, fuel consumption at that RPM is 7.5-8.5 GPH (each)

My 1997 7.4L carbureted Mercruiser (according to my fuel flow gage) burns 10GPH at 3000RPM.

So your difference running as above would save you approx 3-5 GPH using the Diesels vs the twin Yanmars.

I don't know what it would cost to repower a twin inboard boat from MIE 454's + associated transmissions to twin Yanmars with transmissions but I am going to assume it would be more than $25,000 per side

Even if you paid $6/gallon for gasoline and only $3/gallon for diesel, your savings would be only be about $15 per hour.

At $25,000 per engine + new transmissions I am thinking the "savings" isn't going to "pencil" out well.......

It might be better to just sell that boat and get one with the diesels you want......

 

[machined]

Hey thanks for the input
I actually have the yanmars and the transmissions
I currently burn about 36gph cruising at ~2900
So theoretically if I can cruise the yanmars somewhere around the same rpm I should save considerably.
The concern is how the hp/torque of the two engines compare at the cruise rpm.

 

[HT32BSX115]

Hey thanks for the input
I actually have the yanmars and the transmissions
I currently burn about 36gph cruising at ~2900
So theoretically if I can cruise the yanmars somewhere around the same rpm I should save considerably.
The concern is how the hp/torque of the two engines compare at the cruise rpm.

What will your Mercruisers turn (RPM) at WOT?, what is your prop pitch and transmission ratio?

 

[machined]

Hey
WOT = 3800, current props are 19x20, gear ratio 1.5:1. Clearly these props have too much bite. This is the set I was planning on running with the diesels.
My standard props are 19x17.5, WOT = 4200

 

[HT32BSX115]

Hey
WOT = 3800, current props are 19x20, gear ratio 1.5:1. Clearly these props have too much bite. This is the set I was planning on running with the diesels.
My standard props are 19x17.5, WOT = 4200

Ok. You're WAY "over propped". Your engines want to turn somewhere around 4200-4600 at WOT. (your owners manual will indicate the proper MAX RPM)

Assuming your tachs are accurate, select a prop pitch that will yield 4600RPM at WOT with the boat loaded ( with fuel, people, gear etc) where you will operate on a regular basis.

This will give you the best all around performance and will likely reduce your cruising fuel flows a bit too. It sounds like you're burning 18gph per side at at an RPM that I burn 10gph with the same engine! I am propped for 4600 at WOT. if your MAX RPM is only 3800, your throttle position to get 3000 RPM will be MUCH higher than mine. Even 4200RPM would be too low. I would try an even lower pitch than the 19x17.5's

You're essentially lugging those engines.

by the way, If I was going to switch to diesels I would go to 400hp Cummins (or equiv) engines. Not 230hp Yanmars. I think you won't be very happy with the performance of any 230hp diesel.

 

[machined]

Thanks for the suggestions. When I ran my 19x17.5's in order to run at 24-25kts I still burned 30-36gpm depending on load. I realize that my current setup is over propped. I swapped to these to get an idea of the rpm/boat speed that I'd be running with the diesel swap.
This may be way off but my thinking is that at 2800 my bbc's can't be putting out more than 200hp and maybe 400lbsft torque... this looks like a real good match up to the 230's.

 

[Scott Danforth]

Diesels burn 0.333 # of fuel per HP per hour. Spark Ignited motors burn 0.4 # of fuel per HP per hour.

if you want fuel economy, get a row boat or a sail boat. if its pushing a propeller, it will use fuel.

the big blocks will be happiest at about 3300 RPM cruise (best rpm found with fuel flow meter). prop for WOT at 4600 not 3800

if your that over propped, your seriously lugging your motors which isnt good, creates detonation which eats the motor from the inside.

I would also go cummins vs yanmar if moving to diesels, however prior to that I would put the right props on the boat.

 

[machined]

YES. I know it's currently over propped..it was an experiment to see the rpm and boat speed id get with the more pitch.
You're response about getting a row boat is brilliant.. Saving some money and getting the reliability, durability and safety of diesels with same performance was the idea. The boat is an x-police boat that was professionally set up with 19x17.5' WOT 40kts 4200-4300. In other words, I have good props for the 454s.
I'm not running out and buying a set of cummins if I already have yanmars that directly fit in place of the bbc's.
Thanks for the input...

 

[HT32BSX115]

I'm not running out and buying a set of cummins if I already have yanmars that directly fit in place of the bbc's.
Thanks for the input...

Makes total sense to use the engines you have!:thumb: And there's nothing wrong with Yanmars!

Just not sure you will be very happy with the performance from a 200HP reduction.

 

[machined]

I agree with that. Hoping the increased torque of the yans at the lower 26-3100 rpm will allow to swing these 19x20's.

 

[Scott Danforth]

I doubt the yanmars have any more torque than the BBCs except under 1500 rpm

A simple cam change in the BBC will increase torque by 80 lb-ft in the mid range as well

 

[HT32BSX115]

I agree with that. Hoping the increased torque of the yans at the lower 26-3100 rpm will allow to swing these 19x20's.

Ok,

Those are 19p 20" dia props? AND you'll be using your existing transmissions? (1.5:1 ratio)

Using http://www.go-fast.com/Prop_Slip_Calculator.htm

If you can get it to turn MAX RPM (3300) with those Yanmars running the 19p props, you'll be propped correctly. If they will not turn that fast in that hull, you'll need to drop the pitch to where they will.

Take pictures! (We love pictures) If those engines have EVER been installed in a 31 Uniflite, someone else will have already done your work for you.
Otherwise, you're in experimental territory!

 

[machined]

Sounds good. Will do.