7.4L Mag MPI RPM/MPH - prop slip?

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I started off chasing a CVX23 and a CSS19. I like the inboard/outboard boats. Don't care for working on outboards.
The CVX sold too quickly and I couldn't come to a price agreement on the CSS (it needed 2-3k in mechanical work).

So I got under the dash today, the tach was set to a 6 cylinder calibration.. Which means either someone monkied with it attempting to get someone like me to buy it via thinking it would run full RPM - or it was never setup correctly in the first place.

It's got norskog instrumentation.

 

[JustJason]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

play with the prop selector some more.... it could be as simple as being over propped

 

[180shabah]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

....but i highly doubt a 23ft glastron is going to hit 65. If it is, it's going to be on a day the water is glass, and the outside air is cold and dense. It's going to be on a top speed prop, that will give horrible acceleration and take forever to get there.

Where do you come up with this S#!T?

multibladed props (more than 3) will offer better acceleration,

in general - it is possible for a 3 blade prop to out accelerate a four blade and have higher top speed.

Also, multibladed would be more than 1

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

play with the prop selector some more.... it could be as simple as being over propped

Why is it that no one believes a Glastron can hit 65? If I said it was a Baja, no one would doubt it for a second..


I've played with the prop selector.
I've got test data from one of the boating magazines that tested this boat in 1998 with a top speed of 68.5 mph. That could have been in cold weather, flat water, and minimum fuel - I don't know... So I'm assuming that the boat should be a 65mph boat, give or take. If it'll do over 60 and over 4700 RPM, I'll consider that close enough.

A 23P puts me to 63-65mph with a little less than 10% slip, so I think that's where it should be at. I do have an aluminum 21 pitch prop, slightly smaller diameter, that came with the boat so I could try that also - just to see if I can get more RPM out of it.

New plugs tonight. I hate replacing the darn GM cap/rotor as it's always wedged in the back of the motor against something. I'll check DTC codes also.

You guys have been great...

 

[180shabah]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Why is it that no one believes a Glastron can hit 65?

There is no reason to believe that a Glastron Can't hit 65.

I do have an aluminum 21 pitch prop, slightly smaller diameter, that came with the boat so I could try that also - just to see if I can get more RPM out of it.

Should pick up a few more RPM's, nut little if any speed.

Either your boat is seriously overweight(saturated foem, etc..) or your engine just isn't making the power it should.

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

There is no reason to believe that a Glastron Can't hit 65.



Should pick up a few more RPM's, nut little if any speed.

Either your boat is seriously overweight(saturated foem, etc..) or your engine just isn't making the power it should.

Agreed with one exception a good SS only, and he has one now, it will be intresting to see what happen's when he get's that engine in form and see if that prop is the right gear or wheel as they say for that for the combo.....

 

[QC]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I saw your post in the Merc section . . . TG is not with me, and of course MikDee thinks it's a prop hub and next he'll have your coupler out . . . :razz: But why are you referring to it as a 7.4L? This makes me think that it may be noted as such somewhere . . . I have often seen 7.4L's called 454 Mags in adds. The 7.4L Model is MPI, but only 310 horsepower, is it possible that's what you have here?

BTW, my boat tested with the exact same engine drive combo at 62 MPH . . . I have never seen above 57 and that was after a lot of playing around with props and trim. With a 385 my 23 footer would run over 60, but 65 is a big leap from where you are now, no matter what is wrong with it

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I saw your post in the Merc section . . . TG is not with me, and of course MikDee thinks it's a prop hub and next he'll have your coupler out . . . :razz: But why are you referring to it as a 7.4L? This makes me think that it may be noted as such somewhere . . . I have often seen 7.4L's called 454 Mags in adds. The 7.4L Model is MPI, but only 310 horsepower, is it possible that's what you have here?

BTW, my boat tested with the exact same engine drive combo at 62 MPH . . . I have never seen above 57 and that was after a lot of playing around with props and trim. With a 385 my 23 footer would run over 60, but 65 is a big leap from where you are now, no matter what is wrong with it

QC, apparently dcg, has found the prop to be ok, so I'm past that issue
If it truly is a Hi Perf 454Mag 385hp I agree it should do 65. I seem to remember that the least hp a standard 4bbl 454 Merc marine motor had was 330hp, you either had a base 350/260hp, or a base 454/330hp for years, not withstanding Mercs Hi Perf line of racing engines. I find it hard to believe that a 7.4 MPI only has 310hp? unless we're talking about different hp ratings over the years.

 

[QC]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I find it hard to believe that a 7.4 MPI only has 310hp? unless we're talking about different hp ratings over the years.

As they say, you could look it up . . . 330 was at the flywheel. An MPI does not "have to" make more than a Carbed engine unless you want it to . . . How're you going to charge more for a 385? Remember, engine manufacturers sell horsepower. There are some mechanical differences as well if I remember correctly.

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

QC, Let me explain, in the mid 90's I had a 24' SeaRay Cuddy that had the 350/260hp Mercruiser, I was desperately looking for a used 454 big block Merc. conversion for this heavy boat, the stock, average, most common available upgrade, was the 454/330hp 4bbl motor. I figured if I needed to, I could eventually build it up to as much horsepower as I needed. Of course I never found one reasonable enough so I gave up on the idea, and sold the boat.

And to think for $500 more at the time I was looking, I had a chance to get the exact same boat with twin 470 Mercruisers (motors in pristine shape) that they were claiming was a 45mph boat! Out in The Great Peconic Bay, L. I. But, after asking around, I got bad reviews on the 470 motors, so I let it go.

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

QC, apparently dcg, has found the prop to be ok, so I'm past that issue
If it truly is a Hi Perf 454Mag 385hp I agree it should do 65. I seem to remember that the least hp a standard 4bbl 454 Merc marine motor had was 330hp, you either had a base 350/260hp, or a base 454/330hp for years, not withstanding Mercs Hi Perf line of racing engines. I find it hard to believe that a 7.4 MPI only has 310hp? unless we're talking about different hp ratings over the years.

It's the MAG version. My brother had the 7.4L MPI (non-mag).
385 vs 310 hp. My understanding is that it's largely a difference in heads.
The 310 MPI is should run 4400-4600, the Mag 4600-5000.

Remember that they changed how they rated HP across the years, so that may account for the "310" hp version.. Dunno... The real numbers are in the torque anyway.

I don't want to drive the boat around at 65mph, but I do want it to run right.

 

[QC]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Just thought I'd cause a double check. It was your use of 7.4L that made me think of it . . . Hmmmm. Back to the drawing board. Do you have any way to check fuel flow? Without proper diagnostic equipment this would be one way to determine if you were getting full power. You'd need to get the revs right, but if she's flowing fuel, she's making the power . . .

BTW, your torque comment is actually not correct, but we we can discuss that later. Just a pet peeve. Peak torque is at a much lower RPM, if that's all that matters, prop her for 3200 . . .

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Are you saying that your motor is ok and you have to much prop?

 

[180shabah]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

BTW, your torque comment is actually not correct, but we we can discuss that later. Just a pet peeve.

Yeah, we can save that for another thread

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Are you saying that your motor is ok and you have to much prop?

No, I'm not saying that. I'm saying according to prop calculations and tested top speed of this boat, the prop is a little under pitched for the tested top speed of this boat. I'd expect the motor to turn it.

I don't have a way to check fuel flow, short of putting in one of those GPH sensors. I believe (from tuning stand alone EFI on cars) that if I didn't have enough fuel, I'd go lean and the motor would run badly...

It seems to be purring along well.. Although the one caveot is that I might be on 7 cylinders... My V8 ear isn't that well developed.

I'm basically guessing that this point.. We'll see what the DTC's (trouble codes) come back as.

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Yeah, we can save that for another thread

Well you cant have tourqe without rpm, kinda like which came first the chicken or the egg......and i do have a difinitive answer for that PM me if you want the thruth.........

 

[QC]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Well you cant have tourqe without rpm,

Well actually . . . you can't have horsepower without RPM . . . remember that you your ownself can apply 300 lb ft. to main bolts, you just can't make any horsepower to speak of because you cannot maintain any RPM

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Well actually . . . you can't have horsepower without RPM . . . remember that you your ownself can apply 300 lb ft. to main bolts, you just can't make any horsepower to speak of because you cannot maintain any RPM

Ummmm no


Quote:
Originally Posted by Tail_Gunner
Well you cant have tourqe without rpm,

Well actually . . . you can't have horsepower without RPM . . . remember that you your ownself can apply 300 lb ft. to main bolts, you just can't make any horsepower to speak of because yuo cannot maintain any RPM
__________________
If I had any dignity, that would've been humiliating - Adam from Mythbusters

I said Tourqe

Now how's about the chicken vs the egg....


There's something else going on here that i cannot explain, it's either gearing, the hull, or the cam's, that is if the timing is setup properly....Something is dragging that thing down, could you post some pic's from the stern forward???

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Timing: Fair guess. So the timing was off slightly (I didn't ask how much) - probably from me replacing the cap and rotor, but not retiming it afterwards.

The deal was that she's got a bad injector. I asked how they diagnosed it and they said that their diagnostic tool allows them to fire each injector individually and they watch for a fuel pressure drop. Apparently the latest Mercruiser ECMs will self-diagnose this, dropping one cylinder at a time and record the RPM drop. No RPM drop = bad injector.
My MPI motor doesn't have the self diagnostic.

I could have taken a mildly educated guess at that from the one plug that didn't look the same as the others, but it would have been a long shot for me.

Dang Mercruiser injector is $175 (for one). I asked for an auto equavalent - they said there isn't one, which is probably not true - but they don't know the specs on the injector.

We'll see how she runs with a new injector.

 

[QC]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

At least they found sumpin. Awesome!