7.4L Mag MPI Performance Issues

[dcg9381]

I've got a 1998 Glastron / Carlson boat, 23.4 feet, CSX-23
It's powered by a 7.4L Magnum MPI motor.
The boat looks nice, has 125 hours on it. I'm the 3rd owner.

I bought it in "guaranteed" mechanical condition - after some testing a GPS and an inductive tach, I found that I had a problem - top end in 52mph range. I gapped and changed the plugs, ran compression, new plug wires, new cap & rotor - same issue, top end of 52mph at about 4600 rpm (off the top of my head).

I didn't have the tools to run the EFI diagnostics. I took it in to my local marine dealer, that rechecked all my work (nice, eh?) and found that it had a "bad injector". Replaced that injector and the boat ran much better. Top end 61-62 in calm water, full tank of fuel.

My guaranteed boat went silent after a $600 repair bill.

Results now:
4700-4800 rpm, 61-62 mph, 23 pitch Mirage Plus prop. Bravo-I outdrive.
Calculated prop slip is 11%. This is good performance, but not to spec.

This boat tested out per a boating magazine at 68.5 in rough water fueled. Same motor - same outdrive. I'm well shy of that mark in smooth water. I'm thinking that the 6-7 mph difference is probably 50hp or more on a boat like this, so I wonder if I don't have something wrong still.

I know how to work on EFI systems - I just have no idea of what I need to do on a boat, as I don't have an 02 sensor to look at for output and don't really have the expensive Mercruiser diagnostic tool....

My suspicion would be the injector set - the injectors are not a recirculating system like they are on cars, so any crap that gets sent up in the fuel is going to stay in the injectors.

I can pull them, have them professionally cleaned.. But I'm just guessing, and I hate to guess... Any ideas?

 

[tommays]

Re: 7.4L Mag MPI Performance Issues

In the mag TEST you can be sure there was NOTHING in the boat BUT 1/3 of a tank of fuel and at most 2 small people










Tommays

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

Thanks, Tommy.. I can run the thing near dry and it's not 6-7 mph difference.
You'll have to assume I know how to correctly trim the boat, also...

 

[Bondo]

Re: 7.4L Mag MPI Performance Issues

Ayuh,........

2 things come to mind,.......

Rough water makes for Much faster times because you can catch more Air to break the hull free of the water's Drag........
And,.......
A Magazine's Boat Tests are about as reliable as the Manufacture's stated Top End figures,.........
Both are usually Extremely Optimistic............

You may be chasing a Ghost that isn't there......................

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

Well, if that's typical, it's certainly the kind of feedback I'm looking for. I thought boat magazine's should be someone independent (although I do recognize where the ad dollars come from).

I've only talked with one other CSX-23 owner, he said his was capable of 68+ also, but he had the Bravo-II drive.

Anyone think I should be getting more RPM out of a Mag MPI motor? I'm certainly in the power band, but don't know if I should be able to hit 5k RPM.

 

[Bondo]

Re: 7.4L Mag MPI Performance Issues

Ayuh,.....

Have a look at this thread,..........

I'm thinking a PM to 96lt4c4 might lead you to some Insight,......
He really seems to Know efis.......

 

[magster65]

Re: 7.4L Mag MPI Performance Issues

Thanks, Tommy.. I can run the thing near dry and it's not 6-7 mph difference.
You'll have to assume I know how to correctly trim the boat, also...

'Boat test' tested my old Sundancer at 46.5 mph. Impressive but in the real world with fuel, people and stuff in the boat, it ran 41 and change.

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

'Boat test' tested my old Sundancer at 46.5 mph. Impressive but in the real world with fuel, people and stuff in the boat, it ran 41 and change.

Fully loaded, full of fuel, I can understand a 5 mph (or more) difference.

Single person, 1/2 tank of fuel, 7-8mph is too far off...


Bond-o,
I've done a good bit of EFI tuning myself (mainly stand alone EFI turbos and carb to EFI conversions) - I don't expect Mercruiser's tune to be off unless I started messing with the motor itself, which I haven't done.

The issue is that I don't have a good starting point on Mercruisers, particularly the 7.4L Mag MPI - so that's why I'm turning to this forum. I don't know what the typical issues are.

Baja's commonly run these motors in their "smaller" boats, but they boot me off for posting as I'm not an owner... And I thought the Harley guys were bad! Glastron made classic boats, but only did "sport" boats for a few years - not a lot of help with Glastron owners.

 

[newport dave]

Re: 7.4L Mag MPI Performance Issues

Clarify one thing for me. Is it a 454 MAG MPI or is it a 7.4L MPI? The WOT range for the 7.4L MPI is 4200 to 4600, your already over that and would be right on the rev limitier. If it's a 454 MAG MPI the WOT range is 4600-5000, your midway on this one, so either way I think your doing ok.

Dave

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

7.4L Mag MPI (385hp).

 

[tommays]

Re: 7.4L Mag MPI Performance Issues

One other thing is that it has been a LONG time since i could buy real 100% gas on Long Island

If i did not trailer to upstate NY every summer were they still have real gas i would never no the difference





Tommays

 

[magster65]

Re: 7.4L Mag MPI Performance Issues

Well... it might be a bit slow but actual boat weight and minor hull design differences can really add up. My old 23' Baja Bandit (lake boat) gps'd at 69 with a carb'd 7.4 (330 hp) / bravo / same prop as you (23" Mirage) @ 5200 according to that tach. This was in the salt with 2 people, a little under half fuel. It didn't have much deadrise... pretty flat.
One of my bud's had a newer 23' Baja (Hammer I think it was, deep V) with a 496 mag mpi ho (425 hp)... our boats were the same top speed and I could blow his little doors off from a standing start.
I know it's not the technical answer you're looking for but I thought I'd give ya' a real world side by side comparison.

 

[QC]

Re: 7.4L Mag MPI Performance Issues

7.4L Mag MPI (385hp).

Do you have a Serial Number for DonS or newport dave? It is still a little confusing as the 7.4L model nomenclature indicates it is the 330 bhp and the 454 Mag model is the 385, both are MPI . . .

Anyway, in my experience you are absolutely at VERY good speed as is. I don't think there is any more there. Throw the magazine test away. My boat tested at 62, and the best I can do alone with a lot of playing with props and trim, trim tabs, on fumes, alone, no ice chest, no inner tubes, no kid toys, just a beagle puppy of about 7 lbs, a little ripple to help out is 57 . . . twice.

Another little experience/spec watching note . . . almost all test data I have seen from prop tests, engine tests etc. indicate that, contrary to popular belief, the best top speed runs are achieved at the lower end of the WOT RPM range. Definitely not the best for flexibility, but for mag tests etc. maybe so . . .

Do you have your fuel rate at WOT? That can indicate whether or not you are making the horsepower you are supposed to. Also, I believe 6 - 7 MPH above 60 MPH is over 100 bhp, so that should show up in fuel use.

 

[newport dave]

Re: 7.4L Mag MPI Performance Issues

7.4L Mag MPI (385hp).

Trying to help here, but it is difficult to respond with accurate information if you can't give the actual engine model. 385 HP would be a 454MAG.


Edit -

Posted on another forum on 9-23-07;

It's the 454 Mag EFI motor. My brother owned a PowerQuest 22 with the non-Mag version and I've worked on the 350 Mag version.
Photo of the motor that's in this boat:

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

Trying to help here, but it is difficult to respond with accurate information if you can't give the actual engine model. 385 HP would be a 454MAG.

Part of the problem is that I use 7.4L and 454 interchangably. To me, displacement is displacement. Sorry if that has caused some confusion.

If you were to look on top of the motor - it says "454 Mag MPI".

IE: 454 ci, 7.4L, 385 hp, port injected.

Photo:
http://lakebox.dnsalias.com/photos/1998csx23/med/DSC06564.JPG


I do have the test specs on fuel use. I don't have actual "real world" fuel use. I think you're asking me to compare tested fuel use to actual fuel use at a fixed cruise speed. If I'm down on HP, it will show up as added fuel consumption. I can try that - thanks for the suggestion.

I don't understand the "real gas" comment. The motor is rated to run on 87 octane, if I recall (R+M/2) - and as it's fuel injected with a fixed fuel and timing map, I see no advantage to running more octane than required to keep it out of detonation. It's trailered - and it's fed auto pump gas.

I've never seen a dyno graph on this motor, but I know that V8s can make best power/torque earlier in the RPM range. The prop is not OEM. Glastron says that it was equipped with a 23 pitch prop from the factory. Someone added the Mirage Plus (23P) - but I seriously doubt if that's would cause this big a difference.

I can post hull specs and approximate weight. I believe the boat weighs in at 3800-4200.
I see other 23-24 boats in the 350-400 hp range pulling high 60s. Just trying to determine if I've got an issue...

 

[tommays]

Re: 7.4L Mag MPI Performance Issues

"I don't understand the "real gas" comment."


Around here we have had MTBE and when they pulled that E-10 to "help" the motor run cleaner

IMHP thre has allways been a MPG and power hit when using blended fuel



Tommays

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

I'm in Texas - I don't think we have a lot of E-blended fuels at the pump.

Fuels blended with ethanol have lower energy per unit - they result in lower MPG because the EFI systems operate in closed loop - it requires more fuel to get to a stoich (ideal mixture) point with Ethanol blended fuels.

This shouldn't impact boats that don't have 02 sensors - although they will be down on power because the mixtures are fixed don't operate in a closed loop that adjusts based on 02 sensor... The resulting mixture will be lean, away from peak power.

I'm taking my own thread off-topic.

 

[QC]

Re: 7.4L Mag MPI Performance Issues

I do have the test specs on fuel use. I don't have actual "real world" fuel use. I think you're asking me to compare tested fuel use to actual fuel use at a fixed cruise speed. If I'm down on HP, it will show up as added fuel consumption. I can try that - thanks for the suggestion.

Well actually I think you should compare WOT fuel use, and my take on this is kinda the opposite of yours; if you are burning reasonably close to test spec fuel at WOT, then you are more than likely making all of the horsepower. Using cruise numbers will TOTALLY muddy the water. Then your bringing in hull efficiency etc. However wide open throttle runs are basically the same as a dyno run at WOT. As long as you are in the recommended RPM range and hopefully close to the RPM of the test data, these are fairly valid tests that boats do better than cars (where you gonna run 150MPH). Really the only differences then are air temp, density and fuel quality . . . It knows what the Manifold Absolute Pressure is, so it is going to be dang close on mixture even without the O2 sensor . . .

I've never seen a dyno graph on this motor, but I know that V8s can make best power/torque earlier in the RPM range.

This may be why speed is a little better toward the bottom of the range. Probably more to do with engines being a little more efficient when they turn slower (turning all stuff slower). It is also because of breathing characteristics, etc. at higher RPM.

The prop is not OEM. Glastron says that it was equipped with a 23 pitch prop from the factory. Someone added the Mirage Plus (23P) - but I seriously doubt if that's would cause this big a difference.

Can make a big difference. Look at Walleyheds prop testing stuff in the prop forum ^^^^

I see other 23-24 boats in the 350-400 hp range pulling high 60s. Just trying to determine if I've got an issue..

My bet is that you are chasing a ghost. I do not know of a lot of 23 - 24s with 350 - 400 hp that are hitting high 60s with reliable info. Where I boat there are a lot of 100 MPH boats, but there are very few high 60s boats with stock power that you can take your girlfriend in a ride in too . . .

 

[dcg9381]

Re: 7.4L Mag MPI Performance Issues

Well actually I think you should compare WOT fuel use, and my take on this is kinda the opposite of yours; if you are burning reasonably close to test spec fuel at WOT, then you are more than likely making all of the horsepower. Using cruise numbers will TOTALLY muddy the water. Then your bringing in hull efficiency etc. However wide open throttle runs are basically the same as a dyno run at WOT. As long as you are in the recommended RPM range and hopefully close to the RPM of the test data, these are fairly valid tests that boats do better than cars (where you gonna run 150MPH). Really the only differences then are air temp, density and fuel quality . . . It knows what the Manifold Absolute Pressure is, so it is going to be dang close on mixture even without the O2 sensor . . .

I'd like a 2nd opinion on the WOT take; I can run the boat in an attempt to eat up enough fuel for accurate measurement... With a fixed fuel map, I think it's going to be a wash...

The reason I want a 2nd opinion is that at WOT the manifold pressure is going to approach 0 (1 ATM), regardless of mixture. Open that butterfly and all manifold vac simply goes out the door unless you backfire.

REROP; Can make a big difference. Look at Walleyheds prop testing stuff in the prop forum ^^^^

This I buy.. but usually under/over propping will take you out of RPM range.
I've also calculated prop slip at 11%, which seems to be very much within spec.

My bet is that you are chasing a ghost. I do not know of a lot of 23 - 24s with 350 - 400 hp that are hitting high 60s with reliable info. Where I boat there are a lot of 100 MPH boats, but there are very few high 60s boats with stock power that you can take your girlfriend in a ride in too . . .

That's fair enough... And the kind of feedback I'm looking for - I just wish I could find a few more CSX-23 owners. A 60mph boat is more than fast enough.. I just wanted to be sure that the boat is running like it should.

Of course, I'm getting the opposite feedback too - guys claiming 60 mph out of the non-mag motor, full of fuel... The feedback I got from another CSX-23 owner, non-Mag motor:

You must have something wrong somewhere, My boat only has 7.4 MPI and I have no problem hitting 60 MPH with 5 people and full gas tank.Maybe try a bigger 4 blade prop.

Of all the problems to have, I suppose this is a *good* one.

 

[QC]

Re: 7.4L Mag MPI Performance Issues

I'd like a 2nd opinion on the WOT take; I can run the boat in an attempt to eat up enough fuel for accurate measurement... With a fixed fuel map, I think it's going to be a wash...

A 2nd opinion is good. No issue there, but let me take a 2nd or 3rd run at you too. I would ask where does this fuel go if it is being pumped out of the tank at correct WOT rates? Assuming you have good compression, no big broked parts, nobody with a vice grip on any of your drive parts and no fuel leaks, unless timing is off, the only place for low power is an intake air restriction or some sort of fuel supply problem. In those last two cases then you would have low WOT fuel rates or I guess crap fuel . . .

at WOT the manifold pressure is going to approach 0 (1 ATM), regardless of mixture. Open that butterfly and all manifold vac simply goes out the door unless you backfire.

Ahhhh, but you're forgetting about "absolute pressure", you have to count Atmospheric pressure too, zero is Gauge not absolute . . . that's how it knows how to adjust for altitude which is another good question . . .

Edit: I see your 1 ATM there now, but I am not sure why this is relevant? O2 sensors are there to provide perfect stoichiometric burn ratios so that you don't foul a catalyst. We are talking teeny weeny tiny variations of rich/lean ratios if the ECU is mapped properly and the MAP sensor is good . . .