Need RPM’s on 383MPI 1970 SeaRay 24’ project

[jrglombo]

So I got ambitious last winter and converted my 1970 SRV 240 hard top from omc stringer drive to a new 383 MPI and used Bravo 3 drive. I’ve now tried two separate sets of props and still have max rpm at 4400. Engine has a mild cam and upgraded valve train. Otherwise a basic built 383 Vortec. The accessories(computer, electrical, intake, injectors, etc) were off a donor 350 mag engine. I plan to get the computer tuned over this winter to now match the 383. I’m skeptical I will have what I’m looking for from the tune as it seems to run well as is minus the 4400 max rpm. Here’s what I’m getting for numbers:

22p set - 4200 RPM, 41 mph
20p set - 4400 rpm 38 mph

Any suggestions to get closer to the 5200 mark? The engine was built and dynoed at 5200 so I’d really like to get this thing up there but I’m out of prop pitch and suffering on speed. Drive ratio 2.2. I know these hulls aren’t built for speed and are heavy but I feel I’m missing out on a couple more mph’s if I can get the rpm up. Boat sits in a well all year so maybe hull is full of water? I’m taking it to a scale after the season is over to get a weight spec. I’ll also do some inspecting under the floors. Pictures just to show WTH we did to this old boat lol

 

[Grub54891]

What condition was the transom in when you rebuilt it? Was there rot?

 

[alldodge]

You said new and motor looks new, but did it come from Merc or someone else rebuild it?

You have a 555 ECM so no one can really change the tuning

 

[Scott Danforth]

do you have a boat weight? not what the specs say it weighted in 1970, what the actual scale weight is?

is your tach accurate?

what ECM did you have on the dyno?

 

[Scott06]

I believe that MPI just does fuel mapping in open loop off the throttle setting- doesn't have O2 sensors in the exhaust system to give feedback to ECM?

If so you likely are running lean would check your plugs ASAP and not lean on it too much.

I think you need to find a way to reflash or replace your ECM with a 383 version and make sure the injectors flow what is needed on a 383. Not sure if 350 and 383 use same injectors. No sense messing with props until this is figured out

Is the intake stock or aftermarket? it looks like bare aluminum vs the factory painted.

Since you are needing to tune for the displacement and cam change you can also stab in a AFR meter. Not sure how they do it with dry joint manifolds as the only O2 sensor plates I have seen are for wet joint exhaust and aftermarket exhaust. I have found this pretty invaluable for tuning on a couple different engines

 

[jrglombo]

Thanks for reply guys.

Alldodge - engine is rebuilt from local reputable rebuilder known for MPI and Vortec builds. He recommended I get the computer tuned since it’s from the stock 350mag mpi

Scott Danforth - I do not know actual weight but when I pull it out of the well in a couple weeks I plan to do a light/heavy cat scale with trailer and trailer boat combo.

Scott06- the intake is a stock MPI intake and injectors. The intake was just powder coated to look that way I’ll check plugs and consider finding a way to to check AFR.

I have not run it hard yet. I have 35 hrs on the setup and usually run at 3200 rpm for a nice cruise speed of 30mph.

Thanks again for the input! I do not mind sending the computer in for tune/correct for the 350mag to 383.

 

[alldodge]

Alldodge - engine is rebuilt from local reputable rebuilder known for MPI and Vortec builds. He recommended I get the computer tuned since it’s from the stock 350mag mpi

Guy might be a reputable rebuilder but should not be selling MPI's with the wrong ECM. Having the wrong fuel tables loaded will not produce the power it should and can damage the motor.

He sold you a new motor and it might cost another $2500

Kind of wonder if it's actually a 383 with correct rotating assembly and cam or just a boarded out 350.

Does it have the flat distributor cap or something different?

 

[Scott06]

He recommended I get the computer tuned since it’s from the stock 350mag mpi

I’ll check plugs and consider finding a way to to check AFR.

Thanks again for the input! I do not mind sending the computer in for tune/correct for the 350mag to 383.

Retuning this for the 383 is not an option. You are running this lean for sure in an environment that is pron e to detonation.

Check your plugs before you detonate this thing to death, which no engine builder will cover.

What was the hp/torque curve and AFR from the dyno ? Should have made at least what the stock 383 did which is 340 hp and like 425 lb ft TQ. I am assuming on a dyno they would have checked AFR and not released it to customer? Doesn't sit right how they could have loaded the motor up with what off the given info is the wrong fuel mapping ?

Does it utilize knock sensors ? If so you with the wrong fuel mapping it may be pulling back timing. Bottom line work out the fuel mapping before doing anything else

 

[jrglombo]

Sorry I need to elaborate…I had a donor 350mag mpi and purchased a 383 long block. I used all my own accessories, he dynoed with his own intake and a 2 bar. Carb application. Idk why he dynoed with a 2 barrel. However, the computer was my own. I’ll try to find the report. It has one knock sensor near the remote oil adapter.

 

[jrglombo]

It has a flat cap and yes the block was clearanced for stroker crank and rotating assembly. I know and trust it was built as a stroker.

 

[Scott06]

Sorry I need to elaborate…I had a donor 350mag mpi and purchased a 383 long block. I used all my own accessories, he dynoed with his own intake and a 2 bar. Carb application. Idk why he dynoed with a 2 barrel. However, the computer was my own. I’ll try to find the report. It has one knock sensor near the remote oil adapter.

Ok so wasn't a full set up (or power) dyno. Not unusal to use the dyno owned carb as the builder wont have all accessories like fuel pump etc that go on car. Most guys have known carbs so they wont waste time chasing owner provided carb issue or ruin a motor running lean.

He used the 2 bbl cause that's what he had...

So what you are likely seeing here is the 2 bbl limited the air flow to 500 cfm or so maybe less, but the jetting was probably reasonably correct to provide good AFR for the airflow the carb allowed..

Going through this you are 100% guaranteed to be running it lean. You are also 100% guaranteed with the way the builder provided the motor to not have any warranty if you destroy the motor due to detonation (they would likely cover wrong bearing clearances or ring gap issues etc).

If you have run it 35 hrs and not checked the plugs given the mismatch between MPI system and engine size you are asking for trouble. Check your plugs and use a borescope to look at pistons for signs of detonation.

If it were mine I wouldn't use it until the fuel supply is worked out as you are not putting enough fuel in and probably ECU is yanking timing out, which is a good thing under current conditions.

 

[Scott06]

You said new and motor looks new, but did it come from Merc or someone else rebuild it?

You have a 555 ECM so no one can really change the tuning

Is this the same 555 ECM ? https://www.cpperformance.com/p-81018-ecmpcm-555-custom-calibration.aspx

I think these guys list offer a 555 to MEFI conversion

 

[alldodge]

Sorry I need to elaborate…I had a donor 350mag mpi and purchased a 383 long block. I used all my own accessories, he dynoed with his own intake and a 2 bar. Carb application. Idk why he dynoed with a 2 barrel. However, the computer was my own. I’ll try to find the report. It has one knock sensor near the remote oil adapter.

This does clear things up

Dyno with 2 barrel would have also used a different distributor

Was the original distributor installed correctly


I would scan the ECM, it could be it's retarding the timing because it's running lean

Is this the same 555 ECM ? https://www.cpperformance.com/p-81018-ecmpcm-555-custom-calibration.aspx

I think these guys list offer a 555 to MEFI conversion

Yes, that's the 555 and from just looking at your pic that's whaat you have. Check your connectors to make sure you have 2 and not 3 connectors

 

[ROY WILLIAMS]

So I got ambitious last winter and converted my 1970 SRV 240 hard top from omc stringer drive to a new 383 MPI and used Bravo 3 drive.

Boat sits in a well all year so maybe hull is full of water? I’m taking it to a scale after the season is over to get a weight spec. I’ll also do some inspecting under the floors. Pictures just to show WTH we did to this old boat ..........

the boat is the draining of the vessel plug take it off in the winter .... also the wooden boat is the damaged also the plywood .
the polyester resin fixed it of the wooden boat ... 3 times .... also the fiberglass .

did you painted it off the wooden surface ?

my sister was the 1970 SEARAY 24 FEET .... THE 1980'S MY SISTER 10 YEARS ..

 

[Scott Danforth]

I agree with Scott, check plug color and get the motor tuned before worrying about props.

if your dyno guy has a kenne bell boost-a-pump DC/DC converter, you can feed the fuel pumps with 16-17 volts and get a few more PSI out of them to see if that helps until you get a tune.

 

[jrglombo]

Thanks again for the suggestions! I have the ECM555 for sure. I wasn’t aware you can convert to MEFI. Attached the Dyno graph here…

I have that same document for when I installed the distributor and I’m confident it’s installed correctly. Additionally I can get a scanner plugged in for a test ride to check timing, but on first run I believe it was advancing properly but I do not remember the value. Is there a value I should be looking for timing advancement.

Roy - I’m sorry your sister was a 24’ SeaRay! I’m not too sure what you’re asking though, I did not paint any wooden surface. It’s all fiberglassed over.

 

[jrglombo]

Looking at the graph it doesn’t really show much…this was more of a break-in run from what I’m reading

 

[Scott06]

Thanks again for the suggestions! I have the ECM555 for sure. I wasn’t aware you can convert to MEFI. Attached the Dyno graph here…

I have that same document for when I installed the distributor and I’m confident it’s installed correctly. Additionally I can get a scanner plugged in for a test ride to check timing, but on first run I believe it was advancing properly but I do not remember the value. Is there a value I should be looking for timing advancement.

Roy - I’m sorry your sister was a 24’ SeaRay! I’m not too sure what you’re asking though, I did not paint any wooden surface. It’s all fiberglassed over.

According to this it made 244 hp, and torque dropped way off at higher rpm.

Stock merc 383 is 340 hp so comparatively something is really wrong.

Roy means his sister had a 24 ft Sea Ray in the 80's for 10 years....

You can google the tuner companies give them your cam grind and they can set you up with correct 383 ECU. Do the 383 and 350 use the same injectors? Willing to bet the part numbers are different.

Get this stuff sorted out and you will be happy, its a nice looking boat. I'm on my second Sea Ray- 1991 170 bowrider that my parents bought in '95, kept that until 2016 and got a '04 200 sport which is perfect for the lake we are on.

 

[alldodge]

It should show hp, torque and time but the agree the chart is doesn't show much. Torque drops way off and HP climbs oposite, doesn't make since

I have the ECM555 for sure. I wasn’t aware you can convert to MEFI.

You cannot using the same harness, would have to change harness

 

[jrglombo]

I don’t have it in the report but I was told it made 360hp (advertised as such too) maybe the dyno company can give me a better report.

Thank on the compliment, we really enjoy the boat and the new reliability aspect. Just working out the things like this. Believe it or not the injectors are the same part number for both engines…