Lost rpm
 

I have an issue that is driving me crazy. Boat has twin 540's with Prochargers. Ran great two seasons ago but sometime last season we lost rpm in one motor. It was time for the top ends to be done so figured that was the issue. Did both motors this past winter and both were run extensively on a dyno. Made great power on the dyno. Put them back in the boat and we can still only run one motor up to 4,000 rpm - when they should be running 5300. The other motor runs to 4500 and starts making boost (2 psi) but the other motor will only run 4000 and makes no boost at that rpm so I feel that one is holding the other back. I've tried grounding the motor, checked voltage at the coil (12 volts +) and run off an external 5 gallon tank - nothing changes. I've pulled everything off the 4000 rpm motor's battery and am running it right from the + battery terminal to the starter and the - to the ground on the block. Nothing has changed on the boat, same props, same drives, etc. Boat used to run 85 and now I'm lucky to top 60. What can I possibly be missing? Any help would be greatly appreciated.

at this point,I'd almost be inclined to pull motors and switch positions and see if its really a engine problem or a hull problem.

Hull problem how? Like one side of the boat is heavier than the other?

Anybody within towing distance with a prop dyno

what parts where used on the dyno not used in The boat

Pretty much everything I've tried - fuel supply and electrical. I've checked throttle plates are fully open.

Old sc belts or low tension or other reasons for slipping or trailer still connected to boat :)

i’m assuming compression and fuel psi and timing correct

Haha didn't check for the trailer!
All belts are tight and in good condition. Planning to verify fuel pressure with a mechanical gauge - boats gauges show pressure is good - 50 psi. Compression was checked when doing top ends. The baffling part is everything ran great on the dyno.

Assuming Holley Carbs. Check the inlet fuel screens-filters in the carb, Also the water separating fuel filters.

Mark

no these are efi motors. Did not replace the water seperators but again, nothing changed from the dyno to install.

valet mode.

Well, things like fuel tank , gas in it, and supply line and fittings wheren't at dyno.

a cracked spark plug or two from engine installation not uncommon

Can a drive tighten up without scattering?

I would also check the fuel tank vent hose for a kink or blockage.

What EFI is it ? if its TB have you swapped them ?

ive eliminated that by running off an external 5 gallon tank with the exact same results A cracked plug is possible but not likely That motor went back in first and there's plenty of room.

I thought about that but I think with the HP I'm pushing it would have exploded by now

They're 500 efi's that were punched out to 540's. I have not tried swapping TB's yet . . .

They're 500 efi's that were punched out to 540's. I have not tried swapping TB's yet . . .

I was thinking they were carb style efi, MEFI 3 , 4 ? who tuned it ?

I believe they're MEFI 3. Tuned by the builder. We used the exact same program that was used before the top ends were done. Tuning was originally done on the dyno and then fine tuned in the boat using welded bungs in the headers.

what are you running on inlet side of prochargers, ive seen them suck filters partially closed and create a big restriction. happened to a friends 35 cig, wouldn't do it on dyno, but did it in boat

The inlet sides are open - no filters other than a thin mesh

are the intercoolers clean,spider webs,,mouse nest,.......

I'd put a scanner on it and see if your tripping the knock sensors.

No

Internal exhaust flapper stuck partially closed somehow??

Not really familiar with prochargers. Is it normal for them to not make any boost until 4500 RPM??

I did check the intercoolers and they are clear of any instructions. Knock sensors is something I need to check

There are flappers in the tailpipes but they're just the rubber ones from CP Performance - I don't know how they could not be forced open. Something to look at though.

It is normal to start seeing boost around 4200 - I'm seeing around 2 psi on the other motor @ 4500. The Prochargers are based on rpm.

So over the weekend I swapped blowers and intercoolers and the problem has not changed. I put a smaller prop on the side that I'm having an issue with and was able to run the rpm up to 4800 but again saw no boost. At that rpm I should be seeing close to 5 psi and running that smaller prop with ANY boost I'd have hit the rev limiter easily. I think I've ruled out pretty much everything. I'm leaning towards an intake leak but how would that go bad from the dyno to install?

There’s no way that the tensioner is acting goofy and the belt is slipping is there? When I had m3SC’s in my BT I had to tighten the port blower belt to what seemed like overkill to get it to match the stbd engines boost. Sepentine Belts, not cog belts. I think you’ve ruled out about everything else.

i put new serpentine belts on both sides over the weekend and tightened them up to the point I was afraid to go any tighter and no change. Hooked a computer up and it shows zero boost @ 4000 rpm.

Where is it picking up the boost pressure at ?

Mechanical gauge right in the plenum as well as reading it off the MAP sensor

Bravo boat? Bad coupler maybe?
I know you said the lazy engine won't rev. But on the flip side, no load no boost for the most part.
Other than that, fuel issue? Are all the fuel components you dyno'd it with the same as in the boat?

Yes Bravo boat with brand new couplers. Engine will spin up to 4000 rpm and at that rpm should be making around 4 psi boost. If the procharger is spinning at that rpm it HAS to be making boost - its a mechanical pump that's driven by a belt off the crank. Anyone know if there's a way to actually test the procharger? We've done a leak down on the motor and everything checks out perfect. Either the blower isn't producing boost or its being lost somewhere. I've pressure tested all the tubing and the chiller and there's no leak.

You already switched blowers from side to side if I'm reading right, correct?

And no it doesn't HAS to make boost. Put a open ended tube from the supercharger just into open air with a pressure gauge in the tube, spin the thing 50,000 rpm and Ill bet you see no "boost" on the gauge.
So the boost seen in the past on your gauge on that engine was the gauge pressure seen at a typical rpm with a typical load at that rpm on the engine.
Got a turbo'd diesel truck with a boost gauge? Drive along flat ground at whatever speed, make note of the boost, hit a steep hill and don't downshift or let it downshift, try to maintain that same speed, watch the boost rise.

Otherwise, have you tried at least to run both N/A with out the belts, see if you can achieve the same rpm on each engine with the same props?
And yeah I'm aware the tune may be off running N/A

"Bad" side isnt making boost at 4800... What RPM is the "good" side making boost?

Yes blowers, chillers and all connected tubing was swapped from sure to side

i have not tried running them N/A but I will try that. The problem is actually on both motors - not just one as originally thought. We have hooked them both up to a computer and one will spin @ 4000 rpm and reads zero boost off the MAP sensor and the other motor will spin to 4300 rpm and reads 1.7 lbs boost off the MAP sensor.

The fuel you are using is crap and the knock senors pulling them back? Just throwing out stuff because I don't think there is much left