How to coax out a few more RPM

davidbrooks

Now that i have the smartcraft gateway connected i have a ton of information at my fingertips.  It is almost a little overwhelming.  I am trying to figure out the best way to coax out a few more RPMs from the old gal without doing any major surgery so fire away with ideas.  Just as background i have an 03 342 with MAG MPI 350's.  The serial number has them as a 2002 vintage.  I am running with Hill 4x4 22P Props. Inline fuel filters were changed and according to Mercruiser i dont have the cool fuel module to worry about so at most i might have a screen on the pump itself.
Rotor, Cap, wires and plugs were replace June of 2017.  I opened the distrib cap and cleaned off the contacts with emory paper.  They didn't look worn or corroded at all though.  

Currently at WOT i can get up to 4300 on Port and 4050 on Starboard engines and i am maxing out at 36 mph.  This is with 2 people, FULL fuel tanks and empty bilge and water.  I ordered some new plugs but i put in the NGK Iridium ones that should last longer than 3 yrs and 170 hrs.  Watching it suck fuel at WOT was interesting.  Each engine was reading 18.7 GPH. That is a thirsty girl.  I am cruising at 30 mph at 4000 RPMs.  The one thing i was thinking was that my throttle cables seemed to have very little impact from about 80% to full forward.  Is there an adjustment on the cables that i should tweak?

Dream_Inn

You may want to check on your engine while moving the throttle to see if maybe you aren't getting full throttle.  I'm betting your cables are pretty old and could use a replacing if they've never been done.

Michael T

I believe you are over-propped for those engines and they are not operating in their WOT rated RPM band or possibly even near them. Your RPM seem waaay off any RPM bands I have seen for 350 Mags. If so you are lugging those engines and that is not good. If however as @Dream_Inn suggests your throttles are at fault that would be an easy fix. Who suggested you go to 4 blade props?

davidbrooks

@Dream_Inn any recommendations on what to look for there?  I was searching for a video on how to properly adjust the throttle cables.  Does this look right to you? https://www.youtube.com/watch?v=kYn7VJ2y7fI ?  if not can someone recommend a video or doc?

davidbrooks

@Michael T i have had these for 1 1/2 yrs now.  This was also what was recommended by Hill when i talked to them.  My 3x3 props that i took off where 24P.  I feel like i was getting more RPM out of them before.  Even while on the Hill props.  I also have almost a 300 RPM difference between engines.  Props wouldn't account for that.

Michael T

Actually, improper propping could account for a lot more RPM than 300 RPM but in your case - but because I assume your engines were running within their normal WOT RPM band with the newly installed Hill propellers the change has been subsequent to that and indicated something other than propellers as the culprit. BTW I hope you tested your WOT RPM bands as part of initially testing your new Hill props to ensure they fell within the OEM WOT RPM band - if not Hill misinformed you. 

davidbrooks

I did. But my Fairia tachs wernt the most accurate. I wasnt hitting 5000 but was close

Black_Diamond

The Faria tachs are close to useless if you want accurate rpm.  WOT should be 5000-5200. Exactly why I went 20P on that same set up. 

You do have a fuel cooler, it is just not the Cool Fuel II that all the problems.  No screen. Just the in-line filter and the water separator filter. 

Certainly worth checking the throttle cables / adjustment. No way you should have lost rpm going from 24 to 22P though. 

Michael T

@Black_Diamond.....yes BUT if he went from three blades 24 to 22 three blades - I agree THAT would increase RPM but there was also a change to 4 blades, no? Sooo many times friends would buy a new "super prop" but fail to realize the huge differences the big 5 can make: # of blades, pitch, cup, rake and diameter. Change any of these and the game changes. When Merc brought out its High Five prop my buddies were snapping them up it was whoa WTF - big changes in RPM, squirrely behaviour, etc. etc. Changing the number of blades is huge. Yes, RPM changes if all else stays the same OK that should be linear.....change anything else then different storey. IMO putting a 4 blade on most cruisers.....no way.

Dream_Inn

MT, it was all ok, until that last sentence.  🤗

Black_Diamond

The prop options on a B-III are super limited though. Merc 3x3, Merc 4x3, Hill 4x4, Solas.  You really do not have the Big 5 to play with: # of blades and pitch (the 4x3 does have a slight dia difference with the 3 a bit smaller than the 4). 

With my set up, the 4x4’s were a total game changer on total performance: docking, getting on plane, running performance, gph improvement, engine WOT rpm and audible less engine strain. 😁

YYZRC

Agreed with @Black_Diamond on all changes. My top end RPM did not change. Still at 5000. Down 2”’ pitch and plus one blade. 

aero3113

YYZRC said:

Agreed with @Black_Diamond on all changes. My top end RPM did not change. Still at 5000. Down 2”’ pitch and plus one blade. 

Same here

YYZRC

davidbrooks said:
Currently at WOT i can get up to 4300 on Port and 4050 on Starboard engines and i am maxing out at 36 mph.

Curious - is this WOT or WOT while attempting to get up on plane?

My motors both max out at between 5000-5100rpm whilst on plane but when attempting to plane my starboard motor is about 200rpm behind the port motor. I’m hoping a new cap and rotor solves the issue. 

davidbrooks

This is fully while on plane. Holding 35 mph

YYZRC

I’m 4300rpm at 35mph as well. 

Dream_Inn

I went down 2 on pitch and up a blade also (I know, different boat).  I still hit my 4800 rpm with just a little less on top end.

Little off topic, but MT would be proud.  Pulling out of Kent narrows yesterday, with my buddy in his 400 in front of me.  Very busy area at times and lots of current flow.  Well, we had a mid 35ish footer SeaRay behind me but not happy with our 6 or 7 mph speed thru the slow zone.  He went out of his way under the bridge and tried to get around me and then got squeezed in the channel, right up on my swim platform.  My buddy radioed to me to pull in front of him to get on plane first and he'd follow (I tend to be his GPS).  Well, as I did that, the SR pulled up with me. I told my son, these SR guys just don't think we are fast enough, just wait.  He got beside me while I got up on plane.  I shocked him with my 6 seconds to 35mph!!  Then about 1 minute later my buddy blew past him as well!  Sure was a great feeling.  Then he tried to follow in our wake, but he slowly drifted back! These props are awesome!!  I feel like MT blowing these guys away just doing my normal cruise.

Oh, and then on my way into my home river a guy was crossing off my side, & went up to about 90% throttle to get across quickly.  I looked down and literally in a couple seconds at most, I was up to 41mph!!  Only saw 4350 rpm and slowed back down!  I'm gonna need to take her back up and see how she does at open throttle.  I think I've worn down some of my paint bottom now and maybe can get 45mph again!

Oh, and then the admiral commented "you know, that trawler you want some day...you're not going to be able to do that!"

reneechris14

Make sure you have no blockage on the exhaust side. The 2002 350 mpi should be able to over 5000g my 2005 shows 5200 at 41mph but that is on my faria tachs with stock 22's. My 2002 270 350mpi I put 20 pitch hill prop on had a piece from the flappers blocking the exhaust made think the props were bad. Once that was removed a new boat was reborn.

davidbrooks

@reneechris14 Good idea. I replaced them a few years back but it doesn't hurt to check that also.  The one thing i failed to mention is that i am really don't have a lot of complaints about it.  It seems to respond pretty well up to 4k rpm.  I was just thinking that it was very little to no response the last 25% of the throttle to WOT.  @Dream_Inn
got me thinking with his suggestion to check the shift cable.  Just trying to see what else i may need to check out.

212rowboat

davidbrooks said:

@reneechris14 Good idea. I replaced them a few years back but it doesn't hurt to check that also.  The one thing i failed to mention is that i am really don't have a lot of complaints about it.  It seems to respond pretty well up to 4k rpm.  I was just thinking that it was very little to no response the last 25% of the throttle to WOT.  @Dream_Inn
got me thinking with his suggestion to check the shift cable.  Just trying to see what else i may need to check out.

That last 25% should be where you see most your improvement... the hull is already on the pad and most the work has been done- the engine should respond to throttle well at that point.  You've got something else going on.  Exhaust obstacle is a great place to start.  A vacuum gaige will help you find out if you dont want to do surgery... 

For those of yall wanting just a little more push throughout the range, roller rockers respond well on these engines... not because of higher lift or slightly increased duration, but because of reducing parasitic drag on the engine.  Most these engines run 1.5:1 ratio stamped rockers... bumping to 1.6:1 will bump into vortec maximum head flow and bump power maybe a point... but the rollers allowing for less drag will bump another 5 to 8%... and throughout the band... from off idle all the way to WOT... cost? $300ish... time? 2 or so hours including tuning them (lash)... worth it? Up to you. 

davidbrooks

@212rowboat that sounds like a good upgrade.  After a quick google online as to the process i quickly realized it wasn't something i wanted to tackle by myself.  This to me is something i would want an expert to do or at least supervise. 

212rowboat

davidbrooks said:

@212rowboat that sounds like a good upgrade.  After a quick google online as to the process i quickly realized it wasn't something i wanted to tackle by myself.  This to me is something i would want an expert to do or at least supervise. 

You can do that.... or.... you CAN do it yourself.  Not much to it... 

You'll want the pedestal variety, not the ones requiring guides.  No more than 1.65:1... 1.6:1 is best... the head flow on vortec heads maxes out somewhere between those two ratios with the cam profile mercury uses most often, ive read.  

Take off valve covers, unbolt a rocker, replace it with new rocker... go to next one... go to other side do same... tighten them to 20# I think it is, and loosen them a turn.  Place cover on far valves but not bolted down... get a towel ready... fire up engine and adjust the lash by ear (no clanking).  Go to other side do same... when you're done all should sound smooth as silk.. tidy up. Enjoy.  

Its NOT that this is a power adder, per say, its that it frees up the resistance added by stamped rockers.  It does add a little power, but the loss of the parasitic draw is its real benefit.  The engine will rev smoother and faster, adding noticeable oomph throughout the range.  You'll likely add as much as 250 to 350rpm to your WOT, and, you'll get there noticeably faster.  

LaRea

@rowboat212 would the 496 be a candidate for this procedure?

Dream_Inn

LaRea said:

@rowboat212 would the 496 be a candidate for this procedure?

@LaRea, you do it first and let me know the improvement.  Rockers are a pretty simple change.

I used to have an aggressive cam with solid lifters and rockers, she ran pretty well in my old street rod.  That engine went in my old man's boat to buy me a crate motor.  I do miss that motor sometimes.

LaRea

Hardin has these for $380 a set.  I have no idea whether they would fit my engine, but it's a scientific fact that the gold coloring adds another 10 HP.  

https://www.hardin-marine.com/p-27048-ultra-gold-roller-rocker-arms-big-block-chevy-716-17-ratio.aspx 

davidbrooks

Came across this video that seems to describe the process. @212rowboat is this what you were thinking?  Except go to 1.6?  https://www.youtube.com/watch?v=702aV5plVs4

212rowboat

LaRea said:

@rowboat212 would the 496 be a candidate for this procedure?

Any pushrod v8 will be.... the small blocks respond better and more noticeable,  and because they don't produce the low end torque their big brothers do.  

You'd free up roughly the same amount of torque translated to power, but, that would be a smaller percentage with the big block and you'd not feel the push like you would with a small block. 

You'd still increase throttle response and step up your rev, though... just like with the small block.  

I did it to a diesel as a for instance... it was peaking at 1187# of tq prior.... i didn't even notice them because the tq was so immense prior... but i did notice a particular metal count drop out of my oil analysis and to the point Blackstone pointed it out.... meaning the friction/wear was tremendously reduced. 

Its just a real simple and fairly cheap way of getting more out of what you have... stay close with ratios so as not to cause funky geometry issues (a pushrod hitting the sides of its journal on the head or a valve smacking piston).... stay under 1.65:1 and that won't be an issue... max valve lift would still be under 505 or so... with 1.7+ you'd be nearing 515 to 525 and will possibly have inference problems.  And..  you'd be surpassing the heads flow rate anyway, which causes turbulence/flow issues and may hurt more than help.  The 'game' here is to eliminate parasitic loss of power, not really add anything... 

Anything being driven off the crank that doesn't translate directly to the gearbox is waste- somebody mentioned one engine revving lower than the other... my question is of the power steering pump on the slower engine is the one being used? Or if an alternator on one is being used and not the other?  Those are parasitic drags and directly off the crank where its easiest to sap its energy (before mechanical leverage)... its the same thing the roller rockers addresses- simply making it easier for the engine to provide power to where you want it by not wasting it somewhere else.  

Roller rockers are the "low hamging fruit" of that class of sappers.  Easy. Cheap. Beneficial.  

Dream_Inn

I'm not sure it's worth the effort on the big blocks, but definitely could be on small blocks, especially boats that need a little more to go.  I do get there are other benefits as mentioned too.

212rowboat

davidbrooks said:

Came across this video that seems to describe the process. @212rowboat is this what you were thinking?  Except go to 1.6?  https://www.youtube.com/watch?v=702aV5plVs4

Yup... thats pretty much it.  You MAY have to remove the baffles in your valve covers... you DON'T have to rotate engine, but you can if you want.  They need to be adjusted while running to ensure proper lash.  Relying on torque is dangerous and a good way to score a camshaft.  Simply adjust them to no clanking, and add an eighth turn is rule of thumb... observe oil flow... move to next one.  1.5 ratio is stock.  1.5s are fine.  1.6 will offer a little bit of power, but again that isn't really the benefit.  Reducing parasitic loss of power is... 

My mercruiser built 357 4bbl vortec is running 1.65:1 rollers.... but that is only because that is what I had on the shelf... they are pedestal mounted self aligning (the rollers braces extend past the bearing disallowing them to roll off)... stay away from those requiring guides or shafts and you'll be fine.  

Those big blocks MAY have shafts already, which makes this simple simple job more complex and likely not worth it. 

davidbrooks

212rowboat said:  They need to be adjusted while running to ensure proper lash.  

That is the part that makes me nervous.  Any videos out there that demonstrate this technique?

212rowboat

I'm sure ypu could find some.  It's the most common way ots been my experience... a matter of fact its the way those who go by torque alone return to when they try torque alone.  

You want to slightly (or better said firmly) seat the pushrod into the lifter... holding it firmly but not compressing it noticeably... you simply can't do that unless the lifter is charged... the only way to ensure the lifter is charged is to run the engine.  

It sounds scary or at least worrisome... it isnt.  Turning the rockers to torque and moving to next one just about ensures they're TOO tight... youll be loosening them until they start to clack a bit, then reversing and tightening until they dont, then add an 1/8th turn... done... youll have the valve covers off and oil will be flying (not that bad) while you do this... this process will take perhaps 3 minutes per side.  If you got the locking style caps (you probably should get them though they add stack height) add a minute per side... 

You'll be amazed how easy it is, but how impressed your uninitiated friends will be when you explain it to them.  Their really isn't much to it.... its the king of bolt on power adders and definitely the low hanging fruit of most engine mods you can do.  Easier than headers by a far cry... easier than undersized pulleys... easier than port matching... and applicable for boat engines where those others aren't (except for port matching and bowl blending, but that is a decided move toward more advanced).