F75 TO F90 conversion

[CatCam]

Well, the question is why NOT. Seems that everything is the same, compression ratio, all internals, displacement, etc, etc.

Is this the only combo (75HP & 90HP) that share all of this?

So, if all this is the same the only thing that could be different is the airflow, fuel and/or the cam....

What am I missing?

[rodbolt17]

mmm so your telling me its raining?
this is a simple speed density EFI setup.
speed density,unlike Mass Air Flow, does NOT monitor air flow and has no way of knowing the amount of air past the throttle plate.

yes the ECU is that stupid.

the ECU, based on inputs and internal mapping, adjusts the injector pulse width and ign timing.

at key on the MAP sensor takes a barometric pressure reading(air density) this reading is locked in for the rest of the key on cycle.

as the throttle is opened the TPS sends information as to throttle angle,the MAP sensor gives invormation on intake manifold absolute pressure,the crank position sensor gives inputs on crankshaft rotational speed.
the intake air temp sensor gives inputs on air temperature in the intake manifold,another density measure.
the coolent temp sensor gives inputs on block temperature.

note, nothing so far as to actual air flow.

now,based on an internal map,simple truth table, the ECU looks at all the inputs to decide throttle angle,engine load and engine speed and makes ign timing and injector on time calculations.

since it has no idea you added extra air by removing or modifying the intake system, it CANNOT add extra fuel to maintain a 14.7/1 air to fuel ratio.
its simply to stupid.

now we get to rail pressure VS injector on time.

out of all the EFI sub systems fuel rail pressure is the most critical.
however its not monitored on most 4 stroke EFI speed density setup.
the ECU simply assumes fuel rail pressure is correct.

this is the basic operation of the speed density type 4 stroke EFI, as injector on time is varied that controls the amount of fuel sprayed into the intake.
the whole function is to maintain that 14.7/1 air to fuel ratio that gasoline burns the best at.
much above 16/1 and the mix becomes to lean to burn and burns to hot.

much below about 13/1 and the mix becomes to rich to burn.

so you did not add fuel,you did not add ign timing advance.
you added more air and made a magical 15 HP out of thin air.

I feel the rain.

[99yam40]

Is that rain kind of yellow in color?

[madharchod]

i appreciate your theory and your concerns but i will not agree with you as iam here in Fiji have turned a F80 to F100 and its has already done 700+hrs and a F75 turned F90 has already done in excess of 3907 hrs which will very soon go for a lower unit rebuilt ,as far iam concerned all your theory is out of the door and nothing has happened to the motors ,pls see for your self when every part and all electrical units are the same the only difference is the air flow restrictor why would the engine go bad ,here is the proof a full running motor no yellow rain or burnt piston
cheero

[99yam40]

I just read it on the internet so it must be true,
they cannot put anything on the internet that is not true.

Where did I hear that?
I read it on the internet

[rodbolt17]

cause you did not actually gain any horse power.
if you had your engine would have over speeded until you got a bigger prop.
the F75 was rated at 75 HP at the propshaft at 5500 RPM.
if you made it an f90 and changed nothing your max RPM would have climbed.

that and like I explained.
to make more power you cannot simply add air.
you must add fuel to keep the air to fuel ratio correct.

dont care if your in fiji.
dont care if its a carbed,efi.
dont care if its a flat head,Fhead.Lhead or overhead cam engine.
articulated rod internal combustion engines still have to burn gasoline at about 14 parts air to 1 part fuel.
its why aircraft that fly much above 15,000 ft have to be supercharged.
its why when using NoS to make power you MUST add gasoline seperatly from the fuel induction.
NoS is an oxygen bearing gas,inert by itself.
when burned the oxygen seperates from the nitrogen.
thats why the extra fuel is injected.

biggest restictor in the F 75 or most EFI motors is the throttle shutter.

you can remove the airbox entirely and it wont change anything.
if you restrict the airbox it goes rich .
like I said,the Yamaha EFI 4 stroke cannot monitor air flow.

you see why I am skeptical?
you added 20% or better to the HP output and only added air,no fuel.

[CatCam]

I'm not trying to "fuel the fire" here as I agree to everything said from Rodbolt on the dynamics of the internal combustion engine but the key element is the control of the fuel/air mixture. Laws of Thermodynamics (see Otto Cycle) are not violated as the compression ratio is the same in the F90 and the F75.
So, simply changing out an air breather box will not add more HP unless the fuel/air ratio is changed.....however there was somebody, a "One post wonder" back in 2006 who says they changed out the control module along with the intake assembly and got a lot more power:


http://www.yamahaoutboardparts.com/f...on-th5461.html

[THerbert]

more Fuel - NASCAR uses a restrictor Plate to limit the HP on the Big Tracks, Also with the plate removed could that change the pressure inside the manifold and the MAP sensor would compensate? Just a thought.

[almetelo]

on my interpretation from that member in 2006 was that he changed the ECU and intake silencer assy like I posted before. I'm sure thats the right way to go...

[madharchod]

you all can go and write what ever you want as i have done it and its working so whats the big deal if ever you come to Fiji i will show you as i have known yanks are a very hard headed people to to conceive in to any new ideas as they think of them selves very superior ,but in this case i have to say sorry you guys are little bit behind in modifying things as your don't have time to muck around with things ,but here in Fiji we don't wear watches so time means nothing as if we can save few thousand dollars on bigger motor and change a few on the smaller ones we do it very nicely as there is no warranty on these motors anyway,if it ****s up its our problem so we try it any way ,and this case we have done it and its working and you guys and throw all your theory to us it is a lot of rubbish.we go through each and every part number and physically check each item on different motors before we proceed to modify ,and it might come as a surprise to some of you that F100 gear box seal is the same as an enduro 60 hp 2 stroke and the list goes on
hahaha

[rodbolt17]

no
here in yankee land we just watch left turn racing and drink watery beer.

however you can only convince 1/2 of us that you can magically make 20% more HP out of air.

going back to the previous left turn racing comment.

that simply proves my point,even though the poster has no real clue on how or why.

nascar engines are currently carburated.

the engines,like the F75 and other F motors of the same displacement, burn gasoline and air.

remember the ****en rule is 14 parts air to 1 part gasoline.
works world wide.

on a carbed engine there are basically two ways to control power output.
restrict air into or exhaust out of the said AIR pump.

an articulated rod internal combustion engine burning gasoline MUST adhere to the 14/1 A/F ratio, its a simple law of thermal dynamics.

much above about 15/1 its to lean to burn, much below 13/1 it becomes to rich to burn.
to say you simply added air,unlike carbed engines yours is speed density EFI which cannot and does not monitor air flow, you can magically make an additional 15 HP.

makes me remember why I eat lucky charms.

that so called air restictor you removed,is simply a silencer and has nothing to do with intake air.

but I knew Nascar would come up .

[99yam40]

I do not believe he can convince even a few of us much less 1/2, but who knows for sure what some will believe..

Looking at the parts breakdown of the F75, I see that the #6 part is the THROTTLE SENSOR ASSY and I do not believe the motor will do much with out that part.

And he magically solved the lean fuel mix problem by changing the prop

i have just turned a F75 EFI to F90 EFi just by removing the airflow restrictor which is number 6 in the parts catalog from the throttle body assy ,there is a lean fuel mixture at high RPM but to compensate it i have used a higher pitched prop

I had problems with my sons when they were young trying to explain to them how some things worked and the laws of physics that con*****ed them.
But once they received some schooling in these areas by other people it became a lot easier for them to understand some of it and not just have to believe what Dad told them, but I still have problems with them from time to time with some things I would think was simple logic.

I have a feeling madharchod is just trying to see what he can stir up, so I am finished with this thread

[THerbert]

Thought you would like the NASCAR referance. Did anyone ever explain what the differance is between the 75 and the 90? I sit back and drink another beer with Rodbolt.

[madharchod]

mate as much as you people like me to spill the beans i have few secrets down my sleeve as i have been in this **** for quite some time when few of of you were in your nappies and weren't even born ,i traveled extensively to different parts parts of the world visiting outboard motor plants and working on them since early days when Johnson and evinrude were the same engine except the paint,then came with mercury and marina ,then it continued into four stroke ,so my experience with the old motors when we had to tune manually the air intake and valve clearance gave me enough knowledge and experience to just look at a motor and find the similarity and hey presto iam in it to change the hell out of it ,i still am a member of outboard engineers from Mexico where these outboards were built in the early 70's and am very familiar with the four stroke built and electronic procedures ,just a word of thought by making the ECU starve of power leads it to readjust to new perimeters,this should ring few bells if you know what i mean
ok guys have to get back to stripping a E-Tec evinrude ,a new homework
cheers

[ausnoelm]

There is a small "glitch" in the parts comparisons on a parts list, lots of manufacturers list one part for two different (but similar engines) but in reality the originals were different parts, why? to keep their spares inventory lower, so you could get a (say) 75HP Yamaha, and compare it to a 90HP Yamaha and find little/no difference, because if something fails later in ife, the 90HP part are supplied, as they will fit both, but the 75HP componets do not fit a 90 (well not if you want to maintain your 90HP) get what I mean? I also find it a bit hard to believe that if a 75HP is cheaper than a 90, but has one more part (the restrictor) then why someone has not had Yamaha in court for some sort of scam! Charging more for exactly the same motor, that has one less part.