Air Intake Cover

[StingRay_90V4]

I read a post on another fourum about removing the air intake that is over the carbs. And that removing it allowed the engine to breath better and gained a small amount of power . is there any truth to something simple like that at all ? I have a 90hp rude crossflow v4.

 

[robert graham]

Might gain a little power and a lot of intake noise....I'd leave it on there...

 

[StingRay_90V4]

Is there anything worth doing engine wise ?

 

[F_R]

Removing the air box cover may surprise you---with LESS power. That thing is part of the exhaust tuning. Makes use of sound waves to get the fuel/air mix into the engine. Worse than that, removing it may change the air/fuel ratio, possibly resulting in engine damaging lean condition. Bottom line, don't be trying to second guess the engineers that designed it. They were highly paid for what they know.

 

[batman99]

Hi StingRay_90V4

Within a few different posts, I noticed you are trying to give your engine more power. re: Optimal engine height, optimal prop, low cost engine mods, drilling holes in prop, etc. etc. Always trying to make it go faster.

As a suggestion, review the factory plate on your boat and advise the MAX HP engine size your boat can take. For example, my Bayliner Bowrider can take MAX 120 HP. If wondering, my boat was sold new with Force 85 HP or Force 120 HP. In thinking of buying brand new, perhaps your engine is the "smaller" size - when it was sold new. Instead of trying to make a small size engine have more power (travel speed), perhaps it might be better in the long run to "swap out" existing small OB engine with a different MAX size HP engine. For example, replacing existing 90 HP with 120 HP (assuming your boat takes a MAX 120 HP). Something to think of....

Advise from a wise mechanic: Always buy +1 size large size engine (in vehicles, trucks, boats) than needed most of the time. When you need that extra power (like towing 2 x tubes at same time), the engine won't be over stressed. But if using a small engine at max and pushing it even harder under heavy loads, it will be way over stressed. And over stressed means things break. Always best to run throttle under 3/4 throttle and leave that upper throttle range when really needed (just like with auto vehicle engines).

Hope this helps...

 

[sutor623]

Yup you'll run her lean like that. No bueno.

 

[oldboat1]

Removing the air box cover may surprise you---with LESS power. That thing is part of the exhaust tuning. Makes use of sound waves to get the fuel/air mix into the engine. Worse than that, removing it may change the air/fuel ratio, possibly resulting in engine damaging lean condition. Bottom line, don't be trying to second guess the engineers that designed it. They were highly paid for what they know.

Agree it's not wise to run without the cover -- assembly is engineered to control intake air flow and air temp, and running without it will change the mix for the worse and likely the intake air temp (higher temp would mean loss of power).

My understanding is that the purpose of an intake cover/resonator is to keep sound in (intake and vibration noise), let non-heated air in, and keep water out. But the use of sound waves to move air or control the mix? That part has got me curious. Thinking the sound dampening structure (ribbing, for example) probably also directs or redirects air flow, so probably serves a dual purpose -- but not getting the part about sound waves. Would like to hear more on that.

 

[F_R]

We don't know the specifics of what motor the OP is running, but OMC made big hoopla about their "Pulse Tuning". Have you noticed the long intake runners on many cars? Part of the pulse tuning, making use of the sound waves, or whatever you want to call it. Lemme tell you a story: Back in the day, we sold a new 6hp Evinrude to a sailboater. He ran it in a compartment to keep the sound down. (Sailboaters hate outboards). And then he complained about exhaust smoke rising around the motor and choking it out. So I got the bright idea of constructing a hose from the carburetor intake, to fresh air outside the compartment. Great installation, but that dang motor would barely run. So, must be too small hose, right? Redid it with larger hose, and still would barely run. There certainly was no air restriction through that hose. Only possible conclusion: Sound pulses. Think of a trumpet or trombone. There is a reason for the shape and length/design.

 

[StingRay_90V4]

First off... let me thank you all for all your input. I do appreciate it. With that being said . i will get a bit more in depth here so you can understand me and what im up too a bit better. So hold on... Here we go.

I am a 40 year old that has my mechanics liscene. I am very capable obviously of working on engines. However i am NOT at all familier with marine aplications at all. However.. My current car is turbo charged from the factory. I have the cars computer re-programed by myself with a tuning program called hpt. My car currently sits at 345hp and 385lbs of trq . As tested by a dyno. All in a car that only weighs 2800lbs. I race it at the drag strip and autocross the car at a local auto x course. So this love for power and speed has of course traveled over to my love of boating. I'm an extremely good water skiier. I have been skiing sense i was 12 years old. I only own 1 ski. Lol... As well as a tube and a very expensive wake board . My parents have a cottage on a large, wide and very long lake. Sense 1987 we have owned a Sunray boat with a 60 hp Evinrude. It had an ok hole shot and could hit about 32-34mph tops. That old boat served us well for many many good years and NEVER let us down. But the time came to sell the old girl for a boat a bit bigger. So we picked up a 1996 StingRay 501ZP. It has there v plane hull. Which is apperantly a really well designed boat for speed. So i am told. The boats max hp it can handle on the transom says 90hp. So our new boat is mated with the largest motor it can handle. It has as siad a 1996 90hp crossflow v4. The boat has much better hole shot then the 60hp we had. And hits 38-40mph. On occasion with only myslef in the boat i have hit 41mph several times. The boat no doubt works every bit as good as any other 90 hp boat does. But like most boats. It can be fine tuned for a bit better gas milage as well as top speed. The first thing i am going to address is the fact that the ac plate is 1 1/2 inches below the bottom of the boat. Thus dragging some excess gear case through the water slowing me down. Im sure rasing it up one hole will net me a few more mph.

 

[StingRay_90V4]

Continued.....

Along with raising the engine up one hole in a few weeks. I figured why not try and fine tune the boat as a whole. My father. The best any man could have. Accidentally hit something in our lake that has never been there before. So our prop is currently destroyed. And i mean destroyed. One blade is ripped in half. And another bent totally in half. We have an OLD prop left over from our 60hp rude. That is the same 13.25dx17p as the omc that WAS on the 90. But the old prop is a Solas. Same specs. Different make. Once we put the old Solas prop on. We instantly noticed a fair amount MORE whole shot. With the same 38 to 39 ish mph top speed. So that really got me thinking of how to fine tune our new boat even more. Which brings us to all of my current posts and all my reading to learn more about the boating side of things. As a side note. We also have a 13dx19p prop which gives us an ok.. But not super hole shot and hits about 40mph top speed. I was thinking about drilling holes in that prop in another thread to acheive a better hole shot with the 19p prop and have a 42ish mph top speed once the engine is re mounted one hole up... So there you have my story as best i can explain it.

 

[oldboat1]

F_R: venting, sailboaters and trombones. This is good stuff. Needs a thread -- really. OK, not sure about sound waves. (think that sailboater needed a dedicated bilge blower pumping air into that engine compartment. See if he is still alive and sailing.)

StingRay -- maybe a hint there for your motor. Get some air up into the engine compartment, sort of a soft turbocharge -- but leave the intake cover in place. Sounds like you are only looking for a small increase in power. Just run it without the motor cover and see if you get a difference. Might do some research on pulse timing and see if there is a way of getting creative.

Did you try a cupped prop for your '92(?), as was suggested in one of your earlier threads?

 

[ondarvr]

If you get serious about getting power out of that motor you can, but you need to find some old guys that raced them, we had one here for a long time, he hasn't been around for 3 or 4 years now though. Do a search for Dhadley, he raced and built them for years, might get some info from his old posts.

The old crossflows aren't the best motors for performance and mileage, but were fairly reliable until a ring breaks (various reasons).

There is far more stuff available for Mercs, but the 3 cylinder 90 wasn't one that people liked to modify, mostly people started with much bigger motors.

I raced motorcycles for a couple decades, rebuilt motors, ported them, built pipes, etc, everything you can imagine...I leave my outboards stock, want to go faster, get a bigger motor.

 

[F_R]

No Title

I'm not a performance engineer, and maybe not too good with words, but I have had schooling on outboards and worked at a dealership. And I was taught that the exhaust and intake pulses, as well as intake and exhaust porting design (lets just say total airflow through the engine) is all related. I'll just let it go at that. But here are a couple of blurbs from an old sales brochure. The guy that wrote it probably wasn't an engineer either. BTW, stingray, you probably are smarter than I am. But I won't hold that against you. Oh, and the sailboat project is long gone. Thank God.

 

[oldboat1]

F_R -- analogy or not (sound waves), it's good stuff. Seriously. Tuned exhaust in relation to intake pulses starts to make some sense to me, and didn't before.

btw -- Dhadley. might go way back to '02 I think, when I came on. I need to do some searches too -- Thanks ondarvr. StingRay -- need to go to AOMCI and look around for more Merc guys into modifications, here too. Ideas. Maybe you have a background and interests similar to those fellows. Sign up (How did Belushi/Bluto put it? "...need the dues.")

edit. will leave it alone, but had a turbo/fuel supply issue with the old Mercedes diesel last week driving to Michigan. Trying to get a grip on boost pressures and related stuff. Know this -- get a 20% increase in fuel economy if you do a lot of coasting and creeping up hills. But all pretty flat -- until back to western NY. Outboards are more fun.

 

[Chinewalker]

A friend of mine who raced the little 15hp Evinrudes did extensive testing to squeeze as much out of his stock motor as he could, within the rules. He was running almost 60 mph with a 15 when he got out of it about 6 years ago!! Anyway, he made a small plate with a bolt and nut "jack" that he mounted in front of the carb to test the pulse tuning effect on his motor. He could screw the plate in or out from the carb mouth and note what it did, while the motor was running, With a test wheel and tach, he noted about a 100-120 RPM increase between where the factory cover would be and no cover at all (peaking near 7000 RPM). Moving the plate inwards brought the RPM up to where it would be with the cover, then went downwards as he brought it in closer still.

 

[StingRay_90V4]

I really don't want to get THAT deep into trying to squeeze a few extra hp out of the engine, I thought maybe if there were a few easy things to do that would net a few extra ponnies then I would give them a try. I have decided to leave the engine alone as is. It works great and has excellent compression on all 4 cylinders. So the only thing I can really do for the boat to get anything more out of it is to raise the motor up ONE hole . It currently is on the very bottom hole. My goal is 42mph top speed. Im currently at 39 to 40 ish mph. GPS confirmed . My garmin will always show 39 mph everyday basically. Sometimes it hits 40mph . And on a few different times she has hit 41mph on my gps. Im aimg for a steady 42mph on my gps by raising the engine up a hole. Hell , If i need to im gonna throw my seats and my emergency kit and all overboard. Lol... less weight.. more top speed. ;-)

 

[oldboat1]

Try it without the engine cover/hood.

 

[bonz_d]

When I recently purchased a 1994 30hp Johnson there was no air box on it. Having had many others that always did I thought that the previous owner removed it and didn't put it back on before he sold it. So I started looking for one only to find that on that engine there never was one made for it. Guess it was OMC's way of cutting costs.

 

[oldboat1]

same experience with a similar motor a few years ago ("took a look" at a neighbor's motor for him). kind of dropped the issue after checking the manual, but good you jogged the memory -- would also be curious now why some motors had no air boxes/covers.

Can't be the lower hp, as 9.9s and 15s of the period have covers. (??)

 

[StingRay_90V4]

I may try it then for a few seconds just to see what it does. But im inclined to think that of Omc pit that mich reasarch into pulse waves and port matching. Then i would say that it is a needed item.