90 Hp Crossflow , Removing the engine cover gains 1.5 mph ??

[StingRay_90V4]

Well... Just like the title says. While out in the boat this long weekend I just happened to remove the engine cover so my father could use one of those laser heat gun sensor things. To check and compare the two banks for heat. Just to be totally sure we have a bad switch in the head and not an over heating engine. Anyways.... We putted around the lake for a bit. Then we did a few wot pulls and got up to 44 mph to 44.5 mph as always. With the two of is in the boat. Then after about 15 mins we stopped. Removed the engine cover and checked the two banks. They were extremely close in heat. So for what ever reason my father wanted to check them both while driving. So we left the cover off and we went a bit more. How ever. The results of leaving the cover off shocked us both. For some crazy reason the boat would hit 46 mph and gained about 150 ish to 200 rpm Some where in that area. This wasnt EVER something I planed or thought of. Or would ever believe that it would go faster by removing the engine cover. Now I understand the whole cold air is denser and devils mpre power thing. I took my mechanics course and was a mechanic for 5 years before switching professions. I am not up to speed on marine application's. But still... I didnt see or think that I would gain 1.5mph and a few hundred rpm by removing the cover. Has anyone else ever noticed that with there engines ? And why would it really make such a big difference. I mean.. That is the fastest it has EVER went. 46 mph at almost 5700 rpm. And as soon as I but the xover back on. 44 to 44.5mph at about 5500 to 5600 ish rpm. So... How do I make that permanent. Aside from throwing the cover in the lake for good. Lol... Cut and drill big hokes in a second cover ? But where. The sides. Close to the intakes breathers. Or in the very back and front to allow cool air in and allow it to push the hot air out the back ? Thus cooling the engine more and giving the inake a cooler air charge ? Any thoughts at all ? Thanks ...

 

[boobie]

You may want to start looking for the exhaust leak you most likely have.

 

[rothfm]

Agree with Boobie......

may want to ensure your plug wires are not arcing to any cover latch components. That arcing can be random/intermittant and faster at high speeds with vibrations--that Cyl will stop firing, then start, resume. All happening quickly. Just an outside thought.

 

[StingRay_90V4]

Exhaust leak ? I dont think I have an exhaust leak. I'm familiar with exhaust leaks in cars of course. But I dont hear one in my boat. And really isnt the only place these engines could have an exhaust leak is on the back cover around the gasket. Which I wouod be able to hear. As well as water come from it as well ? Or am I missing something. Also...if it WAS an exhaust leak. Removing the engine cover wouldn't help the engine to be more powerful or go faster ? Or am I missing something again. As for the plug wires. They do not appear to be arching . a miss like that would cause a bit of bucking and what not I would assume. Which I do not have one. I'm inclined to think it all has to do with the motor being able to breath a bit easier and the and OR the air it is getting is cooler which allows it to develop more hp and trq then when the cover is on and it is pulling in hot air from around the engine. I am wondering. If buying a second engine cover and cutting holes out of it will help it to breath better as well.

 

[flyingscott]

Are the hood vents plugged?

 

[Chris1956]

You could have an exhaust leak where the powerhead meats the midsection, which deprives the motor of some oxygen, leading to power loss. You don't often hear the exhaust sound, like on an automobile, but can see the smoke.

Additionally, some motors have exhaust bypass hoses under the cowling. These can leak, leading to the same symptoms. If she idles rough after a high speed run with the cowling on, that is also a symptom of an exhaust leak.

 

[StingRay_90V4]

I wouldnt say it runs rough per say at all really. But maybe I just havnt noticed it. Ever sense we rebuilt the carbs last year . We gained 5 mph at Wot top speed. But I did notice sometimes after I come to a full stop after a wot run that the engine will hang at near 1800 rpm for a few seconds or so before dropping to 1000 rpm idle. Where should I look for exhaust leaks and how would I tell of it has one or not ? I was really assuming the extra power/top speed was from the engine being able to grab more air and air that was much cooler that allowed it to wined out a bit more. How do I look for a leak in the exhaust ? I dont think my cowling is plugged at all. And lastly. Why would removing my engine cover IF I had an exhaust leak allow my boat to go even faster all of a sudden if it was a leak. What would removing the cowling help at all ? Thanks.

 

[interalian]

I think I need to try running mine with the cover off - maybe I'll crack 50mph.

 

[StingRay_90V4]

I think I need to try running mine with the cover off - maybe I'll crack 50mph.

I'm not claiming huge gains.. I gained may be 1.5 mph and 100 to 150 ish rpm. The thing is I don't know why that would be ? I know my boat and what it gets at wot. It has never hit 46 mph in till I took the cover off. I tried it 3 times. Every time with the cover on. Its 44 to 44.5 mph. With the cover off its 45.5 to 46 mph dead on. It has to be something aboit the way the engine is breathing better and being able to grab colder denser air. That's all I can really seeit being ? I have seen people before with home made breathers on there engine covers. Maybe they were after the same thing ?? What are you getting out of your 90 to 140 hp conversion at top speed. Must be near 50 as is. What I'm asking is WHY does it gain that much speed from no cover ?

 

[Chinewalker]

Why would removing my engine cover IF I had an exhaust leak allow my boat to go even faster all of a sudden if it was a leak. What would removing the cowling help at all ? Thanks.

An exhaust leak inside the cowling means you're sucking exhaust back into the motor. Removing the cowling gives it clean fresh air again. Given that you're not gaining a whole lot, any exhaust leaking you're getting is slight, at best. Could also be a slight restriction in the air intake at the back of the cowling, under the grab handle (if it's a newer style motor). Wasps and spiders love those crevices.

 

[StingRay_90V4]

Yeah , it is not a huge gain. 1 to 1.5 mph . I don't feel any extra few or pull from a dig. It just seems to breath a bit better up top and runs a bit faster at top speed. The engine is a 1991 Evinrude... I dont think there is any restriction in the breaker hole. But I will double check though. I don t see any smoke coming from the engine anywhere at all except the exhaust ports them selves in the lower unit. Could it just really like the open colder air at all ??

 

[interalian]

An exhaust leak inside the cowling means you're sucking exhaust back into the motor. Removing the cowling gives it clean fresh air again. Given that you're not gaining a whole lot, any exhaust leaking you're getting is slight, at best. Could also be a slight restriction in the air intake at the back of the cowling, under the grab handle (if it's a newer style motor). Wasps and spiders love those crevices.

I would argue that gaining 1 to 1.5mph is a pretty significant difference in power on a boat.

 

[interalian]

I'm not claiming huge gains.. I gained may be 1.5 mph and 100 to 150 ish rpm. The thing is I don't know why that would be ? I know my boat and what it gets at wot. It has never hit 46 mph in till I took the cover off. I tried it 3 times. Every time with the cover on. Its 44 to 44.5 mph. With the cover off its 45.5 to 46 mph dead on. It has to be something aboit the way the engine is breathing better and being able to grab colder denser air. That's all I can really seeit being ? I have seen people before with home made breathers on there engine covers. Maybe they were after the same thing ?? What are you getting out of your 90 to 140 hp conversion at top speed. Must be near 50 as is. What I'm asking is WHY does it gain that much speed from no cover ?

My 90-140 change gave me a highest observed speed of 48, where it would only do 41 at this altitude (3300') with the 90, even when newer. It would do 44 max at sea level with one person. So if I added 50 flywheel HP to gain 7mph, you might be getting 7-10HP more by removing the cover, for whatever reason.

 

[StingRay_90V4]

My 90-140 change gave me a highest observed speed of 48, where it would only do 41 at this altitude (3300') with the 90, even when newer. It would do 44 max at sea level with one person. So if I added 50 flywheel HP to gain 7mph, you might be getting 7-10HP more by removing the cover, for whatever reason.

I know it sounds crazy. But I promise. I'm not makimg it up. I can and will video the difference if I need to and anyone truly doesnt beleive me. I dont have the faintest clue why cover on. 44 to 44.5 mph. And cover off. 45.5 to 46 mph. Maybe like 45.75 ish. Lol... Idk why. i was really hoping someone would have a good explanation and I would just buy an old cowl and cut it apart to let the engine breath better. I am at damn near sea level. The ocean is all of about 20 mins away from us. I cant hit 48 mph. Nor do I think this boat could ever do that with out a rebuild like yours. However... Keep in mind. The stingrays 501zp I have is an amazingly fast hull design they say. P.s...according to Google my cottage is only 43 feet above sea level. That's it... Not even a full 100 feet above sea level. So a 90 like mine is ALWAYS going to produce more power then one at 3500 feet above sea level. lso our boat is a bit lighter then it was when we first got it.

 

[jimmbo]

It may be because of the air being prewarmed under the cover, it may be due to spark loss when cover is on, it may be due to some exhaust gases getting under the hood, it may be from a restricted air inlet in the cover(unlikely though, but...). Maybe a combo of the forementioned. From your description of the symptoms you have been offered a variety of possible causes. Does your engines cover have the sponge/foam sound deadener? If so is it still properly attached?

 

[StingRay_90V4]

It may be because of the air being prewarmed under the cover, it may be due to spark loss when cover is on, it may be due to some exhaust gases getting under the hood, it may be from a restricted air inlet in the cover(unlikely though, but...). Maybe a combo of the forementioned. From your description of the symptoms you have been offered a variety of possible causes. Does your engines cover have the sponge/foam sound deadener? If so is it still properly attached?

It does indeed have the foam in the cowl. And it seems to be fairly intact. I think anyways. I will of course double check. I'm curious about the spark lose you mentioned. How or why would that happen with the cover on. Especially where it is fiberglass ? I dont think it is arching anywheres that im aware of. Would you think it could or would be the simple reason of more air flow . With said air flow being a fair amount colder and densor ? Or would that really affect it at all. I dont think I have an exhaust leak. But I guess never say never.

 

[jimmbo]

It was mentioned in an earlier post. That cowl has the rear latches pretty close to the spark plugs and wires. there have been many instances where the voltage intended for the plugs jumps to the latches which are also grounded.
Your engine gets its air through a couple of vents at the upper sides of the cowl, does it also get air from around the tilt grab grip?

 

[StingRay_90V4]

It was mentioned in an earlier post. That cowl has the rear latches pretty close to the spark plugs and wires. there have been many instances where the voltage intended for the plugs jumps to the latches which are also grounded.
Your engine gets its air through a couple of vents at the upper sides of the cowl, does it also get air from around the tilt grab grip?

I didnt notice that mentioned in an earlier post. Sorry. How ever , last time I looked the wires seemed to be in good shape. Mind you that was a few months ago. Maybe I will double wrap them in electrical tape and see what it does. And then buy new ones over the winter as the boat is coming out of the water in a few weeks for the winter.

As for the tilt grab ? I have no idea. I thought the obly place the engine got air was from the top of the cowling at the hand grab location up top. I have never noticed any other openings at all. However I have never looked overly hard for more as well. But one would think they would be obvious.

 

[Faztbullet]

It does it because the mixture leans up.....

 

[StingRay_90V4]

It does it because the mixture leans up.....

Meaning it is running to rich with the cover on ? And my carbs need to be adjusted ? And when it leans out . To what extent ? In a danger zone of leaning out ? Or just enough to produce a tad more power. I wouldnt be surprised if the carbs where sending too much fuel into the engine. Not to the piont of fuel wash. But the carbs where rebuilt this year as well as a new timer base installed. And I have wondered if it needs to be fine tuned ?