power vs displacement?

[jfbiv]

Re: power vs displacement?

What's your deal Sloopy? There are serious inquiries here about getting 10 or 15 more HP via carb replacement or mod. Nobody (except you) is talking about trying to get 600 HP out of a 150. Be serious for a second - Jango asked about getting a bit more out of an 85 and you told him to buy a 250. !!??!<br /><br />Jango - I doubt you need 14 years of college, (notice that I spelled college correctly), to coax an extra 10 or 20 HP out of an outboard. Yes it may be cost prohibitive, but who knows for sure until you check all the options and spend some time to understand the engineering of your particular engine. <br /><br />Again I'll cite another model-family: the 75 and 90 HP Merc engines sold on Tracker boats are same bore, stroke, etc. just slightly different carbs. Is anyone really sure there is something more than carbs that are different? Is there really a port size/timing difference? I don't know for sure...I'm trying to find out. One has to wonder if the owner of a 75 couldn't maybe find a carb for the 90, slap it on, and say cool man, I just picked up 4 MPH! Along the same lines, I would guess that the 90HP owner is out of luck as far as carb tricks - 90 is all the factory guys could really get out of that powerhead, so you're likely to be pissing in the wind to try and juice that one up.<br /><br />It's not that 10 or 15HP is that significant performance-wise, it's just the challange of trying to squeak it out without spending a lot of dough.<br /><br />Sorry if I missed the joke Sloopy, but some of us are seriously wondering why the outboard companies can sell exactly the same power head in 4 different HP ranges. And sell it at four totally different prices to boot. It makes us think there are simple ways to get a little more out of our hardware.

 

[sloopy]

Re: power vs displacement?

 

[Greg Lawrence]

Re: power vs displacement?

if i was going to replace the pistons on a motor at rebuild, which does happen occasionally, you could get the forged wiseco for the extra money, and maybe mill the head slightly, and could drill and tap the water jacket so it would drain from the top of the head, and increase the tell-tale hole to a slightly large diameter with certain fittings.. that may get you a little more. the bump in compression and maybe some port work. of course you might wanna run a little higher octane.<br /><br />hrmm on another note, wonder if you could take crank and offset grind it and use a little longer stroke rod, or find something compatible? it could work. ive built junkyard strokers before for fords. you dont have to buy stuff new but there has got to be a way.

 

[Eldiablo]

Re: power vs displacement?

sloopy, sloopy, sloopy..<br />how can i reply without attacking you personally? The miss budweiser racing boat series are some beautiful boats with some master workmanship on the engines. but of course you know that all the engines must meet certian spec's in each class. and the hulls, where the streamlining and ability to float on air is a work of art. this all makes my eyes water when i think of these pieces of art. However, again i must ask you to maybe first read the thread, then digest the train of thought, then, and only then, try, oh please try to make a remark that has something remotely to do with what we are talking about. No one is boring blocks. No one is taking there motors to machine shops to put a new crank in it. Now, without your opinion, and stating only fact, can you point out the fault in my reasoning?

 

[JB]

Re: power vs displacement?

Hi, jfbiv.<br /><br />Port timing in a 2 stroke is a VERY rough equivalent of valve timing in a 4 stroke, and, like increased overlap of an aftermarket cam, you can't have anything without giving up something else.<br /><br />Port area is a rough equivalent of valve lift in a 4 stroke. Same effect.<br /><br />By modifying port timing and/or area you can get higher rpm with an attaendant increase in peak hp., provided that the chain of factors in charge flow, cylinder scavenging, ignition timing and exhaust scavenging support the increased rpm.<br /><br />It is very much like putting a hot cam in a car. It doesn't do much good without improved breathing and exhaust flow.<br /><br />So, with the engineering knowledge required, one can get several levels of peak hp at different rpm peaks out of the same displacement block by making compatible changes in intake breathing, port timing and area, cylinder scavenging, compression ratio, ignition timing and exhaust scavenging. Very few part numbers or variations in one part are required to do this.<br /><br />The carb is an important part of intake breathing. It requires a jet size that provides the best mixture (NOT the most gas).<br /><br />Finally, every increase in peak hp reduces the flexibility of the engine at lower rpm.<br /><br />Hope that makes sense without being too wordy.

 

[Sand Bass]

Re: power vs displacement?

Good post JB. That makes sense.

 

[MartyE]

Re: power vs displacement?

Hi Guys,<br /> Good forum. I've learnt a heap here, so I'll wade in with a post <br /><br />Not that I know much, but after buying a new outboard (85hp Yamaha) just recently. I checked out all brands and Hp to suit my boat. Apparently the 60Hp Tohatsu can be changed to a 70hp with a pair of sidecutters. The only difference would be the cowling and re-sale value when tried to explain to the prospective buyer that it really is a 70!

 

[orca]

Re: power vs displacement?

G'Day. I think that I will wade in here. I reckon the 75 Merc and the 90 use a different cylinder block. The 9.9 merc and the 15Hp use different exhust tubes. I have often wondered about HP and engine displacement, Why don't the engine manufacturers sell their motors with engine size as a criteria. After all, motorbikes are sold by CC. 500cc, 250 or whatever. You never see bikes marketed with HP in big letters. If you have a good look at specifications you can see a lot of differance between different outboard manufacturers. the 40HP merc 3Cyl is 935cc, the 70Yam is about 800cc. The johnson comes somewhere in the middle. Sorry if this appears to be rambling on but it is an area that has interested me for some years.

 

[Bob_VT]

Re: power vs displacement?

JB is right on the money (as usual ) The primary factor missing from the original question is WHY?<br /><br />Yes, we all tend to have that "More power" in our DNA but more power does not equate to much. If you search the forums JB posted a "speed formula" that is VERY close to being exact.<br /><br />Run the formula with 25 vs 35 hp and you will see tiny differances if any. This basically holds true with almost any outboard used for normal use.<br /><br />Save the money spend it on more enjoyment not trying for more power. If you really want more power then re-power with a newer larger motor.<br /><br />Not really worth the investment.<br /><br />Bob

 

[ivar]

Re: power vs displacement?

Great post from JB (as usual)<br />A 2-stroke is basically a self-propelled air pump. <br />Breathing equals power.<br /><br />There are many ways to get different ratings from the same engine block: smaller carbs , intake restrictions like different reed stop specs , lower compression , different timing , different exhaust tuning , and port timing/area.<br /><br />I have a yamaha pro-60 , same block as the 70hp. <br />So far I have found out the 60 have different carbs and 2 degrees more timing than the 70 (indicating more compression?)

 

[jfbiv]

Re: power vs displacement?

JB - Thanks. Good stuff.<br /><br />Back to [what I percieved to be] the original topic: Do the outboard manufacturers sell 3 or 4 HP models of engines with only a carb change between them? Well, I did even more research because I wanted to make sure there were not different pistons, (could change compression), and that there were not different blocks, (could presumably change port area and timing). For my original example of 84/85 Merc 18XD/25XD, the block and piston part numbers are the same for Merc 18/20/25. Matter of fact, the ONLY different part number in the whole lot was the carb for each.<br /><br />JB - I'm just asking, not contradicting: Doesn't it appear that Mercury is claiming to produce 3 different HP ranges with only a carb change? And if this is true for these little mid-80's models, couldn't it be true for larger late models?<br /><br />If someone has serial numbers for the 75 and 90 Mercs I talked about before, they can get on www.mercurypartsexpress.com and check to see if powerhead assy's are identical part numbers. I would find that interesting.

 

[sloopy]

Re: power vs displacement?

 

[sloopy]

Re: power vs displacement?

notice that I spelled college correctly

Thanks, you make me feel so much better

 

[Jango]

Re: power vs displacement?

Thanks JB for your info. Kind of refreshing to hear some theory on 2 cyc operation instead of REASONS WHY IT CAN'T BE DONE. I suspect spending a few bucks for some porting stones and some gasgets, Additional power can be had. If and when my motor needs some internal work, I may attemp it. (I have done this many times on a 4 cyc.)<br />Thanks all for your interest.

 

[jfbiv]

Re: power vs displacement?

Sorry Sloopy. I thought you were making fun of my ignorance when you posted 4 messages in a row laughing about how expensive and silly it was to try and increase horsepower - so I was just lobbing one back at you. Truce, ceasefire, peace, or as Eldiablo said, "put away your swords".

 

[sloopy]

Re: power vs displacement?

 

[David R]

Re: power vs displacement?

jfbiv - I've a 70hp from late 80's, so I looked it up on the Mercury's site.<br /><br />There is a separate block listed for:<br /><br />70/75<br />80/HANDLE<br />90 with WME-10 carbs<br />90 with WME-15 carbs<br /><br />Reeds sets and reed blocks for:<br /><br />70<br />75/80/90 grouped together<br /><br />Mercury gives a part number top, centre and bottom for each carb, in each power level.<br /><br />I hope all that makes sense. I've always wondered about this, as I had chance to strip carbs etc. off a trashed 90hp once, but didn't as I've enough projects.<br /><br />David R

 

[sloopy]

Re: power vs displacement?

 

[G DANE]

Re: power vs displacement?

G day<br /><br />I studied this exact subject and came to this information:<br /><br />Block, cranck and pistons is exactly the same on all three models<br /><br />The differences on the 1980 thru 88 20 - 35 motors are mainly:<br /><br />From 20Hp to 25HP another cylinderhead with increased compression, different jet size.<br /><br />From 25 to 30 HP Exhaust through hub, changed exhaust design in leg.<br /><br />from 30 to 35HP, different intake housing and carbureator.

 

[G DANE]

Re: power vs displacement?

And I forgot : 3 deg extra ignition advance on 35 HP