Anyone know why the hp difference in same size motor

[dbs52a]

I just bought a 2004 Yamaha 20C twenty h.p. outboard. I noticed that the specifications for the 25C is exactly the same but 5 h.p. more. Every aspect is identical from displacement to engine dimensions. Maximum rpm is also the same. My question is, where's the increase in power coming from? Is it the carbs or the CDI? Would the exhaust be the same in both engines as well?

 

[Ray Neudecker]

Re: Anyone know why the hp difference in same size motor

You would have to take the parts list and compare to determine why the difference. Different carbs and exhaust tuners as well as head would be the logical differences.

 

[dbs52a]

Re: Anyone know why the hp difference in same size motor

You would have to take the parts list and compare to determine why the difference. Different carbs and exhaust tuners as well as head would be the logical differences.

The two engines I'm comparing are the Yamaha 20C and 25C two strokes. So, I think there would be no difference in the head. Compression ratio is the same. What's an exhaust tuner? Is that similar to an expansion chamber?

I've been trying to find parts lists on the internet to compare but not doing too well in my search. Is it possible that carb jetting would make the difference? What about the CDI?

Thanks for the reply.

 

[steelespike]

Re: Anyone know why the hp difference in same size motor

You might do a search for parts like you were going to buy them.Compare part numbers.Carbs,reed valves,Cylinder blocks,Exhaust legs.Ignition parts
carb linkage and stator parts.
Generally you are better off to simply upgrade to the bigger motor.
If your mod info is wrong you could waste your money or worse end up with a frankenstein motor.
At 5 hp the change could be small.I believe on a suzuki model it is simply a gasket change.

 

[lexkyboater]

Re: Anyone know why the hp difference in same size motor

Try www.yamaha-motor.com, click marine, parts and service and service and you can get the parts breakdown. The OMC guys have said for the OMC motors with different horsepower and same displacement it's carb and carb restrictor plate (intake manifold). I believe that OMC usually beefed up the drive shaft and possibly other drive components too when they upped the HP. So, if you're thinking about taking your motor and putting the bigger carb and restrictor (if that's all there is to upping the hp) you might stress the rest of your motor.

 

[dbs52a]

Re: Anyone know why the hp difference in same size motor

Try www.yamaha-motor.com, click marine, parts and service and service and you can get the parts breakdown. The OMC guys have said for the OMC motors with different horsepower and same displacement it's carb and carb restrictor plate (intake manifold). I believe that OMC usually beefed up the drive shaft and possibly other drive components too when they upped the HP. So, if you're thinking about taking your motor and putting the bigger carb and restrictor (if that's all there is to upping the hp) you might stress the rest of your motor.

Thanks for the link. Checked it out and was a little disappointed. It only lists US models. No Canadian models and it appears that the US didn't import 20s, just 25s. So, the search continues. I called the local dealer to see if they'd give me a parts list for each engine. Nope. Real nice folks. Not that I was asking much. They have it on computer. All they'd have to do is e-mail me the PDF file for each. No can do. But, if I wanted to, I could go down to their shop and have them provide me the numbers I'm interested in one by one. I don't think that would go on for too long.

Personally, because every physical aspect of both the 20 and 25 appear to be the same, I'm wondering if the difference could be something as simple as a restrictor plate. Could the CDI also have an effect on horsepower output.

 

[lexkyboater]

Re: Anyone know why the hp difference in same size motor

I wouldn't think CDI would be different. All it's doing is providing spark. Buat that's a guess.

Maybe try a different dealer. You'd think they'd help you if they expected future business, but there are some cultural differences between the US and CA.

 

[dbs52a]

Re: Anyone know why the hp difference in same size motor

I wouldn't think CDI would be different. All it's doing is providing spark. Buat that's a guess.

Maybe try a different dealer. You'd think they'd help you if they expected future business, but there are some cultural differences between the US and CA.

Another dealer would be alright if there were one. Next Yamaha dealer is 150 miles away. That's the problem with living in a "one horse" town. Thank God for the INTERNET.

I just got the engine a few days ago and I'm more interested in the hows and whys between the two models. I bought it in good condition used. Looks a little carboned up from too much slow speed work. I have a can of Sea Foam pressed into service already. I put a few drops into the cylinders to soak the rings and the rest of the can into a couple gallons of gas that I will run through it this weekend. Looks like a well made engine. I also have an old 1986 Suzuki 15 hp that has been the best engine I've owned. Still runs like new and hasn't lost any power over the years. If the Yamaha is anywhere near the same, I'll consider myself fortunate to have it.

Thanks again for all your help. If there's any addition info you can provide, I'll be waiting for it.

 

[Boilermaker]

Re: Anyone know why the hp difference in same size motor

I'm not familiar with these engines, but are they the same year of manufacturing???
Difference in HP could be bigger carb, better exhaust breathing, different port timing, different cyl. head design (yes even 2 stokes can have high performance heads) piston design, better reeds, ect. ... ALL are tricks for improved performance.
It could also be the same exact engine & parts. OMC had MANY hp ratings from the same engines (88, 90, 100,110, 115 ect).
They do have a 'tolorance' to the advertised hp spec. (they can't dyno each engine they make & them lable its H.P.).
I hope this helps.

 

[grid]

Re: Anyone know why the hp difference in same size motor

Generally you'll find a 5hp difference will be in the carb throat size, necessitating a different intake. There aren't restrictor plates. Different exhaust stacks become more prevalent in the upper HP ranges, or in OMC's case, the racing 15 & 40. The difference in price between the 20 and 25 will probably reflect the price of a carb and intake. At that engine level, it becomes too expensive to be using a bunch of different CDIs, heads, stacks, etc. Your '86 Zuki 15 and the 9.9 were identical except for the throat of the carb and the amount of throw the magneto plate gave the linkage. If you had a 9.9, you could get 12.5HP by removing the screw!

Back to your 20/25...First off, when a dealer becomes a dealer, he pays through the ying-yang for factory stuff. What you think is easy for him to send is proprietary information--which may or may not be 100% correct--and may open him up to dealer cancellation, or a lawsuit if you elect to order something that (for instance) ups your HP to 25, while your boat is rated for 20, and your actions cause an accident. Lawyers will go for your pockets, the dealer's pockets, and Yamaha's pockets. Your possible future business probably doesn't carry a lot of weight in this case. Additionally, the engine is an EPA engine, with certified emissions. If you modify it and are caught, you're sublect to a $5000 fine--the first time. When I started working on marine engines in the late 50s, legal ramifications were never considered. everything was done on a gentleman's handshake and agreement. My, hasdn't civilization advanced....

 

[dbs52a]

Re: Anyone know why the hp difference in same size motor

I thank everyone who gave replies. I took the 20 hp '04 Yamaha out to camp last weekend and took the '84 15 hp Suzuki off of my fishing boat (14' fiberglass Excel). Before making the change, I took the boat out to take a w.o.t. reading. With the 15, I achieved a speed of 23 mph. I switched to the 20 Yamaha and managed an astonishing 25.5 mph. So, for a 33% gain in hp., I was rewarded a whopping 10.86% increment in speed. Props are pretty much the same. Stock units on both. Seat of the pants estimation is that the 20 has a bit more bottom end torque and gets out of the hole better and the Yamaha idles and runs a trolling speed much better. For twenty years difference in age, and the difference in engine displacement of 111 ccs in favour of the Yamaha, I must admit that old Suzuki is still an impressive engine for it's size. The Yamaha is a fine engine nonetheless. Seems more refined. That could be due to having dual carbs. As far as mechanical condition goes, it like new. It hardly any time on it and I doubt that it is running below it's intended rating.

Now here's something interesting. I put the 15 hp Suzuki on my 14' Mirrocraft aluminum boat. The Excel has to weigh around 300 lbs. the Mirro weighs less than 150 lbs. After getting trim set up just right I did a few w.o.t. runs with it and the 15 hp. I was disappointed when the GPS showed the same 23 mph in speed. Half the weight and same speed? Can't figure that out. The Excel though is a tri-hull design with a very low silhouette and it does like to run flat and stable. The aluminum has a higher profile and is twitchy compared to the Excel.

Does anyone have any comments re the marginal increase in speed for the horsepower difference and why the 15 hp on a much lighter boat didn't achieve higher speeds?

 

[dbs52a]

Re: Anyone know why the hp difference in same size motor

Made a trim adjustment and managed 26 mph with the 20 Yamaha. Any comments re my previous questions in my second last post.