18hp vs. 20hp Any noticeable difference in performance?

kfa4303 · Jul 30, 2011

Hi all. I've been looking to upgrade my motor form a '61 10hp to an 18hp-20hp of similar vintage. I found a great looking 18hp runner today, but I was wondering if y'all think it would be worth it to hold out for a 20hp to come my way. Will 2hp make much of a difference one on a 14' aluminum Starcraft-style runabout converted to a tiller drive fishing boat. Thanks for any advice.

BonairII · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Probably not worth the trouble to find a 20hp. A good running 18hp motor NOW is better than waiting around to hopefully find a 20hp in the future.

JB · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Agree.

ultra353 · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

I don`t think you would notice a difference. Alot depends on the condition of the motor- compression, timing, carb properly set-up, prop. boat set-up. 18 to a 25 you would notice. Those 22 cubic inch motors seem to be one of the best motors produced. I have a `69 johnson 20hp on my 14ft starcraft seafarer and i am very impressed with this motor. Fantastic holeshot, gets on plane with 3 adults quickly and top speed is 27 or 28 ( haven`t gps it yet but ran along side my friends with same boat and `67 20hp mercury and pulled away from him. he gps his at 24 mph) I agree if the 18 is avail now go for it and fine tune it for max performance.

watergecko · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

ultra353 said:
I don`t think you would notice a difference. Alot depends on the condition of the motor- compression, timing, carb properly set-up, prop. boat set-up. 18 to a 25 you would notice. Those 22 cubic inch motors seem to be one of the best motors produced. I have a `69 johnson 20hp on my 14ft starcraft seafarer and i am very impressed with this motor. Fantastic holeshot, gets on plane with 3 adults quickly and top speed is 27 or 28 ( haven`t gps it yet but ran along side my friends with same boat and `67 20hp mercury and pulled away from him. he gps his at 24 mph) I agree if the 18 is avail now go for it and fine tune it for max performance.

Maybe a mph at full load.

kfa4303 · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Glad to hear it. I think I'll go check it out. It looks like a mid-late 60-something Evinrude 18hp. Hard to tell from the pic on c-list and it's been camo painted. He's asking $300, but hopefully I can talk him down a bit. Any tips for the best way to test the compression and what sort of psi I should be looking for. Thanks for the input.

Daviet · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

You are going to have to use a compression tester. I would look for at least 75-80 psi compression, if it has been sitting for a while, squirt a little oil in the cylinders and rotate it by hand a couple of times and then retest.

kfa4303 · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Thanks Daviet. How many pulls on the rope would give me a good average reading? I was thinking maybe 5-10, or so? The ad says it's a runner, so hopefully she'll turn over fairly easily.

Daviet · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

I usually go about 5 or 6 pulls. Should be within 10% of each other.

jamesgduratech · Jul 30, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

this is just an idea but if you put a racing prop on it you might get about a few mph more.

kbait · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

If the ad says 'runner' make SURE you have him barrel test it! It may be one of those 'it ran the last time I used it' which means about 17.4 years on average. Make sure it's pumping water at low idle, and shifts normally. And after test, pull the lower gearcase drain plug - if it's milky, or just water, it's got a leak somewhere..

If he won't barrel test it, assume it's a non-runner. Check spark, compression, and gearcase oil. If that's all good, you can take a chance, but NOT @ 3 bills.

194269 · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

If you are comparing a Evinrude 18 to a Johnson 20, no difference the same engine.

kfa4303 · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Well got a speeding ticket this morning, so I think the motor is gonna be on hold for a while :/ I know they used the same block for several different hp models, but I couldn't remember which were the same. Did they use he same size block for the 25hp too, or was that the bigger block they used for 25hp-35hp? Thanks again for all the tips.

194269 · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

The 25 used the same basic block as the 18 -20's, the 25 had different port timimg. It also used a different c/shaft and pistons, I believe the the reed stops and carb where also different. The later 25 -35's where a hole different animal, 31 cu/in and 22 cu/in on the earlier models.

kfa4303 · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

Thanks for the info. Well whata ya' know?!? I stumbled across TWO matching 60s-something 20hp motors for...........$100 total!!! Apparently, neither is running (yet), but assuming one has good compression, I'm confident I can get it going, plus it comes with a parts motor. Best of all, $100 + Speeding Ticket from this morning still costs less than the 18hp I was looking at checking out! LOL!!! Hopefully, the guy will get in contact with me before someone else gobbles them up. I'll keep y'all posted.............(does this addiction ever stop?........I hope not

)

milliesdad · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

At one time, I had a complete running 20 recoil start, 25 elect. start, 2 complete power heads (both runnners) 2 extra carbs, 2 universal magnetos (complete with good coils, condensors and points) and too many other misc. parts (could probably make another power head).

Sold my 14 ft tiller control with the 25 (bad tendonitis in left arm) and bought a 16 ft. console steer.

By the time I sold off boat, extra motor and all parts, the new boat cost me $150.00.

There can be light at the end of the tunnel.

kfa4303 · Jul 31, 2011

Re: 18hp vs. 20hp Any noticeable difference in performance?

I know what you mean. I think if I play my cards right, I can come out even or slightly ahead. I'm actually going to try selling off a couple of other boat things in hopes of streamlining my collection of toys, but it may get worse before it gets better

Still no response from him yet :/ I hope it wasn't too good to be true. As much as I love tinkering with these old things it will be nice to have one nice simple, strong running, reliable motor that I can count on. Although, a nice matched pair of 20's might be nice too............. LOL!!!!! Thanks again for all the input.

18hp vs. 20hp Any noticeable difference in performance?

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