Upgrading Suzuki 15hp to 20hp

[Duggie12]

Hi all was just wondering if anyone knows if it's possible to upgrade a Suzuki fr 15hp to a 20hp
It's a carb model not efi (2011)
I wasn't sure if it was as simple as a carb swap
I might trade my engine in and get a new 20hp, but if it was simple to upgrade I'd consider doing that
Cheers !

[Martin]

For what I remember at the time I was swatting up to buy mine,
The df9.9 and the DF15 was the same block unless you have the high torque..
200cc vs 327cc I think, don’t quote me in those figures lol,

However, as far as i’m aware, the df20 was a different block sona carb swap would be out of the question.

If you have a carb version 2011 it’s actually a 2010 as they started producing the efi in 2011.

Have a chat with Paul B, he has my old 10” stainless prop. If he’ll sell you that, it’ll give you a hell of a boost for a lot less money than a new outboard.

Martin

[Paul B]

Correct , I could be persuaded to sell the the prop, My engine is a 9.9 converted to 15 with a carb swap, however it doesn’t have to torque to spin the bigger prop at WOF as it’s the smaller capacity block, not the same as the engine Martin had,

[Andy B]

Paul,
Did you have to make any adjustments to the ignition timing when you did your upgrade?

[Paul B]

Andy, Not sure mate, as it was my dads motor, he had it converted at Pennine Marine, and on the invoice it doesn’t say anything about timing, just carb swap I can ask him if he knows what they actually did

[Duggie12]

Ah right I thought cos 15 and 20hp were the same weight they would be same block
How come a prop change makes such a difference? ( sorry I'm clueless about these things)
Cheers

[James]

I could be wrong, but my experiences with 9.9 and 15HP is they tend to share the same powerhead.
20HP's have a different powerhead

[DouglasW]

Hi Duggie, I run a Yamaha F15C on my F-Rib 330. This is the same powerhead as the F20C so potentially I could upgrade it. However as the 330 is small and easily driven, at WOT the F15C reaches max revs which are the same max revs as for the F20C. So the 20 with the standard prop on my boat would not go any faster. To make it go faster I would need to fit a prop with a bigger pitch. The F15C would not be able to hit max revs with this but the F20C would. I am a great believer in fitting a rev counter to an outboard so you know what revs it will hit at WOT. All boat designs/sizes will be different but there is almost no information on the internet about what size pitch of prop is best. Elsewhere I have posted a photo of my uncle’s heavy Steady workboat/Suzuki 40 HP. It would not hit max revs at WOT so we fitted a smaller pitch prop. An outboard will only develop its maximum power near the top of its rev range which is why having the right prop is very important.

A bigger HP version of an outboard should help you get on the plane quicker but this does not apply to all manufacturers. It depends on what they do to limit the power of the power head down from say 20 to 15. My friend Ronnie who is the Yamaha dealer in Castle Douglas tells me that in the case of Yamaha the power difference is only apparent at higher revs and so a 20 would not get out the hole any quicker than the 15. He also said that if I had a 20 and fitted a bigger pitch prop to it, although top speed would be faster, the 20 would then be slower out the hole than the 15 with the standard pitch prop because the power curve in the bottom half of of the rev range is near identical in both motors. This does not necessarily apply to other manufacturer’s designs.

It is a pretty complicated business and in practise only experiments with your own combo and measuring the results with a rev counter and a gps will give you a definitive answer.

Have fun, Douglas

PS The small Chinese rev counters on eBay don’t last long. The Gurnard, my friend Bill and I all spent about £50 on the Yamaha one and all three have been reliable.

[DouglasW]

Here is why my F-Rib 330 and Yamaha F15C would not go any faster by upping the power unless I also changed the prop to one with a greater pitch or diameter.

On what was possibly the one glassy calm day last summer I did some speed tests using my Garmin GPS. I managed to get 6 good readings:

rpm 3800 4230 4780 4950 5600 5880
km/hr 27 30 34.5 36.2 41.7 42.9

The Yamaha full throttle operating range for both the F15C and the F20C is 5000 -6000 rpm.

In practice my combination is unlikely to hit that top speed ever again as it is so rare to get glassy calm conditions on the Solway, the boat gets very skittery at top speed and it slams so hard on even a small ripple that my back really hurts. For interest I am just a little older than Paul but similar height and weight and with similar back problems. I know exactly why he is changing to a combination which will give a softer ride at speed.

Douglas

[Paul B]

Paul,
Did you have to make any adjustments to the ignition timing when you did your upgrade?

Spoke with my dad, and it was just a carb swap, no need to alter the timing