Speed increase from Nissan 9.9 to 15 hp conversion?

I've got a 9.9 hp Nissan 'C' model that I'm going to convert to 15 hp. by changing the throttle cam. I've got two questions: 1) Is changing the cam the only change I need to make, and 2) how much speed increase will I get from my 14 ft flatbottom. I'm presently going about 11-12 mph (meas. by gps) now.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

BobGlover said:

I've got a 9.9 hp Nissan 'C' model that I'm going to convert to 15 hp. by changing the throttle cam. I've got two questions: 1) Is changing the cam the only change I need to make, and 2) how much speed increase will I get from my 14 ft flatbottom. I'm presently going about 11-12 mph (meas. by gps) now.

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I rechecked the speed after having the 9.9 tuned and a new prop installed. It now goes 17.1 mph on my 14 ft. boat as a 9.9hp motor.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Yes, the cam is all you need. 362637180M THROTTLE CAM $7.57. WOT speed will depend on propping and boat loading, etc. Roughly 50% more power, but that does not necessarily mean 50% more speed. If it goes 17 now, expect 20+ after.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

A 9.9 would be rated for about 21mph on a light reasonably fast hull.
If we knew the proip size and the wot rpm we may be able to tell if your setup is working.
An induction tach or optical tach and a gps will work.Typical 15hp speed would be around
25 again on a light reasonably fast boat.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Hi Bob,
I know that I am comparing apples and oranges but I have a 61 14' starcraft with ply floor and carpet. At sea level I get 21.6mph
with a 15hp 05 Mercury that is setup with 9 1/2" X 10" prop. Don't forget that motor height, bottom condition and trim is very important. Be sure you post your final top speed back here, just qurious.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

I have a 14' Grumman with a '10 Merc 15hp and I've hit 24.5 by myself with about 3 gallons of gas. Pretty light rig though. But I'm a ways above sea level here so it would be quicker at the coast.
Looks like I should have gotten the 9.9 and a $7.57 cam. LOL But it was a gift from my father, so I'm not complaining a bit!
Anyway to get a 15 to a 20? Just kidding... I don't want to go any faster on my tin can than I already do. It's rated for 15 and I don't want to overdo it. I hardly go WOT anyway.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Star said:

Hi Bob,
I know that I am comparing apples and oranges but I have a 61 14' starcraft with ply floor and carpet. At sea level I get 21.6mph
with a 15hp 05 Mercury that is setup with 9 1/2" X 10" prop. Don't forget that motor height, bottom condition and trim is very important. Be sure you post your final top speed back here, just qurious.

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Wll do! I haven't ordered the part (it's been too cold to fool with any motors!) but I hope to get it sometime next week and installed sometime after that.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

I finally received the throttle cam and had it installed. I checked the throttle butterfly and it's in the maximum position at full throttle, unlike the old cam. However, I checked the speed and it's still the same--from 16.8 to 17 mph with the same boat and load--me & my lab. How much speed increase should I have gotten & what should I do now? The motor runs well, idles great, accelerates smoothly, etc.
Will the only advantage be that it'll keep the same speed under a heavier load?

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Get a tach on it and determine your WOT RPM. You may need to change props. Verify jack height and trim. Twice.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

i get about 26mph in my 1436 with a Tohatsu 9.9B2, same motor as the Nissan 9.9B2, converted with the throttle cam to 15hp. I'm happy with it. My buddy has a 25hp e-tec on a heavier 1548 and mine will outrun it.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

pvanv said:

Get a tach on it and determine your WOT RPM. You may need to change props. Verify jack height and trim. Twice.

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Where can you find those portable tachs and about how much are they? Would advancing the timing do any good? It's running well now and doesn't appear to have any hesitation issues, etc. I've got a new prop that I purchased from a Nissan dealer, but don't know the size. I didn't know they made different pitch props for 9.9 & 15 hp. motors.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Some tech issues assuming it's a 2 strokes model.

Don't work with Nissan brand so know nothing about ending letters, comparing to Tohatsu brother engines you have 9.9 & 15 D & D2 models, being the D2 the latest upgrade. Both have different timming marks ranging from 20? to 25?, to start with, what's your actual 9.9 C advance timming factory set at ? Do you know how to check it ?

Depending if set at 20?/22? could advance the timming by playing with stoppers untill 25? (15 HP timming) is matched. iI you have a throttle arm/roller over actuation at wot with new cam & advance setting, will need to re adjust arm shaft height properly. Higher timming will give about 300 + revs at wot and if properly matched with correct prop N? will see/feel the difference between both.

Both Tohatsu 9.9/15 HP engines comes factory delivered sharing same N? 8 prop, you have other different N? to pick for different boating applications. As stated by Paul place an induction tach and chech your max rpm at wot on that set up, will need a tach that reads 2 pulses per revolution, some of them as Tiny Tach, Hardline will add second spark plug induction to your reading counting double, it's not a matter to divide mentally by 2 because most tachs works till 10 K and will turn cracy over higher revs as you will be working around 5800 at wot with 25? advance timming or around 11,600 rpm if counting double.

Happy Boating

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Both Tohatsu and Nissan models have the same suffix letters (B, C, D, D2, etc.) Yes, a 9.9 B2 is a 2-stroke; no throttle cam on the 4-stroke 9.9.

You MUST determine RPM, and verify timing and pitch before jumping to any conclusions.

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Paul,

Assuming a 9.9C model has same 250 CC as 15 models, would there be a mechanical issue to advance timming to 25? to match the 15 HP model once cam has been swapped for the 15 cam ? Asking this as we don't import 9.9 or 15 models to physically see timming difference or have a look on older C versions, just work with 18 horses which are out of this world champs. Have so much fun leaving any brand 15 so much far behind...

Have been upgrading Mercury's 25 which are the C2 Tohatsu versions, will only need to swap carb gasket and advance + 5? timming to have butterfly opened at 180? and perfect update to a 30A4 which costs a bit more. On this model both cams are same shape.

Happy Boating

Re: Speed increase from Nissan 9.9 to 15 hp conversion?

Luis, Just as you mentioned... The de-tuning of the 30 is accomplished by restricting the intake throat (with a restrictor plate). The 15 is de-tuned by limiting the throttle opening with the cam. Of course, verify WOT timing after any work to the linkages.