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I have been researching the old 400r compared to the 2018 ex400.
One of the items that caught my eye was the fact the older model had 34mm throttle bodies on it while the newer model has 32mm throttle bodies on it. I was previously reading a post (I believe by @Engine-R) concerning the flow characteristics of the 32mm throttle bodies and how they were limited above 9k rpms. Does anyone have any feedback /thoughts for the installation of 34mm throttle bodies on the newer model ex400?
 

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This is a good question and tbh I can't give any real qualified answer, I can only guess that the reason for this could be
1. Emmissions, like with all the newer models, maybe it's because Euro4.
2. Better rideability at lower rpm's with taking into account that the higher revs are to be avoided.

From the technical p.o.v. we must look at venturi, but also the speed of air 'inhaled'.
The numbers look the following
32 mm throttle
6000 rpm = 50 m/sec.; 11000 rpm = 91 m/sec.; 12000 rpm = 99 m/sec.
34 mm throttle
6000 rpm = 44 m/sec.; 11000 rpm = 81 m/sec.; 12000 rpm = 88 m/sec.
The speed in the best case is between 45 m/sec. and 100 m/sec.

Lower revs means, less noice and lower consumption and please don't forget that the 'little' Ninjas are more Touring-Bikes than SS.

For a better knowledge we would also need the data from the injectors.
But what we can already see, is that with given setup at 12000 rpm there could be a danger of running too lean (must not, but???).
 

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I have been researching the old 400r compared to the 2018 ex400.
One of the items that caught my eye was the fact the older model had 34mm throttle bodies on it while the newer model has 32mm throttle bodies on it. I was previously reading a post (I believe by @Engine-R) concerning the flow characteristics of the 32mm throttle bodies and how they were limited above 9k rpms. Does anyone have any feedback /thoughts for the installation of 34mm throttle bodies on the newer model ex400?
I touched on this in another thread:

From my p.o.v. the 36 mm should be the optimal TB for racing, a 39 mm is only good for the redline and not very much lower, the lower range with this would be catastrophic.
For a street-only-bike I would even go only up to 34 or 35 mm, but please take this for information only.
What we also should not forget is about the mean piston speed and what that means, I for myself would never rev the EX400 up to 12,000 rpm (there the m.p.s is already higher than 20 m/s and that for a 2-cyl-twin).
Realistically, though, that means the peak power gain is 3hp with 39 mm, and slightly less outside of peak (which occurs at around 10k rpm I believe). So, the stock size is probably a good compromise. I have enlarged TB's on my 14R, and have used them on a 251 hp naturally aspirated race engine, as well. The gains are much more significant on those engines.

If anyone wanted to try enlarged TB's, there are a few places that can do that work. The problem is that as you enlarge them, you lose the taper. Not having looked at a set of 400 TB's, I am not sure how significant that would be.

As far as piston speed, it is nothing to worry about on the 2018 Ninja 400, imo.

Comparison of piston speeds, parallel twins:

4078 fpm Ninja 400 51.8mm stroke @12000 rpm
4179 fpm Ninja 300 49mm stroke @13000 rpm
4186 fpm Ninja 500 58mm stroke @11000 rpm

V-Twin:

4764 fpm SV1000 66mm stroke @11000 rpm
4114 fpm Superhawk 66mm stroke @9500 rpm

As you can see, the 2018 400 has the lowest piston speed of this group.
Keep in mind, also, that there is more to intake technology than TB size. Port design, valves, taper, cams, etc...all contribute. Everything functions as a system. As any modern cylinder head porting company will tell you, a bigger port (or TB) is not always better. Now, if you are porting the head, changing the cams, exhaust, rpm limit, etc...well, then all bets are off.
 
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