Atkinsoning motorbike engines

[mechman600]

Originally Posted by renault_megane_dci

Prius is only 13:1 static.
This gives 8.2:1 dynamic with the advanced cam phasing but only 6:1 with the late cam phasing.

On the GPZ (pretty much set on this now) I was aiming for restoring the standard dynamic CR wich is 7.261.

Don't forget that when your motorcycle engine is at peak torque, the intake and exhaust are "in tune" and VE is 100% or greater, your dynamic compression will far exceed 7.26:1. Tuned intake and exhaust harmonics effectively supercharge the intake charge, resulting in a dynamic compression ratio equal to or even greater than static compression.

The Prius cam phasing and intake/exhaust design prevents this from happening.

[renault_megane_dci]

I can only wonder what a tuned intake will do whith altered cam timing and a fair blast of the charge out of the cylinder.

Tuned intake is build on valve closure event wich will be delayed.

I have two cam phasing options :
A :
phase the opening as is done on the shorter cam / lower power derivative (keep a fair chunk of overlap as seen in 100hp / liter engines)

or

B :
phase it even later as seen on the Prius Otto phasing wich should place the opening just a few 10° before Prius Atkinson closing.

A advantage is it would need only 0,75 mm off the head to restore dynamic compression ratio.
B is much more involved since it requires more skimming (closing event delayed even more) but is more hardcore Atkinson (and less overlap wich I believe would help lower RPM grunt)

A is more reversible if need would arise.

Mechman600, would you advise on a lower Dynamic CR ?

Not skimming anything would be good.
I could also fit the big inlet cam to the lower power and lower CR derivative wich could set me down to Prius Atkinson CR more or less ...

[nemesis]

I already raised my static compression in my zx10r from 12.7 to 14.0 and I have 3 intake cams to play with, 300 duration, 296 duration or 292 duration. On the exhaust side I have 292 duration cam and 280 duration cam as well.

[renault_megane_dci]

Let's agree that high CR only works with big duration cam (to stay within acceptable dynamic CR limits), precise timing and optimal fuelling.

When building the GPZ and ER5 engine, Kawasaki applied basically the same dynamic CR to two differently cammed engines.

I am not a huge fan of the brand but I'll give them credit on building reliable engines and will stick with the target dynamic CR.

[renault_megane_dci]

All right folks, I was on the lookout to find out the target MPG improvement and power while atkinsoning a GPZ 500 engine.

I found out in a thread in the car section that a 23% improvement is on the cards.

For power I made a simple cross between useful compression stroke and power and with the corrected stroke.
It does not take into account the exhaust cam.

Anyway, the result came out at 22 hp compared to the previous 58.

(edit : let's not forget the quoted HP for the Prius is obviously in its Otto mode 76 compared to the strict Otto version at 106)

Sounds OK to me .

How would you calculate it ?

[beatr911]

Could you please show your math? I would like to understand how you arrived at 38% of the original power output where the Prius is alot more. I can be a little slow, if a picture shows the cross you mention well please show it if possible.

We are working along the same path of only adjusting a fixed gear intake cam, and on a similar engine (mine just has two more cylinders).

Do you have access to engine building software that can predict engine output based on varying cam timing and/or compression? That is what we really need here before we start cutting, grinding and moving gears.

[renault_megane_dci]

Originally Posted by beatr911

Could you please show your math?

I will show it tomorrow, it is on my work computer (based on valve closing event wich I can"t remember right now)

Originally Posted by beatr911

I would like to understand how you arrived at 38% of the original power output where the Prius is alot more.

Prius is advertised with its Otto cycle power as I stated before.

Originally Posted by beatr911

I can be a little slow, if a picture shows the cross you mention well please show it if possible.

The calculation is crude :
GPZ power per liter for the useful stroke on compression gives 180hp or so per liter
Later inlet closure gives a reduced capacity wich multiplied by the specific power gives 22hp.
It is relevant with cars output althought older school engine.

Originally Posted by beatr911

We are working along the same path of only adjusting a fixed gear intake cam, and on a similar engine (mine just has two more cylinders).

Is yours water cooled ?

Originally Posted by beatr911

Do you have access to engine building software that can predict engine output based on varying cam timing and/or compression?

Unfortunately not.

Originally Posted by beatr911

That is what we really need here before we start cutting, grinding and moving gears.

I still have to bite the bullet and buy a bike (then an ER-5 exhaust cam) and then dare to attend ruining this fine piece of engineery with technical ingenuity ...

[renault_megane_dci]

Off the shelf sprockets improvements are as follow :
14 x 41 => 17 x 38 wich is a 29% improvement

I am not too sure the bike can handle both Atkinsoning and sprocket change

[renault_megane_dci]

my mistake, I calculated the output of a single cylinder, it should be 44 hp, that is a more acceptable power reduction (standard EX is 58, lower power derivative is 50)

[beatr911]

OK that power reduction looks about right.
Yes, my Concours 1000 (GTR1000 in your country) is of the same design family of the EX500, just two less cylinders. 108hp, water cooled, etc. but shaft drive so gearing change is extremely limited.

For what it's worth, I found a discussion on incorrect cam timing on the Concours.org forum that is interesting. The symptom was that it didn't rev past about 4000 rpm. Turns out the poster had mistakenly installed the intake cam 3 teeth retarded. No mileage indications but apparently it idled fine and ran fairly well under 4K.

Possibly going 2 full teeth retarded would be a significant step toward what we are after. I think the cam sprockets are 38 teeth for 18.95* per tooth or a retard of 37.9* for two teeth. Adding the 37.9* to the stock 54*ABDC intake closing results in closing at 91.9* ABDC. Wow, not much compression left there. The Prius retards from 72 to 105 ABDC so this is right in there to get a mild effect.

I normally get low to mid 50s mpg so am expecting to see mid to high 50s to consider it a valid improvement.

Once I finish the suspension refresh on the Ford Focus, I'll be in to the cams on Big Red. As part of the regular spring "tune-up", of course.