Cylinder head chamber shape

[Steve Best]

Spent a wonderful Saturday swapping heads:

The front right is my modified head that I have been running for a while now.
The rear right head is an original GrubeeGT5A head that was prone to detonation.
Note the squish band is out to full cylinder diameter and in further on the modified head.
The squish band is 3 times wider on the modified head.
Compared to stock, no detonation, more low end torque and more power at all rpm.

On the left is an aftermarket head I recently bought. Notice something missing? No bolt holes!
They were drilled almost all the way through from the other side.

What should I do? Call the seller angrily? No, I laughed and drilled them through.

So I took some white grease and a flat plate, greased the head gasket surface:

Then filled the chamber up with water to the bottom of the threads with a calibrated syringe.
This syringe is really too big for this job, but I just wanted approximate numbers accurate to 0.5cc.
A bit of dish soap in the water will help the syringe slide smoothly.

Tip the head up so the water is square with the sparkplug threads and squeeze just enough water.
Now read the amount of water off the syringe:

Later I did the same thing but instead of a flat plate, I used a piston with a bit of grease on the chamber taper.

Look carefully in this picture and you can see the water at the bottom of the plug threads:

So what did I find for volumes?
Stock head with flat plate = 6cc, with piston = 4cc
Modified head with flat plate = 6.5cc, with piston = 3cc
Aftermarket head with flat plate = 8cc, with piston = 3cc

Whoa! See the problem here? Just telling you it is a 6cc or 8cc head does not tell the whole story.
You have to measure the chamber volume with a piston, not a flat plate to get the true story.
Seeing as everyone is talking about 5cc chambers, I don't think they are doing it.

How did they work?
Stock head not tested because previous use gave overheating, detonation and blown head gasket.
Modified head works well, top speed 57 kph (today, it varies)
Aftermarket head MUCH more low end torque, starts better, accelerates better, top speed 53 kph.
WHAT??? I did the old double swap to be sure and true enough, only 53 kph on all runs.
I actually got 58 kph on 2 of 5 runs with the modified head.

Why? I believe chamber sweep, quench losses and burn speed.
The deep chamber does not clear exhaust gasses well, the extra quench area invokes pumping losses and the faster burn speeds of the more turbulent mixture peak out too soon and probably need a delayed spark like from Jaguar's CDI unit. More testing will tell.

Meantime, I'm running this head because I really like the low end torque. It will pull from low speeds and gets me up to 53 kph quick. It pulls strong on the hills too. I like it.

Steve

 

[HeadSmess]

hi quality machining followed by lax QC...um...they didnt drill the holes all the way? who let that one go?

i would have said something nasty to the supplier... i aint paying for something that wasnt finished!



mmmm, low end torque.

about all i noticed on my nsr when i rebuilt the top end and tightened the squish from 1.8mm down to 0.8 mm was the way it wont rev right out to redline when running higher octane fuels... screams on 91, feels like a slug on 98... but the low end improved somewhat, regardless of fuel. might pop the top off and have a peek inside soon, see how it looks.

hard to say if the compression gain was from the reduced squish or the new piston/rings/barrel... anyways, way better than when i first got it, and definitely better than the last two weeks after having to dig the rings out of the piston after seizing it... have a feeling the issue had existed before i even got it. completely different engine sounds now. smoother.


the issue with modifying your only vehicle...you gotta keep it running! just got myself a kawasaki as a main bike. nsr is for weekend thrashing now

 

[AutOtaku]

I dont mean to derail this a tad as it was a very interesting read, but doesnt the aftermarket head overheat after long rides? theres not much material there so i would believe it would.....

 

[Steve Best]

I dont mean to derail this a tad as it was a very interesting read, but doesnt the aftermarket head overheat after long rides? theres not much material there so i would believe it would.....

It actually weighs at least double what the stock head weighs.
I've only rode it a few miles yet so no hard shakedown test. So far no sign of overheating, but weather is cool yet.

Steve

 

[Steve Best]

Gave the head a good test today, top speed on my test track was 55kph today, same setup as my last post. I kept on going and put some miles on it, running full tilt for mile after mile after mile. No problems with overheating. Temps today are about 20c or 68f. My long steep test hill needed no pedaling and there was no detonation or overheating even as the speed and rpm dropped under maximum load.

So time to try more "stuff". Exhausts! I bought a few to try:

The bottom three are Grubee/motorizedbicycle.ca parts.
The black pipe comes with the engine and has a catalyst. It has been tested before and is noisier and less top speed. Both worse with the core removed.
The small chrome is a $20 replacement and is slightly shorter. Quieter and slightly higher speed when tested against the black pipe.
the long straight pipe is the "Fancy Poo Poo Exhaust Pipe"
and the banana pipe is off EBAY.

NONE of these are a bolt on. All needed slight bending to fit. Without a vice and a torch you will need to be creative.

First test was the snakey Banana Pipe:

As you can see from the picture, it barely let the pedals spin and my feet hit it.
I noticed slightly more take off power and it accelerated well, but didn't feel like the tuned pipe on my KTM.
It ripped up the long steep hill with no problem at all and maintained a good speed (not recorded).
I was kind of shocked when I looked at the GPS.
Top speed was 60.2kph! 5kph faster than my regular pipe and 10kph faster than this head did earlier this week!
So what will it do with the faster head? 62kph!

Next up, the "Fancy Poo Poo Exhaust Pipe". Unlike sham-poo, this is the real deal!

Oh-oh! I could not pedal as is, but wait! I didn't have to. Enough low end torque I could start away wit no pedaling.
Not as perky as the Snakey Banana, I was suprised to see I was doing 58kph!
Speed up the long steep hill was not as fast as the Snakey Banana but it was unstoppable.
Great gobs of low end torque. Surprisingly, it wasn't all that quiet.

So what did I stick with? My $20 shorty silver because I need to pedal.

Otherwise it would be the Snakey Banana for sure.
As crude as it was it offered more low end and top end.
Very slightly more noise was a problem. Worse, it is in the way. I cannot live with it. I'll have to make my own.

Steve

 

[Frankfort MB's]

You should also try MZpartsmiami's expansion chamber it's not that loud (when your riding it)
It seems to have great mid-high rpm power and sounds awesome on a bicycle

To other people it sounds like a pissed off weed wacker lol

 

[Steve Best]

Actually my head was from MZpartsmiami
The snakey banana pipe was from Boygoesfast.

 

[Deleted member 12676]

Nice report. Your bike needs a Jaguar CDI if you are increasing the cranking pressure.
To the volumes of head with piston touching the squish area you need to add in the area that would be there in real life. Use solder to measure the squish clearance and then use the volume formula. (=2 x 3.142 x bore/2 x squish clearance)

 

[45u]

Nice wright up thanks! Yes a good pipe with a expansion chamber is the way to go. I have the MZ pipe but have not tired it yet.

 

[Fabian]