Steve Best
Well-Known Member
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
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