500cc J-engine compression reduction

[sam2019]

To reduce the compression all we needed to do was take off a part of the head that made it smaller in the 350 stock configuration. This was done with a dremel and including washing the head it took under 40 minutes. I then decided because we had the head out and the engine has 20k km on it to also change the valve guides and the valves themselves - which are very cheap here, the entire package costs just little over 5 euros. After another 30 mins the head was re-assembled with new valve guides and valves and all of it did cost me 40 euros. Can't beat that! Compression now is 170 psi (down from 200) and knocking has disappeared. 170 was what we measured on the stock new classic bullet.

 

[sam2019]

Here you see the stock head - the red line shows to where we cut the material, head with new valves and re-designed chamber next.

 

[Dool]

Valve Seats or Valve Guides?

 

[sam2019]

guides - fixed!

 

[Dool]

If I do get around to modifying a J to larger displacement (have buy one first) I would be inclined to keep the Heart shape and just enlarge to the bore size retaining the "Squish" effect.
Whether this is any better I do not know.

 

[sam2019]

I would be inclined to keep the Heart shape and just enlarge to the bore size retaining the "Squish" effect.

only for half the circle then - the squishy part can not be enlarged. thats another reason I did do away with it.
And although I could feel a bit of an effect with the fuelx pro+, and quite some with the cam and of course a lot wit the BB kits I felt absolutely no difference after the reduction from 200 to 170 psi - although I expected some loss of power and technically there must have been some, just not enough to register on my behind.

 

[Dool]

The squishy bit has a purpose, it squishes the gasses toward the Centre of the combustion chamber and spark plug.
How much of an effect it has I am unsure, Himalayan/Scram does not have this. But on them with a Big Bore and stock head/Combustion chamber, there is a Squish created by the mismatch of size, It may help?

 

[Ikak]

So you say it's either adding additives to 95 gasoline to reach 100RON, or reduce compression by removing that part?
What about 98 gasoline?

 

[sam2019]

What about 98 gasoline?

dont know, try it. but the head mod seems the safest way. in my experience at least.

 

[Dool]

What is your compression
Does it knock on hard acceleration?

 

[Dool]

This was the first cut to accommodate the 84mm bore.
This is where I am now with my latest build.

Compression a good number.

PT4 allowed me to set at 35% more fuel than stock and ignition left stock for now.
The Dyno tune is still months away.
Runs good with constant 13s showing on the AFR meter,fuelx pro is set at 9 .

 

[Dool]

Using an adapter from the compression tester kit, I glued in a borescope which connected to my phone. (With poster putty) and watched the piston in ( almost) real time.

I set the Cam timing by eye

So I was not required to remove the rh subframe clutch cover and do an oil change

 

[Ikak]

Here you see the stock head - the red line shows to where we cut the material, head with new valves and re-designed chamber next.

Have you calculated the change in volume of the ignition chamber? From what volumn to what volumn?

 

[sam2019]

Have you calculated the change in volume of the ignition chamber? From what volumn to what volumn?

No, I just measured the compression in PSI which is all I need to know - compare stock with current, then adapt current to match stock.