Trials Australia Forums

Thanks David and Greg.

I guess conditional on the head fitting, the other issue is compression ratio. My guess is that if I measure the volume of the existing head ( let's say fill it with fine sand) and compare this with the DT250 decompression head, I'll discover any concerning differences.

Alastair.

Oh no Tat ty... surely you're having a lend of us?!
SAND?! That'd result in a very Tat ty bore wouldn't it?
(I started to go through the measuring procedure for comp ratio when I realised you must be kidding).

tat ty wrote:Thanks David and Greg.

I guess conditional on the head fitting, the other issue is compression ratio. My guess is that if I measure the volume of the existing head ( let's say fill it with fine sand) and compare this with the DT250 decompression head, I'll discover any concerning differences.

Alastair.

The hot ticket for easy and accurate head volume measurement is to use blue-coloured kero and stick a piece of clear perspex to the head sealing surface with grease. The perspex has a little hole at one side to put the kero in and to let the air out. Measure the kero into the head with a laboratory burette. The blue kero is because it's easy to see when you've got all the air out.
The perspex allows you to do the volume measurement without a meniscus affecting the accuracy or having to hold the head completely level.
This technique came from a master engine builder and I used it recently when adjusting the squish and combustion chamber shape in my 210cc TY175 head.

Yep that's one way of doing it... Or you could do it the traditional way...
Assuming you want to do a 'proper' compression ratio / swept volume measurement -(not just head volume).

Jools wrote:Yep that's one way of doing it... Or you could do it the traditional way...
Assuming you want to do a 'proper' compression ratio / swept volume measurement -(not just head volume).

What's the traditional way to measure head volume?

Yes, I'm sure you know mate. (Knowing the head volume is only part of the story eh? It's always good to check the whole thing. Who knows what PO's have skimmed, bored etc). Just need a burette - or very accurate means of measuring liquid.

With head off Bring it to TDC and apply a small amount of oil around piston... just enough to seal the rings. Put the head on and lean the motor so that plughole is level.
Ensure it's at TDC and hasn't rotated, now using burette, pour water into the plug hole so that it comes to the lowest plug thread. Measure what you've put in.
Next: Move the crank slowly to BDC. Now pour more water in until it again gets to the lowest thread of the plug hole. Now measure what extra water you have put into the plug hole. What you've measure here is the swept volume.

Now you know for sure exactly, the total volume, the swept volume, and the head volume (compression space).
If you divide the Total volume by the head volume this is your actual REAL compression ratio.

Make sense?

Jools wrote:Yes, I'm sure you know mate. (Knowing the head volume is only part of the story eh? It's always good to check the whole thing. Who knows what PO's have skimmed, bored etc). Just need a burette - or very accurate means of measuring liquid.

With head off Bring it to TDC and apply a small amount of oil around piston... just enough to seal the rings. Put the head on and lean the motor so that plughole is level.
Ensure it's at TDC and hasn't rotated, now using burette, pour water into the plug hole so that it comes to the lowest plug thread. Measure what you've put in.
Next: Move the crank slowly to BDC. Now pour more water in until it again gets to the lowest thread of the plug hole. Now measure what extra water you have put into the plug hole. What you've measure here is the swept volume.

Now you know for sure exactly, the total volume, the swept volume, and the head volume (compression space).
If you divide the Total volume by the head volume this is your actual REAL compression ratio.

Make sense?

Pardon my ignorance, but when you move it slowly to BDC won’t the existing water and any that you add fill the crank case via the transfer ports and also fill the exhaust system.
Unless you are measuring a 4 stroke that doesn’t have transfer ports !.

In my defence, my plan was to use fine sand with the comparative heads OFF the engine, sitting on a bench along side a bottle of Cooper's Best Stout.

Hi Everyone,

tat ty wrote:In my defence, my plan was to use fine sand with the comparative heads OFF the engine, sitting on a bench along side a bottle of Cooper's Best Stout.

Alastair, would the Stout be dark enough to see against the aloooominium background?
Is this "Swept Volume"?

Doh! of course you're right Phil, A 2 banger is a different fish-kettle. It would be down to bore and stroke calc to get the swept volume.
But it's still a good way to measure head volume, so useful for establishing comp ratio.