WBX combustion chamber cc checks

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937carrera
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Re: WBX combustion chamber cc checks

Post by 937carrera »

I have been doing some measuring and head scratching today, a few thoughts

1. Can you measure the head volume without the fire ring, I'm expecting it to be 5.9cc less. I'm not exepcting anything different to turn up, but it'll just confirm something against my spreadsheet

2. None of the SCR actually tie up to spec. I was looking at the deck height and how that would need to change to get the CR back "into spec". The DG would need to have 0.66mm deck height (before adding 7cc to account for the flat bowl, see below as I started out thinking the DG and DJ pistons were the sasme shape), the DJ -0.4mm and the MV -0.49mm. None of that makes sense to me as my understanding is
  • Conrods are common
  • Barrels are common
  • Cylinder heads are common, German ones being 23.3cc including the fire ring / HG volume
  • Compression height of the pistons must be 3.55mm smaller on the DJ/MV engines If the only adjustment is for stroke
  • Piston dish volume for the DJ is 35cc, MV 46cc. I have not done any measurements but have scaled from images and back calculated / estimated that the DG must be around 42cc (any chance someone can measure that, I don't have the gear for the right accuracy)

3. If the compression height was as calculated above the piston to head clearance would be 0.44mm for a DJ (nowhere near enough). I quickly measured one of my DG pistons and got a compression height of 40.27mm, can you check that and also the DJ measurement as I have no DJ pistons

4. I measured one of my used fire rings, it was 0.84mm

5. Gowesty has some updated information at http://www.gowesty.com/tech-article-details.php?id=82. On the MV engine they get a CR of 8.64 which is the same as my 8.61 calc as far as I am concerned. They also have the view that the top of the piston is level with the barrel, but state that the head clearance is 1mm, when the fire ring is only 0.85mm, so there's 0.15mm floating around somewhere, the 1mm clearance can only be true if the deck height is 0.15mm :!:

6. I then measured from the top of compression ring number 1 to the top of the piston and compared that against the witness marks on the barrel. That was interesting as the piston was about 9.05mm and the barrel had about 8.6mm of "unscraped" wall. That's 0.45mm negative deck height which is just about the right value to get the SCR into line with factory spec for all engine configurations. Coincidence ? I don't know, I don't have enough experience of working on WBX to know where the deck height normally is, it does seem to answer the SCR question, but that would leave just 0.4mm of piston to head clearance which is nowhere near enough. 1mm would be just fine, as Gowesty measured

Hopefully this adds to the sum of knowledge. If the deck height assumption is incorrect (probably), then one of the other measurements must need reviewing. There's only 2.4-3.4cc reduction in compressed volume required to get the SCR as factory spec

Here's me measuring the fire ring

Image

and the unscraped part of the barrel

Image
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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937carrera
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Re: WBX combustion chamber cc checks

Post by 937carrera »

I found a 1cc syringe this morning so have done a volume check on the DG piston.

Now I know it's not the same as using a perspex sheet over the top and the right fluid, but I did add some washing up liquid to break the surface tension of the water. On that basis I reckon a DG piston has about 42.7cc in its dish. Spurious degrees of accuracy perhaps.

That's actually quite interesting as it means the SCR for a DG comes out at 8.24, but the dek height requirement to get back to 8.6 SCR is 0.44mm, which is bang between the equivalent 0.4 and 0.49 for the DJ and MV. Alternatively the combustion chamber volume needs to be between 2.8 and 3.4 cc smaller

Here's a photo, the water level did look more up to the level of the top of the piston when I looked from the side, bit of a balance between adding fluid to the point where it is full, but not overflowing or high, and allowing a bit for the remaining surface tension meaning the water level is a bit lower on the outside edges.

Apologies for the splitty, I haven't got a T25 mug. :( Now it has water and wahing up liquid in I suppose I had better wash it :rofl

Image
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: WBX combustion chamber cc checks

Post by silverbullet »

I suspect that the origin of the original SCR figures may come from using the early small-valve DF chamber volumes, which were significantly smaller than the later heads. I could be wrong though!

I dont have any 1.9 parts left (other than an odd crankshaft) so I can't comment on the DG piston bowl volume.

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937carrera
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Re: WBX combustion chamber cc checks

Post by 937carrera »

silverbullet wrote:I suspect that the origin of the original SCR figures may come from using the early small-valve DF chamber volumes, which were significantly smaller than the later heads. I could be wrong though!

I dont have any 1.9 parts left (other than an odd crankshaft) so I can't comment on the DG piston bowl volume.

In that case we'll have to use my number until someone else does the same exercise. :)

I got the comression ratio data from http://vwt3.com/t3-tech/t3-data/engine- ... 3-1-9l-dg/ and http://busman.be/syncro-information/t3a ... ginetypes/ so that would I thought have been using the correct heads.

If you get a chance to cc the head excluding the compression ring I would be really interested in that value :wink:

You may notice I have gone back to doing some investigation work on my own "unknown" engine today, more detective work, but at best can only be 2/3 of the fun you are having
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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937carrera
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Re: WBX combustion chamber cc checks

Post by 937carrera »

Well, I finally got round to buying some perspex and cut it to shape using a dremmel. It's the first time I have done this in anger, the 2mm perspex was probably not thick enough and the surface tension of neat g12 coolant was probably too high.

The spark plug couldn't be wound in all the way as the earth electrode was contacting the base of the perspex. The above photos were taken after 17cc of g12 was put in. You can see that in the first photo there were a couple of small airlocks, and some fluid had migrated under the perspex in the flat head area. The second has the perspex taken away and not all the liquid is contained the the valve recess.

Based on what I saw as I filled the chamber, and adjusting for the spark plug not being fully would in, I reckon the head volume is 15.0-15.5cc

I'll see how that fits from a maths perspective later

Image

Image
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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Re: WBX combustion chamber cc checks

Post by silverbullet »

Bubbles contain more volume than you'd think, they have to be purged.
Yoy have also found out why I drilled the filling hole so that it was directly over the spark plug!

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Re: WBX combustion chamber cc checks

Post by silverbullet »

Update. Useful online SCR calculator:

https://www.omnicalculator.com/everyday ... sion-ratio

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