WBX combustion chamber cc checks

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WBX combustion chamber cc checks

Post by silverbullet »

I have been meaning to do these measurements for a while as part of doing some static compression ratio calcs, finally got around to it and they might be of some use to other 025 tuners.

All done with the same WR7 triple electrode spark plug (horrid things but there are a lot in circulation, makes next to no difference in the real world in terms of cc) and a used (compressed) top barrel seal:

Standard VW Germany head
Image
AMC head (some carbon deposits)
Image
VW Germany "low compression" head
Image
DJ piston (OEM Mahle, Germany not a short deck Brazil type)
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Re: WBX combustion chamber cc checks

Post by silverbullet »

Something has jumped out at me from these measured volumes.

Static compression ratio is calculated as

(Cylinder volume + piston bowl + combustion chamber) divided by (piston bowl + combustion chamber)

For a 2.1 this gives

(526.25+35+23) / 35+23) = 10.07:1

Not exactly 10.5:1 :?

Did I accidentally pick up a DG piston? Its the only standard loose one knocking about my workshop now (CJH had the last DJ set) and the ring widths are defo 2.1 but I have nothing to check it against :lol:
Last edited by silverbullet on 19 Feb 2016, 08:26, edited 1 time in total.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

Of course if it is a DG piston, then the numbers come out thus

1,915÷4 = 478.75cc

(478.75+23+35)÷(23+35)
= 9.25:1

Which isnt 8.97:1 :?:

Unless 8.97 is actually the ratio for the DF? Its hard to work out what info is correct, the Book of Lies is always there to trip us up...
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Re: WBX combustion chamber cc checks

Post by CJH »

silverbullet wrote:Of course if it is a DG piston, then the numbers come out thus

1,915÷4 = 478.75cc

(478.75+23+35)÷(23+35)
= 9.25:1

Which isnt 8.97:1 :?:

I don't know what a DG piston looks like, but the bit that you can see in the photo certainly does look like a DJ piston.

But if it is a DG piston, then assuming the other numbers are perfect it wouldn't take much of a difference in your measurement of 35cc to get a ratio of 9.25:1. I think 35.03 would do it.

It would have to be 32.4 for the DJ sums to make 10.5:1.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

I reckon my volumes are accurate to 1/4cc, I was careful to work from the same point on a full syringe and purge the bubbles Nursie style.
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Re: WBX combustion chamber cc checks

Post by nicq »

How far down the bore do the pistons sit. They are not flush with the head.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

The oem pistons come flush to the top of the barrel (Brazil pistons fall short by a good 1mm)
Thats why you have to allow for the crushed barrel seal when measuring the head cc.
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Re: WBX combustion chamber cc checks

Post by CJH »

silverbullet wrote:I reckon my volumes are accurate to 1/4cc, I was careful to work from the same point on a full syringe and purge the bubbles Nursie style.

I was being a numpty. The piston volume would have to be 32.4cc to make a DJ compression ratio of 10.5:1, or 37.1cc to make a (DG?) compression ratio of 8.97. Both of those are more than 1/4cc away from your measured value, and a 1/4cc error in the other measurements isn't enough to get to either of the expected values. Odd.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

Its suprised me too and suggests that until you accurately measure, you can't trust any published data about these engines.
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Re: WBX combustion chamber cc checks

Post by CJH »

Do you need to consider the volume around the edge of the piston, down to the depth of the first ring?
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Re: WBX combustion chamber cc checks

Post by silverbullet »

That volume is so little its never measured afaik, once the engine warms up its down to something a thou' or so radial clearance.
My money is on lazy publishing / basic errors, maybe using the original DF chamber volume and back-calculating for the larger capacity.
I dont have any small-valve heads to check but I am pretty sure the chamber is more compact.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

I am having a few thoughts about developing a stablemate engine to go with the 2500, but still a stroker.

Using an 82mm crank and 94 bore will give circa 2275cc and with a DJ piston bowl a comp ratio of 10.8:1 which would be great for lpg, which likes the higher ratios to burn well.

Subtle head/chamber mods could find a few cc (similar to the "low compression" heads) to drop the ratio back to 10.5 or so if its a petrol-only engine.

It would certainly be good and punchy!

They would only be a couple of hundred quid cheaper than the 2500 (no need to bore out the case for the bigger barrels, no special seals each end) but not everyone wants ultimate capacity.

The pistons will still be lighter than the hefty oem items as per the 2500 so that will be good news for big end bearing and bolt longevity.

2300E
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Re: WBX combustion chamber cc checks

Post by silverbullet »

Almost one year on and I remember to cc a MV piston :lol:

46cc :shock:

So with a 23cc combustion chamber as before, an MV actually has a SCR of 8.6:1
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Re: WBX combustion chamber cc checks

Post by 937carrera »

A bit more than a year on now....

I've just acquired what I believe is a DG to strip, hoping I've got really lucky and it's a DJ, I doubt it. Anyway I hadn't measured the pistons before I returned home so started looking at the specs and realised that the bores are the same for DG/DJ, the difference is in stroke. Now I've had a bit of prior experience messing with customising pistons to correct an "engineers" erroneous block shaving, so just as a refresher and out of intellectual curiosity I decided to do some quick calcs for the T25.

I back calculated the compressed volume from the compression ratio and then did a google search to see if I could find the piston volumes, which is why I am here.

Taking the DJ, by deduction compressed volume must be 55.5cc, and if you take off piston volume of 35cc and head volume of 23.3cc as stated above, you are left with a deck volume of -2.8cc. This equates to a deck height of just -0.40mm so the top of the piston is just proud on the barrel...... assuming all the measurements and calculations are accurate. BTW I normally separate deck height proper from head gasket height, but that's not so appropriate for our engine design, I understand why you left the compression ring in place.

Duplicating this for the MV the deck height should be -0.49mm and for the DG, with a CR of 8.6:1 (I think you mentioned 9:1 above ?) the deck height is +.66mm, but I just assumed the piston volume was the same as the DJ, probably wrong, 42.5cc would give the same deck height.

That's the theory, I think you have done the calcs on the assumption that the top of the piston is at the top of the barrel, maybe it isn't and I have stripped my engine without measuring so have no further insight at this point. I can see where your calcs came from however :)

What is clear is that small changes in compressed volume do change CR quite a bit. Did you happen to measure head face to compression ring so that piston clearance can be evaluated for each engine or gudgeon pin to top of piston, that could possibly provide further light ?

BTW, I've just gone through another exercise with changing head gasket thickness on an engine because a water leak (i.e steam) eroded part of the block. It's a turbo engine so we wanted to get back to the same CR. The engineering shop had different measurements for piston volume compared to the piston manufacturer. Make of that what you will.
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Re: WBX combustion chamber cc checks

Post by silverbullet »

From what I have seen, any wbx with oem Mahle German (not Brazilian) pistons will bring the piston squish flush with the top of the barrel with remarkable consistency.

For the record, I cc'd a 3.2 wbx6 head today (the 3.2 has bore and stroke of a 2.1), which inexplicably had been built up with 98mm fire rings from a 3.7

Volume with 98mm ring: 24cc
Volume with 94mm ring: 22cc

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