Casa 185 compression

[Knowledge]

I have heard that the CR on these kits is too high out-of-the-box and the combustion chamber needs enlarging to get it right.

Has anyone got any information on this? Darren Scott?

[ULC Soulagent]

Think Benji has some info on this kit( VeganSdyney)

[vegansydney]

The Casa 185 v1 (pre-2015) has a geometric compression of 10.8:1 with a 1.1mm squish clearance. The volume of the combustion chamber with the piston at TDC is 19ml and with no squish (piston taped to head) it is 16ml.

I've heard from very reliable sources that v2 of the Casa 185 (2015-current) is the same, as is the Scootopia 185, but I have not independently verified this.

[Knowledge]

Hmmm. 10.8 CR sounds a tad high. I would have thought 8.5 would be more like it.

I haven’t got the kit in my hands yet, but I will have to assess whether the combustion chamber can be enlarged and whether that will have any impact on the width of the squish band.

I wanted this to be a plug and play build, but I guess there is no such thing. Measure, measure, measure and then do the leak down test.

[Fast n Furious]

10.8:1 (uncorrected) should be perfectly ok providing other characteristics are matched to suit, such as fuel quality, ignition timing, carburation, exhaust bleed and an efficient forced air cooling system.

[ToBoldlyGo]

I was told off for suggesting the compression was high, on the basis that they wouldn't have supplied that head if it was wrong. Maybe, I don't know. But they went on to suggest that the front sprocket also has a big effect on it.

[Knowledge]

I finally got a chance to measure this and the figures are:

10.2 uncorrected
7.0 corrected.

The squish is currently about 1.5mm, it could benefit from being 1.2mm. With 1.2 mm, the compression would be:

10.7 uncorrected
7.3 corrected

Any comments, please?

[vegansydney]

Nothing substantial to add to the discussion except for this confirms the heads on both v1 (pre-2015) and v2 (2015+) of the Casa 185 are the same.

[Warkton Tornado No.1]

With compression ratios, which to my way of thinking should be thought of as geometric, there are other, interactive influences to consider.

Super unleaded will tolerate higher compression. The ignition advance will be instrumental upon the avoidance of pre-ignition. The plug grade is important. The piston itself & it's underside's ability to transfer heat, as well as it's mass. The best pistons are most likely those with a waffled underside. The number & configuration of transfer ports impacts upon the cooling of the crown. Then, of course, there is the exhaust. Any decent exhaust, which will be an effective expansion chamber, will cause the higher compression engines to start incurring pumping losses & the loss of any chance of over-rev which is a good thing to have with the restrictions of a four speed box when going down hill An exhaust in conjunction with a 'kitted' engine needs a bleed of an inch bore for a 200, & that formula should be applied to the varying capacities IMO for an engine to run cooler.

However, the ability of the cylinder head to do it's job of shedding heat relies upon the tightest, safe squish possible & that should be contoured to suit the piston crown, not just a straight cut that is 'near enough' as is so often the case. With great care, the profile of head to crown can be matched by attaching some fine abrasive paper to the crown (a ring, or 'polo' shape works well) & lapping the two together in the head to cylinder assembly. Obviously, the head to cylinder should be spigotted or dowelled for positional accuracy.

What shouldn't be overlooked is that with the generic overall size of an OEM type head is that as a consequence, a 125 can run considerably higher compression than a 225 as it will have greater mass.

Ultimately, without the luxury of a dyno to hand, which some of us can ill afford, a keen ear will detect pinking. A keen eye will read the plug.

My rule of thumb is to aim for 9.5:1 maximum for the large block engines with fixed ignition, decreasing with capacity rise, though an alloy barrel helps with the conduction of heat. About 10:1 maximum for a 175 Indian alloy barrel, though the time involved in making them run well is colossal......

[Knowledge]

Well, thanks for the last sentence WT1.

Perhaps I should have asked a better question. Something like, has anyone fitted a standard Casa 185 kit and found the as-supplied CR too high?

I had heard that it is sensible to reduce the CR. I was thinking down to about 9.5 uncorrected.

[dickie]

Well, thanks for the last sentence WT1.

Perhaps I should have asked a better question. Something like, has anyone fitted a standard Casa 185 kit and found the as-supplied CR too high?

I had heard that it is sensible to reduce the CR. I was thinking down to about 9.5 uncorrected.

No, I haven't, but I have built 2 x scootopia 185 kits as supplied, both with tight squish; I aim for 0.9mm but no less. Neither was hard to kick over and had front sprocket changed to 16; both LI150 boxes.

Neither has had any reliability issues related to cylinder.

I only mention this as I'm told that the scootopia kit is a direct copy of the casa 185 kit. Sorry if I'm just muddying the water.

[Warkton Tornado No.1]

I forgot to mention what may be obvious, but whatever is decided as a safe maximum should take into account the inevitable tendency for carbon build up.

As for pumping losses, I learnt the hard way that an engine that pulls like a train out of a corner or hairpin can hit the equivalent of a brick wall when it should be over-revving down hill to gain good inertia to climb the inevitable upcoming hill with some gusto. Little wonder that the Japanese had variable compression heads on their top road racers.

[Storkfoot]

I wish I’d stumbled across this thread when I last had my Casa 185 top end apart.

Mine, with a 19 front sprocket, is hard to kick start. Daft as it may seem, initially I put this down to the kickstart shaft being a tight fit in the bronze bush in the casing. Increasingly, in the last 6 months, or so, I have started having my doubts about the compression.

Why would a kit which is meant to almost replicate a standard 200 performance be created with a high compression head

[Warkton Tornado No.1]

High compression - & high exhaust ports - are both a relatively easy way to increase the power, but, as is usually the case, there is a trade off.

We probably all know by now that the exhaust system is critical to the amount of power a given top end can produce & the manner in which it is delivered.

For the harmonious, practicable production of power & torque in an engine equipped with a four or - at most - five speed gearbox, besides the inlet, transfer(s) & exhaust durations, there needs to be serious consideration of what occurs not only within the top end of the cylinder but also the bottom end.

Whilst it’s quite simple to establish the top end compression ratio, the bottom end is likely always to have more primary compression than necessary in our engines, compared to modern two stroke thinking. That’s why I don’t use the full circle HPC crankshafts & endeavour to incorporate longer con-rods with packing plates. The other advantages of such set-ups is an increase in volume of the fresh charge & lower pumping losses.

Unfortunately, unless substantial modification to the OEM crankcase bottom end is done, it is unlikely the ideal lower primary compressions will ever be realised.

Nonetheless, we can at least ensure that the top end compression ratio is a parameter over which we have control. I continue to go with the geometric compression ratio & for a 200+ cc capacity set that at no greater than 10:1. As I won’t deviate (much) from a selected squish, nor resort to head gaskets, it can be very time intensive to get things spot on

[Storkfoot]

I’m just running this with a standard GP200 box exhaust, PHBL25 through a remote filter and an AF 4 speed close ratio gearbox. It probably replicates, more or less, the performance of a standard 200 and I’m happy with that.

At some point, I’ll strip the top end and assess the top end compression ratio before, if needed, getting the cylinder head machined to lower the compression ratio.

Does that seem reasonable?

[Adam_Winstone]

I finally got a chance to measure this and the figures are:

10.2 uncorrected
7.0 corrected.

The squish is currently about 1.5mm, it could benefit from being 1.2mm. With 1.2 mm, the compression would be:

10.7 uncorrected
7.3 corrected

Any comments, please?

Both higher than I would like to see on a general use.

The geo appears surprisingly high and the corrected confirms it IMO.

I know that you can counter this by reducing ignition advance but that would seem to be a bit of a workaround for the questionable high comp ratios.

It's not unusual for Italian kits to start with higher compression than we might apply in the UK, which is supported by the initially higher compression of the Quattrini kit too, then application of decompression hole.

Much of this comes down to personal preference of setup, which is always a compromise of one sort or another... in the opinion of others.

[vegansydney]

Given the similarities with the Scootopia 185, here a some some measurements which might be of interest. These were shared with me by a good friend/mentor. I've only built Casa 185s and although I've put some klicks on a friend's Scootopia 185, I've never had my hands on one.

Combustion chamber volume with zero squish = 17ml
Spark plug thread depth = 18mm
GCR for 58mm stroke with 1mm squish = 10.24:1
GCR for 60mm stroke with 1mm squish = 10.55:1