Yeh, I'd like to think so....but that's what the calculator gives!!Or am I being an idiot! Either way, the official AA site says 10cc, so the figure is out either way!!
So, just to be clear, the combustion chamber (Chamber Volume) is simply the bit in the heads where the ports and spark plugs come in, right?
Think about it - you have the piston dishes the same as the head chambers which they are pretty obviously not. Yes, unless the heads also have a step around the edge which you could calculate as additional deck height or add to the head chambers.
Yes, measured from where the cylinder meets the head. It's quite significant. Have a Google for "CCing heads" for how to measure it.
Yep, your right. Did the calculation with the diameter, not the radius as required. Doh, but I am in a freezing Scottish shed do brain is slightly frozen. That changes things! 14.7 CC instead! It's all making more sense. Just with the deck height of the new AA's being 0.33mm v the original Mahles at 0.65mm, if it could do with a spacer, I'd prefer to do that now! I'll do the chamber volume tomorrow with a syringe, but for the time being, that improves things!
If your final calcs come out like that and you have the opportunity to raise the compression a bit I would. It will be ok like that but it'll be a bit more snappy if it were nearer 8:1 or even a bit higher.
Is that gasket thickness of 0.7mm in the calculator the head gasket? If it is you need to delete it and add the dimension to the cylinder base spacer. Maybe you didn’t read the 1990 VW Technical Bulletin, this is what it says: The following improvements have been introduced as of remanufactured Engine number 89000. To eliminate seizing of pistons, increased piston/cylinder clearance from 0.03mm ± 0.008mm to 0.045mm ± 0.005mm. Eliminate aluminium seal 021 101 341A between cylinder and cylinder head. Eliminate paper gasket 021 101 341A between foot of cylinder and crankcase. Install 1.6mm aluminium shim 071 101 341, with adhesive D 000 400 at foot of cylinder to compensate for removed paper gasket and aluminium seal. To improve heat transfer, oil groves on both sides of the connecting rod to allow oil splash to cool bottom of the piston.
He might not be able to increase the CR with those pistons. Assuming the 0.7mm is the head gasket and 0.33 is the piston to cylinder height that’s a total deck of 1.03mm (40 thou in real money) – I wouldn’t want to reduce that by much. 8.0:1 would be a negative deck.
I hadn't looked at the details - it'll still go ok, it's only a bus. Funny really, the usual problem is getting the CR down, not up.
Interestingly, these are the figures I get for the stock set up that the engine was running on, given that the chamber volume of 56cc is correct - Very happy to be pointed to a reference source for that, as havent found one written down anywhere yet.
@Zed ... Yeah but yours runs ok with a deck height a badger could hide in doesnt it?.. .. and wouldnt trust AA's value for dishes just meaure it much safer a few cc's makes a difference to compression ratio
It does, 3.5mm or something ridiculous, but even so it has high compression for the stock cam. I forget what but IIRC it's over 9:1.It runs well simply because it's big - it's definitely not clever.
That bulletin 1.6mm shim only applied for standard parts. AA pistons and cylinders aren’t standard and neither are Mahle. Looking at the combustion chamber and piston dish volume you’ve quoted the problem will be getting the compression ratio high enough but you need to measure the chamber and dish volume, don’t trust what someone else might tell you. Don’t fit cylinder base or head gaskets for your measurements and try to aim for 8:1 CR but you should have a 1mm deck to be safe. If the engine was running with a 0.65mm deck and only had a CR of 7.1:1 it would have been sluggish and if the AA pistons have a larger dish volume it will be even worse.
Those piston dishes are for bad low octane fuel that we cant buy here... and keeping the engine cooler by deleting power.....like the same capacity T4 engine without dished pistons was 90HP in a T4 car and 70HP with dishes in a bus..
So what is the stock piston? As far as I can tell from Haynes etc I have the originals, certainly in form if not it make? As I understand it, both are faithfull copies of the originals but this aspect is all new to me, so what do I know. As things stand, I'm thinking of returning (if I can) the dished variety, and getting flat pistons version plus a spacer to try and get a decent compression and deck height combination. Never had to do this level of thinking as regards cc before, so all a bit of a learning curve here, but as it stands, I don't think I can live with just 0.33mm of deck height.
No, you can't live with 0.3mm deck - when it warms up the pistons would hit the heads, if not when cold, which they probably would. Do the calcs before ordering is the best advice. Have you cc'd the heads yet?
@Shep Woof Woof If you find you require 0.7mm spacers or some other type 4 odds and ends, I have these: 6 unused plastic case to case half vibration dampers 5 used but look reusable bearing dowels 4 square port heat exchanger to cylinder head gaskets all new 4 base shims 100 x 109 x 0.7(?) all new 4 cylinder head spacers 93 x 104 x 0.7mm all new I very much doubt I'll need them in the future & when I get home I'll check exactly what is in my tubs.
I've done a rough cc measure (as in not using perspex to take out the bulge and it's looking like being in the region of mid 50's, so yeh, the 56cc I've used so far aint going to be too far away. Got a bit of perspex coming, cos I'm now intrigued enough to get it right. Basically then the current AA set up is no good then, even though it was supposed to be an identical replacement. I guess my old Mahle set up must have had a pretty poor performance but with just enough deck height to get away with it and I knew no different. It pulled well enough mind! So, if I replace the dished with a flat version of the same pistons, and put in the 1.6mm spacer (entered 1.4 in the calculator to allow for losing the 0.2mm gasket currently in place) it now goes up to 8.245:1, which I guess will be a bit too much. NB I know I should add this to the deck height in the calculator, but it's easier to see by putting in the gasket thickness box and it has the same result. Can you get larger spacers than 1.6mm, as a 2mm spacer would give me 7.9:1 which sounds like it would be bang on the money!
What you need to take into consideration is that too big a deck height using flat top pistons will also rob you of power as you loose the swish effect. As you see from the calculation I posted earlier in this thread I managed 7.7:1 CR with mahle dished pistons and it has plenty of power. Certainly more than the 1911 i had in before. Another alternative if you can run to it are Keith Black pistons which have a smaller dish. Just to clarify my total deck height is 1.6mm as the extra 0.8mm which is the AMC head lip i calculated into the head combustion chamber volume. The head lip does not apply to your build unless you are using AMC heads.
