It’s 1.5mm deep, 3mm wide, U shaped. The tech bulletin had 1.5mm deep, 3mm wide, V shaped so I have a little more area. Here’s another pic.
2022, over two years since I contributed to this message and still havent finished the engine, but I have moved house during that time and all the fall out, stress and upset that moving incurs. I wasnt sure which would finish first, my marriage or the house. To remind you all, its going into a Karmann Ghia. I've completed a mock build up including the fan, alternator etc on top, just to make sure everything lines up and looks ok. I'm back to where I was in 2019 regarding selection of compression ratios. Still got the following: Type 4 2056 with 41 x 34 valves heads AA P&C's, with valve relief in top of piston. Fully balanced SCAT C35 IDF 40's with 30mm venturi's Stage 1 flywheel to match T1 G/B E5 97/98 octane Street use only. Type 4 CR current state, static and dynamic. Please note Deck Height currrently 0.2-0.23mm........... I "think" the Inlet valve ABDC angle of 47 Deg or could be 51? for a SCAT C35 couldnt find anything definitive.........Having read other similar topics on here theres some conflict with 7CC for valve relief volume some say 3-4cc? In the house move I have lost all my working out and records for head combustion volumes and valve relief. If all the data was correct for running its not a bad start except for deck height. Deck Height 1.2mm ABDC angle of 51deg Static 8.6 dynamic 7.5 To increase the deck height i had thought of machining the material of the piston head. PROS. easy to do, gives accurate deck height, and will alter combustion volume not sure how much. Cons....need to rebalance the pistons. Use 1mm shims under the barrels? To dig out and grind the heads is a last resort. So what dynamic CR should aim for to run on 97/98 octane? Thanks for reading this John
Removing material from the top of the pistons will give you the exact same result as shimming the barrels ie reducing the CR. I don't think the reduction of the small x-section valve pockets will make up for it in any useful way. You'd get a better result skimming the heads because the combustion chamber you'd be reducing is far bigger than a valve pocket. I'm trying to work out what I set my wasser crank conversion to as it went like hot snot with similar valve sizes and same cam. 9:1 static rings a bell... but... it needed revs, it did naff all below 3,000rpm and really only got going at 3,500rpm
Welcome back. You definitely do not want to increase the head combustion chamber volume; the static CR is already too low for a C35 even with 0.2mm deck. I wouldn’t go less than 1mm deck. You need to measure the valve cut out volume but 3 or 4 cc is probably closer than 7cc and you might find that machining 0.8mm off the piston to get 1mm deck will give a decent SCR, but you won’t know until you know the correct volume. I run just under 9.0:1 with E5 98 octane and a C25 cam. For a C35 you want more then 9.0:1 static.
Not exactly, it will also reduce the valve pocket volume, not a lot, but something. Skimmimg the heads would help but the deck would still be too low. There was someone else recently having problems with AA pistons with valve pockets but I can’t remember who it was and I don’t think they ever posted what they did. I’ll have a search for the thread.
It comes down to loosing overall combustion chamber volume, 1mm off the valve pocket vs 1mm off the combustion chamber - no contest, the combustion chamber is many times the area of the valve pocket. whether you gain deck by shimming or loosing piston height makes no difference, both will loose exactly the same CR, it's where you gain it back that counts.
The AA pistons have 3cc "dish" if you can call the valve cut out that. I'd have thought a shim would be the easy off the shelf fix for deck height. my 2056 in the dokka runs a deck of 1.3mm , CR of 8.9, DCR of 8.3 on a Webcam 73/86, more than it's supposed to like but don't seem to bother it much .
Back. So, I measured the valve pockets on the piston crowns =2cc, I was surprised. I have assumed the Air close, 51deg ABDC the data I found looked like a copy and not official SCAT T4 C35 spec sheet see attached. changeing the angle changes the CR significantly. Deck heights were 0.2mm Ive assumed a 0.8mm will be fitted to give 1mm DH. NOW, I measured the combustion chambers using petrol, they vary from 48 -51 cc I didnt think this was much until I looked at the valvle cut out top of piston which is 2cc, so this looks a big deal? ???? The heads have had the raised edge (where he gasket would have gone) machined off, there is a slight lip where its been undercut a little bit to much. I think im getting close if not close enough ? I ll wait for your opinions, small car 97/98 RON so, things look like this for static and dynamic. commenat as usual would be appreciated. thank you.
But if you use 48cc combustion chamber volume the static CR is 10.0:1. I’d increase the deck to get the static in the 9.3:1 range. Actually, I’d balance the chamber volume first, 3cc difference is a lot. Or is that what you intended to do?
I was hoping NOT to have to, machine/dig out material from the combustion chanbers.As you say 3 cc is a lot. I wouldnt know where to start thats safe, without penetrating air or exhaust spaces. Im going to remeasure to make sure I'm right. At a DH of 1.2mm 48cc combustion space volume, its static CR 9.575 dynamic CR 8.494
With 1.2mm deck, 48cc combustion chamber plus 2cc in the piston (50cc total volume) I make the static CR to be 9.8:1. To balance the combustion chambers, unshroud the valves out to the cylinder sealing face and/or remove material from both sides of the spark plug, but leave the chamber roof alone. Pic is my 1800 head opened up to 62cc.
As Westy.. each side of the spark plug there's a fair chunk to go at and smooth around get a few cc off that Sent from my SM-T580 using Tapatalk
arrrh i see now, you cut/grind away the sides of the combustion face towards the sealing face (of head to barrel) either side of the spark plug hole. dremil? stone or steel arbor? CR 9.757? should we be that far apart, are my figures correct?
A rotary cutter in a flexible drive or direct in a 240v drill will easily open up the combustion chamber. A Dremel will work but it might be a bit lightweight. If you only want one cutter this one, or similar, will do the job. https://www.pferd.com/products/mill...k-dia.-6-mm/shank-dia.-6-mm/hss-l-1225-6-z3/# I use the CR calculator on the JMR website http://johnmaherracing.com/calculators/engine-calculator/ and using your numbers - 96mm bore, 71mm stroke, 1.2mm deck and 50cc combustion chamber volume – the CR is 9.8:1. I’ve just tried the CB calculator, same result. http://cbperformance.com/v/enginecalc.html The calculators round up the CR so your 9.757 is close enough to 9.8. If you balance the chamber volume to 51cc so the total with the valve cut-outs is 53cc the CR is 9.3:1. I’d run that but you might be happier with a lower CR and you could increase the deck to 1.3mm – available shims will determine what deck you can achieve. I ended up with 1.6mm deck and a slightly lower CR than I wanted. Also bear in mind that the deck on one side of the engine may be different to the other – I have a thicker cylinder shim on one side to compensate so that the CR is the same on all cylinders.
So glad you explaind this in detail, In my head I was thinking of attacking the 51cc chamber to get it to 48cc duh OMG
Using Omni and 96mm bore, 71mm stroke, 1.2mm deck and 50cc combustion chamber volume – the CR is 9.757:1. The calculators are all saying the same thing, Omni is just more accurate. Open the smaller chambers up to 51cc and you’ll have 9.331:1 using Omni or 9.3:1 with JMR.
.. just in case its not been mentione I could only find cylinder shims in units of 'thou so .254mm steps in size.. that might effect your choice.. unless you find others.
This^ It’s what I meant earlier when I said ‘available shims will determine what deck you can achieve.’ CSP have a reasonable selection but there are still large gaps in sizes and inevitably it’s a compromise between available shim thickness, deck height, combustion chamber volume and compression ratio. https://www.csp-shop.com/en/engine/barrel-shims-101-385-400026-13679a.html
