I've had some fun measuring deck height for the first time today and want to check the compression ratio with this top end rebuild I'm doing. It's like doing maths homework with my daughter. I thought I was gonna spend the day putting on the new p&b's and heads but have just been measuring and calculating instead! Should I do the calculation for each of the cylinders or are they all going to be about the same? The deck heights were 1.3, 1.2, 1.2 & 1.4 (mm). A little low maybe but ok? I've got the Cylinder displacement figure. I'm just waiting for a cc measuring tool that I've hired from CoolAir to arrive so I can check the cc in the heads and then calculate the ratio. Will the stroke be the same in each cylinder? It was 68.8mm in cylinder 1. If so that means the Cylinder replacement calculation will be the same for each cylinder. Deck height was marginally different and I presume the cc in the head will be the same for each chamber? So what Compression Ratio am I looking for? It's a stock set of 1600 p&b's and stock classic line twin port heads. I've seen 51cc as a stock measurement which would give me 7.8/1 ,7.8/1, 7.8/1 & 7.7/1. How's them figures? No one I know wants to talk engines any more.
http://johnmaherracing.com/calculators/engine-calculator/ Have a look on here for the CR calcs. For the deck height you have a minimum of 1.0mm and a max of 2.0mm, you want to be aiming for around 1.5mm approx. Our type 4 is 1.52mm with 58cc per cylinder, they are all spot on, we took the time to do all four pots. We have a nice healthy 8.1:1 compression ratio. Tom Wilson gives all of the specs in his book, its well worth having on the bookshelf!
Are you measuring deck height and stroke on the gudgeon pin axis to minimise dimension discrepancies due to piston rock? Stroke (standard 1600 type 1 is 69mm), deck height and combustion chamber volumes ‘should’ be the same on each cylinder or as close as you can get so each cylinder does the same amount of work. 7.8:1 CR is okay with a standard cam.
Yes thanks. The Tom Wilson book is my bible at the minute but I'm not sure it actually deals with compression ratio. It just focusses on deck height as far as I can see. Because mine was low I called to order some shims but was persuaded to measure cc and work out the CR. That calculator is going to save some time and margin for error!
Its well worth the time and effort in getting each cylinder as close to one another as possible, we ended up with a healthy 96bhp at the flywheel, and a nice driveable motor!
Yes I used a vernier caliper depth gauge at the centre of one of those deck height tools. TDC by eye which I hope I think/hope I got right.
We didnt need to shim our, straight out of the box we got 1.52mm - which was great. You can use shims if necessary, the heads too, they came straight out of the box and didnt need gaskets as they had the little step machined in. We have got Verniers (both calipers and depth gauges) - with the deck height we too a series of measurements all round to check for consistency across the piston top
Check the cylinder lengths are the same on all four, either have them machined to the shortest one or adjust with shim thickness but I wouldn’t add shims unless absolutely necessary. The deck height you have doesn’t need increasing (I set mine to 1.0mm (on a type 4)). If you add shims squish is reduced and CR is lowered, both decrease engine efficiency and it’s so poor anyway you don’t want to make it worse. Your calculated CR is okay but check combustion chamber volumes to make sure they are equal. A degree or so either side of TDC won’t make a significant difference to the dimension but I questioned how you measured the stroke because it should be 69mm.
I used the vernier caliper depth pin that sticks out the bottom. Measurements were 82mm fully in and 13.2 fully out (TDC). Take away 12mm as I had the deck height tool with that width attached which I measured from. So 70mm and 1.2mm (deck height) and so 68.8mm. I'm not sure I'm right, maybe I misread it and it was 69mm, I'll check again when I get back in there.
I hope the vernier caliper was either made out of metal with engraved scales or cost a good deal. Because the £10-£20 ones are probably less accurate than a 0.5mm marked steel ruler, featuring backlash and random electronic offsets. I think my £12 Trago Mills special can see and is upset by fingerprints on the plastic cover on the scale, despite the fact the active part of the scale is a printed circuit board under the plastic bit.
I'd do it again using a steel rule in the gudgeon pin direction, and feeler gauges. It will vary across cylinders. You'll also need to "cc" your heads to find the combustion chamber volumes, otherwise you can't calculate CR. You could fiddle and trim for ever, but I doubt whether it's worth it on a stock bus. As I recall, variation in stock type 1 heads was 2cc (50 - 52cc), so it could get a bit obsessive.
I'm guessing a lot of your inaccuracies are the way your measuring it . Ideally you need a dti and a depth micrometer Digital calipers even decent ones are not hugely accurate or easy to use like your doing ...
Your not quite right on the £20 ones Mike. We have a cheapo one at work, it's as good as a £300.00 Mitutoyo one and they both pass UKAS calibration down to class 0.5 year in, year out. It's all down to looking after them, not abusing them and trusting them with pride. I'll look what make it is in the morning & report back
OK I cc'd the heads and they're all 55cc. That's 4cc more than I expected but I think it will be alright because my deck height is low. Cylinder Deck Height Stroke CR Cylinder 1 1.1mm 68.9mm 7.5 Cylinder 2 1.2mm 68.8mm 7.4 Cylinder 3 1.1mm 69mm 7.5 Cylinder 4 1.2mm 69mm 7.4 After the discussion about accuracy I remeasured everything. It may be that I am one or two tenths of a millimetre out on the DH & stroke but it doesn't make much (if any) difference when I change the CR calculation by that amount. I'm thinking 7.4 & 7.5 are fine and I can just go ahead and stick it all together?
Thanks 77 Westy that gives me some confidence. Also, in terms of accuracy, using the vernier caliper was quite a bit more accurate I think than using the twisting bolt method which the tool provides and which I was going to use for a while!
Pea sized bit of car filler on top of the piston and squash it when the piston is at the top - 10 mins - gives you a nice penny sized disc to vernier. Oil surfaces to stop it sticking.
