Compression Ratio's Type 4

Discussion in 'Mech Tech' started by Deefer66, Nov 24, 2019.

  1. Back, bedroom decor done, wardrobe doors fitted, back to more important things.

    I now have 4 heads with combustion space volumess of 54cc each, with no metal removal needed. Its about doing the measurmnets accurately first time and getting the maths done correctly DUH . The piston cut out for valve clearances are 2.5 cc each, total combustion volume 56.5cc. /cyl.

    CR static is now 9:1 (dyn 7.9) with deck at 1mm. By just reducing the deck height to 0.75mm (0.010") CR rises to 9.2:1 (DYN 8.102) 0.010" across 96mm = 0.2 increase in CR, this amazed me. is this correct?

    I have also noticed the face where the cylinder mates with the head is anywhere from 0.001"-0.oo5" undercut, see pics. Would it be prudent to have these heads fly-cut to even up the face alignment and also adjust the CR? I was thinking just to clean the casting marks proud of the cyl/head mating faces. I would estimate if, 0.010"x 96mm deck height reduction = 0 .2 CR increase how much should I have flycut? I understand flycutting the casting marks will reduce the combustion volume AND maching the mating faces of cyl to head will also reduce the volume.

    I hope this makes sense.
    IMG_4160.jpg

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  2. 77 Westy

    77 Westy Supporter

    Measure twice (or more) and cut once.;)
    That is correct. 9.0:1 will work with a C35 although it is on the low side but I wouldn’t go less than 1mm deck on a road engine – you don’t want the bits that are flying around to hit the bits that aren’t moving. If you want to increase the CR you could machine the piston crowns to reduce the volume of the valve cut outs and reduce the thickness of the cylinder spacers to keep the deck at 1mm.
    I’d only flycut the heads to clean up the sealing face and remove the step but if you don’t have cylinder spacers you could flycut them to reduce the combustion chamber volume – or reduce the cylinder length of course.

    Are your measurements with no cylinder base shims and no head gaskets, or spacers or whatever it is snotty calls them?
     
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  3. Zed

    Zed Gradually getting grumpier

    If you are going to fly cut to clean up the mating surface you might as well take a bit more off to increase the CR? Very difficult to pre-judge how much effect that would have on the combustion chamber volume, but I would not go too mad here. Remember taking off the raised part within the cylinder dia to make it flat will actually lower the CR so there's that to take into account too.

    On the subject of combustion chamber cc and deck height, that groove where the cylinder sits must be making a nonsense of your calculations unless you've measured it's depth and taken it into account, so yes, I'd get them skimmed flat as a minimum if only to make your calculations more reliable.
     
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  4. 77 Westy

    77 Westy Supporter

    But it would only be as flat as the sealing face so removing the step and cleaning up the squish face reduces the chamber volume and increases the CR.
    EDIT: Wait, he said undercut so you’re right. But there shouldn’t be an undercut, the cylinder sealing face and the squish faces should be flat.
     
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  5. Zed

    Zed Gradually getting grumpier

    Quite confusing how it is, particular when you consider what a big difference 0.2mm makes to the CR Bearing that in mind, if nothing else, getting the heads skimmed flat would be a good start point for more realistic calculation? If it were me, at this point I'd actually just guess and have 0.5-0.75 mm skimmed off then start fresh with the calculations!
     
  6. 77 Westy

    77 Westy Supporter

    Yes, the deck measurement is really 1mm less 0.001"- 0.005” which will change the CR. And with that sort of difference there isn’t much chance of the head sealing.

    Having a closer look at the pics the heads appear to have an undercut, possibly caused by excessive lapping, and then been grit blasted. I agree machining them flat would be a good start point and reducing the chamber volume would help to increase the CR.
     
    Zed likes this.
  7. Great response from everybody, thank you for your time and sharing your knowledge and experience.

    What is the reference point (if any) to ensure each head is machined to the same depth and is parallel to the cylinder face?


    77Westy That is correct. 9.0:1 will work with a C35 although it is on the low side but I wouldn’t go less than 1mm deck on a road engine – you don’t want the bits that are flying around to hit the bits that aren’t moving. If you want to increase the CR you could machine the piston crowns to reduce the volume of the valve cut outs and reduce the thickness of the cylinder spacers to keep the deck at 1mm.

    All measurements taken without shims or head gaskets, so no shims to reduce if pistons were machined, good point I hadnt thought of that. Current deck height 0.24mm.

    So the only option is to flycut the heads and shim to achieve 1mm deck height?

    "Zed If you are going to fly cut to clean up the mating surface you might as well take a bit more off to increase the CR? Very difficult to pre-judge how much effect that would have on the combustion chamber volume, but I would not go too mad here. Remember taking off the raised part within the cylinder dia to make it flat will actually lower the CR so there's that to take into account too.

    On the subject of combustion chamber cc and deck height, that groove where the cylinder sits must be making a nonsense of your calculations unless you've measured it's depth and taken it into account, so yes, I'd get them skimmed flat as a minimum if only to make your calculations more reliable. I did not take the under cut into consideration DUH. I have since measured the cc's @0.005" deep =0.252cc @0.003" deep its 0.147cc, not a lot but still needs to go.

    I did some rough maths regarding how deep to fly cut.

    I attached 1mm solder wire around the circumference of the lip of the combustion chamber, removed it and measured it = 258mm. This was now the circumference of a circle, from which I calculated the area of the combustion chamber withing a 96mm diameter circle and calculate the area of the cast face.

    Using the CR calculator, I needed a combustion volume of 52cc to give me 9.3:1. So 2cc's needed to be removed across a diameter of 105mm.

    Results: I need to remove material to a depth of 0.0145"

    Now previously, reducing the deck height by 0.o1o" equates to a rise of 0.2 CR, so 0.0145" equates to 0.3 increase. Rough I know.

    Please feel free to comment...................................

     
  8. Zed

    Zed Gradually getting grumpier

    Honestly, though I am a bit of a tit, I would work back using a calculator to see what the ideal combustion chamber cc's would be with a sensible deck, compare to what you have and guestimate from that how much to skim to get in the ball park. Even using some squared/graph paper to make a rubbing of the combustion chamber and counting the squares would get you close to the area that a skim would effect - you should then be able to work out depth of skim for 1cc and take it from there? I don't think there is an easier way other than trusting your machinist to keep taking a bit off and measuring the chamber cc's as they go. A good one that actually specialises in machining these air cooled VW heads may have the experience and numbers at hand to do this without too much bother - it is a common necessity but maybe not every machinist out there is going to be able to do a good job of it.

    As for ref points, the only bit you can trust is the sealing surface, the rest is cast. Your man will clamp it down, level it up and do them both to the same depth. This should be pretty accurate even though it sounds a bit trial and error. Here's mine. :eek: You can't go tooooo deep or the top fin on the barrels will hit the bottom fin on the head. You might want to mock that up, they get close even as stock. On the pictured one, that fin got milled down too, but it's not recommended, there is a lot of strength on that fin.
    IMG_20151110_153926079_zpsm2oymvvy.jpg
     
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  9. 77 Westy

    77 Westy Supporter

    A decent engineering workshop will set up the heads to machine both chambers to be flat and the same height relative to each other – the machined area outboard of the cylinder sealing face is probably as good a reference point as any.
    If the current deck is 0.24mm you either need to machine 0.76mm off the pistons or fit 0.76mm cylinder spacers (at the crankcase). Or you could machine less off the pistons and use a thinner cylinder shim but one way or another you should increase the deck to 1mm. And bear in mind the limited choice of available shim thicknesses.
    You probably should machine more than 0.0145” off the heads. Theory is one thing but what is achievable in practice is another. A machine shop might not be as accurate as you’d like and shims that you need might not be available. I had to compromise both CR and deck height because I couldn’t get the shim thickness I needed.

    Incidentally, are you measuring both sides of the engine? They may not be the same and what would be perfect on one side could be a bit off on the other. Even VW didn’t always get the crankshaft centre line exactly on the crankcase centre line. And if the crankcase has been machined it will almost certainly not be the same on both sides.
     
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  10. This is statement is wrong. "reducing the deck height by 0.o1o" equates to a rise of 0.2 CR, so 0.0145" equates to 0.3 increase" Deck height equates to 96mm piston dia, the 0.0145" equates to 105mm dia across the inside of the head. My head hurts.


    "Zed,"]Honestly, though I am a bit of a tit, I would work back using a calculator to see what the ideal combustion chamber cc's would be with a sensible deck, compare to what you have and guestimate from that how much to skim to get in the ball park. Even using some squared/graph paper to make a rubbing of the combustion chamber and counting the squares would get you close to the area that a skim would effect - you should then be able to work out depth of skim for 1cc and take it from there? I don't think there is an easier way other than trusting your machinist to keep taking a bit off and measuring the chamber cc's as they go. A good one that actually specialises in machining these air cooled VW heads may have the experience and numbers at hand to do this without too much bother - it is a common necessity but maybe not every machinist out there is going to be able to do a good job of it.
    I used 3 differnt CR calculators, and reversed back using 9.3 CR Static) to arrive at a combusiton chamber volume of 52cc's. Even then, there was a descrepancy of about 0.5cc. Dont think I'm smart enough to work back from first principles, I really struggled with yesterdays simple maths.

    As for ref points, the only bit you can trust is the sealing surface, the rest is cast. Your man will clamp it down, level it up and do them both to the same depth. This should be pretty accurate even though it sounds a bit trial and error.

    When I measured the volumes, I rested the head on the face for the rocker cover gasket and it was very close, until you have to move it to release the trapped air bubble. Would this be a good datum to work from? With respect to your comment re the reputable VWAC machinist I'd hope he would know his stuff.

    Here's mine. :eek: You can't go tooooo deep or the top fin on the barrels will hit the bottom fin on the head. You might want to mock that up, they get close even as stock. On the pictured one, that fin got milled down too, but it's not recommended, there is a lot of strength on that fin. Point noted, I'll check this out before machining, any idea how much of a gap there is stock?

    77 Westy, post: 1885389, member: 491"]A decent engineering workshop will set up the heads to machine both chambers to be flat and the same height relative to each other – the machined area outboard of the cylinder sealing face is probably as good a reference point as any. see above re rocker cover gaskets?

    If the current deck is 0.24mm you either need to machine 0.76mm off the pistons or fit 0.76mm cylinder spacers (at the crankcase). Or you could machine less off the pistons and use a thinner cylinder shim but one way or another you should increase the deck to 1mm. And bear in mind the limited choice of available shim thicknesses.
    You probably should machine more than 0.0145” off the heads. Theory is one thing but what is achievable in practice is another. A machine shop might not be as accurate as you’d like and shims that you need might not be available. I had to compromise both CR and deck height because I couldn’t get the shim thickness I needed. This will need careful thought, considering/juggling the available shim thicknesses with respect deck height required.

    Incidentally, are you measuring both sides of the engine? They may not be the same and what would be perfect on one side could be a bit off on the other. Even VW didn’t always get the crankshaft centre line exactly on the crankcase centre line. And if the crankcase has been machined it will almost certainly not be the same on both sides. I have had the block decked, all barrel lenghts are the same and the deck heights work out ok, please see attached.

    The way forward:

    Understand the method to achieve 1mm deck height with available shims/ machining piston crowns. Take into account reduction of valve recesses if machining is required.

    Source a sympathetic VW machinist to flycut the heads, and equate quantity of material to remove to achieve 52cc cylinder head combustion volume.


    I have heard of Aircooled Hut UK, any comments https://www.aircooledhut.co.uk/ ? I live in the Midlands, Leicestershire.


    Thank you for your time and for sharing your valuable knowledge and experience.

    Comments always welcome.

    IMG_4174.jpg
     
  11. Zed

    Zed Gradually getting grumpier

    "Less than you might think", it will be the smallest gap between any two fins. You need to see for yourself. Probably not a problem but best to be aware before you make it one.
     
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  12. 77 Westy

    77 Westy Supporter

    I don’t know what stock is but 2.3mm/0.090” is generally considered to be the minimum for good airflow. This is mine.

    IMG_6559 (Medium).JPG
     
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  13. 77 Westy

    77 Westy Supporter

    I’d search for a good engineering workshop rather than an AC specialist but you’ve had the crankcase decked so you know where to take the heads. The rocker cover face may be a good datum but I’d probably use the machined land outboard of the cylinder sealing face. The one Zed had machined away for the big cylinders but you can see it in his pic around the unmachined combustion chamber.

    Don’t get too hung up with a 1mm deck. There are those that are happy to run less and many that have far more. I aim for 1mm to 1.6mm – 0.040” to 0.062”.
     
    Last edited: Apr 25, 2022
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  14. So, the heads are at Chesmans in Coventry to have the heads cleaned up to give 52cc, I estimate 0.0010"-0.0015"

    Now, with 52cc combustiion volume 1mm deck and 2.5cc pistion volume the CR is static 9.34, dyn, 8.12.

    The current Deck ranges from 0.2mm 0.25, 0r 0.008" - 0.0010"

    To achieve this do I,

    A) machine the piston crowns by 0.8mm -0.003", thereby achieving the 1mm deck and increasing the CR to static 9.39 and Dyn 8.18. Ive asummed 0.8mm will reduce CC approx from 2.5 to 2cc. Is the moderately higher CR more benificial?

    or

    B) Fit a shim of 0.8mm....but I cant find any 0.8mm shims. The nearest will be 0.75mm or 0.9mm giving a Deck of 0.95min - 1mmmax or 1.1mm - 1.15mm

    1.15mm deck = 9.18 8.0Dyn

    0.95mm deck = 9.37 8.16Dyn

    Thank you for your patience.

    John
     
  15. 77 Westy

    77 Westy Supporter

    Option A is the better solution but shims are cheaper than machining and I’d probably fit a 0.9mm shim. The C35 would work better with the higher CR but you’d only notice the difference on an engine dyno. If you were racing the difference would matter, for a road car it doesn’t.

    However, don’t machine the pistons or buy any shims until you have the heads machined and you can trial fit to check the dimensions. And the combustion chambers might need a tweak so they are equal volume. Also, the shims advertised size might not be the actual size.
     
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  16. Back. Head faces machined to 52cc, piston crowns machined 0.9mm. Deck clearance now 1.1mm. Static CR 9.4, Dynamic 8.2. Head on and all torqued up.

    Im now researching rocker geometry. When I rotate the engine with the cam follower push rod and rocker gear in place it will wipe the running in grease of the cam, can I avoid this?

    Im going to attempt this using the bullseye bubble spirit level method, incorporating a DTI to confirm the mid stroke of the rocker.

    When I have the final results can I assume any changes required could be applied to the other rockers without the need to set up and check.

    ANY advice, comments, storys of experience etc would , as usual, be much apprecited.
     
  17. Zed

    Zed Gradually getting grumpier

    You want the adjuster centre line in line with the valve centre line at half lift. That's it. If it's not far off I wouldn't worry unless you fitted a cam with a daft amount of lift.
    Most people would mock it up before the final build, but you could at least remove the pushrods except your adjustable one and, if you had one, fit a weak spring.
     
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  18. 77 Westy

    77 Westy Supporter

    It’s easy enough to pull the followers and reapply the assembly lube before final assembly and I crank the engine without the rockers to build up oil pressure on a dry engine.

    I’ve never used the bubble method to set the geometry but it’s as good as any other way. I check every rocker and measure the ideal pushrod length, then average the results and cut all the rods to the same length; others cut each rod to the ideal length.
     
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  19. This is wot i dun... not saying its right of course.. but as above mid stem at mid sweep of rocker at mid lift, bullseye levels were ok getting the dial guage to stay on the valve somewhere not in the way was the hard bit..

    Dont think youll wipe any more grease off than first turn of key at start up anyway




    Sent from my SM-T580 using Tapatalk
     
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  20. As usual lots of good feed back thank you.

    It looks like ive put the cart before the horse, the heads are all built up, stiffer springs, but no rocker gear. My enthusiasm got the better of me and I fitted the push rod tubes but they can easily be removed.

    Zed
    You want the adjuster centre line in line with the valve centre line at half lift. That's it. If it's not far off I wouldn't worry unless you fitted a cam with a daft amount of lift.
    Most people would mock it up before the final build, but you could at least remove the pushrods except your adjustable one and, if you had one, fit a weak spring. C35 cam 0.381" lift. as well as side view do you also include looking down from the top at the valve stem (as if the engine is fitted in the car), Im aware some acceptable off set fro this angle exists to turn the valve when running?


    77westy, I check every rocker and measure the ideal pushrod length, then average the results and cut all the rods to the same length; others cut each rod to the ideal length. Do you use the same size shim all round? Is there a method to calculate the required shim size?


    Deefer66 This is wot i dun... not saying its right of course.. but as above mid stem at mid sweep of rocker at mid lift, bullseye levels were ok getting the dial guage to stay on the valve somewhere not in the way was the hard bit.. Dont think youll wipe any more grease off than first turn of key at start up anyway Great photos, I see so much more from a photo rather than reading. Did you consider fitting a solid rocker shaft, I have wavey washers and I'm seriously considering going solid. Elephants feet for adjusters? never thought about the first turn of the key, good point.

    Further questions.

    I have aluminium push rods, I also have a set of steel rods from a hydraulic tappet engine. Should I consider steel depending on length requirements?
    Solid rocker shafts appear to have a strong following when using stronger springs and higher lift cams, any thoughts?
    Uprated adjusters elephants feet, (porsche 911)?

    Thanks again John


     

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