Chris Jones Posted September 19, 2002 Share Posted September 19, 2002 How is the CR ratio derived. I know what it is, but wondered how you calculate it from the engines dimensions. I'll relate this to the k series as that is my application. 😬 I'd like to know what is necessary to reduce the CR from 10.4 ish down to 9.2 ish. Can this jsut be done with shorter rods or would I need dished pistons too? There must be limitations where the rods would be too short or the pistons too weak. Any idea where these might be?  And what would the implation on the torque to weight ratio be... just kidding  Cheers Chris Link to comment Share on other sites More sharing options...
Highlander Posted September 19, 2002 Share Posted September 19, 2002 Someone with a sense of humour! Excellent. As I understand it, the CR is the ratio between max and min cylinder volume. Seem to remember someone suggesting using a thicker head gasket to reduce the CR, as it'll increase, (slightly) the minimum cylinder volume. As for effect check this out here Might be useful. as to torque/weight, I couldn't possibly comment. Link to comment Share on other sites More sharing options...
Joe 90 Posted September 19, 2002 Share Posted September 19, 2002 The difficult bit is determining the combustion chamber volume, i.e. the space above the piston at TDC. This is traditionally done by adding together the volume in the head, the volume in the block (recesses in the piston) and the head gasket volume. The volumes are determined by measurement, as the complicated shapes make calculation too error prone. If you are absolutely sure about your existing compression ratio you can deduce the extra volume needed, by the following calculation. 1. Swept volume per cylinder is ccs / number of cyinders, e.g. assuming 1600cc engine 1600 / 4 = 400cc 2. Combustion chamber volume = Swept volume / ( CR - 1) 400 / (10.4 - 1) = 42.55 cc now 400 / (9.2 - 1) = 48.78 cc desired 3. Increase in combustion chamber is 48.78 - 42.55 = 6.23 cc It's best to remove material from the head or the piston, because that way you preserve squish, but that may not be possible on a modern engine. Replacement dished pistons would be better than short rods.  99,000 miles so far Link to comment Share on other sites More sharing options...
oilyhands Posted September 19, 2002 Share Posted September 19, 2002 You'll have fun lowering the CR on the K... you can excavate some material from the combustion chambers, but machining the pistons tops is bad news , the pistons are a weak spot, you can shorten the existing rods by 2mm or so but that will up the rod angle which is bad news. QED sell a lower CR piston/rod combination for blown engines. Swopping rods between engines of different sizes doesnt work because the 1.6 and 1.8 have different journal sizes, a decompression plate is a recipe for disaster on the 'K'. Turbo Technics do a shorter rod and hypereutectic piston combo that gets the CR down to about 8.4. Oily Link to comment Share on other sites More sharing options...
Mole Posted September 19, 2002 Share Posted September 19, 2002 Sorry if this is a dumb question but what is the benefit of reducing the compression ratio 🤔 SV 52 CAT - the Mole is flying Link to comment Share on other sites More sharing options...
Chris Jones Posted September 19, 2002 Author Share Posted September 19, 2002 One thing we have looked at, but have little information on is the new turbo k series. Has a compression ratio of 9.2 apparently. With luck the parts for this (on the off chance they're available), should be more reasonably priced (if ultimately not as strong) than the QED items. I'm just trying to ascertain all the factors and limitation involved so we can consider and evavluate as many solutions as possible (without spending any cash!). How much material is "available" in the combustion chamber? Significant or not? Cheers Chris Link to comment Share on other sites More sharing options...
AMMO Posted September 19, 2002 Share Posted September 19, 2002 Chris's mate is planning on supercharging a K. How did he get on with that 'phone number I gave you for the Opcon Autorotor guy? I can have pistons made in small batches relatively quickly if you draw a blank down other avenues. AMMO Link to comment Share on other sites More sharing options...
scooby dooby doo Posted September 19, 2002 Share Posted September 19, 2002 on this topic.. my head has just been skimmed by 8 thou as it was slightly warped. what effect is this likely to have 🤔 ie do i desparately need a remapping, is it merely a good idea, or will i not notice 🤔 8 thou is 8/1000*25.4 = 0.2mm ....a few calculations later.... assuming 10 to 1 compression and therefore a 40cc combustion chamber... so my compression ratio has gone up to about 10.06 should i care? or should i just be extra sure to run on super unleaded even though normal unleaded used to be fine 🤔 HOOPY R706KGU See you at Cadwell Link to comment Share on other sites More sharing options...
Chris Jones Posted September 19, 2002 Author Share Posted September 19, 2002 Ammo, He's been just a bit busy putting his car back together since the weekend! I'm sure he'll be in contact soon though! I'll let him know about the pistons. Cheers Chris ps. Any progress on the head?? *smile* Link to comment Share on other sites More sharing options...
oilyhands Posted September 19, 2002 Share Posted September 19, 2002 I have a K head here that has already had one chamber excavated for a superchared application, it dropped the CR by over one point. Oily Link to comment Share on other sites More sharing options...
Chris Jones Posted September 19, 2002 Author Share Posted September 19, 2002 So that would equate to about 2 points by the time Lorne has finished with it then  Can you take a piccy of said chamber with a standard one next to it when (if!) you get a moment? Many thanks Chris Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now