Doof 11 Posted March 21, 2008 I'm trying to figure out the compression ratio for my engine but i'm struggling on a few of the dimensions. I'd like to understand a bit more about it rather than putting it together on peoples word saying "this will fit, that will fit" etc. Bore 87mm Stroke 88mm Deck Height = ???? (my piston spec sheet says 1.539 inches (39.0906mm) for comp height, anyone know what a standard piston is?) Head Gasket thickness = ??? whats standard? Piston top volume = 0 (flat top pistons but then presumably valve cut outs count for something? How do i measure them!? Combustion chamber volume = anthony thinks 40cc, can anyone confirm? What if i get the head skimmed? Finally, as I have to go for a custom gasket to work with the 87mm bore, i can alter the CR slightly. What should i be aiming for considering the spec? And who can supply a suitable MLS gasket in 87mm bore? Spec 2092cc S16 1.9 Mi injectors to increase fuelling. are these 214cc compared to about 196cc for S16 injectors? Petert stage 1 camshaft camshaft spec : Advertised Duration 260 Duration @ 0.050” 226 Maximum Lift 0.350" Lift @ TDC 0.065" Lobe Centre Angle 109 3 angle valve seats GTi6 inlet and standard management. I'm wanting to be able to use normal super unleaded but will use optimax where i can. Standard CR is 10:1 right? How much more can i, or should i go for? Thanks for any answers! Lewis Share this post Link to post Share on other sites
Doof 11 Posted March 21, 2008 Just spotted on my piston spec sheet the following, could be used to calculate the valve cut out volume? if i knew what TE meant... Pocket diameter 41.27500mm 34.92500mm Angle rotation 26 24 Depth from TE -5.9944mm -5.9944mm Share this post Link to post Share on other sites
Doof 11 Posted March 21, 2008 (edited) Just had a bit of a read of A Graham Bells book, Four stroke performance tuning which states the following: "A good rule of thumb for a fuel injected engine is to multiply the MON (motor octane number) by 0.118" Based on that, and 97 RON fuel (89MON) i get 10.5 as being ideal. Which is lower than i was originally aiming. Interestingly though he goes on to state that engines running very sophisticated engine management, and operating over a narrow power band with lots of testing can use a factor as high as 0.146 (this gives 13:1 on 97 RON fuel). Thats quite a large difference, and i wonder what his 0.118 engine actually was? I mean, i'd imagine the Mi16 to be on better management than some american V8's which are referenced alot in the book, so perhaps its ok to take a slightly higher value, if the race engine can be as high as 0.146? Thoughts? I'm thinking 10.8:1 or 11, but this is just guessing. Edited March 21, 2008 by Doof Share this post Link to post Share on other sites
B1ack_Mi16 67 Posted March 21, 2008 Std comp on S16 is 10.4:1. Your piston will sit 0.1mm above deck at TDC if you're keeping the standard length 152mm long rods. Compression will be 11.6:1, head volume 40cc and assumed 2cc valve pockets. This is with 88mm bore headgasket which is 1.3mm thick. Share this post Link to post Share on other sites
Doof 11 Posted March 22, 2008 Can i ask how you got to that deck height? Also, is it recommended to go for 1mm oversize on the head gasket? Thanks for the help. Share this post Link to post Share on other sites
B1ack_Mi16 67 Posted March 22, 2008 Can i ask how you got to that deck height? Also, is it recommended to go for 1mm oversize on the head gasket? Thanks for the help. 235 - 152 - 44 - 39.1 = -0,1 When the pistons sit above deck @TDC you should use a gasket that's at least a little bigger than the bore, 87.5mm would do it too, but 88 should be fine. Share this post Link to post Share on other sites
Doof 11 Posted March 22, 2008 Right, so 235 is the distance from crank centreline to the deck. Share this post Link to post Share on other sites
Doof 11 Posted March 22, 2008 Well, just measured the vulume of the valve cut outs and they total to 4.5cc...which brings my CR down to 11.1:1 If i can source a 1.5mm compressed head gasket then i'm at 10.85:1 which sounds better. I'm just worried that A Graham Bell states that its bad for power to have an overly high CR relying on the knock sensor to retard the timing. Share this post Link to post Share on other sites
B1ack_Mi16 67 Posted March 22, 2008 Well, just measured the vulume of the valve cut outs and they total to 4.5cc...which brings my CR down to 11.1:1 If i can source a 1.5mm compressed head gasket then i'm at 10.85:1 which sounds better. I'm just worried that A Graham Bell states that its bad for power to have an overly high CR relying on the knock sensor to retard the timing. Cometic can make gaskets in lots of different thicknesses. Share this post Link to post Share on other sites
James_R 3 Posted March 22, 2008 a std gasket clears 87mm pistons, as they're not exactly 86mm holes in them, it's only 88mm pistons you need a lardy dar headgasket for. The pistons were spec'd to be 11.5:1 on a GTI-6 head so with a s16 head and the bottom end the same it's about 11.2:1 Hope that helps Share this post Link to post Share on other sites
Doof 11 Posted March 27, 2008 Anyone know what a typical head skim would take off? I have no idea really and just want a rough guide. Thanks. Share this post Link to post Share on other sites
