I ran the calculations for piston displacement numerous times on several CR calc's and figured it to be ~ -11cc (-10.85 by my own math) But the eta block I have sitting in the garage definitely has flat tops in it. I am really curious about the eta dimensions. Piston Comp Height:35.7mm (Eta) 36.2mm S-Eta)ĬR: 8.8:1 (US 87-90) 9.7:1 (euro spec 85-89)Ĭan anyone fill in the blanks and correct any errors please? Please correct me if I'm wrong but this seemed a likely place to post this:ĬR:9.0:1 (82-87 Eta)200 casting, 8.5:1 (87-88 Super Eta) The latest Metric Mechanic catalog lists 143mm rods on their S38 stroker engine. They were going to do some marketing to their clients and it seems to have worked. The sales guy there helped me get 6 matched rods (as opposed to the 4 for Honda engines) and was very interested in the outcome. I worked with Scat for several months to make sure these were going to fit. I got these to go with my 4.0L S38 because it was actually cheaper than rebuilding the stock rods. The Scat rods are rated to something ridiculous like 500 HP per cylinder. There is no reason to buy new rods unless you're building a performance engine. They sell the same rods in 143mm for Hondas for about $160. The static and dynamic compression ratio calculator This calculator will calculate both Static and Dynamic compression ratio Compression Ratio is the ratio of an engines cylinder volume vs. Eagle, Carillo, Scat all sell 4340 H-beam BMW 144mm rods for about $1500. The 1mm difference in length doesn't matter since most of us get custom pistons for the S38 (especially if you're changing rods). I use them with Honda bearings and they fit without any problems. Honda rods are 143 mm and BMW rods are 144. Once the effective stroke length is known, the dynamic compression ratio may be calculated the same way as the static compression ratio.Honda rods? Ok, I'll bite, please tell me more, knowing you, there has to be a reason. The equations above show the expression for the loss of stroke, l loss, and the effective stroke length, l loss. As the intake closing angle increases, the value l loss increases to a maximum of the total stroke length. The cross in the red circle marks the crank centerline, and the cross in the blue circle marks the rod centerline. The figure below quantizes all values that relate to l loss. This can be done using basic trigonometry. Engine Compression Calculator Calculating compression ratio is EZ Use our compression ratio calculator to try piston specs and calculate compression ratio. To do so, we must find the vertical movement, l loss, the wrist pin makes from BDC to when the intake valve closes, which is the loss of stroke. The loss in compression from the open intake valve can be accounted for by finding an expression for the effective stroke, l stroke,eff. The intake closing event is given in degrees of crankshaft rotation, θ, after BDC. To calculate the dynamic compression, we must find out how far the piston has moved up the bore by the time the intake valve closes, sealing the cylinder. This is because the intake valve is open for some time while the piston is moving up the bore in the compression stroke. However, the uncompressed volumes are different. The compressed volume is the same in both static and dynamic compression.

The dynamic compression ratio is similar as it is the ratio of volumes. Calculating static compression is simply the ratio of the total volume in the cylinder at bottom dead center (BDC) to the compressed volume at top dead center (TDC).

Compression ratio calculator