First/Second Gen Pistons & 3rd Gen Rods
Author: SupraCharged007 from Sixthsphere Original Post: Located here
In light of my new engine build, it has become apparent that this information needs to be saved somewhere. I’m not the first person to document all this, but I want all of the information needed available to you guys for this project. It seems that the little information that is available out there is scattered and mostly incomplete. With that said, welcome to the 91-98 Piston / 99-02 Rod Combo Thread.
First and foremost, this has been done a few times ahead of me. I’m not the real pioneer here. From the information that I’ve gathered, there was a guy named Tom awhile back on TSN (TSN Archive) that did this. And most of us know Mike at SDA repeated on a build not too long ago. I guess this would be a good time to thank him for all his help and suggestions that he gave me along the way. I’m just trying to solidify what’s already been done.
Before you cut anything, there is some math to figure out with this. You will need a few measurements to get an accurate assumption of your compression ratio. I have done a lot of this measuring, but I would urge anyone doing this to do your own measurements. You never know when there will be some variance and I’d hate for you to waste your time and money because you just used my numbers.
Some measurements you will need:
Bore: Stock is 82mm(I’m using +1mm pistons, so my numbers reflect an 83mm bore.) Stroke: 90mm Cylinder Head Gasket Thickness(CRUSHED!): 1.27mm Cylinder Head Gasket Bore: 84mm(I’m not certain on this one, I really can’t remember) Cylinder Head Combustion Chamber Volume: 43cc (This is the one you really need to measure yourself. My head has been ported and polished. It’s a 3rd gen head for the record. I would assume your number to be pretty close, but for your safety I would get an accurate reading from your own head)
To get an accurate reading of the cylinder head combustion chamber volume, flip the head over with the valves, and spark plugs installed. Cut a piece of plexiglass at least a few inches larger than the combustion chambers and drill a hole in the center. Spread some vasoline around the combustion chamber and place the glass over top of it. Fill a buret(I used a syringe) with alcohol. Slowly add the alcohol into the head through the hole in the plexiglass until the chamber is filled. This might take a few tries so be prepared to stick with it. I found it easier to place the hole in the glass near the edge of the chamber to help with those pesky air pockets. *
Piston Protrusion: X This is the number we are solving for. Just placing the pistons on the rods, your protrusion will be .130”(3.302mm) That’s WAY too much. With a head gasket thickness of just .050”(1.27mm) it’s clear that Just for the engine to spin, you have to take off at least .080”. However for the sake of clearance and expansion, I wouldn’t recommend any more than .030” protrusion. Subtract .030” from the .130” we already have leaves you with cutting the piston .100”. On the other end of the spectrum, you can cut the piston too far. Mike at SDA cut his .120” which should have left .010” protrusion. For simplicity sake, I wouldn’t recommend cutting more than that. At that point the upper ring land was as thick as the stock 99-02 piston. I definitely wouldn’t recommend going past the deck, cause then your piston is going to start getting really thin. With that in mind, our variance for piston protrusion is .010” - .030”. That’s plenty to get a wide array of desired compression ratios. Also, I would avoid cutting the head if possible. If you have already cut the head, or you will need to, I would make sure to limit the amount of piston protrusion to .050” – what have you cut off the head. So if you already cut the head .040”, I wouldn’t go beyond .010” piston protrusion.
Ok, so now we have all the numbers we need. First thing we need to do is find our displacement, per cylinder.
- Displacement = (Bore/2) ^ 2 * Stroke * pi
Using my numbers from above, the equation looks like this.
D = 41.5^2 * 90 * pi = 486955 Divide by 1000 for Cubic Centimeters = 486.955cc
Next thing to figure is the volume of the head gasket, same formula, different numbers.
Volume(hg) = (Bore/2) ^ 2 * Thickness * pi Volume(hg) = 42 ^2 * 1.27 * pi = 7038 Divide by 1000 for Cubic Centimeters = 7.038cc
Since we know the piston is going to sit at least even with the deck, we don’t need to calculate any other values for our total combustion area. At this point, we can combine our head gasket volume and cylinder head combustion chamber volumes. 43cc + 7.038cc = 50.038cc Total Combustion Volume
If the pistons are cut flush with the deck(.130”) this would be your final combustion chamber volume. But since we will have the piston slightly above the deck, we need to subtract the amount of piston above the deck from this number to get our final answer.
You can do this next part a few different ways. I would suggest a trial and error approach since you probably don’t really know what final static compression ratios you will have at your disposal. Since we know we are working in a range of .010” - .030” piston protrusion, I would setup three equations here to find displacement of the part of the piston above the deck. The same displacement formula is used again.
- Displacement(piston) = (Bore/2) ^ 2 * protrusion * pi
.010”(.254mm) protrusion Displacement(piston) = 41.5 ^ 2 * .254 * pi = 1374 Divide by 1000 for Cubic Centimeters = 1.374cc Subtract from Total Combustion Chamber Volume Final Combustion Volume = 50.038cc – 1.374cc = 48.664cc
.020”(.508mm) protrusion Displacement(piston) = 41.5 ^ 2 * .508 * pi = 2749 Divide by 1000 for Cubic Centimeters = 2.749cc Subtract from Total Combustion Chamber Volume Final Combustion Volume = 50.038cc – 2.749cc = 47.289cc
.030”(.762mm) protrusion Displacement(piston) = 41.5 ^ 2 * .762 * pi = 4123 Divide by 1000 for Cubic Centimeters = 4.123cc Subtract from Total Combustion Chamber Volume Final Combustion Volume = 50.038cc – 4.123cc = 45.915cc
Now we just need to plug these numbers in to the Static Compression Ratio formula
CR = (Displacement + Combustion Volume) / Combustion Volume CR(.000” Protrusion) = (486.955cc + 50.038cc) / 50.038cc = 10.73:1 CR CR(.010” Protrusion) = (486.955cc + 48.664cc) / 48.664cc = 11.01:1 CR CR(.020” Protrusion) = (486.955cc + 47.289cc) / 48.664cc = 11.30:1 CR CR(.030” Protrusion) = (486.955cc + 45.915cc) / 48.664cc = 11.61:1 CR
Since these results are linear, you can pretty easily find any compression number you want between your high and low piston protrusion numbers. Here’s a link to an Excel file that will quickly and easily calculate all of these numbers for you. All you have to do is provide the correct measurements.
A very special thanks goes out to TimT who executes on all of my crazy ideas perfectly, aka thanks for cutting the pistons for me Tim