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DISH DOMED FLAT
CHOOSING A COMPRESSION RATIO:
FOR HELP IN CHOOSING THE CORRECT PISTON OR TO PLACE AN ORDER CALL (205) 823-7278
In these days of low quality gasoline, the correct compression ratio for your engine is crucial. A ratio that is too high will cause detonation and possible damage to the engine. Conversely, a ratio that is too low will result in poor efficiency, low gas mileage, and anemic performance. Most late model engines fall into the "too low" category because only low compression engines have been able to meet the nitrogen oxide emission tests. A boost of one or two points in compression will make a tremendous difference in both the performance and economy of these engines.
Matching Pistons & Cams:
One must also keep in mind that the "smog" engines (produced from 1971 to present) have been produced with extremely conservative camshafts in order to minimize the undesirable effects of the lowered compression ratios. If more than a one point increase in compression is made to these engines, then the camshaft should be changed also. Leaving the "smog" cam installed will not allow full performance and may the cause the engine to "ping" because of the conservative valve timing.
As a rough rule of thumb, street engines do well with a compression ratio of 9 or 10:1 and a Stage 1 camshaft. Slalom, autocross, and really hot street engines require 11:1 compression and a Stage 2 camshaft. Racing engines require 12 and up compression and Stage 3 or 4 camshafts.
Making Custom Pistons:
Contrary to popular opinion, it really doesn't cost any more to make up a set of custom pistons than it does to make up listed pistons. However, it is a little more troublesome, and we will need help from you on this in the form of three items:
1. An old stock piston . 2. A mold of a combustion chamber (or two molds if the engine uses mirror image chambers) 3. The cc volume of the combustion chamber.
If you don't know how to make the mold or measure the cc volume, call us and we will walk you through it. If you prefer that we make the mold and cc the chamber, ship the head to us and we will do it for you. There is no extra charge for this.
A Common Error:
If you have an overhead cam engine (Datsun, Jaguar, Alfa, Honda, Mitsubishi, Toyota, Porsche, etc) DON'T try to raise the compression ratio by milling the head. The reason is that the cam is driven by a chain or belt connected to the crankshaft. When you mill material from the head, you decrease the distance between the crank and cam gears, which throws the valve timing off. What you gain by the increased compression is lost many times over by the changed valve timing.
Some pushrod engine heads can be milled to raise the compression ratio a moderate amount. The rule of thumb is that you raise the compression approx 1/10th of a point for each 0.010" milled off. Since many heads won't take more than .050" milling, the most you can usually expect to gain from this technique is 1/2 point increase in compression. The only reliable way to substantially increase compression is to install high compression pistons.
Just a word here about wristpins. The strongest pistons are no good if the pins connecting them to the rods are weak and break at high revs. Chrome-Moly pins are very strong but they are not much lighter than the stock steel pins. Titanium pins are both very strong and very light, allowing more power at higher revs with greater reliability. They are ideal for all-out racing engines.
CHROME-MOLY PINS -------- $40.00 EACH TITANIUM PINS ---------- $150.00 EACH
Our piston sets normally come with circlips that fit into grooves at the ends of the pin bores to keep the pins in place. For racing applications, teflon buttons are used instead. They eliminate the possibility of a circlip coming loose at high RPM and damaging the cylinder wall, and are self-lubricating.
TEFLON BUTTONS ---- $10.00 EACH (These are made to order)