Reher-Morrison Racing Engines' David Reher and Darin Morgan answer your questions about cylinder heads or anything else concerning your racing engine building problems. Remember who you're are dealing with so no soft-ball questions.
7/7/06
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Neil,
Lightening the piston will produce more accelerative power just so long as you do not sacrifice the structural integrity of the piston in the process. Taking 100 grams from each piston (800 grams total) is like taking a piston out of the engine. Think how much more power an engine could make if it had to accelerate one less piston up and down the cylinder going down the race track! Any time you take weight from the reciprocating components you gain accelerative power but there is a point of diminishing returns. First off, let’s look at static verse accelerative power. Envision two identical engines. One has twice the reciprocating mass than the other. The engine with the lightest reciprocating weight will make more power when accelerating down the race track and will accelerate quicker off each gear change. However, If both engines are held at a static RPM, the difference in power will be negligible. The faster an engine accelerates, the more a lighter reciprocating weight shows up in your ET slip. There are limits as to how light you can go before ugly things start to happen. It is possible to lighten the piston to the point where it distorts under dynamic loading. This distortion of the pistons shape ultimately affects the compression rings ability to seal against the cylinder wall and seal itself in the ring groove. This condition lets cylinder pressure bleed by the ring and a great deal of power can be lost. Be careful when lightening pistons and don’t go overboard. If in doubt, let a professional perform the task. Companies like CP Pistons have had decades of experience with a multitude of different piston designs for various forms of motor sports. If you order your pistons through a major piston manufacture such as CP Pistons then you’re in luck because they can lighten them for you. Arbitrarily taking out metal for the sole purpose of saving weight while at the same time disregarding the over all design and structural integrity of the piston is a big, big mistake. Be careful and if you are in doubt, consult a professional.
Now let’s look at raising the ring .100. If you have a piston with plenty of material to support raising the ring and by raising the ring you will not protrude or get within .035 of the intake valve relief, Then buy all means do so. Just be aware of the fact that when you optimize a piston for a particular camshaft by pushing the ring location up as high as possible, you can’t go with a bigger cam in the future without replacing your pistons. When everything is as tight as possible an increase in duration of as little as 2° on a cam lobe can throw things out of kilter. Now for the reasons one should raise the ring as high as possible. We all know it makes more power but why? Well, there are two reasons. The first reason is that the volume in the top land is reduced making the ring react and seal the cylinder faster against rising cylinder pressure on the power stroke. Just a few milliseconds of reaction time can bleed cylinder pressure and decrease power. This is the same reason why gas ports work so well. The second and least important reason is the over all volume of air fuel mixture in the land area is reduced. The air fuel mixture in the top land is burned but burned to latent to help create cylinder pressure. By raising the ring we force more air fuel mixture into the combustion chamber to be burned during the power stroke. Although the actual amount is infinitesimally small, it is still enough to produce a small increase in power.
David Reher asked Darin Morgan to answer this question because it is his area of expertise.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147
Visit our web site at http://www.rehermorrison.com
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