![]() ![]() There is a happy balance between compression and ignition advance, and unless you want to retard the advance to run on street gas, I would tend towards the lower compression. Even that is pushing it, with 10.0:1 being safer. "With today's street gas, I don't think I would go over 11.0:1 in our motors. The wonders of modern engine management let us run closer to the edge with less risk, less need to fudge. And, in the end, we can't run as much compression as we would like because of these sub-optimal conditions coming and going at various rpm's and throttle positions. We can't run as much advance as might be desirable at other rpm's and throttle positions. As a result, we can't run as lean as might be optimal for other throttle settings / rpm's. Also, somewhere in that range, ignition will be as advanced as practical without leading to detonation. Somewhere in the operating range, at some throttle settings, will be the leanest running condition mix will not be nearly as consistent as we would like. We need a lot more "fudge factor" in setting these up and tuning them. The trouble with carbs and mechanical advance is that we simply cannot control things as closely as we can today. I'm not sure that the HYM has anything that fancy, but it certainly has injection and ignition "maps" that are optimized a great deal more than old school carbs (or early injection) and mechanical (or early electronic) ignition advance. The various feedback loops talking to the "brains of the operation" keep the mix optimal at all times and the ignition timing just short of critical, where the motor would start to detonate. Modern engine management systems also allow more compression, and leaner running, than ever possible before. It's all about volumetric efficiency - how well the cylinders fill with each fresh gulp of air/fuel. ![]() Combined with port shape, valve sizes, exhaust system scavenging efficiency, induction system differences, and other factors, two very similar motors even with identical static ratios can have vastly different dynamic ratios at different rpm's. ![]() Dynamic compression ratio varies with rpm and is very much dependent upon cam timing. We are really concerned with two compression ratios, static - the number we read in the ads - and dynamic, or what the engine actually see when it is running. While the motor is a newer derivative of ours, it would be my guess that much has changed (in addition to the heads you mention) to make that kind of compression viable.Ĭam timing is a good example of a factor influencing how much compression a motor can tolerate. There are a lot of factors that weigh in on just how much compression is appropriate. you are kind of out of luck.Ĭlick to expand.Interesting, but probably completely irrelevant to our motors. You can get there with a re-map if the pistons are in there. That, and if you ever get the chance to run a high compression motor on race gas and lots of advance. Just be aware that one is dependent upon the other. There is a lot of discussion over which combination "feels" snappier - high compression / less total advance or low compression / more total advance in motors making about the same power. With today's street gas, I don't think I would go over 11.0:1 in our motors. High compression raises the specific output (as do higher rpm's and other factors) which leads to the higher operating temps. It's really not so much directly related to compression as it is the specific output of the motor. That's really the crux of the problem with our air cooled stuff. They have much, much better control of head temps. Water cooled motors can run this kind of compression all day long. Cylinder head temps will very quickly get out of control on a motor that is expected to run lap after lap, and with that, detonation. On a road racing bike (or car) I just can't see it. They don't (nor are the expected to) live very long. I've built Bug motors and Harley motors with that kind of compression, but they were pure drag motors. As far as race motors, only as a tear-it-down-every-weekend kind of proposition. No way on earth in an air cooled motor under any circumstances on the street. We are well into the realm of race gas only and some pretty critical tuning I hope they aren't touting those as streetable pistons. That 13.5:1 compression, though - holy smokes. I know lots of guys that do with complete satisfaction. I have never used Wossners, but I would have no qualms about doing so. The factory has even used their pistons in some race motors, choosing them over the tried and true Mahles. Wossner does have a very good reputation in Porsche circles. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |