Okay, it's rainy and I think we need to discuss the subject of piston speeds, and what that means in relation to our Bullets.
There's been some discussion lately about pistons, and why certain piston characteristics are important, including the mass of the piston and pin.
So here we go.
A long time ago, there was this automotive engineer by the name of Laurence Pomeroy.
He came up with a standard called the "Pomeroy Dictum". The Pomeroy Dictum states that for good longevity, the piston speeds in an internal combustion engine should remain under 2500 feet per minute.
Okay that's a relatively older standard, but it's still relevant, and it was an accepted premise when the Bullet engine was originally developed in the 1940s/50s.
Why is this important?
An engine has reciprocating parts that move very quickly, and this causes stress to them. If speeds of these reciprocating parts exceeds what they are capable of withstanding, they either distort(stretch or compress or twist) or they break. Sometimes this can lead to metal fatigue, and eventual failure, even if they don't break right away.
What's the part at biggest risk?
The part at biggest risk in our Bullets is the connecting rod, although the big end bearing and the main bearings are also somewhat at risk. And the crankshaft itself is at lesser risk, but if it is flawed or defective in some way, higher piston speeds and also rpms can bring it into failure more quickly.
So, what do we do about it?
Well, for users of the stock rod, and especially when changing to a larger and heavier piston which increases moving mass, we need to observe rpm limits, so that we stay in a relatively safe zone for the parts to handle. If we've increased piston mass, but haven't done anything to improve rod strength, then we must observe a lower rpm limit for safety. But this often goes contrary to what the user wants to do. Often, the user wants to put in a bigger heavier piston for performance purposes, and also wants to rev higher for performance purposes. This can lead to a problem. Not always, and not necessarily right away, but the issue does loom. If we use a stronger connecting rod and better bearings, we can hope to avoid the problems, but it's not guaranteed to do so. But, it is very likely to allow more revs with heavier piston mass if you do make the improvements to the other parts involved.
What are the piston speeds in the Bullet?
Well, being a long stroke engine, with a 90mm stroke, they can get pretty fast at lower rpms than a shorter stroke engine. And this is why our redlines are not as high as Japanese engines, or other short stroke designs.
For example, with our Bullet if we wanted to stay within the Pomeroy Dictum
2500fpm with a 90mm stroke = about 4250 rpms. Not very high, is it?
So, we already exceed Pomeroy in stock form with our engine, because we redline around 5000 rpm.
How fast is that?
5000rpm with a 90mm stroke = about 2950 fpm, or 20% over Pomeroy. Getting a bit touchy, but our engine can handle it.
How about 5500 rpm?
5500rpm with 90mm stroke = about 3250 fpm. that's 30% over Pomeroy.
6000rpm?
6000rpm with 90mm stroke = about 3550 fpm, or 40% over Pomeroy.
Now, some comparisons.
Many street cars today, still remain under Pomeroy's Dictum, and perform very well for a long time with piston speeds less than 2500 fpm.
But what about hi performance cars?
Well a Honda S2000 which is a fast production car with overhead cams redlines with a piston speed of 3850 fpm. This is a very modern engine that has all the latest technology in it. And it's not an awful lot higher than if we try to do 6000 rpm with our old antique design.
An extreme case might be the Ferrari Formula One car. It does 18000 rpms, and has a piston speed of 4890 fpm, almost double the Pomeroy Dictum. But this represents the highest level of design and construction of about anything on the planet. That's not where we are with our engines.
So, where does this all lead us?
Well, we need to understand just how much we are exceeding the Pomeroy Dictum with our revving. And we know that we can use an everyday limit of around 4250 rpm for daily driving, and have good longevity with our Bullets. And this has been proven over the years, and holds true to form for Pomeroy.
When we want to go fast, we can rev to 5000 rpm, and exceed Pomeroy by 20%, and this is okay on an occasional basis, and maybe even frequent basis. But it will yield some additional stress, and may shorten engine life some. But, the engineers apparently felt that it would be okay for a redline of that amount. I don't think they had it in mind that people would run around at 5000 rpm all day, or even do it at all every day.
The rest is a "call shot" on the part of the owner, as to how far above Pomeroy's Dictum or the designed limits that he wants to risk with his own bike. 5500rpm is an additional 10% over what the Enfield engineers envisioned as a redline. I think it's do-able. I don't think it's prudent to "live" up there in those rpms, but I think it can be touched when you want to ride hard on an occasional basis, and accept the risk.
6000 rpm in a Bullet is 20% higher than the Enfield engineers designed for, and is pretty much pushing the limits. It's well over Pomeroy, and well over what Enfield had in mind when they made these parts. So, that is gettting pretty darn risky with standard parts.
Add a heavier piston, and you are really pushing it, and I'm not certain it would hold together very long at those speeds. But it might do it occasionally. Maybe. But it might break the first time you try it, too. That rpm is a risk with the stock rod, and especially if you have a heavy piston in there.
This all is a long way around to get to why I selected 5500rpm as a limit for people who want to retain the stock rod, when doing high performance mods.
And why I selected a limit of 6000 rpm for people who want to use the aftermarket Hitchcock's performance crank/rod/bearing package. It is much stronger and can tolerate over 6000 rpms, and tolerate it regularly. But it's expensive. And worth it too.
So, in light of me trying to bring out some speed parts, and reading some of the questions and comments about various combinations, I thought it would be relevant to discuss the "whys and wherefores" about the limitations of engines, and why you can't just do anything with any parts and get away with it. Prudence dictates that if you want your engine to stay together, you must observe the limits of the designs. Or if you want to exceed the limits of the design, you must improve the items which will see additional stress from doing so.
I want you guys to get enjoyment and pleasure out of your performance mods, and not go out and blow up your bottom ends.
You can get plenty of fun and performance at 5500 rpm with a Bullet on the street, and if you want do do more, then please get the bottom end parts that will allow it.
A word to the wise.