Type 4 mechanical compression ratio, Why is the factory compression so low? Options

[Beebo Kanelle]

OK, here's a fun one for the novice to ponder and you experts to answer:

Why is the factory mechanical CR so low?

and for reference, what is the highest mechanical CR that can be physically achieved? assume no octane limitations -

I'm just looking for the design limit for the highest CR that can be / has been in a Type 4 engine using available technology.

Thanks

[Dave_Darling]

Assuming you aren't actually trying to burn anything, and are in a vacuum to boot, the maximum compression ratio is infinite. If you have zero volume in the chamber when the piston is at the top of its travel, you will have ((swept + 0) / 0):1 compression, which is infinite or undefined (depending on who you ask).

We need some space up at the top of travel for the air and fuel, obviously. We also need space for the slop in the rod bearings, wrist pins, etc., and for things to grow as they heat up. So there is a minimum amount of space needed, but how much depends on a lot of factors.

The stock 914 compression ratio is relatively low because it is set up to avoid pre-ignition, or detonation, or pinging. Much like all engines.

The engine has a pretty primitive design. The wedge-shaped combustion chambers are OK, but not great, so you can get detonation more easily than with a more advanced shape.. The cam is very mild so you don't have compression pressure "leak out" on overlap, which means you need a lower compression ratio.

The engine management system is primitive (D-jet, L-jet) or even more primitive (carbs) so you can't tune it closer to the edge and rely on the engine management to save you from detonation.

An air-cooled motor cools the way it wants to, and it is hard to get parts of it to run cooler than they want to. Water will absorb heat a lot better than air will, and you can run fine passages where you want them to go, and control the heat in various parts of the engine better. Having specific things get hot will lead to detonation more easily than keeping them cooler, though of course there are things you want to be hot for efficiency's sake.

The 914's compression ratio is not out of the normal range for the time the motor was designed, as Mark said. There were cars that ran higher compression, and some that ran lower.

Today, there are gasoline-powered cars that run over 14:1 compression, but those are pushing things! The highest I remember hearing about in a 914 motor is about 12.5:1, and that required race gas and had a big lumpy cam and cubic dollars sunk into the heads.

--DD