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Saw laurieaw suggest increasing the compression ratio for improved fuel economy. Just because I change my own oil does not mean I could try to change the compression ratio. I suspect that would be much easier on a 1969 VW Beetle than any of today's vehicles. It seems that timing has to be adjusted along with the compression ratio....

I recall that each of the three flavors of the Honda CRX had different compression ratios: The HF had the highest fuel economy in part because it also had the highest compression.

This is probably easier said than done.

The new r18 in my Civic has 10.5:1 ratio. Along with advance ecu they have also used a stream of oil spraying up in the piston to cool it to help with knock.

Saw laurieaw suggest increasing the compression ratio for improved fuel economy. Just because I change my own oil does not mean I could try to change the compression ratio. I suspect that would be much easier on a 1969 VW Beetle than any of today's vehicles. It seems that timing has to be adjusted along with the compression ratio....

I recall that each of the three flavors of the Honda CRX had different compression ratios: The HF had the highest fuel economy in part because it also had the highest compression.

This is probably easier said than done.

i said that? hmmm, one of us must have been on drugs, 'cause that's way above MY head :)

I knew there was a reason I felt so good today... :D

I've been mulling over what I would do on a hypothetical project car (or just a project engine for my own car.) High compression is a good place to start but can be tricky due to the aforementioned knocking/detonation risk. Modern day engine management systems can deal with it by retarding the ignition timing once knock is sensed, but that only works up to a point. If the compression is too high you can run into a situation where the engine won't run on regular gas anymore without eating itself from the inside out. Where the limit is, I can't say.

Efficiency can be gained at the expense of power via "atkinsonization" (if that word exists). In theory, you would start by modifying the bottom end with high compression pistons; much higher than stock. But these "high compression" pistons really result in a very high expansion ratio. Then you would need to replace the stock cam with one that is ground for an extremely long intake valve opening. You would match this intake valve timing to the native compression of the engine, so that they slam closed in time for the true compression ratio to reach something high but reasonable, on the order of 10.5-10.8:1. This is the same thing the big boys have been doing for years, so we're not even close to uncharted territory here. And, if you really do this right, the stock engine management system ought to be able to handle these modifications without too much trouble.

I've been mulling over what I would do on a hypothetical project car (or just a project engine for my own car.) High compression is a good place to start but can be tricky due to the aforementioned knocking/detonation risk. Modern day engine management systems can deal with it by retarding the ignition timing once knock is sensed, but that only works up to a point. If the compression is too high you can run into a situation where the engine won't run on regular gas anymore without eating itself from the inside out. Where the limit is, I can't say.

Efficiency can be gained at the expense of power via "atkinsonization" (if that word exists). In theory, you would start by modifying the bottom end with high compression pistons; much higher than stock. But these "high compression" pistons really result in a very high expansion ratio. Then you would need to replace the stock cam with one that is ground for an extremely long intake valve opening. You would match this intake valve timing to the native compression of the engine, so that they slam closed in time for the true compression ratio to reach something high but reasonable, on the order of 10.5-10.8:1. This is the same thing the big boys have been doing for years, so we're not even close to uncharted territory here. And, if you really do this right, the stock engine management system ought to be able to handle these modifications without too much trouble.
;;0
see, chuck? he took the words right out of my mouth

brick confirmed what I suspected, the microcomputers would make modifications more challenging.

I'd love to find a way to increase the compression on the Insight, shed 200-300 pounds so maybe narrower tires could work, have the know-how to upgrade the Assist motor to one from a Civic, install MIMA.

I wonder if higher compression would present the same problem as going diesel - more pressure - more chance of cracking the engine block?

Actually, I think that the microcomputers would make life relatively easy. The stock metering equipment (air mass meter, o2 sensors) will keep fuel flowing at the proper rate, and the knock sensor ought to keep the ignition timed properly as long as the compression ratio isn't too wild. IMO, the hard part is getting the mechanicals right so that the engine burns fuel and produces power the way you want it. The wrong valve timing, a combustion chamber that's not designed properly, or any other number of factors that we wouldn't normally think about could result in a big disappointment. It might work, but you want something that works well. Attention to detail during the design phase would be well worth it by the time everything is bolted together.

And, yeah, I wouldn't be inclined to convert a gas engine to diesel. Peak cylinder pressures can get pretty ridiculous, and the components of all but the most robust gas engines would be under excessive strain. Just think about how much torque those things produce! Actually, didn't GM try that with a small block V8 at one point?

BUT- I don't mean to say that you can't make modifications to a stock engine that would considerably increase fuel economy. Anything is possible with the right knowledge and a heathly does of patience.

I don't think the pre-01 Honda D-series engines have a knock sensor at all, by the way. This would be an experiment with a very high potential for engine failure. Also keep in mind that even on engines with a knock sensor, this means it can only pull back timing AFTER it has already started to suffer noticeable engine pre-ignition. Also, this means it will be un-doing your CR gains by pulling out ignition timing. A slightly less drastic modification would be to buy an ignition timing gun and just twist the distributor two degrees advanced and see if that helps you at all. This of course is referring to the del Sol, since it's one of the old style D-series that has a traditional distributor ignition. Oh, by the way brick, the D-series doesn't have a mass air meter either. They just use a manifold pressure sensor and a thermometer. It's known as MAP-IAT style lookup. Pretty hillarious, really. The D-series is an incredibly primative engine they just kept making work, year after year.