[catchtheroadrunner]

What is the maximum compression a 4.63v can run given that pistons, crank, and cylinders are prepped with a typical aftermarket performance package and the only available gas is 91 octane.

If you know of a site, I'd really like a good primer on compression. What are the dynamics which govern how an engine design handles compressions?

The Hayabusa's compression is 12.5, the ecotech v6 is 10.0 and boosted, and the s2000 has 11.1 and can apparently still handle a little boost. The s197 compression 9.8. I was wondering why.

[TheUNZippee!]

I think some of the design is governed by efficiency factors as well. Lower compression is easier to spin which can make for better fuel mileage.

[catchtheroadrunner]

usually the "birds" in your avatar are not sporting feathers.

What you said about free spinning makes perfect sense. All of the engines I listed make their power up high, and an article about boosting an s2000 said that anything above 7 psi boost will make the engine detonate faster than a
"B" movie at lower rpm's.

If you find a link sometime, send it my way.

[TheUNZippee!]

Birds, oh, haha!

Thank you Jesus!


The Internal-combustion Engine in ... - Google Books

[catchtheroadrunner]

Zip you rule. I just ordered that book. In just the preview they note that the longer the compression cycle, the more likely the engine is to experience detonation (our engines are not high revvers in stock trim). They also noted that temperatures increase (increasing risk of det) as bore lengths or piston surface increases (our engines do have a long bore, but not a wide piston).

So...
One cylinder in the 4.6 is about .55+ liters. Significantly larger than any single chamber (and associated parts) on a superbike, or on the s2000. But not a lot bigger than on the ecotech v6, which has a compression level only slightly higher than the 4.6.

vehicle liters pecking order compression

Mustang 4.6 = largest cylinders = 9.8:1
SHO 3.8 = slightly smaller = 10:1
S2000 2.2 = smaller yet = 11.1:1
Hayabusa 1.3 = smallest = 12.5:1

Are the basics that simple? The larger the displacement, the lower the compression has to be, at least as a generality?

[TheUNZippee!]

Well, I think the basics are that simple.... to a point.

I don't think the compression 'has' to be lower for larger displacement. I think it just 'can' be lower and still make similar power to a smaller displacement/higher compression engine, because the larger displacement engine is burning more air/fuel per stroke and thus attaining more volumetric efficiency based on that factor alone.

So one could say an advantage of bigger displacement is that you can run lower compression, still have good power, AND have decent efficiency (mpg).

But were you to run higher compression on the larger displacement engine, now you are obviously going to realize much higher power potential.

[catchtheroadrunner]

Originally Posted by TheUNZippee! Well, I think the basics are that simple.... to a point. I don't think the compression 'has' to be lower for larger displacement. I think it just 'can' be lower and still make similar power to a smaller displacement/higher compression engine, because the larger displacement engine is burning more air/fuel per stroke and thus attaining more volumetric efficiency based on that factor alone. So one could say an advantage of bigger displacement is that you can run lower compression, still have good power, AND have decent efficiency (mpg). But were you to run higher compression on the larger displacement engine, now you are obviously going to realize much higher power potential.

So is it reasonable to assume that higher compression in bigger bore engines requires disproportionately more cooling and/or combustion chamber polishing to reduce detonation factors.

[TheUNZippee!]

Originally Posted by catchtheroadrunner So is it reasonable to assume that higher compression in bigger bore engines requires disproportionately more cooling and/or combustion chamber polishing to reduce detonation factors.

Absolutely. More compression usually needs timing adjustments as well. More compression = more heat. Period. And so that heat must be dealt with. It also requires higher octane fuels to help deal with detonation, which is why you see things available like racing fuels for serious track cars.... and right on up to nitromethane for top fuelers. Although the top fuelers actually run lower compression.... because of the amount of boost they shove into them.

Usually on track cars, you see high compression on naturally aspirated engines, and lower compression ratios on forced induction builds.

.... I would actually say that higher compression, period, requires more cooling and higher octane, regardless of displacement.

[Notcho]

I'd say higher compression engines are more efficient than low compression engines: You can get a lot more power out of them, but in order to do so, you must use a high octane fuel in order to avoid detonation. That's why race cars are either n/a with high compression pistons or f/i with low compression pistons and a hell of a lot of boost. The more fuel and air you can shove into any given space, the bigger 'bang' you get. But you need stronger parts to be able to handle it.

High octane fuel can be very expensive, as well as stronger engine components to handle the high compression, so high compression is not normally used by auto manufacturers for your typical street car... nobody could afford to drive it.

What I see a lot of people at the track doing nowadays is using E-85, which has a fairly high octane rating, but yet it costs a lot less than your typical racing fuel.

Consider your diesel engine. Some have extremely high compression, like 25:1 but yet have large displacement. Diesel engines are the most efficient internal combustion engines strictly because of their high compression ratio.

[TheUNZippee!]

Not if you're defining efficiency as fuel efficient.

[Notcho]

Originally Posted by TheUNZippee! Not if you're defining efficiency as fuel efficient.

Thermal efficiency.

Heat is energy.. the more you convert into useful mechanical energy instead of wasting it by releasing it into the air, the more "efficient" your contraption is. Internal combustion engines are very inefficient in that the majority of the heat produced is lost..

Ever wondered why trains use diesel engines to turn generators which power electric motors to turn the train's wheels?

[HBK_2007]

i thought higher compression engines where considered more fuel efficent vs lower compression engines, same displacement