High compression engines

[05SportV8]

Questions for tech guys:

Given that you have two engines with similar displacement. You keep them in good shape and give them equal maintenance. One is relatively high compression (10:1) engine and one is normal (9:1).

1-It is expected that high compression engine gives you more HP at high rpms that the other one correct? However at low rpms without variable valve timing high compression engine produces less torque than normal compression engines right?

2-High compression also means more heat. Thus a higher percentage of fuel is used in heat rather than in physical movement (say power to the wheels). Therefore not as fuel efficient as normal compression correct?

3-After both engines have been used for a long time. Which one you think is still performing near new condition? Consider the fact that piston seals and valve seals have some degree of wear and some carbon deposits exist.

4-Which engine is expected to suffer more from imbalances on pressure among cylinders (like old engines do)?

5-For this question imagine 2 different engines. They offer the same max torque and same max HP. One is high compression (smaller displacement) and the other is normal compression (larger displacement). Which one do you rather have?

Depending the answers I will have another question but I

 

[OUTRNNR]

Good questions. I'll try to answer a few of them, based on what I know...

1) It is true that the high compression engine gives more HP through the higher rpms. As for the lower rpm ranges, I'm not sure here but I don't see any reason for them to be different, especially since many vvt systems don't change the cam angle until the higher rpm range, meaning the low end timing was geared in favor of torque to begin with. I don't see the compression ratio playing a large role in low-end torque.

2) Not sure here. Comparing a HC and LC engine assuming the same amount of fuel is being used, just being compressed more to provide a more powerful ignition. It is true, by way of the ideal gas law, that temperature should rise with pressure so I see your point of some of the fuel energy being dissipated as heat, but also more power is being made out of the same amount of fuel, Hmm... One thing to note is that diesel engines make more power efficiently using extremely high compression, but I'm not sure if this is because of the compression or the fuel having a higher energy content.

3) I'd bet that the lower compression engine is running better at high mileage, because of less stress on the rings and piston (not that the HC engine could've been designed for this in the first place) but I'm assuming both engines being identical here. Again, though, I'm not positive

4) I have no clue

5) I don't think this is physically possible. Generally, larger displacement engines (lower compression) tend to have more torque than horsepower because they have alot more rotating inertia about the crankshaft. This is bad for power because it takes longer to rev up due to more rotating mass (cank, pistons, con rods) but once you get it going thats alot of moving energy and will want to keep spinning, which is good for torque numbers. The converse is true of smaller, high compression engines. They are eager to rev up because of relatively small pistons, con rods etc. resulting in better power numbers. Once they get going, however, its easy to "bog" them down because of a smaller amount of energy stored in the moving masses, thus lowering max torque.

A good example of this is the Honda S2000 engine. A 2.0L 4 cylinder high compession engine rated at around 270 HP while the torque suffers a measely 150 lb-ft. Now look at my old Blazer's 4.3L V6. A very large displacement V6 with low compression makes 190 HP with over twice the displacement of the honda engine but makes 260 lb-ft of torque due to coffee can sized pistons.

Those are a few of the theories I have on those questions. I think the questions asked raise a couple of good points between the two different compression engines.

 

[Jeff Kleb]

High Compression


If you are a fan of "high compression" engines, then try driving a diesel-powered truck. My work truck has a DT466 (Navistar) engine and I think the compression ratio is something close to 18:1.

Sorry, just kidding. I could not resist.

Seriously, though, those are some good questions and I know that someone here at T4R.ORG will come up with answers. With a modicum of knowledge about engines, I would say that the compression ratio itself is just part of the equation and operational mix.

You should factor in fuel mixture, valve timing, ignition timing, type of fuel used, lubrication and the type of induction system. You have to run much lower compression ratios with supercharged (blown) engines than with normally-aspirated ones.

Dig out some of those old Hot Rod and Car Craft magazines and check out the iron under the hoods of the old street machines of the '60s and '70s that have been modified with blowers (superchargers). They'll use 8:1 pistons, while typical drag-quick cars without blowers will run 11:1 or even 12:1 pistons.

I am sure there is some formula for RPMs and where the 'torquiest' part of the power curve is as well. Oh, and I do realize that modern electronics play an important role in the performance equation......but maybe I am just a little bit old school sometimes!