Professor

According to the Acura site, the new '04 has a compression ratio over 11:1.... wow, do they have two batteries under the hood to kick it off?

However after about 80K miles, rings will wear, and there will be very little performance difference between an '04 and '99.

Racecars are designed for the high compression... but they are overhauled after every race. For a production units, this ratio may have durability concerns.

Bitium

If anything I wouldn't think it would be that drastic. Yes it is high 11:1, but don't compare a high compression ratio sedan to a high compression race car, there a lot more things than the c ratio involved. Plus when they raced, the engine is pushed to the limit for a few hours straight, this makes a huge difference.

I thought race cars were more in the 14:1 plus c ratio.

T Ho

Increasing compression is an easy way for the manufacturer to increase both power and fuel economy at the same time. A higher compression motor is more thermally efficient, and squeezes more power/unit out of the fuel. The reason that they haven't done it sooner, is that increasing compression also drastically increases NOx emissions. So, until the tech (including very tight and fast engine timing/fuel control, as well as cat design) improved enough, they were stuck with lower ratios. As far as cranking, they can just pull the timing back a few more degrees to ease cranking torque. They could also use VTEC to their advantage to reduce cranking pressure, if need be (but I don't think they do).

The compression will make little difference in the reliability. The '02 and '03 TL-S motor is already at 10.5:1, and I don't expect those to be unreliable, and lose compression after 80K. Wear rates are much more dependent on lubrication design and driving/maintenance habits.

Keep in mind that diesel engines in passenger cars are anywhere from 17:1 to over 20:1, and they run just about forever.

Todd

P.S. The Civic Hybrid runs 11.3:1, and the Prius runs 13.0:1- both on recommended 87 octane!!!

fla-tls

Hey Todd, I agree with you almost comletely. The question I have is, how does reducing timing help with starting an engine? I know that Honda is now getting very clever with their valve opening/lift scheme, but I'm not sure how that would help start the engine.

As far as the Professor's question - again I have to agree with Todd. As an example, the H1 hummer motor had a compression ratio of 23:1. (6.3 liters - 150 hp - 23:1 compression ratio.... you gotta love TURBO diesels - which the H1 is not) You need a good battery to get it started, but once it's going it will run all day.

Admittedly, diesel engines essentially burn oil - which makes them wear better. I don't know how a gas engine will do with there groundbreaking production compression ratios >= 11:1.

T Ho

I'm not sure I'd go with groundbreaking. Many of the engines of the 60s ran 11:1 compression ratios. Some as high as 12.5:1, from the factory.

As far as timing affecting starting- the engine is firing, and igniting the fuel mix as the engine is starting. At extremely low speeds, that firing can sometimes put pressure downward on the piston before top-dead-center, causing the crank to want to rotate backwards- opposite the direction that the starter is trying to turn it. By pulling the timing back, that firing occurs later, and the pressure isn't fully exerted until after TDC, which assists in the rotating (and starting) the engine. This is a quite common practice amongst racers with fully-advanced and locked out timing. They will use what's called a "starter retard box" to pull the timing back just for starting. Of course, the factory can (and likely does) easily build this into the PCM program.

Todd

fla-tls

Interesting. Thanks for the info.

TLKrazey20

:wow: