Isn't Compression more important than displacement?
 

i have read alot about a new "4.0L" vq engine coming out. everyone is making a big deal out of it. i thought that the higher compression ratio you have, the more horse power/performance. i always thought that to get better HP numbers, increasing compression would do more than a larger displacement. some of the old vettes came with 11:1 or even higher compression from the factory and were boasting 450+ hp. can someone plz clarify this for me?

Neither is more important than the other. Instead, they need to be designed to work together to make more power, which is the end goal.

It is true that higher compression equates to increased efficiency which leads to more power. Additional displacement will also create more power. So will increasing RPM's while maintaining torque. So will advancing timing, leaning fuel mixture, etc. All these techniques have advantages and disadvantages, and therefore need to be implemented correctly in order for overall power to increase.

Typically, as you increase compression ratio you also increase the chance of knock, which means higher octane gas is required. Having additional displacement will usually cause less problems (assuming you have already tackled the problem of actually getting more displacement out of the Z).

-D'oh!

Also, I read somewhere that compression ratios were much higher on older cars because of the use of leaded gasoline. (Which we no longer have, of course.) Leaded gas apparently allows you to safely run much higher compression.

Have no idea why. I should research that some time. :)

Lead in gas raises the effective octane rating of the gas. A lot of older performance engines had really high compression ratios, my Buick was 10:1 stock, LS6 Chevelles had 11:1 compression and I think some flirted with 12:1 compression.

No, look at it this way. Which is going to make more power? A technologically advanced 10.3:1 VQ35 or a carburated 502 ci big block Chevy running 8.5:1? My vote is that the Chevy will be more powerful by at least 200 HP. Displacement is more important.

Gran Turismo says "There's no replacement for displacement."

Victor :D

That is a very bold and ignorant comment.:icon6:

As D'oh said, "they need to be designed to work together to make more power"

Want to guess what the fasted street legal car was powered by a couple of years ago (could still be, I haven't researched)? It was a RB26 making over 1300 WHP.

If you don't want FI in the equation, then how about this: Why does a 1.8L honda engine make more power than my 3.3L Pathfinder?

Now, HP is a measurement of TQ, and "Yes", larger displacements tend to offer more tourque than smaller displacements, but it is not the be-all-end-all of the power discussion. If you want more displacment, buy an 18-wheeler, but when I pass you on my motor scooter, don't cry.

yes but the chevy will give you 5 miles to the gallon and constantly be up on blocks in the garage :D

a 1300hp rb26 is street legal??

highly doubtful that it was street legal... but with 1300hp under your hood who cares?.. no popo would dare try and chase u on the highway :)

Well, Street Legal in Japan. It was/is a Veilside project car. Went over 225 mph on salt flats. If I remember correctly, the guidelines for reaching the record stated that it would be a 6 kilometer test. From a start, you must reach top speed within 2k, then maintain for 2k, then 2k to stop. It was a SCC article a couple years ago.

anyone know the torque on the 4.0's

actually it was done in the rain on a country road in New Zealand. The car reached 212mph in 1km.

I have to agree :icon18:

POWER (including horsepower), mathematically expressed, is the rate of which WORK is performed. So if you increase the amount of WORK done in the same unit of time, you increase POWER. Guess what, WORK is the product of FORCE x DISPLACEMENT. So it follows: All else being equal, if you increase displacement (cubic inches), you increase work, if you increase work you increase power.

or.....
 

I always liked.....

The only replacement for cubic inches is cubic dollars.

Yep... I owned a factory motored car that had 13:1 compression. It sucked when the station selling the 100-octane gas went away.

But it's true... no replacement for displacement. Sure, an RB26 made over 1300whp... big woop. Do the same mods and technological advances to a bigger displacement motor and you'll get even more rwhp. Don't believe me? Do a search on top fuel dragsters and see what they're putting to the ground on 500+ cubic inch displacement. No way a 2.6L will ever touch those numbers...

it's not out yet

hell, at this point it's still a rumor isn't it? i haven't seen any info anywhere...

I had always read that the top fuel dragster power ratings were usually estimates. They make so much power that most dynos can't handle it. :) What are they making, somewhere in the 3000-5000 hp territory?

It all comes down to how the engine is tuned. You have to take into account HP and TQ (torque). The small Honda engines are able to make so much HP because of their very high redlines. Like you said, HP is derived from TQ. So even though they have very low TQ, they do it at very high rpms. The formula is (if I recall correctly):


(ft-lbs TQ) * RPM
------------------- = HP
5200


So if you have very small torque (like the small Honda engines), you just need to make very high rpms to get high HP. That's how the S2000 does it: only 150 ft-lbs of torque, but with a 9000 RPM redline.

On the other hand, your Pathfinder engine is tuned for more TQ with a nice, broad TQ curve. This gives you good and useable TQ across as much of the powerband as possible, but it sacrifices the high redline in the name of cost savings and reliability. It's tuned this way because the Pathfinder is made for hauling a lot of weight around. Tuning that engine to spin faster and retain its long term reliability would be expensive. Certainly possible, but when making cars, manufacturers try to save costs wherever possible.

And I can guarantee you that the 3.3 liter engine could make more HP than a 1.8 or 2.0 liter engine if they were both tuned for high-rpm power.

On another note, your 18-wheeler example isn't quite fair. Those engines are tuned *specifically* for TQ and usually have extremely low redlines. (1800 or 2000 rpm sometimes) Thus, even though they make 1000+ ft-lbs of TQ, they only make a couple hundred HP. I imagine this is for reliability and cost savings. Tuning an engine with that much TQ to also have a high redline would require a lot of money and might make the engines prohibitively expensive. Plus, the faster you spin an engine the more stress you put on it. One of the many reasons those big rig engines can get hundreds of thousands (or more) of heavy-duty miles on them is partly *because* they only redline at 1800 rpm. (Not to mention how overbuilt they are, etc.) And they are diesels, which really changes how you tune them.