Topic: Higher or lower compression for bottom end power?

[Rod]

ML - this works for me, I've been trying to get a handle on this in terms of actually riding the bike rather than the maths, and the RPM is the key. The stinkwheels (RZs) would talk about dropping off 'the pipe' at lower rpm and would not only fail to accelerate but actually bog-down. Us 4strokers talk about dropping 'off the cam' at lower rpm, but because our engines still generate some reasonable torque at lower rpm we are also therefore making some bhp and can accelerate. Is that it? and does this mean therefore that I just look at the bhp curve on my dyno read out to know how the bike is going to ride?

[mlinder]

Rod, now we are talking about specific applications of power.
The longer you can hold close to peak torque at the top of the rev range, the more power you will make at the top end. This equals not only a high top speed, (as you are making high power at the top of the rev range), but increases your ability to reach terminal speed more quickly.

Let's take Jons (my riders) Ducati monster 800 as an example.

The bike, stock, makes within 10% of it's peak torque from 4krpm all the way to 8krpm, but drops off considerably from 8k to 9k (nearly 8lbft!)
This makes the bike great at accelerating from 50mph to about 110mph. What is terrible for the track, though, specifically PIR with a 1 mile long straightaway, is that we've reached 110mph 1/3 way down the straight, and spend the rest of the time getting up to 130mph.
Not good.
With some cam degreeing, we will be changing the power curve. Torque will drop in the lower ranges, peak torque will occur later in the rev range (its at an abysmally low 6200rpm right now), the powerband will narrow, and we'll pick up 5 or 6 lbft at around 7500rpm.
This will give us the power we need to get down the straightaway faster, with more of a rise in power towards redline, and more power at the top.
We should be able to reach 107 or 108 in the first 3rd, but reach 130 much sooner, and hold 140 for a bit in the last 2/3rds.
You can see how this would help with lap times.
You need to figure out just how fast you need to go, and where, to build your race bike. Remember that Drag increases exponentially, and if you can't reach and maintain within 10% of your competitors top speed for any significant amount of time, unless they royaly screw up all their turns, you won't beat them.

[bistromath]

I think we just agreed.  I was just trying to point out how the word is generally used by gearheads, and that is to denote low-end power. A motor with great peak torque is generally quick off the line.

Put another way -- the guy at the Chevy lot says "this motor has a hell of a lot of torque", not "this motor has a hell of a lot of torque at 1500 RPM". He means peak torque, and high peak torque generally denotes a motor that's quick off the line. It's the difference between a Chevy small block and the Mitsubishi Evo motor.

[mlinder]

Yeah.
Pretty much. It's too bad that the misconceptions about how it works have become so prevalent.

Here's some numbers for a slightly built chevy small block (found randomly on google)

RPM     tq       hp
3,000   443   253
3,500   458   305
3,900   460   342
4,000   459   349
4,500   444   380
5,000   435   414
5,500   428   448
5,900   405   455
6,000   397   453

Please note that it is making within 14% of peak torque throughout the (recorded, usable) rev range.
This is a well balanced engine.
But talking to someone, saying "460lbft", doesn't mean jack #$%* unless you give the rpm range in which it's measured.
Hell, you'd never even have to say 'horsepower' if all you said was "443lbft at 3000rpm and 405lbft at 5900rpm". We'd know just how fast the car is with that information. (conversely, you could say "253hp at 3000rpm and 455hp at 5900rpm" and be saying the exact... same... thing...  which helps to show that torque and horsepower are, in fact, the same thing, expressed differently.)

[754]

I will admit I am not grasping all being said (skimming through some),the basics are making sense in an engineering sort of way, that is it works on paper.

 We all kow, or some of us do , that a design on paper does not always perform on the shop floor or in this case on the pavement.

 So what Mlinder has said so far makes perfect sense to me re Honda sohc, if you are talking bore kit.. as far as cams or carbs or headwork, I am just not seeing it due to variables and constraints.. a big part of real life..

 You can cam for top end to the point it wont even idle.. so at that point where is the increase, you can port a head so it only works at wide open and the bottom suffers greatly.
 so in a perfect world mlinders  argument may be true, but not in all cases..

 BTW drag racers regardless of wether the engine is bigger on torque or horsepower choose an rpm that works for them and the rest is clutch on the holeshot..low end power does not enter the pic, unless you are having a 1st gear rollon..

 As for the duck at the track, it is a torque motor, bring the right weapon to the given track.. thats why XRs excell at flatrack and not at roadrace.. but a duck would do OK.

 RR on a big track with fast corners is a HP game..

[mlinder]

It's cool 754. I know old preconceptions are hard to abandon.

Just as an exrecise, though, go look at some dyno charts that have both the tq and hp reading, and apply the math. You'll see it's always spot on.

[Sam Green Racing]

I know high compression motors can pour out the power...
But with high CR pistons, most people go with bigger pistons anyways.


In the example of a bike with a stock cam, mild porting, new valve job, and lets say 67mm pistons.(haha, i wish...)
Would there be simply a power LOSS by having lower compression pistons?
Or would it simply shift where the power is produced in the RPM band compared to the same pistons with a higher compression ratio?


While i have your ears....(eyes?)
Is there really a big difference in oil temps with higher and lower compression motors?


l8r

I think you two have exceeded yourselves in taking this thread so far when Soos 1st post was a little dificult to understand.

The title was the question, then he introduced big bore pistons into his example that he had already said were high compression.

Where do the low compression pistons come into what you are asking Soos 

Sam.

[mlinder]

Yeah, sorry Sam.

Soos, since higher compression pistons make more power across the entire useable rev range, lower compression pistons would reduce power across the board in a pretty linear fashion, assuming all else is equal.

[754]

In a perfect world..
 There are limits, run 15;1 on a 750 on the street and tell us about the across the rpm range increase...

[mlinder]

In a perfect world..
 There are limits, run 15;1 on a 750 on the street and tell us about the across the rpm range increase...

That's not a very valid example, 754. Even race gas would detonate at 15:1 in a stock street 750 with only a CR increase of that proportion. Rods wouldnt hold up, I doubt there's be valve clearance, myriad failure points. (but it would increase power across the rev range, assuming no failures...)
But you know that.
Theres a reason I use the term 'usable rev range".