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First Post - Uh? Tag your IT?

OK. Higher compression ratios also generate more heat, a limiting factor in air cooled engines due to pre-ignition. Also due to the additional combustion forces, the faster your bike wears out!
 

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Whether the engine has any compression rate at all is much more important than how much it is. Many an unsuspecting buyer has been hoodwinked into buying a bike without. Of course, without the compression rate there is not enough back pressure in the pipe causing deficiency of vibe.



When buying a bike, the first question should always be whether it has a compression rate. See if the salesperson has any hesitancy or sideways glances in answering, might be you´d have to put a bit more pressure to get to the real truth.



- cruiz-euro

 

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Super Duper Mod Man
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Use the 'seat of the pants' spec sheet on any motorcycle you buy. Ride it first. That will tell you everything you want to know. It's worked for me. All the rest is for impressing the fellows at the Burger Barn.
 

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The spec table seems to help me decide which bikes I'd like to take a closer look at when I'm in the market for a new bike and I've already read a road test or track test review and heard about the bikes' performance, comfort, and handling. Unfortunately, I know for sure that I don't have the skill to reach a bike's potential like the test riders do. So, I'm not sure how much their test ride results translate into which bike will perform and handle better when I'm the person at the controls! But, usually I'll use the spec table to look at the seat height, fuel capacity, dry weight or wet weight to help me compare between different bikes until I get the chance to see them in person. But, the spec sheet kind of gets tossed out the window for me once I get a chance to test ride the bikes I'm considering.
 

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Just keep reading. You'll eventually catch on to the meaning of rake, trail, wheelbase numbers (to gauge the bike's handling traits), compression ratio (relating to the engine's performance characteristics), valve arrangement and adjustment (yet another indication of the engines performance goals), final drive (ease of maintenance, performance intent, etc). Familiarity may bread contempt but it also breeds depth of understanding. Wade in and eventually you'll be swimming in the deep end.



The good folks at Wikipedia have a fairly good definition of Compression Ratio if you care to take a look.
 

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The Toad
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Spec tables can be interesting and quite amusing. Some people from Seattle spend their entire lives in vicarious motorcycle riding entirely by means of spec sheets.



Especially hilarious are the dry weight claims... especially claims by Japanese manufacturers. They usually list the shipping weight which is without battery, fluids (icluding brake fluid) and even tires and then they subtract a few more pounds for fun. Some bikes weigh a hundred pounds more when put on a scale, ready to ride, than they weigh on the dry weight chart. Ignore dry weights.



Horsepower claims are also very funny. The distributers like to fudge the numbers. As an example back in the 60s Ducati's 250s used to magically increase in horsepower each year even though no changes were made to the bikes. Trust only the dyno numbers that you can find in the various mags.
 

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The specifications can be of use if know what you want.



For example, I have a Suzuki DRZ400S. It's compression ratio is significatly less than it's off road only sibling, the DRZ400E. This matters to me, as I like to use regular 87 octane gasoline. 92 octane is recommended for the higher compression engine. I have often found myself in situations where only low octane fuel is avaialable.



This also means that the lower compression engine will be down on power compare to the higher compression engine, but I can live with that.
 

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One possible explanation of the ridiculous weight claims is that the bikes are never weighed at all. Instead, the combined weight of all the parts that make up the motorcycle is summed, and that is used for the "Dry Weight" figure. Since all parts of the motorcycle have an acceptable tolerance, simply using the lightest possible weight for each part results in a highly optimistic mass figure.



The same could be true for power figures, that is, the power is never actually measured. Instead, calculations of flow rates, BMEPs, and friction based on the most favorable possible tolerances are used to calculate theoretical optimal power production.



Of course, this is all highly speculative, and I deny any possible connection to the motorcycle industry.
 

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Aging Cafe` Racer
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The only numbers I look at are at what RPM peak torque and peak horsepower hit. That gives you an idea how usefull the bike will be on the street. For example a GSXR 1000 hitting 161 HP. at 12.5K RPM and 81 lb/ft of torque @ 10K RPM is just not going to be that good a bike to ride on the street day in and day out. You wouldn't be coming anywhere near the bikes potential and you have to suffer with a track oriented riding position that gets old quickly.



Something like an SV1000 with a HP peak of 110 @9K and torque peak of 71 @ 7.5 K would be more usable because though it's still up there its reachable. Another thing to look at is final drive ratios and brake disc sizes, those all give you an idea as to how the bike will perform in the real world.



Ultimately it's how well the bike fits you and how much fun it is to ride that counts as longride said. My own "compression rate' is how hard it is to compress my butt on to the bike and ride it. In that case less compression is better .
 

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Sarnali speaks true. Low-end torque is thy friend.
 

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Agreed, but don't forget that a lack of a compression rate can be compensated for by an adequately-rated Johnson rod.
 

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The Toad
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For dry weight I prefer my "Honda says the new CBR600RR is 374lbs so we gotta claim the new R6 is 373lbs" theory.
 

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Low end torque sure works for me. As does an properly spec'd Johnson Rod.



I'm still waiting for a better spec on ergos. I like the "stickman" overlays that I've seen in a couple of mags - I forget which. Wish they'd make 'em bigger and use more contrasting colors.

 

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I don't agree about wearing out faster; the BMW K1200LT has a CR of 11.5:1 and I have read of several going 150,000 miles with little trouble. Air cooled engines with high CR can be a problem, but today most GOOD cruisers and nearly all sport bikes are water cooled. Over heating is rarely a problem then. Just stay away from HD air cooled motors however, unless image is the one's "Big Thing".
 
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