"Doing it Right" or "How to Build a Functional Café Racer"

[calebacm]

The goal would be a happy medium of both. I'm running stock right now and there's not a lot of low-end power but there's an okay amount of high end. I've taken it to a straight and my top speed before it really could not give me any more was 102. And God was that terrifying. I thought it was going to explode underneath me. So if I could get some more low-end out of it and be able to push it more would be great. I'm in the process of lightening it up too, which I think will help some.

 

[Sonreir]

Take it a step at a time and do the easy stuff before you do anything that can't be undone or takes a lot of time and money (especially if this is your first time undertaking performance modifications).

Personally, I would these things, in rough order (again, one at a time and not all at once):

• Drop one tooth on the front sprocket
• Trim excess weight from the bike
• Increase compression by shortening the stack (thinner/no base gasket or shaving head or jugs) - Be sure to check clearances both before and after to ensure all the parts aren't trying to occupy the same space at the same time. Additionally, changing the stack height may affect valve timing. Check this, too.
• Investigate dyno-proven changes to the intake and exhaust systems.
• Consider a carb rejet (use a wideband sensor on a dyno for verification purposes)
• Increase displacement by boring the cylinders and swapping to larger pistons
• Increase compression by using a different set of pistons (usually with a higher crown, but this will probably require ignition timing changes and check clearances here, too)

 

[calebacm]

Thanks man. I'm definitely starting small. I'll come back with any questions that I have.

 

[mrlvlagic]

A café racer without performance enhancements is just a tractor with a body kit. Do you really want to be one of those kids in a 1.6L Honda Civic with glowing lights under the body panels racing his gutless wonder from stoplight to stoplight? If the answer is, "yes", you can probably stop reading now.

(Snip)

That speaks volumes... I think I need to make that one of my prorities

 

[teazer]

I'll admit I haven't read every word of every post on every page so maybe this got addressed… but I missed it if it got discussed.

In your description of what detonation is, you left out the concept of a delay period. this gets important later so let me ramble a bit here. The mixture in the cylinder is exposed to high temp and pressure. It can only resist this temp and pressure for a given amount of time before it detonates, as in , there isn't a given combination where it explodes, there is a given amount of time for various conditions before it explodes.

A common misconception I see is people thinking that detonation occurs because the flame front moves too fast. this isn't true. Detonation is sudden, everything goes BANG rather than burns. This creates a shock wave that bounces around in the cylinder and makes the knocking noise which identifies the condition. it also knocks the boundary layer off the piston, which removes the insulating properties of the boundary layer and lets the piston heat up, which weakens it which leads to damage. There is no flame front when a cylinder detonates I used to have a text that had pictures of detonation happening. you could see the flame, then BOOM. nothing.

Which was all supposed to tie in to something I saw in another post - that high octane gas burns slower than low octane gas.

My challenge is for someone to post a technical reference, that means not Hot Rod magazine or Wikipedia, etc, that claims there is a significant difference in the flame speeds of high and low octane pump gasoline, or that flame speed is the method used to control detonation.

Here's the thing. Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period. You want the mix to burn as fast. Hence all the other mods get get "more complete combustion" like dual plugs, swirl in the intake charge, MSD ignitions, that kind of stuff. It's all designed to burn the mix faster. You want to burn it all before the delay period expires and it goes boom. Why would it make sense to make the gas itself burn slower? A higher octane gasoline resists autoignition due to high temp and pressure longer than a lower octane gas does. it gives you more time to burn the mix but does not burn it slower. You want to reduce detonation? Lower the temp, lower the pressure, burn the mix FASTER, or make the gas more resistant to autoignition.

To address a few misconceptions I see (not necessarily here) Slow flame speeds do not prevent detonation, they make it worse. Fast flame speeds do not cause detonation, they reduce it. Multiple flame fronts coming together do not cause detonation.

Why should anyone believe me? No reason. So here are some sources I found earlier (and posted on another site) concerning this. Read the references, note the sources, and think about it. Make your own decisions.

*************************

I've seen this basic write up on a couple different race fuel supplier sites.
http://www.whitfieldoil.com/www/docs/171.284/vp-racing-fuel-
Another link from a fuel supplier
http://www.pinux-products.com/octane-rating/
Check out Section 6.3 of this university paper:
http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html
Sunoco Race Fuels says this:
http://www.racegas.com/article/10
Any opinion can be supported by links to internet sites. But it also makes no sense that a slower burning mixture would be better for controlling detonation. Flame fronts, even fast ones, are a controlled burn, detonation is an explosion.
*************

No flames intended here, I've just seen the slow burning gas claim so often and no one ever questions why we would do so many mods to make the mixture burn faster, then put a slow burning mixture in there.

Correct that the only thing that octane number tells us is the resistance to detonation which is an uncontrolled explosion for all intents and purposes.

But what's missing is the fact that many additives that improve knock resistance burn slower than regular gas. So a high octane gas may resist knock and still burn slower than regular gas. But that doesn't mean that ALL race gas burns slowly. Some burn fast and some are slower burning. Specific gravity gives some indication as to relative flame speeds, but the vital data about fuels such as energy content and flame speeds are rarely quoted.

The simplest example of high octane slow burning is Methanol which may need as much as 20 degrees more ignition lead and yet still resists knock better than gas. It's better toi think of race gas as a chemical soup which is why it's so expensive and why there are so many choices.

We found on the dyno that street 93 octane makes slightly more power in our small Hondas than VP C12 and there is less unburned hydrocarbons. It never ceases to amaze me how many fuels VP and Sunoco offer and they keep coming up with new ones. And let's not forget that we want a fuiel that works well at slow speeds as well as high speeds and we want crisp throttle response as well as good power flat out and those need different chemicals to change those characteristics.

Check out the RVP and distillation curves to get an idea of how a fuel will work at lower speeds and when you crack the throttle. There's no direct correlation as far as I can tell, but it tends to indicate a probable response.

Shell used to have an F1 web site where you could play with different blends of fuels for different circuits to see what they used and why. They had it down to three components to suit different parts of the track. But that was just for the one engine they were supporting IIRC. Some engines are nice open combustion chambers and are detonation resistant and other designs burn long and slow and are prone to detonation.

I have read many articles on race fuel and I still have no idea how much I don't know

 

[mydlyfkryzis]

Desmodog has some of the statements a little off. Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states

Making the gas mixture burn slower would make detonation WORSE, not better. It goes back to the delay period.

he is contradicting himself. If the burn is slow, it is by definition, not detonation. However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

Here is a non-wiki reference, from Mikuni on it:
http://www.mikuni.com/tg_detonation.html

(I bolded and underlined the specific definition)

5: Detonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosions that take place within an engine's combustion chambers. It can be extremely destructive, breaking pistons, rod bearings and anything else from the pistons down that a large hammer could damage. It is best avoided.

Pinging is a descriptive name for detonation. Pinging is that high pitch ringing sound that an engine sometimes makes when the throttle is opened with the engine under load. It sounds as though the cooling fins are ringing as they do when you quickly run your finger nail over their edges.

Pinging indicates trouble. Trouble that does damage. That damage can be quick and catastrophic but usually isn't. Most often, detonation occurances are small in energy and the engine is able to absorb the punishment, at least temporarily. However, over time, even light detonation does harm; weakening pistons and overheating the top piston rings.

Severe detonation can destroy an engine literally in a heart beat.

HOW IT HAPPENS

After a spark ignites the air/fuel mixture in an engine's combustion chamber, the flame front travels across the chamber at a rate of about 5000 feet per second. That's right, one mile per second.

Flame front travel for detonation is closer to 19,000 to 25,000 feet per second; the same rate as in dynamite. The difference between normal combustion and detonation is the rate at which the burning takes place and therefore the rate of pressure rise in the chamber. The hammer like blows of detonation literally ring the metal structures of the motor and that is what you hear as pinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal burning has the flame front traveling from the spark plug(s) across the chamber in a predictable way. Peak chamber pressure occurs at about 12 degrees after top dead center and the piston gets pushed down the bore.

Sometimes and for various reasons a second flame front starts across the chamber from the original source of ignition. The chamber pressure then rises too rapidly for piston movement to relieve it. The pressure and temperature become so great that all the mixture in the chamber explodes. If the force of that explosion is great enough --- the engine breaks.

WHAT CAUSES IT

Anytime the combustion chamber pressures become high enough, detonation occurs. Anything that creates such pressure is the cause of detonation.

Here is a list of possible causes, it may not be complete:

* Timing - if the spark happens too soon, the chamber pressure may rise too high and detonation results.
* Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure and detonation are likely.
* Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can start burning too soon. Pressure rises too high and detonation can happen.
* Cranking pressure - Any given combustion chamber has a maximum pressure (before the spark is struck) beyond which detonation is likely.
* High engine tempertures - High chamber temperatures raise cranking pressure and promote detonation.
* Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber, uneven burning, pressure spikes and detonation.

Note that each of these possible causes are relative. That is, there is no absolute timing, mixture strength or ignition timing that is going to guarantee detonation. Equally, there are no absolute settings that guarantee that detonation does not occur.

Motorcycle manufacturers, Harley-Davidson included, spend a great deal of time and money fine tuning their engines to eliminate or nearly eliminate detonation. When we change the engine design in the direction of detonation by, say, raising the compression pressure with domed pistons or milled heads, we increase the chance of detonation actually occurring.

Gasoline quality helps determine whether or not an engine is going to detonate. The higher the octane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase the likelyhood of detonation. High compression pistons, thin head gaskets, some alternative ignitions, some exhaust system designs, etc.

Stock street bike carburetion is very lean for emissions purposes. When the air cleaner and/or exhaust system are replaced by less restrictive components, this stock jetting becomes impossibly lean. The engine does not run well and detonation is likely at some throttle settings. Re-jetting or wholesale carburetor replacement (Mikuni!) is the cure for this particular problem.

If one fits high compression ratio pistons together with an early closing (mild) cam, the cranking pressure may become high enough that serious, engine-deadly detonation is likely. How much is too much you ask?

Well (Rule of Thumb here), Evolution engines are fairly safe against detonation if the cranking pressure remains at 180 psi or less. The TC88 motor can dodge detonation if the pressures remain at 190 psi or less. Keep in mind that these maximums are for fairly stock engines; no porting, no chamber work and no squish areas.

A well shaped combustion chamber with squish effect is much less likely to detonate than most stock examples. The main reason the TC88 engine can withstand higher cranking pressures than the Evo is its better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compression test. This test is done by removing the spark plugs and fitting a compression gage in one of the spark plug holes. The throttle is then held open and the engine cranked with the starter until the gage needle stops climbing. The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure may rise too quickly bringing on detonation. The main reason for having an advance curve built into an ignition system is to avoid detonation. The correct timing for any given engine design (and state of tune) varies with rpm and throttle setting.

Hot spots is more than a night club. If your engine has been running rich or burning oil, it may have thick bits of burned-on carbon. This carbon build-up can literally glow and, under the pressure of compression, start burning before the spark is struck. This leads to severe pressure excursions and, often, detonation.

Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures can escalate pressures and bring about sudden explosive burning. Also, lean mixtures elevate chamber temperatures which, as you now know, can lead to dreaded detonation.

If all this leads you to think that your engine is in imminent peril, then we have succeeded. Detonation is a terrible thing to happen to your expensive Harley engine. The pressures of those explosive events can be enough to hammer rod bearings, pistons and rings into useless junk.

If you hear the tell-tale ringing of detonation next time you open the throttle on a hot day or at low rpm or after a tank of questionable gasoline, back off the throttle and ride carefully until you can find and render harmless this demon visiting destruction upon your motor.

 

[DesmoDog]

The simplest example of high octane slow burning is Methanol which may need as much as 20 degrees more ignition lead and yet still resists knock better than gas. It's better toi think of race gas as a chemical soup which is why it's so expensive and why there are so many choices.

But methanol is not gasoline.... I typically put the disclaimer in posts on detonation that I'm talking about pump gas only, not other fuels. Forgot to here.

 

[DesmoDog]

Desmodog has some of the statements a little off. Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states he is contradicting himself. If the burn is slow, it is by definition, not detonation. However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

A slow burn is not detonation. True. A fast burn is not detonation either, if by "burn" we mean something caused by a flame front. Detonation is auto-ignition. the mixture is consumed due to temp and pressure, not being exposed to a flame.

Normal combustion is propagated by a flame front. I think everyone agrees with that? When detonation occurs, this flame front does not suddenly speed up to five times it's normal speed. When detonation occurs, the unburned mixture auto-ignites, independent of the existing flame front.

So either you consider detonation to be a "fast flame speed" (which I do not) or you consider it to be auto-ignition.

 

[Sonreir]

Desmodog has some of the statements a little off. Detonation by definition is a fast flame speed....The difference between burn and detonate, in a pure definition, is flame speed...

However, when he states he is contradicting himself. If the burn is slow, it is by definition, not detonation. However, if I had to guess, I believe Desmodog is referring the the extra heat caused by a too slow burn that can cause the remaining mixture to actually detonate....

Here is a non-wiki reference, from Mikuni on it:
http://www.mikuni.com/tg_detonation.html

(I bolded and underlined the specific definition)

Interesting read, Rich, but a lot of that information goes against things I've read elsewhere. I'm not saying it's wrong, but I do advocate further research into the topic.

Just a few points:

• The flame front travel speeds quoted by Mikuni seem WAY too fast. Most other sources I've read say somewhere between 20 and 80 m/s.
• Mikuni says detonation is when the mixture ignites before it should. I could be splitting hairs here, but that sounds more like pre-ignition to me. Detonation usually occurs after the spark has ignited the mixture. The increasing heat and pressure from a partial burn causes the remaining unburned fuel to spontaneously combust. That's detonation.
• Mikuni's list of causes, while not inaccurate, is superlative; what causes detonation is heat and (too) low octane fuel. Anything that causes heat can (but not necessarily will) cause detonation. Again, their definition here seems to muddy the waters between detonation and pre-ignition.

 

[teazer]

But methanol is not gasoline.... I typically put the disclaimer in posts on detonation that I'm talking about pump gas only, not other fuels. Forgot to here.

Understood. I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating.

BTW, VP make specific mention of the fact that Octane level is only related to knock resistance. They do not offer any data on flame rates or BTu/# numbers. Billy Alvaarson says that flame rates are in the 30-50k Meters/sec range but doesn't quote his source. Mikuni say 19-25K ft/sec which works out to 5-8k m/s So the data is a little confusing at this point. Suffice it to say that it is not One number but a range and some are slow and some are fast burning.

Detonation by comparison is essentially an explosion where flame rates are in the supersonic range

edited to add the K after the metric numbers that I managed to leave out before.

 

[Sonreir]

Understood. I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating.

BTW, VP make specific mention of the fact that Octane level is only related to knock resistance. They do not offer any data on flame rates or BTu/# numbers. Billy Alvaarson says that flame rates are in the 30-50M/sec range but doesn't quote his source. Mikuni say 19-25Kft/sec which works out to 5-8m/s So the data is a little confusing at this point. Suffice it to say that it is not One number but a range and some are slow and some are fast burning.

Detonation by comparison is essentially an explosion where flame rates are in the supersonic range

25,000 ft/sec is 7620 m/s.

 

[DesmoDog]

Understood. I was using it as an extreme example to illustrate the disconnect between flame rates and Octane rating.

Got ya.

I don't do such a great job trying to explain myself at times but I think you and I (and Sonreir?) agree on things, even if I do muddle an explanation here and there.

 

[teazer]

I think we are all on the same page.

 

[LArzFroMArz]

Interesting comment by Sonreir, something to the effect of "I don't know enough to write a book". I like his style of writing so I decided to strip his posts (and a few pertinent other posts) so I could read it offline. Once the 'fluff' posts are out I get a Word document over hundred (that's 100) pages at over 44 thousand words. Not all contributions are Sonreirs' to be fair but the bulk is what he posted. I was told once, "Readins good"... I look forward to this one. Love to see the "article" assembled and made a locked sticky for just the pertinent thread info (less the fluff).

 

[teazer]

Larz from mars,

CB92 tribute, 65 Cb77, say more please.

 

[crazypj]

25,000 ft/sec is 7620 m/s.


Just a few points:

• The flame front travel speeds quoted by Mikuni seem WAY too fast. Most other sources I've read say somewhere between 20 and 80 m/s.

At 10,000rpm you have 5,000 firing strokes.
60 / 5,000 = 0.012 seconds
It's why high dome pistons are a bad thing, they make combustion path way longer than it needs to be necessitating excessive ignition advance (XR750 needs around 55+ degrees BTDC to reach 9,000rpm - 4,500 firing strokes)
The burn rate really doesn't increase much with temperature or pressure so the time available for flame travel is 'fixed' by the bore diameter and spark plug placement (why several motors used dual plug heads and found more power plus better fuel economy)
someone else can do the math

 

[Sonreir]

That's interesting. I was always under the impression that increased compression and turbulence did have an effect on burn rate. I had always assumed that this was the reason that full ignition advance is usually achieved relatively low in the RPM range.

Any more info on this?

 

[LArzFroMArz]

At Teaser... I don't want to jack this thread. Tell me you where you want me to put it and I'll post about it there. Essentially the tribute bike will a Ca160 based frame, knee-tuck tank made from CL72 scrambler tank. The 160 is being modified for 5 speed and hopefully some more grunt. Other than it it'll be red and silver... and fun and 1/10 the cost of real CB92

 

[teazer]

@Larz please start a new build thread. It would be very interesting to read.

@PJ/@Sonrier, as far as I know, burn rates increase with turbulence (squish) and with RPM Honda discovered that as engine speed increases, flame rates also increase basically in proportion. So at 20,000 PRM it takes less time to burn as say 10,000 which is just as well because there is only half as much time for that burn to take place and still reach peak cylinder pressures by 15 degrees ATDC. If burn rate stayed the same, peak pressure would happen later and later in the cycle.

Let's say that at 10,000 burn starts at 35 BTDC and peak is at 15 ATDC, for a critical burn over 50 degrees. At 20,000 it would take 100 degrees of burn to reach peak pressure but of course the piston would now be at 65 degrees ATDC and could not peak because the volume into which the burn occurs is increasing faster than combustion pressure.

In reality Honda discovered that they could use fixed timing on their GP bikes and flame rates increased nicely with RPM.

PJs point is spot on about high domes. High domes leave a combustion chamber shape like the skin of half a tennis ball that's long and thin and is a very inefficient combustion chamber shape and requires huge advance to get the fire going.

That is why some of us do not use very high dome pistons but use lower domes and flatten the combustion chamber and build in an effective squish band to increase turbulence and flame speed.

 

[crazypj]

Honda have been developing engines which more or less detonate above a certain rpm for at least 25 yrs. (they were only in Belgium for about 10yrs until the engine development was 'leaked')
In all probability something close to detonation always happened and actual flame travel was never really known just assumed?