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

teazer

Over 10,000 Posts
DTT BOTM WINNER
I remember there was an SAE paper or similar way back when. Or at least I think I do....
 

doctorlumen

New Member
Can you guys expand on the "domed piston" concept a bit? I recently finished a CB450 build using some highly domed pistons from Charlies Place thinking I was helping the motor along some. But no?
This is the single greatest motorcycle forum thread on the internet. Thanks for posting it up!
 

crazypj

Split personality, I fake being smart
A high dome ~70mm piston will react as though it's a 'flat' 90~100+mm piston. the flame has to travel from spark plug, up one side, over the top and down the other side.
They actually work very well in low rpm situations and improve torque. ('low' around ~3,500rpm max)
Smaller bore engines have less total distance from plug to edge of combustion chamber so resist detonation better, particularly 4 valve/cyl with central plug and squish or quench area's
I've always found it better to use flatter pistons and reduce combustion chamber volume by milling head (sometimes pretty difficult to do)
Flame speed can only increase so far due to temperature, swirl, tumble, etc before it becomes spontaneous explosion, it is in effect 'static' speed only loosely related to engine speed
 

Mcgoo

Been Around the Block
Love this thread - definatelly needs a re-read or two, but I can feel one of those "brain-growth" moments coming on.. Love learning new (to me) concepts/facts that have a practical purpose. :D
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
A certain vintage road race team bought a cam from me some years ago for their CB77 based racer. It had huge tall piston domes and would not rev. That's when they asked if I could get them a cam like the one I raced with, but their motor would not rev. It just went flat and adding more timing made no difference. The flame took too long to raise pressure in the cylinder.

They stripped the motor and machine the crowns down a bit - slight improvement. Then they repeated that until the crowns were quite low and it was revving properly and making power. On that motor, the combustion chambers is only 54mm wide but the piston is 64 or more mm wide, so it had tall bumps on the pistons to raise compression. More compression = more torque and more power but only if it can burn fast and not detonate.

That's one reason why tall dome pistons work well at low speeds.

Modern race bikes run to 20,000 revs and you can pretty much bet that they do not have tall dome pistons. Old bikes, not so much :)
 

crazypj

Split personality, I fake being smart
Kinda kicks the faster burn at higher rpm into touch?
5,000ft/sec isn't going to go much faster at high rpm, fuel can only burn so fast before it's more likely to detonate
 

Sonreir

Oregon
DTT SUPPORTER
The thing is, the fuel doesn't really need to burn very fast.

Let's say you have an engine with 60mm stroke revving at 20,000 RPM.

One revolution means the piston goes up once and down once; 120mm. 120mm * 20,000 = 24,000,000mm (or 2400 meters ever minute).

2400 meters is 7800 feet or so. So if your fuel is burning at 5000ft/sec (which I think is a pretty darn high number, personally), it's still 50 times faster than your piston is moving.

The number I've read (Bell, I think?) quoted at 80m/sec for flame front travel? And with a 50mm bore, the flame should travel across the cylinder in less than a millisecond. One millisecond at 10,000 RPM is 60 degrees of rotation, though, so the domes definitely matter.
 

crazypj

Split personality, I fake being smart
Ah, but, you need max pressure to occur roughly 14 deg ATDC which gives far less time to complete combustion
I think I may have meant 15,000ft/min as well, (trying to remember without looking anything up ;) ) the slower burn means pressure is pushing on piston for relatively 'long time'
Diesels work by creating high pressure from compressing a fixed amount of air then add fuel from around TDC to keep things burning longer, usually to around 60 deg crank rotation (that's why high performance turbo diesel race trucks blow so much black smoke - mostly carbon from partially burned fuel)

*80m/s is roughly 265ft/sec
 

TijmenK

Been Around the Block
Thanks for this write-up! Especially for the great explanation of optimizing the "intake tract", but it's left me with two questions:

How do I determine the intake duration of the camshaft? For now I used the 221 degrees you used in your example, also because it sounds like a pretty reasonable number, because I couldn't find it anywhere in my Haynes manual and also can't think of a way to calculate it. This is kinda crucial for me because using the 221 degrees I found I need a 22mm carb instead of the stock 26mm CV carb currently installed on my Suzuki GN125. It seems like a pretty big jump, but I found quite a couple of 125cc bikes running carbs this small...

I'm looking for a new carb because I want to get rid of the airbox (I know, I know) and I think a roundslide carb like the VM22 will handle pods or UNI-like foam filters much better, which brings me to my second question. The airbox is what currently gives the intake length, right? So if I would put my pod on a pice of tube to reach a 400mm intake and capture the 4th wave would that lessen the negative impact pod filters have? And does the length of the filter add to the total length of the intake tract? Thanks again!
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
26mm CV carb flows much less air that the equivalent round slide and that one is probably closer to 22-24 of effective area, so that's not an issue. It isn't impossible to get the bike to run OK with a pod and those carbs, it's just harder to them right that a slide carb.

The airbox is not part of the intake length. Pressure waves "see" that as an open space. Where it makes a difference is that the air in their is still and not blowing and turbulent.

To put the whole inlet tract length thing into perspective, it's worth probably 2% power at the peak compared to a similar intake of the wrong length, and loses almost as much off peak and getting jetting wrong can cost 10-20%.

If you really want more power, then a larger bike is probably the easiest route, but failing that, higher compression, larger piston, 24mm carb with velocity stack inside a large capacity filter are things to consider.
 

TijmenK

Been Around the Block
teazer said:
26mm CV carb flows much less air that the equivalent round slide and that one is probably closer to 22-24 of effective area, so that's not an issue. It isn't imp[ossible to get the bike to run OK with a pod and those carbs, it's just harder to them right that a slide carb.

And the only difference would be that slide carbs are easier to dial in, or would they offer a performance improvement too? I know I have to be more careful with the throttle on a slide carb, but I suppose more control would lead to (slightly) more power?

teazer said:
The airbox is not part of the intake length. Pressure waves "see" that as an open space. Where it makes a difference is that the air in their is still and not blowing and turbulent.

To put the whole inlet tract length thing into perspective, it's worth probably 2% power at the peak compared to a similar intake of the wrong length, and loses almost as much off peak and getting jetting wrong can cost 10-20%.

If you really want more power, then a larger bike is probably the easiest route, but failing that, higher compression, larger piston, 24mm carb with velocity stack inside a large capacity filter are things to consider.

A larger bike is out of the question, I'll be counting the days until I can upgrade my license but until then I'm SOL.

After researching pistons and compression ratios for an hour I feel like I'm in way over my head, so that might be a project for next winter, but I want to have as little downtime as possible and think half a year of wrenching and riding will help a lot with big "surgeries" like this.

And which of these four filters do you mean?
My plan was doing something like the last one, but with a UNI-filter. Mainly because my gut-feeling tells me to.
 

DesmoDog

Been Around the Block
Tijmen said:
How do I determine the intake duration of the camshaft?

With a dial indicator and a degree wheel.

You need to know what lift the measurements need to be taken at. For example I'll say 1mm. Use the dial indicator to keep track of lift, degree wheel to keep track of degrees. Turn the crank until the lift is 1mm, note the degrees on the wheel, turn the crank until the lift drops back to 1mm, and note the degrees on the wheel. The difference between the two is the duration.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
Slide carbs fall in a hole when you open the throttle too fast because there's an immediate drop in charge velocity, and as velocity drops, so does the amount of fuel picked up. Consequently it bogs for a moment until it picks up steam.

With CV carbs the slide is operated by pressure differential and as you bang open the throttle, the slides stays where it is until conditions change.

But a slide carb tends to flow better than the equivalent sized CV carb and sometimes that allows the motor to run harder. A carb that's bigger than the motor needs makes less power and runs badly compared to a correctly sized carb. That's a bit of a generalization but think of it this way. If you were to breathe through a small straw and you sat still, no problem. if you get up and walk around, you need more air. As you run you need a bigger straw. You eventually reach the point where you are getting all the oxygen that you need, and what's holding you back now is your lung capacity or legs or whatever, but having more air available won't help.

It's the same with engines, bigger carbs only help if they are the limiting factor. As you make the engine run harder with bigger cams or big bore pistons, it needs more air and at that point a change to larger carb will help. So bigger carbs will not always make more power if that's the only thing that changes.

Hope that all makes sense

The best of those is #3 where there's a proper velocity stack and it's protected by a filter. It would be better if that filter were larger, but who's counting.
 

TijmenK

Been Around the Block
Alright, thanks! A filter at the end of a stack would be even better than over it because it would allow more air in, right? Like this: http://i.imgur.com/fHlekbl.jpg


And are pistons model specific, or would every piston with the right bore fit in the cylinder? I suppose the linkage to the crank could be different? I found a high compression 57.0 mm high pressure piston for a Yamaha 125 bike, and don't see why it wouldn't work, seems like a pretty easy upgrade with a good bang for buck ratio.
 

crazypj

Split personality, I fake being smart
Piston mods are specific, you may have to machine parts for clearance. You can't use two stroke pistons in fourstroke motors (unless you want an oil burner ;D )
There are very few direct swaps when doing big bore conversions.
You should be able to go to around 145cc though, it's major work to go much bigger.
You will need better valve springs to cope with higher rpm, they float, touch something, break off heads and destroy motor
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
The key to the "filter over stack" approach is to ensure that the open end of the stack can get enough clean air from the sides to clean up flow.
 

TijmenK

Been Around the Block
crazypj said:
Piston mods are specific, you may have to machine parts for clearance. You can't use two stroke pistons in fourstroke motors (unless you want an oil burner ;D )
There are very few direct swaps when doing big bore conversions.
You should be able to go to around 145cc though, it's major work to go much bigger.
You will need better valve springs to cope with higher rpm, they float, touch something, break off heads and destroy motor

Sounds like making this a winter project wasn't a bad plan at all, thanks for the info!

teazer said:
The key to the "filter over stack" approach is to ensure that the open end of the stack can get enough clean air from the sides to clean up flow.
Cheers!
 

mydlyfkryzis

当有疑问时踢你的敌人在生殖器上,你可以道歉后
Bear in mind, a CB350 is 325cc, about 20 Cubic inch. STOCK, is develops 36Hp at the crank, with is 1.8 HP per CI. That's pretty darn good..... Think of a Mustang with a 5 liter V8 (305 CID). That would be a 550 HP 305 in the same state of tune as the 350....

A modern 2014 Mustang, with fuel injection, variable valve timing, computer controlled ignition is rated 420 hp from 305 CID. Go back to 1967, a Mustang with the 390 CID (6.4 Liter) was 320 HP. (0.8 HP per CID)...thought to be pretty powerful in the day.

Just illustrating that a CB350 is a pretty highly tuned little engine. Of course you can improve it a little, but it is a lot of work to get the little bit more....
 

Sonreir

Oregon
DTT SUPPORTER
It's not uncommon for CV carbs to be much larger in diameter than slide carbs, so 26mm to 22mm may not be as crazy as it sounds.

I'm not sure on the duration for the GN125, but hopefully someone else can chime in. You'll definitely want to use the correct numbers for your bike.

Finally, the length of a pod filter should not be included into the intake tract, but a velocity stack should be. It's the point at which the intake opens to ambient pressure which is important. So in a bike with a stock airbox, you'd measure from the intake valve to the airbox, not from the intake valve to the opening in the air box.
 

DTT Bike Of The Month Gallery

DTT Light or Dark

www.jadusmotorcycleparts.com
www.cognitomoto.com
https://www.townmoto.com
www.speedmotoco.com
www.lostapostle.ca/
www.sparckmoto.com
Top Bottom