Four-stroke performance tuning?

[F5 Dave]

Cool t hay were, but in reality the 2 stroker had had years of GP development at that stage and the 4's were new - kind of like Hoyosung entering Moto GP today - it may take a few years for them to get up to speed! (be interesting to see what BMW does).

You'd find that same impressive 2-stroke is probably a P.O.S competing with the current electronic masterpieces in MotoGP.

I have read a few books by riders of those 2 stroke GP bikes - the yall describe them as untamed monsters!

At the end of the day if they were still that bloody good they will still be on the track today as you race what wins.

Bring it on.

The 500cc class used to be just that. 500cc of 4 cylinder do what you want. Honda tried to compete with oval pistons, fists full of valves & whatnot - but failed dismally. Make the class 500cc & open to all naturally aspirated engine types & see that the only real racing engines would be what they always were. Until Ghey old men F'd with the class. & where has it got them now? Boring racing & they know they are in trouble.

But the new class had to beat the old class of 500s so they made them twice as big. The F2 class is even more silly, 600cc fours against 250 twins. Gee that's fair.

[AllanB]

Until Ghey old men F'd with the class. & where has it got them now? Boring racing & they know they are in trouble.

But the new class had to beat the old class of 500s so they made them twice as big. The F2 class is even more silly, 600cc fours against 250 twins. Gee that's fair.

I'd forgotten about the 500-990 displacement.....

Silly really as who was going to tame a 900cc 2 stroke

Not sure how they derive their formula for the comparison - presumable due to firing 2 against 4?

An open class would be exciting and innovative for sure but ultimately whoever produced the fastest would probably be copied by the others.

Oval pistons - their original theory was to skirt the racing rules - they were aiming for a V8 engine but still presented as a legal V4 (only 4 cylinders even if they were oval.....). innovative for sure but doomed early on and I think it was corporate pride that allowed it to continue as long as it did.

[speedpro]

Not sure how they derive their formula for the comparison - presumable due to firing 2 against 4?

An open class would be exciting and innovative for sure but ultimately whoever produced the fastest would probably be copied by the others.

They derived the formula based on how big a 4-stroke engine was needed to beat a 500 2-stroke.

As for an open class leading to copying you only have to look at bucket racing to see that NOT happening. Bucket racing probably has the least rules of any form of motorsport and there is still a huge variety. Hell, some people are even racing 4-strokes.

On the more serious MotoGP side, within a set of rules there has to be only one ultimate configuration but there seems to be quite a variety still. Don't forget that even at this level that the marketing guys will be having a say in what gets designed, built, developed, promoted and sold. Racing after all is just a way of promoting your brand.

[AllanB]

at this level that the marketing guys will be having a say in what gets designed, built, developed, promoted and sold. Racing after all is just a way of promoting your brand.

Which makes you wonder just why did Honda not capitalize on the V5 configuration a few years ago in a street version

A question asked may times, and I personally suspect any gains made in the engine over their IL4's on the street were limited.

Either that or they are gay.....

[UNSTABLE]

Which makes you wonder just why did Honda not capitalize on the V5 configuration a few years ago in a street version

A question asked may times, and I personally suspect any gains made in the engine over their IL4's on the street were limited.

Either that or they are gay.....

Quite simple really, it costs more to produce a 5 cylinder V engine compared to an inline 4. Joe punter would have to foot the bill in the end for very little return in performance, as the road version of a 1000cc bike these days is far more than most throttle jockeys can handle...

[Buckets4Me]

http://www.trademe.co.nz/Trade-Me-Mo...-224536122.htm

here you go I'm sure you could have the rules changed to allow this in F$
I mean F4 and then 4 strokes would start winning

and I wonder what the balance factor on that would be

[Kickaha]

So who can tell me anything useful about rod to stroke ratios?

[k14]

The higher the rod to stroke ratio the closer you are to an explosion...

[Buckets4Me]

So who can tell me anything useful about rod to stroke ratios?

Rod to stroke or Bore to stroke ??

not sure but I always thought that a square ie same bore stroke was a good compromise

nissan thought a short stroke was the way to go and toyota like it over sqare a bit

always thought that short stroke more revs (hp) at top end
longer stroke more touque less revs better pull out of corners

thats my uneducated guess

(sits back and waits for people to correct him)

[Kickaha]

Rod to stroke or Bore to stroke ??

Rod to stroke

[speedpro]

Kawasaki used a 2mm longer rod in their ZXR750s they raced at one time and Yamaha went to a 2mm longer rod in the 3LN FZR250 with a correspondingly shorter piston. Due to geometry as the crank rotates the peak piston forces are reduced which if you take advantage of that allows you to use higher revs an not exceed the limits.

If the rod was infinitely long the piston velocity would vary at a times 1 sinusoidal rate with crank rotation. As the rod is shortened peak acceleration and peak velocity increase. I can visualise it but the math escapes me.

Short stroke and long rods allow an engine to rev higher without exceeding mechanical limits. Engineers design engines to take advantage of this with the result that most people think that a short stroke engine is peaky and high revving "because" it it short stroke.

[speedpro]

Short rods will increase piston thrust face pressure as well.

Actually the more I think of this the more I remember there is to consider. Things like pressure gradients in the cylinder versus piston acceleration about TDC versus crank/rod geometry.

A good mod for the V8 boys is to fit a 400 crank into your 350 block. They end up with a longer stroke but have to use shorter rods. The rod length to stroke ratio changes a lot. You end up with a pretty grunty 380ish motor. Grunty partly because of the increase in capacity but also due to the rod/crank geometry. Thrust face forces increase because of the angle the rod is at as combustion forces push the piston down and also as the crank rotates and the piston is pushed back up.

[Ocean1]

So who can tell me anything useful about rod to stroke ratios?

Variable is rod length but critical factor is rod angle.

Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
(or)
Sine of Rod Angle = .5 ÷ R/S



Rod Angle “n” Ratio Examples Comments
13½° 2.142-1 High speed motor with small ports. Best breathing with small ports
14° 2.067-1
14½° 1.997-1 Long rods for good breathing with small ports
15° 1.932-1 Long rods to help breathing with small ports. Responds well to stroke increases (“n” value too large for intake port size)
15½° 1.871-1 Responds well to stroke increases (“n” value too large for intake port size)
16° 1.814-1 Approximate “ideal” compromise between stress & breathing (1.81-1)
16½° 1.760-1 Good choice for motors with good breathing
17° 1.710-1 ”Safe” limit for thrust angle. Approaching practical limit for street motors
17½° 1.663-1 Approaching practical limit for street motors
18° 1.618-1 Approaching practical limit for street motors. Good power due to large intake port
18½° 1.576-1 Limited street use
19° 1.536-1 Good power due to large intake port
19½° 1.498-1 Not practical for street use due to short pistons
20° 1.462-1 Poor peak power. Longer rods are not used in any serious application

[TZ350]

Variable is rod length but critical factor is rod angle.

Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
(or)
Sine of Rod Angle = .5 ÷ R/S



Rod Angle “n” Ratio Examples Comments
13½° 2.142-1 High speed motor with small ports. Best breathing with small ports
.

My rod is 110mm and stroke is 50mm.

Sine of rod angle = 50 / 110 * 2)

Sine of rod angle = 50 / 220

Sine of rod angle = 0.227

..........rod angle = 13 1/8 degrees (13.12 deg)

Have I got this right???? does this make my motor Best breathing with small ports ??????.

Hope so because small transfer ports is what I have on the Suzuki GP125.
.

[Ocean1]

...There's stuff missing.

Here:

Effects of Long Rods
Pro:
Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.


Effects of Short Rods
Pro:
Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

Angle Limitation

The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection (this is not the angle of maximum thrust, which occurs when the rod’ beam axis is at 90° to the crank throw or journal, typically between 70-76° ATDC; however, the math is easy to do). Angles beyond 17° (where the rod axis is 90° to the crank throw at 73° ATDC) promote excessive wear at the piston major thrust surface, and piston breakage could be the result. Before you purchase connecting rods that are shorter than previous or increase the stroke of the crank, calculate the new rod angle. High rod angles will require quality rods that have been checked for cracks and have quality fasteners. Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.