Lets give our comments some boundaries. Lets say we are talking about Mopeds, Gopeds, and light duty motorcycles. Not MX bikes or pocket bike.
> jordan * Wrote:
> So what sort of difference in power would be found between two different
> motors tuned for the same peak rpm, but with different exhaust timings?
> Like in your example, with 178' timing and 10k rpm peak, the resulting
> tuned length is 40.5". With 144' timing and the same 10k peak, we get
> 34.7" pipe length.
Yes, the exhaust length would be the same, but the load each engine could carry would be different The port dimensions of most 144' engines (that I have experience with) will prevent a moderate BDT for a pipe, in favor of a longer power stroke and transfer ports that support a low rpm peak torque.
Depending on the engine size and load, the 178' engine has the potential of being a poor performer because too much of the power stroke was cut out of it. In this case increased BDT doesn't imply more performance.
A compromise for a moped sized engine would be 166 ex dur, 83' EO, 60' TO, 23' BDT.
High HP% improvements come from stacking peak HP on top of peak Torque at the highest rpm possible. This statement implies that there is only a very narrow range of TL that is capable of HP increases in the 45 to 50% range. One pipe can do it all!!
> Would I be correct in assuming that the 144' timing motor would make
> more broad power, but less Max power?
Yes. Less peak hp because of transfer port and rpm restrictions. The type of power depends the design of the pipe and cylinder. But it doesn't really have anything to do with front and rear cone angles. Unless it is just a poor design.
Broad power for a 144' engine with a low BDT can be achieved by good pipe design. Good examples are both 50cc Honda & Yamaha scooter mufflers. Good scavenging mufflers will produce 25 to 30% more HP than an engine with no pipe.
> I know the blowdown time has a huge impact on this. Like if the blowdown
> time was matched between the two motors, would the lower timing 144'
> cylinder be just as peaky? let's say our random blowdown number is 27,
> we'd only have 90' transfer timing on our low timing cylinder, but on
> our big timing 178' cylinder we'd have 124' transfer timing. Wheras in
> more real life practice, that 144' cylinder would probably have a
> similar transfer timing and much less blowdown as a result.
>So the broader power is more a function of how much blowdown the cylinder has?
Keep in mind that the idea of a peaky powerband is different when you compare a dyno graph to a test ride. A dyno graph may show peak HP to only last 500 rpm (compared to peak HP 2500 rpm wide). It is still possible for the dyno graph (with a narrow 500 rpm peak HP) to still have great acceleration, a broad powerband and great peak HP and good power after the peak.
But when we ride a goped/moped, we may make a change to the pipe or engine and discover it has a hard hitting powerband and conclude that we made a good change. So we may also consider a hard hitting powerband to have peaky HP, and that is a good description. The problem is hard hitting powerbands do not indicate an increase in peak HP. (I wish I had bold print for the previous sentence).
Instead it is the tell-tale sign of horsepower that was lost before peak HP occurs. The more HP lost in the low to mid range, the harder the engine hits the pipe.
Bill M. (edited)