MacDizzy's Porting 104 - Time The Roar

The response - I concur on the epoxy thing - I even like to have the secondary transfers aimed at a very slight angle upward followed by a flattening off the very edge (maybe just the liner - depending on how thick), so that they compliment the boost port to some extent and help with the scavenging process. And another way to broaden the powerband is to stagger the transfers. The secondaries can be brought up a tad higher than the main transfer.

Trax310

Really? You let the secondaries open before the mains? I've seen it the other way around in quite a few motors though. I never tried that. But I'd be interested to see the dyno result - a before and after test with it done.

Rick

I've found that productive also. I will open the boost port about 2-4° earlier than the secondaries, and the secondaries about 2° earlier than the mains for more midrange (this after achieving the target duration/area for the mains and the objective powerband). In doing this you have improved cylinder filling due to the slightly longer rear transfers' duration. The mixture delivered through them is improved more than you might think because they are subjected to higher crankcase pressure than if the mains were opened simultaneously. This is quite beneficial since, usually by design, they are more restrictive than the mains even after your best efforts (within cylinder metal limits) to reshape them. This trick usually chops a HP or two from the peak though, due to a bit too early opening at higher rev's and a slight tendency towards momentary stagnation or even slight reversion in those rear ports. Also there may be a slightly increased tendency towards "short circuiting" of the main transfer's fuel/air stream since its' velocity has been dropped ever so slightly, so a prudent balancing of this technique is necessary.

For a more high RPM biased setup, I open them all at the same time and lean towards "just enough" tranny duration to do the trick. Inadequate blowdown at high rev's will cut peak power since the piston is moving much faster and time vs. crankshaft degrees of rotation is decreasing.

For a lower RPM spread of power, I delay the opening of the rear transfers, point them slightly upward and also biased rearward. Same with the boost port... upward sharply and late opening. This compromises cylinder filling at upper rpm's but insures against momentary reversion and dramatically reduces charge loss out the exhaust when the piston is moving more slowly and there is time available for this to happen. In this situation, crankcase pressure has also been "bled down" through the main transfers (which also need to be pointed rearward sharply) prior to opening the rears. The sharp rearward and upward biasing of these windows costs heavily on area though, and it will limit the top end drastically. As always, it depends on what we are trying to accomplish. Anybody ever tried "external transfers"?

High output

I can picture what the flow pattern would look like by starting the rear ports first, and it sounds like it would do just about what you

describe. Sounds interesting. I have cut the area of the piston away which is directly adjacent to the port I want to try different timing setups on. It can save time by not having to cut a barrel.

With some big bore kits, the timing gets mellowed out too much from installing the larger sleeve and is almost impossible to get back unless you add a spacer plate or double up on the base gaskets. A little machine work on the piston can make a big difference. I'm not a big fan of resleeved big bore kits. Upping the displacement too much on some motors - ones that have a small crankcase - usually creates problems running at high RPM's because they don't have enough crankcase volume to fill the cylinder and tend to lean out and lose power. Real small bore motors can have a lot of mixture left in the cases during the scavenging cycle since they typically have a tall piston. A hole or two in the piston - high up on the skirt - lined up with a tunnel in the wall can release more cool charge into the cylinder.

Some watercraft cylinders have more transfer port area than I have seen on other types of engines. They go out of their way to make the transfers wide - so wide in fact that in some cases the piston ring end gap directly crosses the port openings. This makes for transfer ports all the way around the cylinder with very narrow separations to support the rings. For what they are, low RPM 2 strokes usually turning under 7000 RPM - they have very radical port timing. Some stock motors have 194° to 196° exhaust duration with 126° to 132° transfer port duration. It is not uncommon for some of these motors to have very poorly designed transfer port tunnels. Though they are limited by the RPM they can turn by their rev limiters they still produce impressive HP numbers. Without the rev limiters the motor would rev to the moon when the craft hit a wave and became air born. Who knows what might happen to the crank when an over-revving engine's impeller suddenly hooks up again. Their multi-cylinder configurations and larger cylinder displacement in each jug helps them get away with the long port duration while they're exhausting it all into a single pipe (compromised exhaust consideration). These motors have to produce a lot of low & mid RPM torque in order to move a fairly heavy vehicle (5 to 600 lb..) through such a high drag medium. It is very "testy" to make a watercraft motor produce more low RPM power, and increase it's overall speed as well. The extra power has to be transferred to the water by additional impeller pitch to gain the top speed since the maximum RPM is (usually) static.

...And no I haven't done external transfers. Though additional external exhaust I'm familiar with.

Rick

Pie cutting piston crowns is something I love to do too. It is much cheaper to play guesswork with pistons than a sleeve. Cutting the hole on the piston's skirt is a great way to cool down the piston a little more. It also can change the inlet duration if the piston actually closes all ports off and calm a hyper motor. On the big-bore thing, I concur - On some of the older motors, primary compression was everything and the motors produced some serious rpms but had very narrow powerbands. The transfers were very short and when they did open they packed a punch. However, there are better ways now and primary compression is not needed as badly as volume - although there is a equilibrium to be established between the two. I have not done any external transfers either, but would love to one day! High Output, what types of motors benefit mostly from this? Later,

Trax310

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