Boysen ports... BOTTOM,TOP,or both?

joeak47

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Apr 21, 2012
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Is there one advantage over ther other? How about both top and bottom at the same time? I'm designing the jig right now,so I thought I would ask. What diameter should the holes be? If nobody knows,I'll just wing it "Joe style". It looks like they will definitely give/promote a larger air/fuel charge to enter the cylinder. I've already started on my "cut away" cylinder and was able to get 3/8" holes no problem.
This jig will make boysen ports fast and easy. Probably about a minute each total time,which includes time to set the jig. I wonder how much extra power these will create?. I also wonder how well they will work in conjunction with a piston boost port. Hmmmm....
 
Sounds like a wonderful start to me! I honestly have no idea which is better, and what size to make them. I say, just do it what you FEEL is best and report back on us the results I:I
 
If you shoot them to the top, they will enter into the back of the "kicker" in the secondary transfer ports. It will steer the charge coming up the transfer port basically straight out of the exhaust without ever pushing it around the cylinder. Been there, tried that.

Stick to the lowers and keep them angled down where they shoot straight through the "meat" of the mounting ear. The size is a tradeoff between strength and power. At the rpm that the boyesen ports become really effective, charge is whizzing through there pretty quick so the bigger the hole the more it can flow (basically, you can't get one big enough in the stock mounting ears to be "too big") however you start to reach a break even point where you're stuffing the cylinder more and more effectively but you're reducing the strength of the weakest mounting ears for the entire engine. I don't think you're going to get to the point you'll rip one off on gasoline but you enjoy tinkering with your head design an awful lot and detonation can raise the cylinder pressures DRAMATICALLY over what they will be under normal operating conditions. Sufficiently weaken the already weakest two mounting ears and then slap a head on that's going to knock like grandma's hip once it gets hot and you might find out what's too big as far as boyesen ports go.

Alright, I don't want you to take this the wrong way (it will probably come out callous enough as it is so I'm sorry for that) but I have to question what you hope to gain out of the "piston boost port" (I'm assuming your plan is to cut another hole in the intake skirt side of the piston).

Basically, your engine isn't of a high enough tune to need the extra wrist pin oiling you may afford by putting the piston boost port in and I fear you are weakening the already weakest link inside your engine (:o Oh no, here it comes), that cast piston. They're known for shredding intake skirt parts as it is (granted, generally as they begin to slap)....
 
What you are saying about the top boysen ports is/was exactly as I was seeing it too. Just wanted to clarify. I guess cutting some lower boysens ports will show me how much more power they will add. This should be interesting. Stay tuned...
 
sorry i thought you were the dg pipe promoter and user

i like to use the lower boysen ports and smooth out the edges after you drill the hole. seems like it would help with crank case filling and help out bottom-mid range. i dont however use a jig. i have been placing the bit just above the base gasket area and then keep the bit as horizontal as possible without damaging the base gasket sealing surface with the drill or bit, keeping it in the V of the secondary transfer. i would like to build a jig though. never seen the upper boysen ports done on a cylinder.
 
James, I can see that lower is clearly the way to go, but as far as diameter, is there a trade off for crank case volume?
Some of the double 10mm hole boyesen ports would add a lot of volume to the crankcase vs what sort of performance gain?

Steve
 
I keep reading,and reading,and reading,but I never can find a clear cut answer about what the increase and where the increase (rpm range) will be made in an engine with the addition of boysen ports. This is why I'll just figure it out systematically. I can see why the top boysens make no sense. It will blow a large quantity of the air/fuel charge out toward the exhaust port. How much of that would bounce back into the cylinder with the sonic wave from the pipe,right before the piston closes the exhaust port...I have no idea.

For right now I can get two 10mm holes stacked in the right place without issues. I was thinking about a tear drop shaped boysen. It would be larger in volume compared to the two round 10mm holes. I could also blend the two
10mm holes into an oval hole and perhaps gain a bit more volume. Hmmm.

I'll start progressing more on this when I'm not sleeping 13 hours strait on my one day off. I do take my note book to work with me and jot down my ideas and sketches. That way i have something to build upon when I get the time at home. I think that a piston boost port might add a bit more power,but the main reason would be to gain added lubrication for the wrist pin bearing which is a good thing,plus I believe that it will help cool the bottom of the piston crown.
 
I think that a piston boost port might add a bit more power,but the main reason would be to gain added lubrication for the wrist pin bearing which is a good thing,plus I believe that it will help cool the bottom of the piston crown.

Cooling maybe, but not lubrication.

Fuel and Premix are not lubricants, lubrication is provided by the oil that comes out of suspension, pools in the crancase and thrown around by moving parts.

Throwing raw fuel at the wrist pin and the underside of the crown could have a detrimental effect of uneven cooling and lubrication loss.
 
The ONLY engines I have seen with piston boost ports are drag engines running for 30 seconds and usually on exotic fuels.

Blaaster makes a good point, all of the fresh fuel spraying on the small end of the rod may actually have a "washing" affect for the lube that would normally be being thrown up there during operation. Proceed carefully...

Large diameter boyesen ports are a double whammy..... at lower rpm they provide a path for the charge to pass backwards into the intake area towards the reeds and close them earlier than the lower intake windows only (not much POWER gained there but maybe some torque that wouldn't be there otherwise). However that comes at the detriment that it passes some of the crankcase charge into the intake tract instead of forcing it up into the combustion chamber. To that end, you HAVE to make sure that the transfer ports are aimed properly for good scavenging AND the pipe has a sufficient suction value to pull the charge up the transfers too.

At higher rpms, it allows a passage directly into the transfer ports short circuiting the normal process of going through the piston ports, turning downward, passing around the crankcase, and turning upwards into the transfer ports. They also allow the charge passing through the reeds faster than they can close to bypass the quickly closing intake windows (as the piston ports pass up and down behind those windows.

The difference between a cylinder with boyesen's and without isn't that great, I assure you, but they aid a small amount in some of the finer operations of a cylinder properly tuned with them.
 
Neil's present piston and bore are on their last legs.
Before we pull it I am going to insist we drill a boost port on the well worn piston and test.
We can probably do the whole test in less than an hour, looking like tomorrow.
Anyone have any other suggestions to try?

This is before we install Joe's cylinder with the hemi head.
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The high boyesen redirect of the transfer flow would short circuit the flush of exhaust
out of the cylinder, trapping exhaust in the cylinder and directing fuel mixture straight
out the pipe in my opinion. No amount of pipe backflow would compensate for this.
Reduced power and increased fuel consumption would be my guess.

Volume is a bad thing to add to the crankcase, in my opinion Joe.
I'd start small, with a single 8 or 10mm hole and test it.
Then later, pull it and try a second stacked hole.
If it works better, fine! If it doesn't, easy enough to epoxy a hole.
Just my measured one step at a time approach.

These engines come apart so easy there is no need to try everything at once.

Steve
 
I'm not even sure you'll notice any gains by adding very small boyesens without enough runs on a calibrated dynamometer to take out any standard deviation (7 being the generally accepted number) of the engine itself. The change that the small boyesens allowable in a stock cylinder produces isn't that sort of noticable. I've built and rode them both ways and the difference between an engine putting out a lot of power and not isn't the boyesen ports.

Again, it's a function of the fact that boyesen ports are more effective when they're sized and designed into the system during casting AND that the factory blaster cylinder casting doesn't allow enough space to put very large passageways in or angle them up towards the end as to force the charge up the auxiliary transfer ports when the engine is "stuffing" at high rpm's.