Porting Timing for Torque

[Smalltownbassin]

Come on Chaz! Spit it out! I've been learning as much as I can recently and this is one of the topics that really grabbed my attention. There is a YOUTUBE channel called "2 STROKE STUFFING" that I've been following for a while. This guy has 2 stroke theory down pat, not unlike yourself. From what I've learned by following his progress, exhaust port width is key to make the most power possible (obviously other factors at play here but we are talking about width). The ideal width is 75% of bore but only due to the limitations of piston & ring relationship. If ring snag could be avoided, the exhaust port could even be 100% of the width of bore but that's unrealistic due to the current piston & ring designs. The point is to scavenge the cylinder at high velocity by giving it a wider opening to rush out as quickly as possible. Opposed to a narrower opening that slowly release more of the gases as the piston continues to travel and expose more of the port. Again, other factors at play like port timing, shape & resonance but I'm only talking about size. Now since I'm apparently the only one that wants to know the answer, by all means, spill the beans!

[rude tim]

#1) 75% of the width of the piston
#2) As wide as possible without snagging a ring

you must remember that the width of the port is measured around the bore not straight across.

[Chaz]

you must remember that the width of the port is measured around the bore not straight across.

Just walked in .. B-shift was terrible tonight. I had to put up with me the whole time ..
And what I didn't get done .. I'll do tomorrow .. err , later on today ..

So you make an interesting point. But lets look at it from a different angle.

Mr. Impatient bassin in the big time, spoke about a 100% bore to port width.

Lets say the bore is a stock 3.0L @ 3.625" measured straight across in the center.
Now if you did the math to find 50% of the circumference or even the paper around the bore .. shade it with the pensile .. pull it out , lay it flat .. your gonna need more than 100% bore size to make the trip ..
So I'm goin with .. as the crow flies .. vs. bore to establish percentages ..

Mr. bassin .. are those numbers derived via. a few shots of Irish whiskey .. ?

When you say .. 75% of the piston diameter ... All pistons are built on a taper , narrow at the top , wider at the bottom .. where do you measure from ???

[Smalltownbassin]

My % is referencing bore diameter. So 75% of a 3.625 diameter bore would be an exhaust port that's 2.718 wide. As for the Irish whiskey....YES, there's plenty of that (maybe too much) where these numbers come from . I mistakenly mentioned "piston diameter" when i actually meant bore diameter. Sorry bout that. When my gears start cranking my words just come flying out. Now that i've gotten this can of worms completely open, i'm happy to sit back and listen because i have no doubt that we are about get the answer with experience to back it up

[racer]

Chaz you are correct I wanted to keep it simple. As you know we could also go into when each of the of the intake side open in relation to each other. So many different things that effect power and where that power is made.

Martys garage has a read you might want to look at but one must also understand that is on a motor cycle engine with a gear box so power bands can be much narrower than on an outboard. Its IMO just a read that will help explain how one effects the other.

[FORBESAUTO]

Exhaust port width is a major factor, but shape of port and where the widest point of port is located plays more of role where power is made also. I’ve been able to get away with really wide ports with no issues yet, to make low end torque. Have to dig up my notes on width, but a good bit of aluminum was removed from port leading to chest to accept it. Been running a couple years now with no leakdown issues.

[Merc 2.5]

Exhaust port width is a major factor, but shape of port and where the widest point of port is located plays more of role where power is made also. I’ve been able to get away with really wide ports with no issues yet, to make low end torque. Have to dig up my notes on width, but a good bit of aluminum was removed from port leading to chest to accept it. Been running a couple years now with no leakdown issues.

Have you ported a exhaust port where its ported wider ,and only raise "the middle" of the port and kinda blend it in , lol. I know what im thinkin but can't spit it out just rite. Hopefully u get what im askin

[Dave S]

Easy........2.5 150.......

[wrechin2]

Lol. Huh? I wanna learn all I can so plz go in detail in English if no one else does, fir dummies like me , im n process of doin builds for lite boat and heavy boat and just tryin maximize what i got. After light porting my motor degrees at 34% blow down. The 245 measured 30 or 31% blow down stock and rated at 7500 rpm? I know there's alot goin on in there on angles tho. Has anyone got same or close when measuring on 245? Stock 2.5 200 bout 28 29% just curious on others findings

Interesting thread!!! Gonna add some basic info to clear ya up a little 2.5, it is degrees of crank rotation not percentage.

The 245/260 porting is as follows, exhaust opens at 88* and the transfers opens at 118*.
To find blow down you subtract the exhaust timing from transfer timing. For this engine 118*-88*=30* So BD for this engine is 30*. This is the amount of time the exhaust has to escape.
To find the transfer duration subtract transfer measurement from 180* and times it by 2. For this engine 180*-118=62*. So then we times 62*X2=124* So transfer duration is 124*
To find exhaust duration take the blowdown and times it by 2 and add to the transfer measurement. 30*x2=60* Add 60* to 118*=178* So exhaust duration is 178*
Here is some good reading that may help. https://martysgarage.info/reference/...port-duration/ it is about motorcycles but is relevant. As you see the duration to RPM is very close.

To add some reference a 2.5 200 FF has porting of exhaust at 92* and transfer at 120* with 28* of blowdown.

Your engine has exhaust of 86* with 120* resulting in 34* of BD. With this porting I have found that with the ports widened it loses around 300 RPM but with lower porting like 92* it is very beneficial to widened the exhaust port.

Because I make my own sleeves, I can do a lot of testing with different shapes and sizes. I have found that a exhaust port with a heavy radius has a wider powerband and 15-20 PSI higher compression with the same port height timing. I used to make sleeves for Bobby and I came up with a port map that was basically a 175 without idle reliefs and it was a strong performer on heavy bass boats. Just my personal experience.

[Chaz]

Have you ported a exhaust port where its ported wider ,and only raise "the middle" of the port and kinda blend it in , lol. I know what im thinkin but can't spit it out just rite. Hopefully u get what im askin

Ken a feller jus rap sum grindin pepper ronde his fumb an mash it in a middel moore harrder dan the skide's uv idt .. ???


A feller I no axed me did I wanna see his new steel ... I said .... sure nuff ..

I akxed him ifin my frend dave culd foller a long ..

He said , long as he aint no rev-enuer . I told him I thought Dave was retired and that he had become fiscally responsible earlier in life and already had a steady stream of revenue .

Well all ite than ... he ken foller us ...

I hated to be the bearer of bad news .. but the "steel" was really made from "copper". No , not the police (rev-enure) kind .. but that of a metallic listed as # 29 on the atomic elements chart ..




Turns out it was an early model two - stroke ...

The left side tank was the intake port where the fresh mixture entered the system. He kept calling it a "masche pit" .. or maybe a pot , anyway .. I'm too old to go to one of those rave parties .

I did notice that it had a very aerodynamically efficient top with a thermometer in the convergent cone . I knew my suspicion's were correct when he stated : Ya cape got one of tham thar round thangs that geos ta tellin ya ifin ya got yur far jus rite.

There was a crankcase that built pressure in the middle that at the right time
(thump kag) pushed the fresh charge thru the rod slot , across the "transfer port" and down thru a water cooled exhaust.

He kept calling the water jacket a "worm box" and the tube inside a "koolin curl / drip pap" .
Now if I was going fishing , I wouldn't use worms and if I did , I certainly wouldn't store them in that box . They would never make it off the shelf let alone make it down the escarpment without getting sea sick ...



Dave on the other hand .. laid under the exhaust port for about a half an hour , got up and said ..

Chaz~A~roni .. I had the besssss ---- time ... of my liffffffffffeeeee

[FORBESAUTO]

Have you ported a exhaust port where its ported wider ,and only raise "the middle" of the port and kinda blend it in , lol. I know what im thinkin but can't spit it out just rite. Hopefully u get what im askin

Found on heavier set ups where you can’t afford to loose too much bottom end by raising top of port too high, you can fudge the system by widening the port to gain some more rpm. Depending on where the widest point of the port is in relation to opening/closing of transfer ports. The more degrees the widest point is ahead of the transfers the more upper end power. The closer the widest point is to or after the transfers the lower in rpm range the power is, and usually no gains in rpm, just low end torque. Usually use a trapezoidal shape of the port to achieve this on higher widths and traditional continuation of stock shape on lower widths.

[Chaz]

By the time they got the shape right, they quit making two stroke ...

Going back to my original question of port width to bore size ratio for a moment.

Most of the stock low power two strokes are in the high 50% range. As a reference, a 300x has a 2.330" wide port , which is aprox. 64% ..
70% is considered the end of the world .. and to make it live it needs big corner radius's . James spoke on what else that helps with .. so no need to go over that again.



Raising or even "just" widening the exhaust alone , while it might "feel faster" what you have done is changed the timed port area within the blowdown period. With no expansion chamber to benefit the early open / late closing, events, you more than likely will put a dip in the torque curve and only feel anything as it recovers some of it back at high RPM . If your blowdown event lasts 30* then after your done porting , it should remain at 30* ... time to invest in some ninety degree hand pieces.

Roof angles / exit angles / port area ...

Hard to fill a cylinder when the charges run into each other right out of the sleeve. Smart money not only raised the boost port roof , but angled it up and over the exhaust port. First thing it does is show any lingering exhaust gasses the exit door with a push from two well developed columns . Then as it continues to fill the cylinder, it does so without getting in the way of the main and auxiliary ports.
Smokey told Chevrolet that the "new for 1953" small block head was junk and got fired. But to this day, cylinder head designers have followed his lead by making the turn early, get your column going in the direction you want it to go .. giving it enough time and space to build speed . In the case of a two stroke, the two main columns should meet in the middle and rise together , using the little ones to feed dead area's of flow. Without inside radius material to work with ... our outboards are only about a century behind.
By looking at this picture, we see all of that. It also looks as if we have been wasting a bunch of "port space" on the exhaust. I tend to believe it, since it's a lot more difficult to coax fresh air into the game than it is to push / pull the old ... out ...

[wrechin2]

Found on heavier set ups where you can’t afford to loose too much bottom end by raising top of port too high, you can fudge the system by widening the port to gain some more rpm. Depending on where the widest point of the port is in relation to opening/closing of transfer ports. The more degrees the widest point is ahead of the transfers the more upper end power. The closer the widest point is to or after the transfers the lower in rpm range the power is, and usually no gains in rpm, just low end torque. Usually use a trapezoidal shape of the port to achieve this on higher widths and traditional continuation of stock shape on lower widths.

I agree. The trapezoidal shape is easily achieved on traditional square ports. It would be hard to achieve on traditional mercury 2.0,2.4,and 2.5 ports due to widths. Here is a drawing of my "custom" port beside a 200 FF port. Pay attention to the outside edges as they have been lowered. I may draw up a port and try it with the port being a true trapezoidal shape and cut 6 sleeves with it in it for future test. Would have to simply make it the same volume of flow as a traditional port.

Merc 2.5,
On yours I have found at my target RPM the wider port allows a little unwanted flow and some of the fuel charge is drawn out causing the loss of RPM's.