jay smith or prof ob???????

[jerry]

what happens if you mill .030 off the top of the pistons and mill the heads .030 ? is it like porting the motor ?

[Prof. O/B]

Originally posted by jerry
what happens if you mill .030 off the top of the pistons and mill the heads .030 ? is it like porting the motor ?

Yes, it advances the port tiiming by the amount you mill off. This may be OK or not so good, depends on the engine and where the ports are now. Some ports you may not want to increase their height on certain engines. The negative side is you you reduce piston crown thickness (mass) which may or may not be acceptable. It depends on how much power you intend on adding. It also decreases the quench area between the piston ring and the cylinder wall as the ring then becomes nearer the crown of the piston. Again that may or may not be acceptable, depends on what the land area between the ring groove and crown is now. However, reducing the quench area does generate more ring heat and increasee ring expansion. You should compensate for this with more end gap clearance on the ring. Just a thought or two for now, goota go
..................Prof. O/B

[hugh j.]

mi;;ing the pistons and heads would increase the squish by .030 also. i.e a smaller combustion chamber larger squish area probably less compression. milling pistons and decking block would maintain all dimensions other than port timing. this is probably not worth the effort.

kart guy

[us1]

Unfortunately it does increase the squish band volume and thus a greater possibility of detonation.

[Forkin' Crazy]

Originally posted by us1
Unfortunately it does increase the squish band volume and thus a greater possibility of detonation.

That's what was thinking, from what I have read, anyway.

I thought that cutting the pistons was more or less just experimenting before the more permanent porting work.

BTW I have a set of pistons that some one just cut them on the exhaust side.

[us1]

That’s one reason for do it but I have never seen a motor yet that ran good with all the ports raised the exact same amount. You are right though, for experimental purposes it does have its uses.

[jerry]

when you say ALL PORTS you do mean int and ex together ? not all 6 int ?

[us1]

Yes, if you cut the piston top flat it will change the port timing equally on all the ports in that cylinder.

[Terry Jackson]

My thinking is that port timing remains the same as the ring dropping below the port is what opens the port, not the piston height. Dropping the piston height would increase flow volumn at initial opening, but not open the port.

[Raceman]

Terry, in my opinion, that's not completely true. Although there is some initial bleed off when the ring passes the top of the exhaust port, the big flow is when the edge of the piston begins to pass it. Remember, two strokes run relatively tight piston to cyl wall clearances. I used to chamfer the piston only in the width of the exhaust port with a corresponding cut of the heads and found it was an effective way to raise exhaust port timing without cutting on a chrome block. Although a .100 cut on the edge of the piston may not be quite as effective as a .100 raise in the exhaust port (for example) for the reason you mentioned, as well as the fact that cutting the port not only changes timing but volume, it is still noticeable.

[Dave S]

Jerry, just send it to Jay Smith and don't worry about it.;)

[jerry]

jay dont want to touch a 3.0 L

[stvhelm]

Simon Motor sports

[us1]

Yea, send it to Eric Simon.

[Prof. O/B]

Originally posted by jerry
when you say ALL PORTS you do mean int and ex together ? not all 6 int ?

Jerry, I'm sorry I did not correctly read your post when I 1ST. posted. I thought you were trying to say "mill the pistons and the deck", but what you said which was the "heads". And, yes milling the pistons effectively increases the port height of of all the ports. We know this because they will open sooner. But by not milling the deck you will increase the squish band thickness (the distance between the piston crown and the squish band of the cylinder head) as others have noted. Merc has pretty well figured out what the thickness or clearance between the piston crown and and the head should be. It is enough to - effectively cool the squish area with the anticipated rod stretch. Increasing that thickness will result in more fuel trapped under the squish. That leaves more fuel unburned, which other than a distraction from performance in itself is not a bad thing. However, the motors will become more "sensitive" to tuning, and what I mean by that is - if the cleareace is great enough, when you start to lean down the motor and the flame succeeds in burning under the squish band you got problems. You can be tuning and everything is going well, then you go down that one extra jet size and it it suddenly starts burning under the squish. A tempermential situation.

Now if you mill the deck in conjuction with piston you do advance all the port timing why retaining the correct squish if both items (the heads and block have the same material removed from them). Some say this is a bad thing, that once again depends on where your ports are now. If they all can stand to be elevated, then you are going to increase top end RPM. You have allowed the cylinder greater "purge" time to cleanse the exhaust of both the initial and residual spent exhuast from the cylinder, pay particular attention to "residual"; residual is the hard part. That is what necessitates high exhaust (and wide) exhaust ports to start with.

Some mention has been made about some ports should be raised while others left alone. What you are really talking about is "transition timing"; if one would stop to the a minute, its kinda of senseless not to raise all the ports together or try to retain the relation (transition timing) that presently exists between the ports. It would be kinda frugal to extend the exhaust timing window, and then no extend the intake time to load the combustion chamber you have tried so hard to vacate for a fresh, full charge. I try to keep my transition timing flowing together smoothly. If one port goes up, so should the other. Now we are talking in generalities here, in some motors they already have too much intake timing on certain ports to start with, so if you did raise the exhaust, it wouldn't necessarily mean you should raise the intakes because the transition timing is already "skewed". However, on lower HP output forms of the 2.5 Ltr genre this is not the case.

But gaining real power is a lot more than just placing the port height here or there, it makes little difference where the port height is, if you has not optimized proper flow to that port to begin with. That with other considerations such as proper flowing of the exhaust cavity can easily mean as much or more than the raising of the ports themselves. And always remember, increased cylinder purging, cylinder charging and exhaust flow always means fuel curve changes to accomodate these changes.
A motor with all the potential to build REAL HP will always remain just "potential" out the correct fuel curve. It is better to have a stock drag motor that is tuned pretty close from the factory than to have a motor that has the "potential" to produce 40-50 more HP with a incorrect fuel curve. Go to the races, you will see it all the time ...................Prof. O/B

[jerry]

huh , could you say that again with your eyes closed ? HAHAHA
thanks for the explaination, but if the squish area is between the dome of the piston and the top of the chamber , doesnt milling the head the same amount keep the squish area the same ?
I.E if the piston dome is .625 from the top of the chamber ,wouldnt milling the piston .030 and the head .030 would keep the piston dome the same distance from the top of the cyl head ( .625 ) keeping the squish area the same ?

[Dave S]

The 3.0 is a different beast than a 2.5. The heads and pistons are convex, the 2.5 are flat. The 3.0 heads have the piston clearance built in to them. If you mill the heads on a 3.0 you will close the head to pison clearance , if you mill the heads on a 2.5 you change the chamber size the clearance don't change. When you mill the block on a 2.5 THEN you change the clearance. If you mill the heads on a 3.0 it changes the O ring thickness. :eek:

[jerry]

dave jerry does not think you read the whole question , if you mill the head AND the pistons ... and yes ,if the heads were milled they have to be re o ringed

[Dave S]

Hey jerry we all kinda got lost when you first posted the question,being 2.0-2.5 guys. Who did the work? milling the pistons would be tricky as they are domed. What hp motor are you doing?

[us1]

Originally posted by Dave S
Hey jerry we all kinda got lost when you first posted the question,being 2.0-2.5 guys. Who did the work? milling the pistons would be tricky as they are domed. What hp motor are you doing?
That would be tricky.:D

[Markus]

It can be done and is done on V6 Yamahas. Usually, the way it is done is that the edges of the pistons are cut at an angle to simulate the radius, whereas a circle in the center is left uncut.

[jerry]

dave , nobody has done anything yet , just wondering . its a 300 promax ...

[hugh j.]

if you measure port duration in degrees of crankshaft rotation are you really changing the port timing very much by raising all ports .030?
blowdown time would be altered but not very much also. seems to me the modification is not going to produce much change and wouldnt be worth the effort involved. i'm new to this outboard stuff but have some small bore single cylinder experience and it seems to me that depending on what type of boat this motor is on you may want to explore other options besides this one!
this explains it better than i can

Blowdown is the amount of time/distance/degrees between exhaust port opening and transfer port opening.

The absolute most accurate description of blowdown would be... the amount of TIME between the exhaust and transfer port openings. This "measurement" would of course only be accurate at one rpm, so blowdown is commonly referred to in degrees of crank rotation.

If you're referring to engines of all the same stroke and rod length, you can of course, refer to it in "thousanths of an inch", which is commonly done in something like a Yamaha (a typical number might be around .380). This can be measured using a long-travel dial indicator, and simply measuring the distance from the point where the exhaust ports just begin to open to the point where the transfer ports just begin to open.

(Realize though that on an engine with say -- .380 blowdown, the actual blowdown (in time and degrees) would change slightly if you were to raise or lower the cylinder. Distance between ports represents degrees/time only at a given spot in the stroke.)

Blowdown can change the way the engine runs throughout the rpm range. There is no set magic number, though on kart motors, it's typically between 25 and 30 degrees of crank rotation.

Usually (but not always) running more blowdown results in an engine that has the potential to be tuned for more peak power. Peak power not necessarily meaning "top rpm", but rather a bigger HP number at the price of having it over a narrower rpm range.

[Dave S]

Hey Markus,over this way we break out the disk sander.:eek: :D :D :D HeeHee. Hey Jerry did you ask Chip at accurate what can be done or is he out of the HP stuff?

[Prof. O/B]

Originally posted by hugh jascock
if you measure port duration in degrees of crankshaft rotation are you really changing the port timing very much by raising all ports .030?
blowdown time would be altered but not very much also. seems to me the modification is not going to produce much change and wouldnt be worth the effort involved. i'm new to this outboard stuff but have some small bore single cylinder experience and it seems to me that depending on what type of boat this motor is on you may want to explore other options besides this one!
this explains it better than i can

Blowdown is the amount of time/distance/degrees between exhaust port opening and transfer port opening.

The absolute most accurate description of blowdown would be... the amount of TIME between the exhaust and transfer port openings.

Hugh, I read your post with some interest. If I could I would like to discuss a point or two with you. By milling the the crown of the piston, say the .030" mentioned, you are changing the "crown height" of the piston (obviously, the distance from the wrist pin and the crown of the piston). Correct me if I'm wrong but what I think you are saying is that it will have litttle effect, because as you explained you have not really changed the degree of port timing. If I may be allowed, I would disagree with that statement, the reason being is - if .030" is removed from the piston, then at any point during the cycle as measured by degrees the piston will always be .030" more advanced than it would normally be at that crank angle.

Having actually done what is being discussed here, the effects of reducing the piston crown height actually mirrored the effects of having lifted all the ports by .030". I did it to see if it truly would (curosity you know) give the same results and they were identical with the exception I was able to loose an additional 9 grams of piston weight I could not have normally lost. This was done with two (2) later model 2.4 LTR Mercs ('87 or '88). It seems to me that on the 1st. engine where the ports was actually "ground" I sat the Exhaust height at 1.500" (from an original 1.560" or so)from the top of the deck. By manipulating the port heights and compensating for the reduced crown height I effectively made the "decked" motor read the same distances from the deck as did the motor that was ported to height. The nice part was I did not have to advance the transfers, booster or fingers on the decked motor. I of course, did have to raise the exhaust (although not as much as I did on the ground engine) to give it the same exhaust port height as the ground engine. In all truth I really couldn't tell the different in the engines' performance. So from that, it appears, returning to my original statement, the "cut" piston will always be .030" advanced over the non-cut piston at any crank angle.

"Blowdown" - I have a little different take on blowdown than you do. Blowdown to me does not stop when the intakes open. I would describe the difference between when the exhaust opens and the intakes open as "transition" timing. Now if we were to say that blowdown ends at the point - when the intakes open. Then we may as well stop the the bottom of the exhaust port window at that point. We of course never do, the reason we extend the bottom of the exhaust port window virtually to the top of the piston when at bottom-dead-center, is to continue the purging of the combustion chamber of residual exhaust gas. So to me, blow-down stops the moment the exhaust port closes. Because that's the only time it ceases to flow, whether it be exhaust, residual exhaust and incoming charge, or incoming charge only. That's what's neat about a tuned pipe as it has the abilty to "back-stuff" the charge into the combustion chamber. When back-stuffing is correctly used it can make a 2-stroke run at greater than 100% volumetric efficency, I wish he had that!!

When you calculate the area you want in an exhaust port, you do so at "mean" port window; or when the top of the piston is 1/2 the way down the exhaust port window. The area at "mean" port is used to insure it is of sufficiently sized for the intakes. We also do it to reflect the "Time" of the window opening both on the down and up-stroke cycles................Prof. O/B

[jerry]

jerry did not realize the the pistons were domed, cutting .030 off the top will not affect the port timing , just lower the compression. milling the head .030 would only bring the comp back up to where we started . if they were flat top pistons that we could have this discussion . hahaha any idea how or if you can mill those 3.0 L heads , not just o ringed but there is no head gasket , just one big giant o-ring around the deck surface of the head ????

[Dave S]

Jerry I looked at a 3.0 I have ,1995 225 and there is some deck hight that could be milled to get better squish but watch the piston to head clearance.If you mill the block,you don't have to worry about the O ring. The trick on that motor would be to mill the block and the turn pistons in a lath as not to change the dome shape, unless you wanted to. I would think Merc did there homework on the specifercations on that motor. But we allways try for MORE.:eek: :D :D

[jerry]

guess they only get a c + on there homework, cause after the 300 they came out with the 300x !!! hahahaha

[Dave S]

They DID their Homework, thet got ya wanting a 300x......$$$$$$ HeeHee:eek: