2 stroke exhaust tuning

[Mark75H]

OK, guys at the risk of attracting a lot of flames I have to vent something here. Unless it can be explained to me otherwise it looks to me that there is major mis-understanding of 2 stroke exhaust tuning. I keep seeing the same thing over and over.....cut the tuner and relieve the exhaust; as if the factory guys have no clue as to what tuned length and back pressure should be used.

PWC's and PRO(alky) racers use very restricted exhaust and really scoot. Their power to weight and power to engine size are an order of magnitude beyond what applies here...one would think they would be the example to follow rather than BillyBob's engine hop up and hacksaw sharpening service.

Personally I recieved an email from a certain respected factory engineer just this week pointing out that the tuned pipes I have on my engine are probably a little short. {This was in answer to my query about the water injection not slowing the engine down at max rpm.} I whipped out my tape measure and ran to my shop as soon as I finished checking the prediction for correct length of my pipes at my max rpm....guess what...my pipes do appear to be a little short. My buddy also gave me a suggestion to test to temporarily extend my pipes to check on possible improvement.....I'll know after my next testing day 9/21/01



Later on in this thread I'll explain REAL the way to select a tuner for optimum flow.

[ShipBear]

Do you know if anyone makes a Tuner for OMC 235..?
Or any Mods that can be done to the stock tuner.
Thanks, Larry

OMCviper@aol.com

[Raceman]

Sam, you're running individual pipes and that really has no similarity between what goes on with a tuner on the end of a multi cyl exhaust chest.

Ship Bear, unless your tuner is restrictive, which I'm relatively sure a 235's wasn't, you'll waste your time and money changing it.

Tuner change is an absolute waste of time and money unless one of the following exists:

1. It has some super restrictive tuner to enhance low end or restrict horsepower (intentionally choke HP to make an engine marketable at a lower horsepower rating with the same displacement as a higher rated one)

2. The need for very small adjustments (read as VERY SMALL, MINUTE. ALMOST NON DETECTABLE) in how and when the powerband hits. (drag racers or roundy rounders)

The average guy that buys an aftermarket tuner to put speed on his lake toy has just wasted several hundred bucks and several hours time. The people sellin' em try to pull from the results of expansion chamber tuning on a two stroke, but it aint the same. The exhaust tuning in a multi cyl, common chest two stroke has a lot more to do with what's going on in that chest, meaning the relationship between the pulses of adjacent cyl's than what happens out the bottom of the powerhead.

[T-REX]

Exhaust tuning and the exhaust tuner(which ain't really a tuner) ain't no where close to being the same thang...Motorcycles and PWC's are ported to use exhaust tuning to be one of the major factors in it's performance gains...If you cut the ports in an OB like a motorcycle, well, U jus ruined a powerhead, cuz it won't git out of it's own way...I know this to be fact...John Lentzcow haz put together a nice set of tuned pipes for a 2.5, but in my opinion, uesing a Fuel injected motor with all in one ecu, the tuning of the pipes can't be perfected...I think a carbed motor can be tuned to run with the piped better than the EFI motor, cuz you can tune each cyl seperatly...Tuned pipes can't be tuned together(to it's fullest),,,Az Raceman haz said above, tuners make a diffrence in the backpreasure, which moves the torque band around a little on the bottom end, and on all out race applications may(and I said MAY) have a slight change on the top end rpm...I think the "TUNER" on an O/B should be renamed to "EXHAUST STACK", because the exhaust on a multi cyl 2 stroke that dump together can't be tuned together...choke it down, or free it up, but not tuned!!!...My 2 cents, T-REX

[BarryStrawn]

Perhaps that is why Mercury names it a “Diffuser”.

[JFN]

Just going through this subject on the BassAngler.com forum under the post 'firing order'. Pretty good detail on how it all works. All opinions welcomed.

[Raceman]

JFN, I briefly read your thoughts on multi cyl. exhaust port relationships and agree with your idea's there. I can't tell though if you're believeing that changing the tuner length is going to somehow change dramatically what goes on in the chest. The reason that the folks selling tuners have been able to do so is that most everybody has at least a passive knowledge of an expansion chamber's effect on a motorcycle. Somehow they've been able to confuse this into a parallel with tuners (tailpipes). I've seen em all, and run most everything available for Mercs and I'm still of the opinion that the guy that sells the average recreational boater or lake hotrodder a tuner is stealing his money and knows it. As Barry pointed out, diffuser is a lot better term than tuner.

If there's a single thing that could control what happens in the chest in general, excluding port timing and firing order, I think it would be what happens with the bottom cyl. I've always believed that if the relationship between the bottom of the lowest exhaust port and the bottom of the diffuser was roughly the same as the relationship between any two cyl's in the chest, with the volumes being equal in all these area's, multi cyl exhaust tuning would be optimized, at least as far as this type system is concerned. Problem with this theory is, I've seen too many multi cyl. engines run just as well with no diffuser as they do with one of supposedly high performance design. So I've come to believe that if they're not restrictive, once in the powerband they're not relative in the real world.

[T-REX]

Now U no that if ya tuner falls off, there iz a difference!!! performance is tha same, but theres that cute lil rattle in tha exhaust hsg...If U gunna tell it, tell it all...REX

[JFN]

This is how I see it working:
When the 1st cyl opens it's exh port the positive pulse goes two ways. One way is down the the pipe thru the difuser which refracts the pulse into a negative pulse which helps in scavenging that cyl near BDC. Secondly, it travels to it's tuned pair, 2nd cyl, ( cylinder that is simutaniously closing it's exh port), and helps to block and push back-in any over scavenging of fresh fuel and air mix of that cylinder. Back to the difuser. After the gas passes thru this it turns and goes up and is bunced back ( this is the part that is hard to understand) up the difuser to the 1st cylinder and helps with the blocking coming from the last fired cyl cylinder. Lots easier to show on paper. Tuning factors are changing the length from one cyl in a tuned pair to the other cyl by using exh stuffers or manifolds, changing the length of the difuser cone, changing the angle of the difuser cone. If the stinger end (tailpipe) is too small causing too much restriction it can be relieved, but should not be totally relieved. You need some restriction here to keep in some of the heat, depending on what temp your system was designed for. I suppose that adding a tuner to the end of the tailpipe could help, ( could act as a second difuser) but you would have to do the math.
I think any help here would be on a second or third order. Just my thoughs.

[Raceman]

JFN, I still think you're trying to make the diffuser the stinger, and it's not. All the refraction that happens in an expansion chamber isn't going on here. The diffuser is in almost all cases at least as large as the outlet of the port itself as well as the opening in the bottom of the powerhead. In a chamber, the last cone as well as the stinger end up smaller than either the port or the head pipe. There's just no similarity in what's going on between the two. In a chamber the cones/shapes/lengths/sizes are designed to make the exhaust pulse change direction at certain times. This refraction does in fact cause positive, then negative pulses within the pipe which first pressurizes against an opening exhaust port, then the negative pulse helps in scavenging the cyl. In the multi cyl pulse within one chest the result may be the same, but I think it's caused by another cyl's pressure within the chest, then the negative pressure created by this charge going toward the end rather than a refraction caused by an open diffuser. Although it is a constant cycle of positive/negative pressures I believe it's from the pulses of multiple cyl's. There's no refraction (reversal) of the pressure itself as there is in a chamber. I believe the only reason the length/volume even seems to matter is how long a given pulse stays within the system and it probably matters most just in keeping the pulse in the bottom cyl. in tune with the other two instead of just being immediately los (that's of course after the diffuser's design has exceeded being a restrictor) This is where I differ with you on theory and maybe it's just a terminology thing: Your quote: "When the 1st cyl opens it's exh port the positive pulse goes two ways. One way is down the the pipe thru the difuser which refracts the pulse into a negative pulse which helps in scavenging that cyl near BDC." I don't think there's any refraction going on. I believe that the pressure of the pulse of another cyl leaving the chest at this time creates this scavenging effect. I also believe that the initial positive pressure of the other cyl is causing the positive pressure against the beginning to close exhaust port, just as a refraction does in an expansion chambered single. Now here's a question or two I'd like to hear your thoughts on: To use your terminology of "tuned pair", would you think that the pulse of the first cyl in the pair has completely cleared the bottom of the diffuser before the second cyl in the pair creates positive pressure within the chest? Is the second cyl that fires in a given side of the chest necessarily the tuned pair mate of the first? I'm saying all this as theory. When the first chambers were discovered, my understanding is that it was quite by accident and then the engineers set out do figure "why" the additional cone and stinger caused the result. As I'm sure you're aware all the old two strokes used to run megaphones and the first crude chambers were just a short cone with a stinger welded on. There was an old guy who lived down the street from me as a kid, and all the alky racers were sending there megaphones to him and he was welding on a short cone and stinger and he didn't have a clue why they were working.

[JFN]

Raceman;
First I do not think the diffuser is the stinger. But arn't the people putting on these 'tuners' using them as a diffuser? I agree with all your points until you say that "This refraction does in fact cause positive, then negatiive pulses within the pipe which first pressurizes against an opening exhaust port, then the negative pulse helps in scavenging the cyl". This is not correct. The opening of the exh port causes the positve pulse and this pulse stays positive until it traveles through the diffuser. The term 'refraction' is the changing of the positive sign of the pulse to a negative sign. The negative pulse or refractive pulse then travels back and helps scavange a cyl. The cyl that it helps is determined by the distance the diffuser is away from the cyl. In our case it would be the same cyl it originated from. Any time a gas travels through a diffuser there is refraction. In turn, a pulse must go through a diffuser to be refracted.Thats physics.
Your next statement about keeping the pulse in the bottom cyl in tune with the other two, there can only be two cyl in tume at any given time, thats why their're called a tuned pair. In a 3 cyl the tuned pairs are 1-3,3-2,2-1.
Your question: Do I think the 1st pulse has cleared the bottom of the diffuser before the second pulse arrives? Yes absolutly. How can this be? These pressure pulses are traveling at the local speed of sound, thats over 60,000 feet per second. Your pistons are traveling on an average about 4,000 feet per second, much slower. At around 10,000 RPM that means the pulse can travel roughly +/- 6 feet. Since we are working with tuned pairs in a 3 cyl engine that means the pulse can travel about 2 feet. These are rough figures but I'm not to far off.
Is the second cyl that fires in the same side a tuned pair mate of the first? Yes. The guys that build the motors design them that way.

Yes I remember the old megaphones. I started racing snowmobiles in the mid sixties and there were still sleds with them on. The first racer was a 68 gyt kitted Yamaha SS440. It had expansion chambers. I still have a original 69 Polaris 650 racer with expansion chambers. Fun days.

[Raceman]

I guess I mistated my thought on the opening port. What I meant is clear in my mind, but seems unclear or even incorrect in the way it was written. "Pre opened" exhaust port would have been more in my line of thinking. I think the differences in our beliefs are that you see a refraction relating to the diffuser, the only positive/negative pressures I see are related to the discharge through the exhaust ports in a shared exhaust chest and a resulting reversal of atmospheric pressure after the given charge exits the system. I guess a simpler explanation is that I see the pulses created by the firing of the cyl's in a shared chest rather than as a refraction that exists in an expansion chamber. One other thing you've made me ponder, and I've got to give this some thought......do these percussions necessarily travel at the speed of sound? I'm not sure, never considered it. I'm sure the bang, which is an indication of the explosion travels at the speed of sound. The flash travels at the speed of light which is an indication of the explosion also. Obviously we have some restriction as opposed to an open port straight into the atmosphere. Does this restriction effect the exhaust gas speed? Y'know, I don't know the answer to a couple of those questions. Perhaps I look at this after the fact in attempting to figure out the "why" a given set of results work, and here's my case in point: If you take a 2.5 EFI Merc and run any number of different designed diffusers from say, 4" long to about a foot, and then take it off completely, you can't read any difference with a radar gun on a given boat and I've yet to see a credible exception, unless the piece improved upon was restrictive in relation to the needs of the engine.

[BarryStrawn]

Good discussion, I always found the 3 cylinder effect interesting since it is not tuned with RPM. A free lunch more or less.

My understanding is the pressure waves do travel up and down the gas flow at the speed of sound. The "bang" is a set of pressure waves. With doppler effect, the downstream wave travels a bit faster than a return wave. I have seen this accounted for in some of the mathematical models which attempted to explain or predict head pipe and collector performance on 4 stroke aircraft.

Perhaps the return wave(s) from the diffuser, midsection, lower unit, snout, etc. comprise a more or less random selection of waves with a few minor nodes here and there. And variable with RPM. I would speculate these are insignificant compared to the wave coming off the paired cylinder. I would bet they are untuned or diffuse which is why "tuners" have so little effect. All they are doing is moving the flow.

Barry

[Raceman]

Here's my question about speed, and maybe this is a poor analogy: What's the difference between the bang coming out of an exhaust port and the bang coming out of a shell casing, projecting a bullet? Certainly with sub and supersonic ammo's the speed of this pulse or percussion can vary? So why then would hotter fuel, more compression, etc, etc, not propel the exhaust gas faster? With restriction at the bottom of the chest creating restriction or back pressure, wouldn't the speed of the pulse vary over a free flowing system? I've been trying to digest some of JFN's reasoning in my spare time today and there's one thing that keeps popping in my mind: It seems like if there is a refractive element present, (which I'm still skeptical of in any case other than reversal of atmospheric pressure) in all cases an even number of cyl's in one chest, with a diffuser of exactly the same length as the distance between the timed pair of ex. ports would be superior. It also seems that in addition to equal length, uniform shape and volume would cause an equal negative pulse at the time it's helpful to each cyl.

And one other question off the subject: Is the bottom end compression now most frequently referred to as primary or secondary? My old 2 stroke racing theory book from the 70's calls it secondary, but it seems like whenever it comes up now on the board it's called primary.

[CarolinaBurt]

I have always referred to the crankcase compression as primary. It has to exist before the combustion chamber can fill. I wish you guys would solve this problem, I need some flashy pipes on the back of my Merc. CB