GT750 cylinder withou intake divider?

[Suzsmokeyallan]

After looking at the earlier barrels it defies all logic for enlarging that divider on later ones. By all means match the intakes to the tubes to remove the edge and clean up the crap you see as poor factory cleaning of the castings.
I like knife edging the leading edge of any separator, Suzuki did this a lot back in the 70s on the works motocrossers etc and I firmly believe it cant hinder flow.
For intakes now I have the media blaster I can texture the surface after cleaning it to the desired volume as it gives such a decent finish over what the tool can do. You do wish to polish the exhaust port through.


Cleaned up intake by the spigot on a stock barrel


The view of the intake ports on the cylinder side, chamfering needs to be re done correctly as it was not 'right' from the factory. Honing of the barrel is done last if its within spec. You usually find a lot of issues on the upper and lower sections of the ports where the liner and port casting do not match correctly.


Looking down the ports at the knife edging on the separators, the silly CCI brass tubes are in the way and slightly scuffed, they need to be replaced.


The finished product of a reworked intake port on a stock barrel. This back lighting view looks very decent for smoothness of flow compared to an oem one.

[tz375]

We don't actually know what Suzuki were thinking at the time, but they started out with a short divider to ease flow over the port bridge and then made them longer. There must have been a reason, but one can but speculate at this late stage. It is probably to act as a flow divider to ensure that flow to each side will be equal. With a short divider, flow will be biased to one side. A longer divider will tend to reduce that. Each port window on a GT intake flows differently into the bottom end and into the bottom of the transfer ports. My guess is that they were trying to change that flow distribution inequality.

FYI, port dividers should not be knife edged. I know that seems like a good idea but they flow less like that because it forms a wide pressure wave either side of the divider. The leading edge should be rounded - which is not exactly easy to do. Think of an airplane wing for inspiration there. The ratio of the area of the port compared to the area of the carb body is also important to get right, but that's a whole other story. I don't follow how changing surface finish changes the volume of a port though, unless we are talking about changing boundary layer thickness to change the effective diameter.

Back to the story. Most of us make the bridge shorter because we can and because we read it in an old article, but no one has bothered to test long dividers on a GT750 versus short for flow or power that I know of. Some racers remove the bridge completely and it doesn't appear to hurt top end power, but whether that would be detrimental on the street is an interesting question.

The problem with a GT750 is that the ports are all rotated but the intakes are rotated less than the transfers, so it will tend to result in uneven flow to one side which hurts power when the two streams are not balanced/matched.

[Vintageman]

Suzsmokeyallan

Beautiful

TZ375.

Thoughtful stuff

I think divider is low rpm performance. At that time Honda was the one to compete with. Suz did well and making a 2 stroke grunt low

I have a pair of lates 70s Rotax 440 F/C jugs still somewhere. They were canted just like GT750/500. I'll see what they did and maybe take a pic. These for Fan Cooled engines were tuned well.

[ja-moo]

I'm with TZ on the port dividers, the sharp edge actually creates a V wave causing "bad" turbulence. A divider should actually be thinned behind the front edge to create an airfoil shape, so the flow sticks to the divider for a straight flow into the port window.

The longer divider allows the disturbance (anything in the flow path causes a disturbance) to happen sooner in the flow path, so it has time to smooth out by the time it gets to the port window. Also the split port acts like two smaller ports increasing velocity. (like a 4 valve over a 2 valve in a 4 stroke)

[Suzsmokeyallan]

Suzuki did some mods at the factory level to their motocross bikes back in the day and were able to squeeze higher levels of HP from the already heavily ported stock barrels.
In the formula they used, they knife edged the intake separators, the transfer separators as well as the cylinder base section to the inner port walls.

Does this mean it works?? well it did for them back then, does it work today?? possibly. What I'd like to see is hard dyno evidence on two similarly ported barrels, the only differences being one has rounded leading edges while the other has knife edged ones.

FWIW, Any aircraft that travels at extremely high speeds does not have a rounded leading edge wing, its extremely thin cross sectionally.
Almost everyone has heard of the F104 Starfighter capable of over Mach 2 from the 1950s. The only way they could get it to go extremely fast was to fit wings that were no more than thin aluminum strips bolted onto each side of the fuselage. They were approx four inches thick at the root tapering to around two inches at the tips.
Anyway you look at that, its super thin.

[tz375]

Air in a port doesn't travel as fast as an F104 though does it? In fact gas velocity at peak revs in an intake port is around 500-600ft/sec which is close to mach 0.5 which is where air stalls in a port because of "bow waves" and it starts to choke. We don't have supersonic or even transonic flow in an inlet or transfer port, so F104 or stealth fighters or YF12 technology does not apply.

We all used to knife edge dividers/bridges but that doesn't mean that it's a good idea. If anyone did it back in teh day, they probably were not going to share it with the rest of the world. What would be interesting is flow bench data at different gas velocities with sharp and rounded edges to see how the flow differs and then take a barrel with rounded divider and run it on a dyno and remove it to sharpen it up and try it again back to back.

Modern car port design favors long dividers with rounded or concave leading edges. One argument is that longer dividers make the flow closer to laminar and less turbulent than in a single large port. One would need to test with a swirl and tumble meter and smoke trails and pressure probes to prove or disprove that. That's where the pros live.

[oldjapanesebikes]

Useful discussion !

So I'm just going to match up the rubber boot and inlet ID's and clean up the splitter along with elsewhere - always easy to remove more material later, but harder to put it back on once its gone The dyno runs this summer will tell the tale - I have four sets of pipes now to compare against the same state of tuned engine and carburettor combination. I'll post the results - good bad or otherwise - over in the other thread I've already started.

[Suzsmokeyallan]

What would be interesting is flow bench data at different gas velocities with sharp and rounded edges to see how the flow differs and then take a barrel with rounded divider and run it on a dyno and remove it to sharpen it up and try it again back to back

That is what I proposed, and I would like to see those dyno figures if someone had the time to do it,,,I somehow believe on a cylinder design such as the GT750, you wont see much of a difference using either edge design given the limitations of the cylinders twisted and rather mild porting layout. This is of course thinking along the lines of using whats there and retaining the piston port design.

The thing is the whole design was flawed,,, originally they should have shortened the points sub shaft assembly and pared down the alternators width which could have given them at least three inches of total engine width reduction.
Now you can untwist the porting and have a symmetrical porting layout while the engine gains back that lost width. The difference is at least now the added width is in the cylinder section and not in the massively protruding alternator and points covers.

Ians on the right track, clean it up, smoothen up any suspicious areas, get things looking nice and you will have a cylinder thats as good as it gets for what it is.

[Vintageman]

One designer during this same period early 70s and up to 1979 that did canted was Rotax (Ski Doo) in their Fan cooled line of twins. They were tuned a little hotter than Suz for CC/HP not a whole lot for FC and prone to melt down. The 440cc was around 47 -48hp single carb stock I forget details of RPM

They used one carb in production models and had a plenum to divide it,
http://gallery.volvo240.org/images/1/to ... g_0673.jpg" onclick="window.open(this.href);return false;

Race used two carbs.
Early dual carbs did try to point carb straight into cylinder but one was above and below the other for Tillotson carbs and you could do that. Still some bend. Can’t find pic anymore

Later used Mikuni VM and were level and manifold made a bend.
http://www.bing.com/images/search?q=vin ... tedIndex=1" onclick="window.open(this.href);return false;

carbs and extra intake hose obviously not stock though
http://gallery.volvo240.org/images/toys ... g_0721.jpg" onclick="window.open(this.href);return false;

Well no divider even up to 1979 when they dropped canted design. Intake port like the later H1s

http://i266.photobucket.com/albums/ii25 ... us/031.jpg" onclick="window.open(this.href);return false;
http://i266.photobucket.com/albums/ii25 ... /024-1.jpg" onclick="window.open(this.href);return false;
http://i266.photobucket.com/albums/ii25 ... /026-1.jpg" onclick="window.open(this.href);return false;
http://i266.photobucket.com/albums/ii25 ... us/028.jpg" onclick="window.open(this.href);return false;
http://i266.photobucket.com/albums/ii25 ... us/030.jpg" onclick="window.open(this.href);return false;

[tz375]

Useful discussion !

So I'm just going to match up the rubber boot and inlet ID's and clean up the splitter along with elsewhere - always easy to remove more material later, but harder to put it back on once its gone The dyno runs this summer will tell the tale - I have four sets of pipes now to compare against the same state of tuned engine and carburettor combination. I'll post the results - good bad or otherwise - over in the other thread I've already started.

That should make for interesting reading Ian. It takes time and dollars to do back to back testing and just to test a few pipes can take all day. I have three pipes to test sometime:

Modified J&R (v2.0)
Bassani
Modified Strader (v3.0)

But I also need to run the bike first with the Jemcos to be sure that the current carb and timing set up is not producing significantly different results to last time. I cannot just compare the latest runs with past runs unless I have the same pipes to test in each set of runs. need a baseline, and of course data from one dyno can't really be compared to data from another dyno, but we all do it anyway..

On my motor the divider is reduced to 1/2" and that's the same on the second motor I'm working on - different porting guy though. With the next one after that I may leave the full length divider in place even though that makes porting more complicated.

[oldjapanesebikes]

I cannot just compare the latest runs with past runs unless I have the same pipes to test in each set of runs. need a baseline, and of course data from one dyno can't really be compared to data from another dyno, but we all do it anyway..

That is a given Richard - I have the first run ( its it this link viewtopic.php?f=5&t=9323" onclick="window.open(this.href);return false; ) and once the engine is rebuilt I will re-run the same test on the same dyno with the same pipes. That is as near as I can get to a baseline comparison, and so presumably any delta will be attributable to the engine rebuild.

Then I will play with the pipes: Strader, Gibson, Bassani and also J pipes. Should be interesting at the least !

[Vintageman]

OK my reply got lost in the future dyno day ahead. Yes. Should be good reading and a great event. I'll have to find that post.

See if you can do better than this 750 (850?)cc liquid cooded triple 2 stroke -
https://www.youtube.com/watch?v=_y5jY76bqp4" onclick="window.open(this.href);return false;


Now that I had a few hours sleep, my plan now is to take a looks see at the total area of the passage paths on GT750. If it starts equal to carb area and grows nicely to 15% -20% wider in total area I won't mess with divider, just remove sharp edges and blend to match intake boot. Too much volume may cause lag for street riding and carb flicking response. Open it up and then put stuffers I say. Given length of intake and addition of CV carbs this may be a reason as well for divider. As usual just guessing. But, I did sleep at a Holiday Inn last night!

Couple more questions please
1) GT750 intake. Do all years have the same width intake liner window size correct? I mean they did not change width like the T500 did when it added full intake divider out to carb. Has this been verified?

2) In the Rotax intake pic I show or as another example the later H1 intake, what is the purpose of the little casting nipple or stalactite hanging down from top center of intake port liner. It must somehow help keep piston from snagging? For a single port window it has decent area and uninterrupted flow path


Thanks

Mike S

[tz375]

The little blob hanging down is to support the rings at BDC

[Vintageman]

Tz375

Thanks.

Do you know about the Intake window width if it changed on GT750?

I could measure mine of someone has one without full divider

[tz375]

I have a few barrels out there and I have to admit that I have never measured the width of the bridge but I think they are the same +/- manufacturing tolerances.