Ports - GT750

[tz375]

We probably all have copies of port maps published over the years and have a good idea what the port dimensions are on the JKL versus later MAB models. The question I have is who has actually measured those ports and what did they find.

The reason I ask is that I have measured a number of sets of barrels and they do tend to vary a bit from one to another, but the big differences seem to stem from where the casting cores were placed or how they were machined. That and how far down the piston is at TDC (Deck height). Conventional wisdom seems to be that the piston is right at the top (zero deck height) or is 0.5mm down (negative deck height), depending on who you talk to.

We recently measured another two barrels and found that the ports were more or less the distance down from the top that we all expect, but the pistons were almost 1mm down at TDC, so the real port timings were significantly different to what we thought we were measuring.

So we assembled two test motors and hooked up a degree wheel and dial indicator and sure as fate, the ports are 0.9mm higher relative to the piston that we had thought. One motor now has a .125 spacer and we'll have to machine some off the base of that barrel to bring the transfers back down to where they should be.

Just wandering what anyone else has seen in terms of deck height.

[ja-moo]

That always seems to be a "forgotten" point of reference. The Kawi triples are horrible about deck heights, up to 1.5mmm off. So mosty all the port specs that use a measurment from the top of the cylinder are not correct to the actual degree. It's why you need to use a degree wheel or a puter program to get the correct timing.

[two-stroke-brit]

tezer i think this is a dumb question but when you measure the TDC to deck height ,are you using the top of the piston or the edge,?
and on a similar note do you know what year the changed the head gasket thickness.
cheers mark

[tz375]

Mark, the edge of the piston is what uncovers (opens) the ports, so that's what we use.

JA, I just ordered a new 75mm range dial indicator - the others I have are too small a range, and a disk is great but only good to +/- 0.5degrees. I use MOTA and Bimotion software to work though the numbers and my cohort in crime uses Tom Turners TSR software and they generate different answers which makes for a lot of fun making sense of things. They also take different input just to make life fun.

TZ's are usually so close it's not an issue and when I work on TD3's I always have to juggle pistons and gaskets and barrels to get things back in balance with the variable length con rods, barrels and pistons...

[oldjapanesebikes]

We recently measured another two barrels and found that the ports were more or less the distance down from the top that we all expect, but the pistons were almost 1mm down at TDC, so the real port timings were significantly different to what we thought we were measuring.

So we assembled two test motors and hooked up a degree wheel and dial indicator and sure as fate, the ports are 0.9mm higher relative to the piston that we had thought.

TZ - I'm intrigued ! I have never really concerned myself with measuring these as I'd always taken the approach that for a mildly ridden road bike 'pretty close' was OK and I've never been involved in performance tuning. I'm guessing some of the variance you describe would account for why some factory fresh bikes always just seem better running than the Monday morning ones

Asking as a complete novice then - and just for my own clarity: deck height is measured at the outside edge of the piston to the bare top side of the barrel implying that if there are differences then that's due to differences in con-rod length, piston height, or barrel measure from lower machined gasket surface to to the machined head gasket surface - or more probably little bits of all three ? I'd understood that the reference surface for the tops of the ports is off the head gasket surface - that's correct ? And just out of curiosity - does the radius of the dome of the piston or of the cylinder head also measurably differ ?

[two-stroke-brit]

so tezer,i like ian have very little experiance with tuning except for the usual expansion/k&n sort of thing.
but it does interest me greatly.
I hope there is a reasonably simple answer for .
How much did they change the ports between the first(72) and last(77) and how far towards TR750 porting can you go and still be reasonably streetable.
and if you were to do such porting which year cylinders are the best to start with.
(not meaning to hijack your thread)
cheers mark

[tgrogan]

We recently measured another two barrels and found that the ports were more or less the distance down from the top that we all expect, but the pistons were almost 1mm down at TDC, so the real port timings were significantly different to what we thought we were measuring.

I don't think that 1mm means too much for performance port timing, since the ports need to be altered around 5-7mm to give a real performance increase. You are just looking at manufacturing tolerance differences which mean practically nothing. If your aim is to 'make the perfect engine' than have at it with all of the tools you already know about. But 1mm is not significant when looking at a typically mildly ported Suzuki engine.

If you're going to port tham further, it doesn't matter what year you start with. But I do remember something about beefier exhaust ports being available on the M+ models that may help with not breaking thru to the water jackets.

[tz375]

OK so keep in mind that the idea was that the motor was going to be ported and we had worked out what timings/dimensions we wanted so the only question was how much had to come out and that's what started this conversation.

Of the motors we have measured so far, unlike early Yamaha race bikes of the era, conrod and piston lengths don't, so far appear to be the big variable. While it's true that every one of the items that Ian listed will contribute, so far it appears that the biggest variances we've measured are in the port position in the block. All the block vary slightly in overall height and that makes a difference, but as tgrogan pointed out, +/- 1mm isn't crucial on a more or less stock street bike.

In this particular case we're pushing the envelope in terms of transfer timing and exhaust shapes - not so much the overall absolute envelope, but the conventional street tuning envelope. Time will tell how well it works.

Deck height
Deck height is the difference in height between the top edge of the piston and the top of the block - not the center of the crown. In the case of the GT750, that edge is somewhere between 0.5 and 0.9mm down or negative deck height.

Timing measurements
When measuring port timings, the correct way it to measure the distance below the piston at TDC. If the port height is measured down from the top of the barrels as we do with port rubbings etc, they must be adjusted by the deck height to get the corrected timing figures.
Many port diagrams show the measurement from true TDC and often show two lines - the deck and the piston at TDC.

Which Barrels
The JKL barrels all notionally have the exhaust port roof 42.5mm down and later MAB were raised according to Suzuki to 40.0mm down. They actually show those dimensions as down from the top of the barrels so maybe they thought they had zero deck height. Who knows what an engineer at Suzuki was thinking thirty years ago.

I have read that early barrels have more metal above the exhaust port, but the barrels we measured showed that to be inconclusive. On a very small sample, the exhaust port floor in the water jacket was 29-31mm down on an M block and 28-30mm on a early J. In other words, casting cores had more variation than from model to model.

Which is best - undoubtedly the early barrels.. Many articles have been written saying to start with teh higher performance MAB block , but they are looking at saving time in the exhaust, which is fair comment. But they are assuming that you will port the motor to have even more intake duration and that was the biggest problem between the JKL and MAB. MAB with the same exhaust timing as a JKL will make less power below 5500 and more above 5500.

For a high RPM (relatively) motor, an MAB is fine. For a street torque monster I'd go with a JKL with wider intakes and raised exhaust.
Transfers can be raised by 3-4mm depending on the objective, the pipe and the exhaust timing - use a spacer to get that. An exhaust port can only be raised to about 35mm before it breaks through at the corners. With a small radius and relatively flat roof port, it will break through at lower than that. Bigger radius at the top give you a cushion.

If you use say 36mm down and say 16mm top radius, you will probably be safe - that's 4mm up on an MAB and 6.5 on a JKL. If you use a spacer, you leave more margin for error. and that's got to be a good idea.

Heads
They are useless. Early head gaskets were made with a 71mm bore and later gaskets and aftermarket are all around 76mm which is waaay too large. Heads seem to rarely be a good match to the bore spacing dues to build up of manufacturing tolerance. Sometimes people get lucky and heads are more or less concentric on the bores, but that's not all that common. OK on a mild street bike, but not so good if porting is improved and chambers and larger carbs are used. If you raise the compression or fit an insert type head, you must use a correctly sized gasket.

That's how I see it - today.

[two-stroke-brit]

thanks tezer,loads of good easy to digest info there.
cheers mark.

[tz375]

No worries Mark. That what information is for - to share.

[oldjapanesebikes]

thanks tezer,loads of good easy to digest info there.
cheers mark.

+1 and appreciated ! On a (somewhat) related note, now that you got me curious I've gone ahead and ordered a copy of Ray Battersby's book. Should be here in time for my next trip with SWMBO'd, so I'll have something to do while she's out shopping

[Suzukidave]

Richard , didnt you find some surprising difference in the porting of the reed block when yall had it ?

[tz375]

Dave,
That was the first block that Bill measured and the ports were not where either of us had expected them to be. I put that down to it being an early J b;lock which was different to all the blocks I had here. I also had a block measured by another two stroke tuner who I realize in retrospect had made a few assumptions as to how far down the piston is at TDC.

At the time, we both took your block to be an anomaly, and of course it was, but so are a lot of others. So just for kicks I put a J block on a bottom end and measured teh ports with a 75mm range dial indicator and double checked the data with a degree wheel. The ports were not quite the same as the reed block but were not quite where we expected them to be.

Of course a mm or so on a stock mild block is irrelevant and when we port motors, if they are 1mm more or less than we sai it's not all that important in the scheme of things but it does go some way to explain why motors don't all perform the same way and why pipes don't always make peak power where they were expected to.

All it really proves is we keep on learning.

Ian, you will enjoy that book. I need a new copy because mine is all highlighted and written on and dog eared.

[Suzukidave]

Gunnar , used to have a repeating video clip on his GT site http://medial.com/suzuki/" onclick="window.open(this.href);return false; that had the 2 different porting layouts and it really did good to show just what the changes were to the L and the MAB blocks .

[sfront]

There's some porting and tuning info on Smokeriders you guys might find interesting........
http://www.smokeriders.com/Technical/Su ... ocket.html