"Pork Chop" Cranks for GT/TR750

[Kris Bernstein]

Several lifetimes ago I had the opportunity to talk with Gary Nixon about the time he worked with Erv Kennemoto on his Suzuki TR750s. Gary indicated that Erv had Special "Pork Chop" cranks built to allow quicker and higher revs on the old Suzi beasts. Chris Applebee Engineering in the UK is making replicas of this design and the cranks are about six pounds lighter than the stock GT lump.
Chris can also supply Nova's straight cut primary gears and perform any of a number of other mods, including dynamically balancing, these cranks. I expect my crank to arrive in the US next week and will post pictures upon arrival....


http://stores.shop.ebay.com/chris-apple ... QQ_armrsZ1

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[rngdng]

That can't be cheap! Interesting........



Lane

[Kris Bernstein]

$1,400.00 including cutting down the wheels, dynamic balancing and matching all three piston sets to within a gram. Also all new rods, big and small end bearings, pistons, wrist pins, circlips, crank seals, crank bearings, and machining off the crank nose for the starter gear/water pump. Also installing the spacer washers in place of the oil slinger plates as the motor will run on premix only.
Straight-cut primary gears from Nova were $900.00. Applebee had no problems pressing the new crank gear in place of the old helical gear (which went into the recycling bin..). The clutch basket gear requires machining off the old rivets, turning new ones and installing those with the new gear. A crazy labor of love (and stupidity, some would say...) but their my toys and I'll exceed the speed limits as oft times as the local gendarme is not around...

Kris

[Craig380]

I've got an article somewhere from an old copy of Classic Mechanics, it's a big feature on the TR750s with lots of internal pics, I will see if I can find it).

It says in there that the UK factory Suzuki team used what was essentially a stock crank (similar weight to the road bike but with different seals etc).

It also quotes Barry Sheene as saying that "we tried a lightened crank to make it rev quicker, but then it wouldn't pull up the banking at Daytona so we went back to the original".

I've no idea how much weight they chopped off it!

[Suzukidave]

Its a lot of cash but that follows the old racing quote " How fast do you want to go equals how much you are willing to spend "

[Suzsmokeyallan]

I have a problem with those pork chop crank halves in a two stroke engine, thats seriously increasing the lower crankcases effective volume.
If they were milling out portions of the crank halves and then pressing on "covers" so as to return the lower case volume back to what it was originally or even a lower figure, i can see that design working.
Look into any 2 stroke motocross engine of the mid 80s to late 90s and you will see these principals in action, keeping overall crank weight down and the lower case volume to the least capacity figures.
On a buffalo dumping the points sub shaft assy in the left side cover and reworking the charging/ignition system so its not using the heavy rotor would be a simple but effective way of losing at least 6 pounds of rotational mass off the crank assy without spending major money.

[Kris Bernstein]

I've got that article and it is full of incorrect information but some really sexy pictures! The TR bearings were a proprietary item that Suzuki commissioned a Japanese bearing manufacture to produce for ease of rebuilding cranks.
I believe the article did mention the pork chop cranks were used on short circuits as a quick revving motor w/ higher RPM's was needed to keep up with the TZeds. I remember Gary Nixon saying he came in second in an AMA national to Kenny Roberts on Erv's TR750. He also mentioned Kenny's budget was 10 times what he and Erv were getting from Suz. Gary also mentioned that Erv actually began to tear up and cry when Suz told him to bring the bike back to La Hobra and Erv watched the shop mechanic smash up the bike. Took a hammer to the magnesium bodied Mikuni carbs and a torch to the rest of the bike. From there the carcass went to the land fill.

I had mentioned in an earlier post, Dave that my budget for this bike was 10K.
This will be one of the most radical GT/TR's ever built and truly collectible (if not weird...).
Now you cannot touch an H2R, or TZ500/750 or RG500 Mk1-MkV11 for that price. I would love to be able to plop down 25,000 Euros for a Roc YZR500
but I keep missing the lottery but at least six ***** numbers!
Anyway, just wanted to put this project in perspective. I will always have a fondness for the buffalo's and this project reflects that passion. Until I win the lottery, I have to do the best I can to feed my drug habit of choice....

Kris

[Kris Bernstein]

No argument from me on that point Allen. I know the H2 cranks are not full 360 degree wheels and I simply cannot figure out how they work without decent secondary compression in the bottom end. I raced modern TZ125's for a few years and the cranks were the same way but the little buggers ran like radioactive snot!
I remember filling the big holes I milled in my old Titan cranks with cork and epoxy to maximize secondary compression. I didn't like the idea of using stuffers, so stubborn bastard that I am...I had huge holes bored in the crank wheels and stuffed the holes. Crank weight was the same as a cut down "stuffer" crank but without the stuffers. That bike won national championships and had 54% more secondary compression that a stock T500/Titan, now comes along a motor with 50% LESS secondary compression. Will the damn thing work? I honestly cannot say it will with any empirical certainty. I am thinking that using a reed valve top end is part of the certainty the crank will work. Piston port motors are inherently less effective with regard to scavenging charge. Somehow the efficiency of the reed motor does not rely on secondary compression to effect scavenging.
Any theories on that hypothesis, Allen?
Stay tuned for this potential disaster as it unfolds.....Ha!

Kris

[tz375]

Modern 2 strokes rely more on harmonics than on high primary compression was was proven to create narrow powerbands. High primary CR is NOT a good way to make power. It's early sixties technology.

Low primary CR reduces charge loss and allows ports to be angled quite differently. It allows for more efficient cylinder filling and that makes for more power.

Of coiurse modern pipes are needed for modern ports to create a suction wave that reaches down and sucks the gas out of the crankcase. To do that requires a larger crankcase volume.

Think of it this way. With a small crankcase volume there isn't a lot of mixture available to move up to the combustion chamber. With larger volume available, port dynamics can draw more gas into the combustion chamber.

Kevin Cameron wrote an article of that in a sled mag years ago. In fact if I recall correctly, the XCR triples had too much volume and did not respond to tuning until they went up in bore size and the lower volume became a better match to what the motor needed.

[Suzsmokeyallan]

Theres lots of ideas on lower case volume size and how it affects the way the engine performs for sure.
I based my reasoning on this from what i have seen working on the RMs KXs YZs and CRs for years, going all the way back to the mid 70s when these engines were piston ported save for the first Kawis which were disc valved.
Looking at the disc valved engines, they made all efforts to reduce the lower case volume even more so than the traditional intake piston port and reed valved design.
This couldnt have been all that bad if the RGs were producing healthy amounts of power for their relative CC size.
The performance figures of the 250 and 500 motocross engines in particular were always very impressive even to the end of production when the four strokers took over.
The same lower case factory design of minimalist volume worked from the narrow powerband 125s to the wider 250s and the extremely wide 500s, so something had to be right for the engineers to persue this level of thinking repeatidly.
Even when case reed induction and then full intake reeds became the design of choice, the lower end volume never got bigger, if anything they made it smaller with ever tighter and closer tolerances.
So how did this effect performance?? well 125s were delivering approx 36hp at 10,500 rpm the 250s approx 48hp at 8,000 rpm and 500s upwards of 68 hp at approx 6,000 rpms
However you look at it, those are very impressive figures for a single cylinder engine with a low volume crankcase design.

[tz375]

Allan,

That's interesting. Those are not bad numbers.

125 36hp, great but not outstanding - an RS125 makes around 48-52
250 48 not so good in relative terms. A good 250 twin makes 80-100
500 68 versus over 200 from a GP motor. Proof enough that small case volume and single cylinder motors don't make great power - relatively that is.

Put another way, 2x125cc should be 2x36=72hp.

4x125 should be 4x36=144hp

As those motors get bigger they become less efficient.

All great numbers but indicative that HP was not the target and that the small volume bottom end design was not the answer.

Interestingly, an RM250 reed motor has way more crankcase volume than a GT750 (stock), so a reed 750 might be the way to go.

I think it was A Graham Bell that had a graph of primary CR versus peak HP RPM and he deduced that dropping primary CR was a good move as long as the ports and pipe were doing the work. he also surmised that when the FIM restricted the number of gears and cylinders that engine design changed and high CR became less of an issue.

I would be interested to see what difference a low primary CR made to a tuned GT750 motor by just swapping out the crank.

That would be interesting. Was that why BS asserted that the small crank motor had less top end on the banking? Lower CR and the same ports and pipes? Or did the reduction in rotating mass offset that in acceleration? Or maybe the lower CR helped that motor accelerate better for gas flow reasons rather than flywheel inertia. Or did it make no difference at all?

Either way, if Kris copies a modern port design he should achieve his HP targets.

[ja-moo]

We have to look at the tech advances and engine design on the last few decades. Street motors designed in the 60's and early 70's didn't have the "advantage" of any "real" expansion chamber capability in stock exhausts. So street motors were desigined with Higher primary compression (crankcase compression) to "push" the mixture through the transfer ports.

I talked to Kevin Cameron about the Kaw triples and their primary compression, especially the H2 with the porkchop crank. He said the H2 had a relatively high 1.5 (ish) to one primary ratio. I know of two tuners that have tried full circle cranks in H2's and found no HP advantage.

I know Honda used porkchop cranks on their later 2 strokes, with tin covers to make them "round". But that was more for smoothing the "flow" in the crankcase at the reedvalve, not to increase primary compression.

Early racing engines of the 60's and 70's had "primative" (to nowdays) expansion chambers That were relatively small in diameter with relatively small suction pulses, and as they increased primary compression, power went up. BUT, the powerbands became razer thin. (Remember the 18 speed tranmissions?... )

As expansion chamber designed progressed, it was found that more and "better" power was produced with lower primary compression. Now, there really wasn't any real need to "push" the mixture through the transfers, as the plpes were now "pulling" the mixture through. And high primary compression caused a resonance effect in the system as the pressure/suction waves "met" and had to stabilize, impeading flow.

With big pipes now, the small clearances needed for higher primary compression is actually an inpediance. Think of a syrenge, pull the plunger out quickly and there is a lot of resistence from the small opening. Cut the tip off and the plunger pulls freely.

So basically, if you have good pipes, decreasing primary compression will increase power.

And good luck, your project is very cool!

[Kris Bernstein]

That is a tough assignment with these old motors. The banned "finless" sand cast TR750 cylinders had bridged exhaust and I agree that might offer some advantages just based on available architecture.
The great thing about these motors is that they are narrow by triple standards but at the expense of transfer area.
Interesting, that Dave Swarbrick's new new TR750 pipes are fatter, have less belly section and a much steeper diverging cone.
I think we would all agree that more suction is always a better thing (no mater what the subject...). We used Dave's old design which was right out of the TR500 era. My hope is that his new design will offer better numbers on the dyno. I plan on using a Ducati 1098 radiator relocated to the area of the rear mud guard (much like they did on the Britton twin). By narrowing BOTH sides of the GT cases and lacking the imposition of the radiator in the front of the motor, my hope is to free up some area for a more modern pipe design. Look at the massive pipes on modern TZ250's and scale that to a three 283cc cylinders...now you know where I am headed.

Kris

[Suzukidave]

For one of these GT750 engines in a race car or maybe side car racer the problem of fitting up proper pipes may not be much of a problem but when 2 stroke tuner and Friend Bill Tripp worked up the spec for a set of prpoer chambers for my project reed bike the bodies were 6" across

[Kris Bernstein]

Any chance you have a dimensional drawing of Bill's pipe?
Did yo finally get all that sheet metal to fit on your chassis?
More pictures please!!!!!

Kris