GT750 Powered Race Car (D-Sports Racer)

[sav0r]

We fought a bit with the water pump gear. Basically, they don't like the RPM we turn and they were junk in the first place. There's a nice press on steel gear in the UK, but after shipping, parts, return shipping, the price is a little silly. Something near $200 all said and done. We are married to the mechanical water pump and adapted a high flow impeller and housing. We've decided against an electric pump as we don't want to carry the extra amp hours. We run total loss, and the battery is way too heavy as is. So i'm going to cut one of these guys out aluminum, but since it will be milled instead of hobbed, we expect it to self clearance a bit. It should work fine, we'll just flush the transmission once after some stand time. I may also cut a sleeve to press over the plastic gears shoulder which should make the plastic gear more reliable, but it will likely stay in the parts bin. After all, spare parts are never used.
 

[sav0r]

We decided against the above hubbed design and instead are going for this hub-less setup. The existing G750 steel hub that's in all the plastic water pump gears will be shrink fit to the new aluminum gear. The benefit of this system is that the the gear will be removable without splitting the cases. All the other gear replacements that I've seen require the cases are split to change/remove the gear. It will be easier to machine too.



We've also picked over our data from the single race weekend and have realized we were geared too short. Top speed was calculated to 107mph. We managed 109mph, it really likes to rev. So we are going a bit longer, it shouldn't hurt us in the slowest corners as we were too short there too. I machined the new sprockets today, looking good! It's basically free speed, we will just plain go faster. I also found in the data that I lost major time in the brake zones, I already knew this as the driver, but the decel curves were frankly pathetic. On a 20 turn course with stone walls the brake zones are pretty important. The new masters are going to be a huge gain.

https://www.pvgp.org/schenley-circuit-map/

[sav0r]

I let the mill run through the night last night. .0625" end mill with a .0625" step down, .001" step over, at just 1.5 inches per minute. Run time was about 5 hours, more than 4 hours to just cut the teeth... I'll face the back side off tonight and then shrink fit the hub. If it works, I'll probably make as second one, there are a couple of small things that could be improved, but not bad for a first try. It's a good thing machine time is basically free.

[sav0r]

Finished this one up as planned. Looking good!

[Evans Ward]

Excellent craftsmanship there! On the original nylon one in your pic, you can see the cracks as it is common and I’ve never seen a used one that didn’t have one or more cracks. When that gear breaks, you lose your tach but more importantly your water pump circulation. This one broke on me 2 yrs ago 60 miles from home. I rode it back gingerly taking it down and pulling the head to see if warpage occurred. It was straight and not warped as Suzuki really over engineered those triples.

[Evans Ward]

Here’s a new Suz nylon one for comparison:

[sav0r]

We actually bought a new one, only to have it be cracked upon arrival. And our gear broke mid race, leaving me to retire.

We have seen 10k on the tach, which is the danger zone big time. The plastic gears just never stood a chance.

[sav0r]

My father has the engine mostly back together. Says the gear will be perfect. Obviously we need to run it, but there are no obvious issues upfront.

He bought a new impeller and housing from the UK, the impeller is missing the key slots. We aren't sure how to make it work yet. I might end up scraping a keyway for it on the mill, that will be a new one...

[teazer]

early style used a woodfruf key and later ones just used a roll pin.  A lot of impellers have a deep slot plus keyway so they will fit wither design of shaft.

I may have a water pump shaft I could swap for yours.  If you need one, please let me know and I'll check my spares box.

[sav0r]

My father ended up fixing the impeller with a scroll saw and some files, he just added the keyway. It has a bit of lash but it will be just fine. He spoke with BDK and they mentioned the old/new situation.

I machined up a bracket for the chain tensioner today. I'd post pictures but without context it's not really worth seeing. Suffice it to say, since the drive train is fixed and the engine is fixed there's a third sprocket that's mounted on eccentrics, the eccentrics are used to adjust the third sprockets location to control chain tension. The bracket was too flimsy before, there is an amazing amount of load on it. We should be good now.

The best news is that my father delivered the head to me today. It's all welded up and ready for machining. I've got the model all worked out. I have the game plan for setting up the head on the mill. Unfortunately my power drawbar is acting up. I might go ahead and do it with manual tool changes, but it's such a bummer running the machine like that. I've got to get some customer work straightened out then I'll make the decision on either fixing the PDB or going ahead with machining. The drawbar repair is like a two to three day affair and I need access to a lathe which is a bit of an issue currently.

[sav0r]

The company that provided me with a day job shut down yesterday as a part of non-essential closures of businesses in PA. As a result I have a lot of free time. For my creative self, this is a win. We'll see how the financial self survives, but for the time being I am going to keep busy and enjoy the vacation.

So, last night I got the stock on the mill that will fixture the GT750 head for machining. For those who aren't familiar with machine tools, my mill is quite small. In fact, in the scheme of things it's basically a toy. Though I've spent a lot of time and effort to make it as capable as it is, and I manage great results on a number of projects. Keeping the size of the mill in mind, to place the 7" wide (tall in Z height) on the mill I had to remove my automatic tool changer. But, I got the stock squared up and looking good, luckily I just needed one tool to get the work done. Today I will be boring all the holes in the fixture plate, tapping said holes, and then surfacing the plate so that it's nice and parallel with my table. I've purchased Mic6 plate for the fixture, which should be fairly flat, but I want to get this right and have a nice repeatable fixture for future head reworking. Since the head will get worked over on both sides the fixture end up using the head bolt pattern and a flipped version of it. Of course the head itself will get surfaced on the fin side before further machining can be done, it's the only way to assure the head stays flat when the combustion chambers get machined. As of now, the combustion chamber cutting will take around 4 hours, luckily the machine runs itself and I don't have to pay any attention to it as it grinds away.

Short video of the stock getting cleaned up - https://i.imgur.com/btu3W2S.mp4

[teazer]

Sorry to hear about the work but good to see you are staying busy.

Is your mill CNC, and can you do more complex combustion chamber shapes such as different squish band angles and diameters and even torroidal heads?

[sav0r]

Yes, it is CNC. The head design is slightly torodial, though not particularly radical in that shape. I put a piston on the mill and measured the arc of the piston top and then matched the squish to be a mirror image of it. My father gave me figures on the design beyond that, mostly squish band width, and I then adjusted the shape of the combustion chamber to give us the volume we need.

But the sky is the limit, really just about any imaginable shape can be made. The nice thing is that no special tooling is required. The entire finishing operation will be done with a 1/4" ball end mill running really slow with really small stepover.

Here's a few photos from the process. The negative of the combustion chamber provides head volume.

[sav0r]

Well I came down with Tonsillitis and that slowed me down a couple of days. On the mend now, tonsils are feeling so much better.

I got the fixture plate machined. Turns out somewhere along the line my model got out of scale and that resulted in the first set of holes not lining up with the head. So, I went back and redid all the CMM because I couldn't find my original data, then redrew all the models. The CAM was good, so the machining went smooth. Literally minutes of machining and everything came out awesome, of course I spent like two hours doing the CMM and at least two more hours redoing and verifying models. All seems great though.









What a ride that was. Now onto the head. Seeing as it's the first one to hit my mill I will be taking my time.

[teazer]

It's really important to get it right before you do my cylinder head....