DRX 50 Build Up

[machwon]

Happy Easter all,

I've been messing around with my daughters drx 50 since October and thought I'd share a little of what I have done. She raced in the 300 ft sand drags last year and at the end she definitely wanted a faster quad. Because we build our own motors with in house porting and machine work, I wanted to do this one from the ground up with the intent of building a national level motor that could compete at any level of 50cc ATV racing. I’ve been really impressed with the results we have thus far. This motor has a whole different tone that is impressive when compared to other well tuned ones.

First we pulled the motor and completely tore it down. The motor got the typical rebuild stuff like main bearings, seals, gaskets, piston kit, wrist pin bearing, etc throwing away all the other parts. From there we made sure the crank was running true, did some extensive porting and finished it off with a race gas head mod for 110 octane. After spending many hours tuning and a couple hundred runs on a couple different bikes, the graph shown is our best runs to date. We have found the stock clutches and belt work really well with 4 gram weights and 2000 rpm red/red springs in the rear. Jetting is stock except for a 17.5 pilot.

We also tried the 20 or 21mm oko carb, larger intake/reeds, advancing the timing, 90 cc belt with cut variator’s and all have lost power on our set up. It seems like I tried other stuff but I don’t remember. So far the porting, head mods and rollers/spring changes are the only good mods. There are clutch component changes that will get a better hole shot on this motor, but the top end speed will suffer.

I also hope to get someone to sell me a slightly different belt, weights and rear spring to try this week. I’ve heard of 4 hp being stock and 6 being around the best so this one is in the ball park. Our testing showed stock running in the 3.5-3.8 range. Hope to find a little more in the future.

[#404's Dad]

Originally posted by machwon
Happy Easter all,

I've been messing around with my daughters drx 50 since October and thought I'd share a little of what I have done. She raced in the 300 ft sand drags last year and at the end she definitely wanted a faster quad. Because we build our own motors with in house porting and machine work, I wanted to do this one from the ground up with the intent of building a national level motor that could compete at any level of 50cc ATV racing. I’ve been really impressed with the results we have thus far. This motor has a whole different tone that is impressive when compared to other well tuned ones.

First we pulled the motor and completely tore it down. The motor got the typical rebuild stuff like main bearings, seals, gaskets, piston kit, wrist pin bearing, etc throwing away all the other parts. From there we made sure the crank was running true, did some extensive porting and finished it off with a race gas head mod for 110 octane. After spending many hours tuning and a couple hundred runs on a couple different bikes, the graph shown is our best runs to date. We have found the stock clutches and belt work really well with 4 gram weights and 2000 rpm red/red springs in the rear. Jetting is stock except for a 17.5 pilot.

We also tried the 20 or 21mm oko carb, larger intake/reeds, advancing the timing, 90 cc belt with cut variator’s and all have lost power on our set up. It seems like I tried other stuff but I don’t remember. So far the porting, head mods and rollers/spring changes are the only good mods. There are clutch component changes that will get a better hole shot on this motor, but the top end speed will suffer.

I also hope to get someone to sell me a slightly different belt, weights and rear spring to try this week. I’ve heard of 4 hp being stock and 6 being around the best so this one is in the ball park. Our testing showed stock running in the 3.5-3.8 range. Hope to find a little more in the future.

Bob, Thanks for sharing, it'd be great if you could over lay a stock graph on your dyno with this graph on your dyno so we could clearly see differences, also an AFR graph would be super.

Thanks for sharing :)

[#404's Dad]

I have a new 788 belt and various springs and weights, some new some used, let me know what you are interested in trying.


Corey

[drr5]

try the 787 belt just a little longer but maintains the same width

[machwon]

I have tried a 788 17 28 belt with a turned down front variator. It has good power on take off, but suffers on top. DRR5 where did you get a 787 belt? I also heard there is like a 782 belt somewhere.

I'll try and get a comparison graph up in a couple days.

i believe the 787 is a 90cc belt

[cheatermtrsprts]

The 787 is a 90 belt. They are good to use instead of the 788 because they are wider. The 787 is the same width as the 780 found on the 50.

[#404's Dad]

I think the ones i have are 788x16.5x30 and the ones on the bike (drr50) are 780x16.5x30 just off the top of my head here so i could be way off too.

[machwon]

Here's a comparison of my stock 50's vs the modified version. 5.82 vs. 3.58 hp and 3.48 torque vs 2.34. I know there was one stock drx that put down 3.8 hp but the run wasn't in my files. Our friends stock DRX which had been clutched and won the jr. class in MI last year put down 3.98-4.08 hp.

On the belt subject, I've used a 788 17 28 off a polaris 90. I ordered a couple more clutch set ups today and they should be on the way to try out.

[#404's Dad]

Originally posted by machwon
Here's a comparison of my stock 50's vs the modified version. 5.82 vs. 3.58 hp and 3.48 torque vs 2.34. I know there was one stock drx that put down 3.8 hp but the run wasn't in my files. Our friends stock DRX which had been clutched and won the jr. class in MI last year put down 3.98-4.08 hp.

On the belt subject, I've used a 788 17 28 off a polaris 90. I ordered a couple more clutch set ups today and they should be on the way to try out.

Those are most deffinetly impressive increases in HP and TQ !!! What all is involved, just porting or more?

Corey

[machwon]

Yes, I've been quite impressed as well, but it was very difficult getting there. I've asked several shops but really only one has told me those are respectable numbers. It seems a few of them have others that build there motors, but know ones saying who does the porting/motor mods. I'm pretty sure it will be reliable as well, but only time on it will tell.

Most of the gains came from the porting the cylinder, head work, and clutching. Between 2 different 50's I know I have over 200 runs since Jan. My goal was to only modify the stock parts and see what this little motor was capable of. If my kid doesn't graduate to a 90 by next year I'm thinking I will try and get a different pipe made. I've also heard the pvl ignition works good too, but it is very pricey.

Does anyone know if you can fit a cobra 50 pipe on a drx?

[#404's Dad]

Originally posted by machwon
Yes, I've been quite impressed as well, but it was very difficult getting there. I've asked several shops but really only one has told me those are respectable numbers. It seems a few of them have others that build there motors, but know ones saying who does the porting/motor mods. I'm pretty sure it will be reliable as well, but only time on it will tell.

Most of the gains came from the porting the cylinder, head work, and clutching. Between 2 different 50's I know I have over 200 runs since Jan. My goal was to only modify the stock parts and see what this little motor was capable of. If my kid doesn't graduate to a 90 by next year I'm thinking I will try and get a different pipe made. I've also heard the pvl ignition works good too, but it is very pricey.

Does anyone know if you can fit a cobra 50 pipe on a drx?

What pipe are you using now? The stock DRR50 pipe is VERY restrictive.

I know what you mean I had one night last season where I put more than 150 runs on one DRR50 just tweaking it and trying to pick up a little here and there.

[machwon]

Originally posted by #8trx450r
What pipe are you using now? The stock DRR50 pipe is VERY restrictive.

I know what you mean I had one night last season where I put more than 150 runs on one DRR50 just tweaking it and trying to pick up a little here and there.

I'm using the stock pipe, pretty much stock everything for that matter. I've tried to buy a different pipe to try but no one had any to sell last fall and most said the stock drx pipe was pretty good. About time we get this all figured out, my kids will need a 70 or a 90. Oh well, sure is fun and I look at it as a challenge.

[#404's Dad]

Originally posted by machwon
I'm using the stock pipe, pretty much stock everything for that matter. I've tried to buy a different pipe to try but no one had any to sell last fall and most said the stock drx pipe was pretty good. About time we get this all figured out, my kids will need a 70 or a 90. Oh well, sure is fun and I look at it as a challenge.

Sending ya a PM.

[hotquads1]

Hi Bob,
thanks for the info , do you have rpm readings for the pulls ? top rpm achieved ? your info is accurate and your power after mods looks good. Is this a w/c 50 with 16mm carb "sunworld and stock metal reeds? We stock 4 different pipes for the DRR and 5 sizes of belts , so you want have to search everywhere . What fuel and main jet did you use for testing? Dont let any one tell you that those are not respectable numbers , they are. thanks, again.
marc

[#404's Dad]

Hey Bob, your PM box is full brutha.

[machwon]

Originally posted by #8trx450r
Hey Bob, your PM box is full brutha.

Ok, plenty of room now. Cleaned it all out.

[machwon]

Originally posted by hotquads1
Hi Bob,
thanks for the info , do you have rpm readings for the pulls ? top rpm achieved ? your info is accurate and your power after mods looks good. Is this a w/c 50 with 16mm carb "sunworld and stock metal reeds? We stock 4 different pipes for the DRR and 5 sizes of belts , so you want have to search everywhere . What fuel and main jet did you use for testing? Dont let any one tell you that those are not respectable numbers , they are. thanks, again.
marc

Hi Marc, yes most of the pulls run best around 9-9200 which is where the pipe seems to like it. I've had a couple clutch set ups that run 9500-10000 but theres really too much belt slipage IMO. The motor is actually A/C with a stock mikuni carb. We dropped the pilot 2-3 sizes and run a stock main jet I think. I'll keep you in mind after I try the next two clutch set ups.

Do you know what does the anonymous national engine builder guy put up for numbers on a 50? Any info would be great. Thanks,