By Walt Gorczowski: Winner of the 2002 HPI Challenge in Elmhurst, Super Nitro class

I've been racing the HPI Super Nitro for 3 years now. For as much fun as this car is, it should be more popular. Let me give you some description of the car.

HPI Super Nitro Basics:

It's a belt driven touring car type chassis. It's basically the exact same design as the 'regular' 1/10th scale HPI belt driven touring cars (the RS4 series), except a little bit bigger (some 20% bigger). The chassis is just a little bit longer, the suspension arms are just a little bit wider, and it uses bigger, 2.2 inch wheels/tires. The car uses the exact same drivetrain components as the smaller HPI RS4 cars, except of course the front belt and dogbones, which are longer. The HPI Super Nitro also comes with a side belt tensioner, which the other HPI RS4 cars do not come with.

The car still uses the 'small block' .12 and .15 engines, although many people put in big block engines using a conversion kit. The car also has a very handy, easily removable radio tray for easy cleaning. The other RS4 cars do not have the removable radio tray (except the new shaft drive version), and few other cars from any manufacturers have this feature, and it is a nice feature to have for as often as you need to clean a nitro touring car.

The car really isn't officially any particular 'scale'. It's just called 'super' scale, or 'super touring'. At 300mm, it's about the same as 1/8th scale (the Super Nitro's wheelbase is adjustable to either 280mm or 300mm). Some call it 1/9th scale, but it seems most accurate to just call it 'super' size. One thing I've heard others say about the Super Nitro is that it is the 'right' size for today's powerful small block engines. I have to agree. The 1/10th scale touring cars were designed back when the engines made 0.3 hp, or were electric. With today's 1+hp engines, the Super size cars just work much better.

It's too bad that this isn't the size that caught on to the masses since it's just such a fun car to drive. Even though it's not a 'full race' car like some of the 1/10th scale touring cars, I've never had anyone run away from my Super Nitro on the track with a 1/10th scale touring car. I've even won many races against Serpent Impacts and other big block 'on road' cars. The HPI Super Nitro is a great handling car, plus reliable and affordable.

Engine:

The car comes with the HPI 15FE engine. This engine will suit you well while you are learning to drive, but will need to be replaced by a quality small block engine if you want to race competitively around here. I run Novarossi engines, and have used versions of the Novarossi engine sold by HPI and Mugen, as well as Novarossi directly… all are fine.

Transmission/drivetrain:

I own a 2 speed transmission for the car, but every time I put it in I just end up taking it back out. This car is plenty fast with a single speed transmission and Novarossi engine for the size tracks we have around here. Even on the 185 foot straightaway at the track I run (Chicagoland R/C Raceway), I don't see much point in the 2 speed. The reduction in reliability does not seem worth the slight extra speed I see with the 2 speed. I've run in 4 different HPI Challenge races, and I have yet to see anyone in the A-main running a 2 speed transmission. I prefer to just adjust the pinion gear (clutch bell) for different size tracks. I use a 13 tooth (stock) clutch bell on tight tracks, and move up to 14 or 15 teeth on bigger tracks.

Do have some spare belts on hand. With the stock 15FE engine (which spews oil and fuel from the front bearing), you will go through rear belts often. The belts don't seem to like being soaked in fuel/oil. With an engine that does NOT leak all over the belts, you'll find that the belts last a good long time, and will only need replacing if a rock gets stuck in one and damages it. I pretty much change my belts once per season, or before a 'big' race (like the HPI Challenge races, where a new HPI kit is on the line for the winner).

The parts on the drivetrain that wear out the fastest by far are the 'diff shafts'. These are the 'dogbone cups' that stick out of either side of the front and rear differentials. These shafts will start to wear little groves where the dogbone cross pin rides. If you don't replace the diff shafts when they start to wear these little grooves, the dogbones (or universals) will start to get stuck in them. At first, this will make the car 'hop' as it accelerates out of turns (as the dogbone gets stuck and forces the suspension to travel up and down as the axle turns). Eventually, the stress will get to the dogbone and it will snap, and a new dogbone will snap too, until the diff shaft is replaced.

I've tried the 'normal' diff shafts as well as the 'heavy duty' diff shafts, and they both seem to wear out equally fast. HPI says to try the Nitro Rush diff shafts, but I have not done that yet.

I have run both the stock HPI dogbones as well as the HPI Universals. I couldn't tell any difference between them. Either will probably work fine. No one makes a CVD kit for the Super Nitro (yet), but even if they did, I wouldn't buy one. I have CVD's in some of my other cars, and they have been, by far, the least reliable part on those cars. The little set screw that holds them together just does not seem up to the task. The only nice thing I've found about the universals is that you'll only get that fast 'diff shaft' type wear at the diff shaft, and not also at the wheel axle end, since that is the end that has the universal joint.

You'll also need to upgrade the stock plastic brake disk. The plastic brake disk will wear fast, but before it's worn out, you'll probably do what I did and simply liquify it during a hard race with a lot of braking needed. The HPI fiber brake disk is a good upgrade.

The other part of the car's drivetrain that I've had reliability problems with are the hex nuts that go into and drive the wheels. On very high traction tracks, with a 13 tooth clutch bell, I've had a lot of trouble stripping the wheel/hex nut combination on the rear (usually the left rear). I changed over to aluminum hex nuts on the axles, and have not had a problem since.

Differential tuning:

Like most nitro touring cars, the HPI Super Nitro reacts well to differential tuning. I have tried the 'one way' front diff in my Super Nitro, but didn't like the off-power oversteer that resulted. Perhaps others aren't as bothered by this as I am, but I hate oversteer under any conditions. Some do say that the front one-way will help the car pull out of corners faster, but I just didn't like it. Perhaps the one-way would be useful on huge tracks where you seldom are off power (and never on the brakes), but these are not the types of tracks we have around here.

I do like the way the car handles with the front differential packed with heavy silicone grease. I use Ofna Diff Lock, which is a super thick differential grease. I find that this helps get the car out of the corners should the inside front tire loose traction, and it also makes the car tend a little more towards understeer, which is what I prefer. I leave the rear diff 'open' (i.e., not packed with grease). I have experimented with rear diff tuning, but find that a tight rear differential makes the car oversteer when off-power, which I don't like, so I just put in a little 'normal' grease to keep the innards running smooth, and leave it at that.

If you're racing, make sure you keep at least two spare differentials in your tool box… one packed for the front, and one 'open' (not packed with heavy silicone) for the rear. You never know when a rock will get stuck in a belt and take out the differential's belt cog, or when a diff shaft will wear a groove and give your car a case of the hops (or start breaking dogbones). One important note about assembling HPI differentials: the 4 little screws that hold the two halves together are NOT in a square pattern. You have to be very careful to get the two halves put together the right way, or only two of the screws will tighten up, and the other two will 'seem' ok, but not be tight at all. This will cause the differential housing to separate and strip the internal gears.

Body:

If you like a very wide selection of bodies for your RC car, than this would be the one reason to NOT get an HPI Super Nitro. There are not nearly as many bodies for it as the 'regular' touring cars, however, it does have many of the most popular bodies available for it, and since they are bigger, those bodies look a lot cooler than their 1/10th scale brothers. Also, the larger car, using basically the same chassis components, means that 'GTP' style bodies fit the Super Nitro much better than regular touring cars.

I have found that the Super Nitro is highly affected by the body choice. I have had very bad luck with the Viper and Lamborghini bodies, pretty good luck with the Porsche body, and great performance from the GT1 and BMW LMR bodies. My favorite is the BMW LMR body. This body has great downforce, especially in the rear (which suits my 'never oversteer' driving style well). This body also allows the engine to stick out for great engine cooling and the fuel tank lid to stick out for easy refueling. It's a great body for racing. I wouldn't worry about the 280mm wheel base verses 300 mm wheel base much. I've tried both, and couldn't tell the difference. The car is extremely stable with either one.

Suspension tuning:

Front: I run the LMR body, which requires the use of the lowered front shock mounting kit. This has a side effect of softening the front suspension a lot as the shocks are very laid down. I find that the purple HPI progressive springs (the stiffest they make) along with 90 weight oil works well for me. I like a car that is neutral or understeers, so the stiff front suits me well. In fact, if I find some stiffer springs than the HPI purple progressive springs, I'll probably try them out in the front of my Super.

Rear: I make most of my adjustments at the rear, as far as shocks/springs go (since it seems that I can barely get the front stiff enough). I generally adjust the rear stiffer if the track is very smooth and the car is very well planted, and adjust a little softer in the rear if I need more rear traction. I usually end up with the HPI black (stock) or white progressive springs in the rear, with 60 or 40 weight oil.

Camber/caster/toe:

The rear toe is not adjustable. The front toe I set to slight toe in. I have found that toe out causes the car to wander a little on the straight-aways.

I use the 8 degree front caster blocks on all but very tight tracks, where I go to the 10 degree setup.

I adjust the camber so that all 4 tires wear evenly across.

I set the ride height so that the front is slightly lower than the rear, and about 3 mm ground clearance minimum to the chassis. A little higher will help keep the car from bottoming out on rough tracks, but degrades the performance slightly on high traction tracks (where traction rolling can become a problem). If the car does start to traction roll, I try to lower the car as much as possible, and then reduce front traction with stiffer springs or a different compound tire.

Tires:

For the HPI Challenge races, only HPI tires are allowed. I've found that the HPI 'C' and 'D' compound tires work very well. I run the 'C' compound unless the tires start to overheat (you can tell because the car will lose traction after a few minutes of hard running). If the 'C's are getting too hot, I go with the 'D' compound. I like the HPI 'low bounce' inserts in my rubber tires. I've had trouble getting the tires up to temperature with the other, firmer inserts. The softer insert does cause the tires to wear faster though. I also like to run the same tires and inserts front and rear, but if I just can't keep the rear end planted, I'll run the soft inserts in the rear and firmer inserts in the front.

If the rules allow, then foam tires work much better than the rubber tires. I use foam tires from www.rc4less.com for the Super Nitro. I have had good success running their Purple foam (45 durometer) all the way around. This was their hardest foam up until recently, and it gives great traction and long wear. However, they just introduced a harder foam for the Super nitro (Red, 55 durometer). I have not tried them yet, but I'd guess that the traction will still be good on prepared asphalt tracks, and the tire wear will be even better.

As with rubber tires, I like to use the same compound front and rear whenever possible, and just rotate the tires front to back and side to side between races to keep the tire wear even between tires. However, like with rubber tires, if I just can't get the rear to stay planted, I'll go softer in the rear and firmer in the front, but you've got to make sure the tires stay close to the same diameter.

For next year at the Chicagoland R/C Raceway, we hope to have an active Super Nitro class with a 'rubber tire' rule to keep the costs down, but we'll have to see how that goes next year. The foam tires aren't very expensive anymore, and work so much better, we might just have to stick with allowing foam tires.

Steering slop:

The Super Nitro has a lot of steering slop. No one part is the cause. It's just a combination of a lot of parts that all move just a little bit, adding up to an overall very sloppy system. The bad news is that I have yet to find a good solution to this problem. The good news is that it just doesn't seem to adversely affect the car that much. Don't let it bother you if your car has slop in the steering, but if you figure out a way to get rid of it, let the rest of us know. By the way, I've tried some of the expensive aftermarket aluminum steering systems, and they were no better than the stock system in terms of slop.

Modifications:

The only other modification I've made to my car is to install a larger, 125cc fuel tank. I use a Koyosho tank intended for a buggy (I think). It can use the stock rear mounting location and puts the fuel filler lid in the stock location. The upper chassis plate just needs to be cut out a little bigger, and a new hole drilled for the front tank screw mount. The stock 75cc tank does have to be put back in for the HPI Challenge races, but for local 5 minute races, it's nice to not have to refuel after the warm up laps to be able to make the entire race. My run time with the 125 cc tank is usually around 10 minutes or more.

Conclusion:

The Super Nitro is one of the most fun cars to drive and own. It will keep up with the top 1/10th scale 'full race' touring cars if you're forced to run in the 'regular' touring car class at your local track. In a 'Super Nitro only' class, you can take advantage of the room for the larger fuel tank and run 8 minute races without pit stops. The car works well with rubber tires, and very well with foam tires. The radio tray comes out after removing two body clips, allowing the car to be hosed down with brake cleaner and blown off with an air compressor for very easy cleaning. The bigger bodies look cooler than the regular 1/10th scale car bodies, and the GT1 and LMR race bodies fit this car well and handle great. Almost all the hop up parts available for the RS4 series cars fit the super nitro, although most of those parts are not needed to make the car work great. You can fit a big block engine in, but don't bother… the car is great with a powerful small block engine.

Pick one up and race with us at the Chicagoland R/C Raceway or your local track, you'll be glad you did.