ENGINE ANALYSIS   by Ed Bridges
  O.S. .18 TZ (S)-T
With all the latest big block engines coming out for the MT class, it may seem unusual that O.S. has released a small block .18. It’s not just any .18, it’s a TZ .18 for the Revo. I must warn you, this .18 is like no other. I’m sure you’re familiar with the saying “big things come in small packages.” Well, that fits the TZ to a T. Read on to decide for yourself if the TZ has the ponies to compete.

CRANK

The TZ’s crank has an outside diameter of almost 12mm. The larger size allows for a 7.7mm inside bore for added fuel delivery and power. This is the first O.S. engine I’ve seen with a crank that had a turbo cut. The leading edge of the crank face also has some new work not seen on other O.S. engines—a pocket cut to reduce weight for better balance and to help swirl the fuel inside the crankcase.

SLEEVE

The sleeve on the TZ is a 5-port rear exhaust design. This 5-port design consists of four transfers and one boost port. The split transfer ports help by adding more surface area for the piston to ride on, so the piston won’t walk out the sleeve on the upstroke. This also allows for equal or greater port area as compared to a 3-port sleeve. Port area can be added by increasing port timing duration, which means it should have a higher volumetric efficiency in the higher rpm and be able to maintain port velocity. The boost port timing is staggered from the transfers, and it has a large port area with a low timing duration.

PISTON AND ROD

The piston is machined from aluminum, which provides a better fit and match to the sleeve once up to temp. Near the top of the piston is an oil groove that retains oil for lubrication. The rod is knife-edged to reduce drag inside the crankcase and to reduce weight. The rod is double bushing, with an oil hole on the large end to improve lubrication on the crankpin area.

CASE

The first thing you’ll notice about the TZ’s case is all the extra webbing it has. This aids in cooling and it provides added strength. The porting inside the case is perfectly matched with the sleeve. The port channels have small diverters that help increase port velocity and direction as the incoming charge goes up the channels to enter the cylinder.

HEAD

The blue anodized head on the TZ is sweet looking, especially with the O.S. .18 TZ name and flames etched on top. The head is a 2-piece design with a separate head button and heat sink. The heat sink is big enough to cool the TZ in the toughest conditions.

CARBURETOR

The TZ I tested came with the 11L rotary carb and has a 6.5mm venturi. It’s a 2-needle carb for easy adjustability and, like all O.S. carbs, reliability. The base of the carb has an insulating ring to stop the carb from absorbing too much heat from the case and thereby causing vapor lock. The tall velocity stack increases low-end throttle response and torque necessary for monster trucks.

TESTING

Ok, testing went down like this. After consulting with my friend Slimbo, I decided to use the stock Revo exhaust. Slimbo, who works at Crispy Critters Hobby Shop in NJ, said that many people were experiencing great success with the stock setup. Really, if you think about it, the majority of people will use the stock exhaust to start. So why not see what the power curve is with the Revo exhaust setup!

I used a complete gallon of White Lightning 20% fuel with the included O.S. P3 plug. After break-in, I replaced the plug with a new P3 for testing, and there was no need to retune. The TZ, from the first run, was producing impressive numbers. At 21,000 rpm, the TZ was pushing 1.5 bhp with 72 in.-oz. of torque. This was impressive, but what really got my attention was the fact that once the load decreased, the TZ just kept climbing. I was shocked to see this .18 push these kinds of numbers.

When it hit 28,000 rpm with 2 bhp, I thought, ok, that’s it; it peaked. Not the case: the TZ kept climbing almost another 4,000 rpm where it peaked out at 2.27 bhp at 31,761 rpm. To have this .18 push out ponies like that with a stock Revo exhaust is crazy.

The torque curve of the TZ was even more impressive. The torque really came on strong around 18,000 rpm with 60 in.-oz. of torque. Torque peaked at 84.58 in.-oz. at 24,000 rpm. At 32,000 rpm, the TZ was still kicking out 70 in.-oz. of torque. That’s a 14,000 rpm power curve, which hits hard and strong in the midrange. I took the TZ up to 41,000 rpm where it was still pushing out usable power with over 1 bhp and almost 40 in.-oz. of torque.

The TZ ran extremely cool throughout testing. The highest reading was 230°F, and the average was 210°F. With my Eagle Tree System hooked up to the engine, it produced an EGT temp of 625°F at its highest, with an average of 575°F. The TZ ran consistently throughout the testing with no hiccups.

CONCLUSION

There’s no doubt the TZ is the strongest .18ci engine that I’ve ever dyno-tested. This thing has so much usable power it’s sick. The power curve of the TZ is in the sweet spot for off-road use, with a broad power curve and plenty of torque. Don’t get me wrong: if you want to go fast, the TZ will do that, and with the right gearing, you can really get your Revo rolling.

Links

O.S., distributed by Great Planes Model Distributors, www.greatplanes.com, www.osengines.com,
(800) 682-8948.

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