Hoca 54mm Cylinder KitI love to recommend big bore kits as the best bang for the buck mod for most 49cc scooters, but how much difference might one see by moving up from a 52mm stock cylinder to a 54mm cylinder? It's time to find out.
I'm using the Hoca 54mm cylinder kit (Part # 169-154) from
Parts For Scooters for this test. As you can see, it includes pretty much everything you'll need to increase your displacement. Of course you still have to supply your own tuning and installation supplies, but it include a cylinder, head, piston, piston rings, wrist pin, circlips, as well as a head, base, and exhaust gasket.
The Hoca kit is the only one I've seen (that doesn't cost a small fortune) that offers auxiliary exhaust ports in a 50+mm cylinder kit.
Here's a closer look at the exhaust port layout. The auxiliary ports help to increase the exhaust port area at the right time. Using this style of ports as opposed to a larger single port can also be easier on the piston rings. A very large port can allow the rings to pass farther into the port and potentially cause the rings to snap. The little auxiliary windows add a lot of width to the top of the port without that unfortunate side effect.
I did no port matching to the case.
The Hoca kit works quite well with the stock 42mm crankshaft. As this image shows, the piston travels all the way to the top of the bore. (These pics were taken after testing, so that's why the piston is suddenly dirty.
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The piston is just shy of clearing the ports at BDC when using the base gasket that comes with the kit.
Port Durations
Exhaust : 178°
Transfer : 124°
Boost : 132°
I installed the kit with just the supplied gaskets. The head gasket supplied is 0.7mm uncompressed. The suppled base gasket is 0.45mm uncompressed.
Once everything was installed, I checked the squish clearance and did a compression test. Squish clearance was 1.7mm. A compression test showed 185psi. 185psi seemed like too much to me, but I wanted to try the kit as it would be if simply bolted on as supplied.
I ended up swapping up from an 85 to a 90 main jet and moving the needle to the richest position, from the next to richest position. The needle setting is actually a bit rich, but I was attempting to increase cylinder cooling with a little more fuel since compression readings are so high. I also swapped up to 7g sliders.
The increase in power output was immediately obvious. The small increase in displacement (from 89cc to 96cc) coupled with high compression and a better port layout makes a nice difference. The engine feels like it has much more torque at lower revs, and that added torque allows for lower throttle cruising and better response from any speed. It's not just low end grunt though, the power increase is steady throughout the revs.
Cruising speeds increased to the very upper 40s and as much as a shade over 50MPH at times. 50MPH cruising was at roughly 8,500RPM. Max speed was 54MPH at about 9,000RPM. It felt like the power was there and it would have the ability to go faster, but the rest of the stock setup just won't support higher revs and the stock CVT just won't let me tune it to go faster.
I purchased a set of 9g sliders to see if I could drop the revs farther and find more speed to no avail. All the very heavy sliders did was make acceleration more sluggish and drop the lower speed cruising RPM. One thing the heavy sliders did show me is that this setup is comfortable at pretty low revs. Even revving at only 4,000RPM in town cruising, acceleration wasn't bad and the engine seemed "comfortable". Once I hit somewhere in the neighborhood of 40MPH, the stock CVT seems to be allowing the belt to travel as far as it wants to as configured. That's why even heavy sliders can't gain me top speed. At that point you are limited by engine RPM and final drive gears.
The biggest problem that I noticed was detonation when operating at high revs. The additional power and timing pushes the engine to RPM that it previously didn't visit regularly. Couple that with an increase of heat from higher compression and you end up with plenty of excess heat, which leads to detonation. When cruising at lower revs I heard no detonation other than when decelerating, similar to what the stock setup has been doing. No big deal.
As long as you have the sense to back out of the throttle a bit here and there, the scooter can easily be operated as-is. Of course adding one extra head or base gasket would likely provide enough of a compression drop that negative symptoms would decrease substantially. Adding an extra base gasket would also increase port durations, which may prove useful if you use an aftermarket exhaust that will support higher RPM. An aftermarket exhaust designed for a 90-100cc engine may also help to keep the engine running a little cooler.
One other issue, albeit minor, is the large cylinder head provided with the kit. The larger head should help to dissipate heat better, but it is a bit big to fit the shroud just right. As the second image shows, it did burn through the shroud a bit, but I noticed no ill effects from this. It's not something I would be too concerned with.
All in all, I'd say this is a great mod. Even without adjustment I rode about 100 miles total while this kit was installed, because my other scooters were out of order. Just keeping out of the throttle a little while cruising kept the detonation at bay. The increase in torque was very much welcomed. Being able to accelerate and cruise into the 40s at lower throttle positions lead to improved fuel economy of 52.2MPG, the best to date. I could have been easier on it than that in town and probably saw even farther improved mileage.
One issue that I have with this kit, or perhaps it's just my nature, but it leaves me wanting more. Yes, it increased max and cruising MPH as much as any other modification to date. Yes, it increased torque and acceleration as much as any modification to date as well. Here's the problem though... It made me want more. This modification, more so than any other I've done to date on this project, made me want to do more to the scooter.
For example, when I installed the Hoca variator and saw the same max speed and a great increase in acceleration, I was content. The setup felt like it was "right". After installing and riding with this big bore kit, I feel like the setup is not reaching it's potential and it leaves me really wanting to see how much more it's got in it with a new variator, pipe, carb, etc... I think it would be very hard to leave it alone after installing this big bore kit, even though it offers performance as good as or better than any single mod I've tried on Project 90 to date. You have been warned, this mod might get you addicted and turn a simple project into a real build.
too. Have you ever had one walk out before this?
too. Have you ever had one walk out before this?
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