EX21 Engine Mods

EX21 Engine Facts & Hop up & Performance tips
There are many misconceptions related to most engines when used for racing or when it comes to modifications. The Subaru EX17, EX21 & KX21 engines are no different. We will try to explain from our experience the different components we have found that can add performance & expel some of the myths.
The KX21 & EX21 engines look the same & use the same bottom end. Engine block, crankshaft, conrod, piston, rings & gaskets.The KX21 Subaru engines are not fitted with a govenor from the factory. The high quality conrod fitted to the Subaru engines from the factory when the engine is left as it was designed to be used.
We have had many customers with jet powered mokai Canoes & EX21 jet powered Mokai Kayaks
The 2 con rods shown are the factory Subaru / Robin on the left & the ARC EX21 con rod on the right. When fitting the ARC conrod the engine block at the bottom of the bore must have material removed to accommodate the extra material on the beam of the con-rod
Due to the wider beam of the conrod this area here needs to be clearanced, “Do Not Grind The ARC Conrod”
This is the other position that we have found needs clearance also when fitting an ARC con rod
This Picture shows the beam of the conrod touching the engine block before the block is ground.
on the top side of the engine block may also need to be ground for clearance
This picture shows the ARC conrod touching the block, this is the area that needs to be ground
the standard conrod passing the area that needs to be removed when the larger arc con rod is fitted
we have machined a hole in an old piston & to double check the clearance a flash light is shone down the bore while the gudion pin holds the conrod at the correct angle
The ARC EX21 & KX21 conrod has an extra oil hole in the top of the conrod to aid lubrication to the piston pin. You can also see the wider big end of the conrod as it sits on the crankshaft journal.
Subaru KX21 Cylinder Head.
The EX21 & KX21 cylinder heads do differ. The KX21 does not use the EX27 cylinder head as some people might think or believe.The KX21 has a cylinder head that has a larger valves from the factory, the combustion chamber is of a different shape the valve springs use an additional washer under the spring to boost spring pressure to take the additional lift of the Subaru Kart Sports camshaft.
We have seen engines that have been modified by owners and engine builders, there have been many Subaru Go Kart engine fail through engine modifications done incorrectly & when the engine is used in a higher rpm range than it was designed for. The EX21 was designed to only rev to 3600 rpm. When the governor is removed and other components are not upgraded it will normally end with a failed Subaru EX17 or EX21 engine.
Remains of an EX21 with the governor removed, 22mm round slide carb, open header pipe, cylinder head machined to increase compression & over revved
Not much was salvageable on the left shows the bent valve. Cylinder head, engine block, piston were all destroyed when this EX21 Subaru engine failed
Ex21 cylinder head had been fitted with KX21 valves & to increase compression it was machined.
KX21 cylinder head note the combustion chamber shape
Genuine 9.2 horsepower KX21 cylinder head, the size of the shape of the combustion chamber differs from the EX21.
If you are going to fit a KX21 cylinder head to your EX21 you will also need.
KX21 cylinder head & valves
KX21 camshaft
KX21Carburetor & intake spacer
or after market carburettor and intake manifold.
If you modify your ex21 engine dont forget that as the engine increases its power output to upgrade other internal & external components like valve springs, engine base plate to chassis mount, clutch with increased engagement speed, engine breather system to stop crankcase pressure build up and use a quality engine oil.
For standard EX17 and EX21 we have had great results with the Inferno Fury with its heavy shoes for lighter drives and junior racing go kart classes. The Inferno Fury is also a good clutch to use in governed EX17 and EX21 engines where the engine produces low end torque & the mechanical engine governor restricts engine RPM to around 3600 RPM.
With mildly modified EX17 & EX21 engines the Inferno Flame is worthy of investigating with its lighter shoe weight, surface cleaning grooves 2 position shoe weight holes for heavy or light weights the shoes can be used in both trailing or leading position. We have found the Inferno clutch when set up correctly engages smooth with out the harsh bang that can happen with some shoe clutches.
The Inferno Blaze Centrifugal Go Kart racing clutch is supplied with both heavy & light shoe weights, a lighter range of engagement springs and has the lightest shoe out of the Inferno range. The Blaze shoe is 25 grams lighter than the Flame shoe & 50 grams lighter than the Fury. Remember the lighter the clutch shoe the higher the engagement as it requires more centrifugal force to move the shoes to make contact with the drum.