Tech:Engine/K Series/How to build a tough K motor

by Super Jamie <[email protected]>

Reasons to use a K motor

DO use a K motor if you are on a budget. these can be very cheap motors to modify for noticeable reward, and also to replace if you stuff up and kill it. that's the best thing about them, parts are cheap, and spare engines and parts can commonly be had for very little or free. however like anything, they can be expensive as well, if you let them. keep it in perspective

DO use a K motor if you can't be bothered engineering your car. want 100hp for as cheap as possible? don't want to spend the money on upgrading the brakes? want to just bolt something in and do burnouts? technically, there's a clause in the ADRs that says if you sufficiently increase the power of an engine to warrant it, you need to upgrade other relevant safety systems, but that doesn't stop your car passing rego. this was one motivation for me

DO use a K motor for something different. at the time i did my motor, it seemed everyone was doing a 4age transplant, or rattling off numbers numbers letters of all these high powered engines which they see in car magazines and will never buy. it's also pretty cool to have a noisy lumpy sounding engine, and you can really impress people when you tell them a naturally aspirated 1300cc did THAT!

DO use a K motor if you just want something that bolts in. i can't weld custom engine mounts either, so i went with the easy solution. i also couldn't be arsed converting my car to an efi fuel system, wiring in a computer and all that jazz. distributor and carbs doesn't get much easier

DON'T use a K motor if you have plenty of money to spend. if you want the best engine for your corolla and you'll spend more than a grand on it, forget a K motor. drop in a 4age or some other efi thing, possibly even force inducted if you want to, and you can drive it without crashing it. take it from someone who's built a $2000 4K, i could have got the same thing (minus the lumpy idle) by buying a $850 4age, with alot of potential for more power as well

DON'T use a K motor if you want to beat things on the street. about the best you'll manage is flogging a stock commodore up till about 100km/h. all your mates probably have really fast cars, and if you want one too, be prepared to either spend alot of money on your corolla, or just buy a car that's fast in the first place. corollas are quick, not fast

DON'T use a K motor if you want to "drift". if you think you have the "mad skillz" like fujiwara takumi, or you are building a "drift pig" corolla because you're a P-plater driving professional, then forget a K motor. the best power you'll reliably get out of one cheaply is less than a bigport 4age makes, and nobody drifts with them

Starting point

Block

Your main choices are usually either a 3K, 4K or 5K. The 3K is easy because it might be in the car now, but really they're too small to make much power, because of the different stroke. About the best you can do is have a little torquey 1200 which peaks at 6500rpm, or a revvy little engine which makes nothing down low. The extra couple of hundred dollars a 4K wrecker motor costs (and really, if you hang around Corolla people long enough you'll pick one up very cheap or free) is money well spent. However, if you do modify a 3K, about the only thing that matters is the camshaft selection, which I will cover later.

The increased stroke of a 4K and the fact that it's a very in-square motor (having 75*73 bore*stroke) makes it ideal for shifting a fairly decent amount of air. A 5K can basically be treated like an oversized 4K, which is handy because extra torque is also very nice and helps make that little bit more top end. However, 5Ks are still quite expensive ($550) from wreckers, and still a little hard to find second hand for a decent price (under $300). If you can get one, go for it, if not, a 4K is almost as good for a cheap thrash motor.

Head

People go on and on about how good 3K bigport heads are. Whatever. A bigport is also well over 30 years old, and probably has a fair bit of corrosion around the water jackets, it might even have developed a crack, will strip at least one manifold or rocker thread, and probably has a few helicoils in it. They also have crappy valve retainers and stem seals, which you need to change to later 4K gear when you make power.

With the amount of money you'll spend having an old late 1960s 3K head reconditioned to a working state, you could have taken a 20 yearold 4K-C head which works perfectly fine, is less prone to cracking, and had a port job done on it which is better than a bigport casting. Don't waste your time searching high and low for the elusive mystical 3K bigport, I have two of them, and they're not that bloody good.

Bottom End Mods

Block Prep

These are pretty well put together little engines. 5 bolt mains and rod bolts as big as a 350 chev. While you've got the thing apart, change all the welsch plugs and take the studs out of the timing cover and rear main carrier and use bolts, it makes servicing the engine (if you ever need to change the timing chain, for instance) so much easier. Check the bearings if you like, but if the engine was working before and hasn't done a huge abount of mileage, you should be right to just slap it back together with a new bearing and seal kit. But it needs new stuff, it needs new stuff. Running bearings one size over isn't that bad, but two sizes and i'd scrap the crank and find another one.

Balancing

One thing I will suggest, if you're going to rev an engine high (8000rpm+) then get the internals balanced. Not only will it prevent the engine from fighting against its' own intertial mass, hence making more power and having less wear, it will also transfer less vibration to the rest of the drivetrain, so the weak little K gearbox will last longer, especially at high speed. Get anything that moves balanced - crank, pulley, flywheel, rods and pistons. They don't need to be balanced together. A balanced component has equal rotating mass on it's own, as well as with other components. Balancing should set you back between $160 and $250.

Pistons

One good thing about 3Ks is that they have japanese Toyota pistons in them. Aussie-assembled 4K motors have these crappy aftermarket Repco pistons, which will crack. You can tell by looking up the skirt of the piston, one side will always have the TEP logo, the other side will have REPCO embossed in it further up on Aussie pistons. If you want aftermarket stuff, standard ACL replacements are just fine, they're actually far better quality metal than the stock pistons ever were. Retail on my set of 060 3K/4K pistons was about $390, but I paid cost which was $160, so you've got a fair bit to bargain for discount if you buy new stuff.

Nobody makes forgies specifically for a K motor, but there ARE motorbike pistons that fit, but you'll have to look all the specs up you need in the freely available ACL catalog, then trawl a bike piston catalog till you find a match. Serco, the Australian Wiseco distributor, offered to have me custom billet slugs made up for the bargain price of US$2000. No thanks.

Lightened Flywheel

Of all the things you can do to a motor besides air supply and camshaft, this probably changes its' characteristics the most. I had my stock 4K flywheel machined down to 7kg (I have no idea what they are stock) and I like the way the engine builds revs quickly. People say you lose torque, like the ability to climb hills, with a lightened flywheel. But the engine also has less mass to spin, so you transmit more power to the wheels. You can make a K motor pretty torquey anyway, I say it's worth it. Doing this will also make your idle more lumpy. Machining of my flywheel cost me $80.

Companies will also make you a custom alloy flywheel, you supply them a stock one, and they make an alloy center with a clutch face, and press the stock ring gear onto it from your flywheel. These things can get down to 4kg, and you'd probably need a fairly high idle (1200rpm+). Expect about $600, though we got down to $400 with a 5 or more group buy. Jun make two billet alloy flywheels for 4K motors, one is 3.7kg, one is 3.4kg. They're $990 and $1100 respectively through Speedworks Auto in Perth.

- camshaft - lifters - oil pressure - oil

Top end mods - head porting - valve retainers - valve seats - bigger valves - rockers and posts

Ancilliaries - distributor - ignition - water pump - alternator - ignition - engine mounts

Air and fuel - stock manifolds - downdraft carbs - sidedraft carbs - exhaust manifold - exhaust size - mufflers

Drivetrain - clutch - gearbox - tailshaft - diff