Engine Limits

The 1.8T is renowned as being a massively tuneable engine and is a wide favourite across many marques and different brands of cars.

They are typically a pretty strong engine, but as with anything, they do have their limits and restrictions depending on how far you want to push power goals.

One of the most noted and widely discussed limit/weakness is the Connecting Rods. Once the Rods are done, then the bottom end is said to be strong and only really needs improvement if you are chasing serious power. After that, the cylinder head is the next weakest with the exhaust valves beginning to melt at around 400bhp. Easiest solution to this is to upgrade to Inconel Exhaust valves which overcomes this issue.

These are the two main weaknesses of the 1.8T engine and generally, 90% of people will be fine with just a set of rods if your going BBT K300 or a Hybrid K04.

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Connecting Rods

There are two types of rods that are fitted to a 1.8T engine.  These are 19mm and 20mm.  This measurement refers to the wrist pin diameter.  Typically, the majority of 1.8T engines have 19mm rods.  The engines which feature 20mm versions are the AGU, AMK, and BAM engine codes.    However, this is only really important when upgrading the rods to stronger ones to ensure the correct fitment.

The standard rods are said to have around 300lbft torque limit before its advised to change them.  However, this is just a ball park figure and running high boost or spikes in boost profiles can also cause bending.  This causes high peak cylinder pressures which can end up bending the standard rods.   There have also been cases of rods having a slight bend to them from standard engines.

There are many different brands of rods out there, all ranging in different prices.  These can also vary in quality in terms of weight, quality of the bushings,  materials used, etc.

One of the key features to look for when buying a set of rod is rifle drilling. This process involves drilling a long hole down the centre of the rod from the big end, to the small end.  This allows oil to be pushed up from the journals on the crank and force oil pressure into the small end bushing/wrist pin to keep it lubricated.   Some people say that there is adequate splash lubrication from the oil spray jets squirting oil up onto the underside of the pistons…others dont.  Both will work, however there is a slight increased chance of premature wear if the engine doesnt see many revs which means a lower oil pressure compared to higher RPMs.

The other key point to look for is the actual small end bushing itself.  There are many rods out there on the market which are rifle drilled, however they feature a small groove around the centre of the bushing and a hole in the very top of the rod.   This allows oil to simply flow around the wrist pin offering minimal lubrication and straight out the top.  Manufacturers will claim that this is ok and that failures are down to wear in the actual wrist pins themselves.  Leading engine builders in the UK will say otherwise and always recommend a solid bushing if going rifle drilled.  Our advice is to speak to your actual engine builder or some of the respected 1.8T builders in the UK and see what they suggest.  After all,  they arent the ones who make or sell a product and the main intention is the get a sale.

When installing a new set of forged rods, its important that you properly check the parts fitting and ensure you use a high quality assembly lube such as Torco MPZ Engine Assembly Lube.  Make sure that you use a micrometer to check the wrist pins in the pistons for signs of any wear.  This will eliminate the chances of any small end rattles in the future.

Also, you will need to fit new bearings between the crank and the big end of the rods.  Ensure that this clearance is ok and there is no excessive wear or markings on the face of the crank which could damage your nice shiny new bearings.

Weight is also a big factor.  whilst machining these days is very good, there is a chance that there could be tiny tolerances in the weight of a piston, pins, and rod which could put the engine out of balance when all assembled.   Make sure that each one weighs the same to avoid this.


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