Posted by: JeremyG 31st May 2019 14:39
Having completed my GT's JTS motor head gasket replacement, I thought you guys might be interested to see how the JTS engine compares to the Twin Spark.
The answer seems to be - the block's the same; and the head differs mainly in the fuel injection system, with a direct injection approach.
Here's what the engine bay of my GT looks like:
And here's what the head gasket looked like once I'd removed the head - not pretty:
(When I ran a compression test, I turned the engine over with all the plugs out - and a jet of coolant came out of cylinder no. 4...)
Here's the head after it had been skimmed and vacuum tested (at Rob Walker Performance Engineering) - valves were all sealing fine, so no further action needed there. Plus there were no ridges on the cylinder bores - pretty good for a 100k engine! From this view you can see the injector ports, and just see where the injectors are inserted into the combustion chamber - it's a direct injection system.
Here are the injectors refitted to the head - with a close up, showing new seals fitted on the right:
... and the fuel high-pressure manifold fitted over the injectors:
And finally the airbox re-fitted, along with the high-pressure fuel pump which is mechanically driven by the exhaust camshaft (at the back on the left). So fuel is delivered at low pressure from the tank via the central black hose (plugged with kitchen roll in this picture) then on to the high pressure pump and then to the fuel manifold via the solid pipe you can see on the left.
Needless to say, access to the injectors is impossible without removing the head... definitely a downside compared to the TS motor.
Aside from that - the block and timing gear looks the same (including the variator, and the water pump) so timing was a familiar process. Other obvious differences are that the exhaust manifold is integrated with two pre-cats into a single unit that runs down and behind the engine; and the water manifold on the front of the engine is made of plastic and not metal...
Now that it's all back together, how does it drive?
First impressions are that it's fairly similar to the TS engine. Although the JTS motor has a few more horsepower (165bhp) the GT also has longer gearing than the TS in my old 145 - so it isn't any quicker, but it is more relaxed - especially at motorway speeds. It's definitely a less raw experience than the 145!
Do I prefer it? The GT is plainly a more modern car... but so far I'd say the 145 is definitely more fun!