The T4 1X 1.9  D engine uses two spring (aka bastard) clips to clamp the downpipe to the exhaust manifold. This is often called  the "toilet bowl" style and it's also used on the less performance orientated Golf models.

The design is compromised as it does not properly isolate the exhaust pulses like a conventional flange type manifold / twin downpipe setup which keeps cylinders 1 & 4 and 2 & 3 separate.

 A petrol engine loses torque because of this design, especially if a longer duration uprated cam is fitted. Diesels are less effected because the valve timing doesn't have such significant valve opening / overlap at TDC.

Google search showed a picture of the T4 Petrol manifold as being a twin downpipe with 6 bolt flange design which is similar to what the performance 8v K jet and Digifant 8v mk2 GTI engines used. Certain Mk3 and Mk4 Golf 8v models also used a similar style although the downpipes can be more centered on some applications.

The downpipe on Mk2 GTIs have a flexi arrangement at the flange to the manifold, if the T4 is similar it may manage to flex enough to allow the T4 downpipe to mate with a Mk2 GTI style exhaust manifold.

Edit: And I recall VW in their wisdom tweaked the 6 bolt flange bolt positions a couple of mm so some downpipes wouldn't mate to some manifolds. Twas only a few mm so opening up holes and using washers under nuts to clamp them up would still work if a fudge is the only viable way because parts are so difficult to source.

My manifold is the toilet bowl flange type, as is the 1x. I have heard that the fitting to the head is the same as mine (AAC). 

I was just wondering that if I fit a 1x diesel manifold onto my AAC petrol engine, altering the front pipe if necessary, will it have any adverse effects on the engine performance. The performance of the aac engine is very poor and inefficient anyway to be honest so I dont want to make it any worse.

thanks again

If your AAC uses a toilet bowl design then the 1X exhaust manifold might well work. The exhaust ports and flange on the cylinder head are the same for 8v petrol and diesels.

Be very careful when installing the head to manifold gaskets if they come individually as they are orientated 1 & 2 going one way and 3 & 4 going the other. It is a subtle difference but they need to be correct to seal properly.

It's difficult to see when doing the job but care needs to be taken as they are shaped ever so slightly different.

A look on ebay found a German shop listing a 1.9D 1X downpipe as also fitting a 2.0 petrol so they might be worth contacting to confirm it will work. Things like if a CAT is fitted to your vehicle might throw a spanner in the works.

I am aware about the gasket orientation. I have the engine on the bench at the moment so access is easy.

I have checked and the 1X and AAC engine front pipes are the same part number so it looks like it should work, fingers crossed.

Thanks again.

I sourced a manifold from a 1.9d 1X engine and it fitted. I had to do a small mod to the exhaust shield and a very slight grinding of the manifold itself to allow inlet manifold to sit properly.

Fit straight to the head and also the front pipe. 

So far so good, she seems to run very well. Time will tell if there are any long term problems.

Thanks for your comments