105hp vs. 130hp 1.9 heads

The difference between the "economy" XU heads and the GTI versions

While I built the second 1.9 engine for my project from a cheap 105hp (DFZ) motor and a GTI (D6B, 130hp) head, I was expecting it to make 120 hp at most, because I would still have lower compression than a GTI, right? Well, not entirely, but I'll get to that later.

Anyway, while swapping the head there was a moment when I had both heads on the bench at the same time and out of curiosity, I started looking for differences. On various Peugeot and Citroen forums I have heard discussions about how to extract 130hp from a 105hp engine (CTI, Rallye) so I thought I'd publish my results and show you all what the real differences are and what's needed to convert.

Note that I will only talk about the mechanical side of things. I have not (yet?) looked into what changes are needed to the fuel injection and perhaps the distributor advance curves, if any, to make use of the extra power. It's probably easiest to use a GTI ECU, injectors and distributor but other combinations might work as well.

By the way, most pictures can be clicked to enlarge. At times, the story can get a bit technical. I will also try to explain HOW I measured things. But feel free to skip to the next section if you don't know what the *** I mean :-)

Looks

As many of you will know, both heads look the same from the outside. All gaskets and manifolds are interchangeable too. But that's superficial. It's actually quite surprising that Peugeot / Citroën managed to cram two completely different heads into the same package.

The most obvious differences

Camshaft

It's widely known that the GTI came with a "wilder" camshaft from the factory. It has both more lift and longer duration. The picture tells the whole story. I probably don't even need to tell you which is which, so I won't :p

I did use a dodgy method to measure lift. Assuming the duration is less than 360 crankshaft degrees (that's 180 camshaft degrees!) the base circle is easily measured as the width of the "egg shaped" cam lobe. Turns out both cams, although very different profiles, measured the exact same lobe width, so I assume this is a valid way to measure camshaft base circle. Then I measured cam lobe height, and subtracted the base circle. It's that simple! Note: it could be that my cams are old and worn, so take the numbers for what they're worth.

Turns out the GTI head has 0.7mm more valve lift on the intake side and 1.5mm on the exhaust side! Hardly surprising, but good to see the numbers.

There are some more subtle differences too. That Allen bolt you see on one of the timing side cam bearing caps in the picture holds a "clip" that deals with end-play. On the other type, the flywheel side (!) cam bearing cap does that job. Also, the "slow" head has an extra lobe on the distributor side to drive the fuel pump on carbed cars. It's also there (unused) on EFI cars with the same cylinder head : "one cam fits all".

It is said that the GTI camshaft won't fit a 105hp head. Could be true, the head casting around the buckets does look slightly different. I haven't tried it though, or tried to measure the casting. With all the other differences I found, I reckoned just a cam swap would not be of much use anyway.


Valve size

The GTI not only has a wilder cam, it has bigger valves too. And to make optimal use of them, the combustion chamber was reshaped to improve flow out the intake valve. I'll provide more details about this in the next section. The valves are about 5% bigger each, resulting in 5% bigger circumference and therefore 5% more flow area. Combined with the extra lift in the cam, this is a large change! Exact sizes listed below. These are not effective valve sizes : I simply measured the outer diameter.

Less obvious changes

Combustion chamber shape

On the "slower" head, there is a "vertical wall" really close to the intake valve. On a GTI, this "wall" is moved away a bit from the valve and sits at an angle. That should help airflow out the valve greatly.


A more important difference is in depth. I thought my eyes were betraying me, but it's true - the 105hp combustion chamber is much deeper than the GTI chamber. Unfortunately I didn't really have a way to measure it reliably, so, using a tape measure, a ruler and repeating it several times, I found the difference is roughly 2.6mm.

With help of a few straight-on pictures of the combustion chambers, I estimated combustion chamber volume, taking area as well as depth into account. I won't bore you with the exact data but I will tell you this:

Given a compression ratio of 8.4:1 for the CTI, the compression ratio of a hybrid engine using a GTI head on a DFZ block.with the standard DFZ pistons(!) would be 9.4:1 - just because of the reduced combustion chamber volume in the head! I know a D6B has 9.6:1 compression ratio, so I suspect there is either still a small difference between both piston types or simply an inaccuracy in my measurements.
(Side note : It would be interesting to see whether a DKZ head is the same since it has a bit lower compression than a D6B. I suspect it is. For reasons I'll explain later, the combustion chamber shape is much harder to change, so most likely the small difference between DKZ and D6B is in the pistons).


Port shape

If you run your fingers down, for example, the intake ports of the 105hp head, you will feel the port makes a pretty sharp turn down near the valve seat. Then try the GTI head and the same bend will be noticeably smoother, even though the rest of the port looks very similar from outside. There is no big difference visible from the manifold side.I have but one explanation for this. Remember the combustion chamber "ceiling" ? It's lower in the GTI head, requiring a slightly longer valve AND leaving more room above the valve for a smoother bend in the port!

Port size

Looks roughly the same. There is definitely a difference down where it counts, near the valve. It is slightly wider down there, but there does not appear to be a difference on the manifold side of the port.

Measurements

Here's the hard data. Numbers. Facts. Figures...
Measurement105hp (standard)130hp (GTI)
Intake valve diameter39.541.5
Exhaust valve diameter33.034.7
Intake valve lift10.511.2
Exhaust valve lift9.711.2
Combustion chamber depth10.68.0

Conclusion

To sum all this up, the GTI head is really a well thought nice piece of work, with the smoother ports, bigger valves, wilder cam and a nice increase in compression ratio all built in. Everything you can think of that will fit in the same package has changed for the better over the "standard" head. You obviously can't just swap over the cam and expect a "slow" head to make 130 hp. You probably wouldn't even get halfway. I think 115 is the most you might expect from just the cam, with a LOT of luck as you will be using a cam that favors an rpm range the rest of the head does not and vice versa. On the other hand, swapping over the whole head, but only the head, DOES probably give you the whole 130hp because it even "fixes" the compression difference! And face it ; a head gasket kit isn't THAT expensive, so just get and swap the whole thing if you had access to a cam anyway. Besides, original valve seals are likely to need replacement (now or soon) anyway and they're included in any good HG kit, plus you will have relatively easy access to them too.

Article written by: Marcel
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