Pistons 818302 / 303 / 342

[Paolo67]

Hi,

I would like to understand if there are different dome on top of pistons for 818302, 818303 and 818342 engine.

Sometime ago Huib said that pistons for 303 are slightly domed respect piston for 302: that means that 302 pistons are flatted on top or just less domed than 303 pistons?

And pistons for 342 engine? They are more domed respect 303 ?

Is there somebody which could post some pics of these different pistons?

thanks

rgds
Paolo

[Huib]

The 302 pistons are pretty flat. Not mathematically flat though.

The 303 pistons are slightly domed. Difficult to see. It is easier to judge from the size of the cut out for the valves.

I have no 342 pistons here at present.

Attached is a picture showing a 302 and a 303 pistons. Both std.

[b20swalt]

Paolo,

And yes, the 342 pistons do have a higher dome than the 303's.

Walt

[Paolo67]

Thanks a lot Huib and Walt,

pics show very clearly (looking at the valve intake, as suggested by Huib) the different dome.

As Walt said, 342 pistons are more domed respect 303...I hope someone can post an image of 342 pistons.

So finally the different Compression Ratio between these engines it depends only on different dome?


ciao
Paolo67

[Huib]

It does indeed seem that the dome on the piston takes care of the differences in compression. I am not 100% sure, there is not something else.

If you do the arithmatic for a 1300 engine, it turns out that a slice of 0,1 mm with a diameter of the piston changes the compression by 0,1 bar.

Some examples.
If you eliminate the foot gasket, which is 0,5 mm, you raise compression by 0,5 bar.
If you use a 1.6mm head gasket instead of 1.2 mm, you reduce compression by 0,4 bar.

If you reduce the foot gasket from 0,5mm to 0,2mm, shave 0,4 mm off the block and 0,3 mm off the head and still use the standard 1.2 mm head gasket you would raise the compression by 1 bar. Theoretically. I would not do it in practice. I prefer the 9:1 compression ratio anyway.

On my S1 818.302 engine I did the reverse trick. I had only 303 pistons in the required size and mounted a so called re-inforced head gasket of 1.6mm to compensate for the dome. The engine has now done 200.000 km and is still going strong. However, if I would have to rebuild it again, I would use 302 pistons and a standard head gasket.

[Charles]

I always wondered what the differences are between the 303 and 342 engine. I've done a comparison using the specs and the parts table (See attached spreadsheet).
They seem to be different in compression ratio. Heads seem also to be different but valves are the same. Valve timing/lift is the same and so are the carburettor venturis (hence flow). So:
- Does the raised compression account for the 10BHP increase in power?
- Has anyone ever compared the heads?

[Ed Levin]

Very interesting comparison; I was so surprised to see that the carb venturis were the same, I had to go check it myself (which of course confirmed it).

The 10bhp delta must be due mostly to the increased c/r, but more compression doesn't work well without more volume. So if the venturis are the same, the 342 head must flow better. Otherwise, why bother with a different head at all? The c/r difference is in the piston dome.

(BTW, you have the 342 exhaust cam timing reversed)

[Huib]

There is also a difference in fixed advance of the ignition and valve clearance.

The crankshaft of the 342 is hardened.

There may be differences in workmanship such as polishing of moving parts, better balancing, flowing.

The values for power, torque and rpm are slightly different on page 01/0510 of the Technical Databook.

[b20swalt]

I have built a Series 1 HF -818.340 & a Series 1, 1.3S - 818.303 among others.

The HF engine had polished ports from the factory and also had a larger clearance between the piston & cylinder bore ( less drag). It also had polished rocker arms, although I don't think that increases horse power.

[Paolo67]

Thanks to all for your interesting answers and opinions (tu)

Very interesting to discover the bigger clearence among piston and cylinder on 342 engine, I didn't know this aspect before.

About the numbers showed by Charles, we have differences between 303 and 342 engines on: power, torque, pistons, head, CR, carburettors.
The carburettors have differences which can be considered as a result of a normal updating. The most important, barrel and Venturi size (31), are the same.

So I think that we have to find the reasons of the HF’s higher power and torque on different pistons, head and CR.

My opinion is that the 342’s higher CR is coming out from the higher domed pistons. I have spent some hours trying to calculate how the combustion chamber volume is related with the different dome shape. Even using an approach very roughly I found that considering the 302 piston as almost flatted, to reach the 303’s CR it is enough to have a dome high 1 mm (around the central region of the piston). Moreover, to reach 342’s CR it is necessary a dome high a little bit less than 2 mm respect to a flat piston.
I think these values could be not so far from the true dome shape…I should be happy if someone could check mesuring the difference on dome !

So in my opinion, comparing 303 and 342 head could be no difference in therm of «geometry». But there are a lot of difference in therms of accuracy on finishing it (e.g. polyshed intake/exhaust duct). 342’s heads were built with HF specifications respect «normal» heads, with the aim to obtain not only better performances on normal road use, but most of all to have a better disposition of these heads on receiving extra-works («Lancia Corse» works or other as Facetti, Bosato, etc.). Probably working on « normal » head it was not possible (or at least it was more difficult) to reach the same results.

At the end, let me share with you some roughly calculations that I have made about CR of Fulvia’s engines. Just starting from cylinder capacity and CR, I have calculated combustion chamber volume: please pay attention to the very similar volumes chamber combustion between 818.100 (30 cmc) and 818.342 (31 cmc) engine. Comments are welcome


Ciao
Paolo67