Inlet to plenum rubber hoses

Road-race engines and ancillaries - general discussion
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Nobby
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Joined: August 24th, 2010, 11:58 am
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Inlet to plenum rubber hoses

Post by Nobby »

On the Fiat Coupe 20vt (and probably numerous other Fiat/Lancia/Alfa R's) there are rubber 'doughnuts that provide the joining seal between the inlet manifold and the plenum.

I've just replaced all of mine after 2 of them had split and were leaking. I replaced them about 16 months ago too. The doughnuts are a factory part only - so off to the Fiat garage it is, and they cost about £7 each too. The thing is I can't help but thing they are of a poor design. They essentially are a rubber tube with a thick lip in the middle. If you use jubilee clips and tighten them too much they will split quickly. The plenum is also bolted to the plenum in 2 places, but the mounts aren't too secure as they also include some give.

You cannot replace the doughnuts with a straight bit of SAMCO either, as you need something with a lip to brigde the gap between the inlet and the plenum.

The problem doesn't stop there though. With everything fully bolted together I don't think there is a smooth transition for the air into the engine. The gap between the inlet and the plenum is not the same shape as the lip. Aside from machining the surfaces to match I can't see any other way of getting round it. You could machine them flat and then weld them together, but it would be very very difficult to bolt it to the cylinder head.

So, a couple of questions:
- Does this problem affect any other TC engines? I've certainly seen Alfa v6 intake rubbers splitting in the same place too
- Will the resulting ridge have a major affect on the air flow into the engine?

I will take some photos this weekend to show this clearly.

Thanks in advance
Chris
Chris Burgess
GC 01
WhizzMan
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Joined: August 13th, 2010, 8:05 pm
Location: Amsterdam, the Netherlands

Re: Inlet to plenum rubber hoses

Post by WhizzMan »

I only use the proper sized Click-R type clips for this reason. If you tighten them that way, they sort-of-almost fit good and there is not a lot of flow lost. Under or over tightening them deforms the rubber lip inside as well, making the fit bad indeed.
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