Burying 3.0 GTV OEM Cats - for good.

[Ascari32]

3.2 JTS
Sent to: le00ns on: Today at 10:46:57 PM
Hi,
I'm sorry not to have picked up on this earlier but the last couple of years have been quite eventful for us. We have moved from East Anglia to Cheshire, My Q4 3.2 JTS has been given to a friend - along with a mountain of new spares - there just isn't room for two cars and I had 3 - my 3.0 GTV and 1.9 GT. The GTV, which I have owned from virtually new, still only has 33,000 miles on her and was only ever used on driving holidays to Italy and the continent generally.

The GTV now occupies my attention with work being done to recover the performance this engine was renownd for, before Fiat strangled it's performance to meet emissions regs for the 164/66 and conveniently neglected to restore it when they put it into the 916 GTV.

Having done so much work with my Q4, it was clear to me - and had been a suspicion for years - the Alfa Twin Catalytic converters were a major issue.

Fitting a complete Supersprint Exhaust System to my 159 including a 100 CPI sports cat, revealed just how restrictive the OEM system is. It really beggars belief that Alfa went to such expense with their OEM system, purportedly to meet emissions regs, when they could have been met with a much simpler, yet effective system such as Supersprints - which sailed through all current emission regs, not just those current at the time the 159 was introduced.

Consequently, I have designed a new single 200CPI Catalytic Converter in conjunction with a Phoenix (allegedly Alfaholics) first box - after the cat, and a Ragazzon DTM Tail Pipe.

There are plenty of tech articles written about Scavenging but none have been particularly convincing when put into practice. And people continue to change out Alfa's OEM system, focussing particularly on cat - back systems, whilst leaving Alfa's Twin cats in place. The claims I have heard about improved performance are really no more than wishful thinking. One can make a exhaust system sound more "Sporty", but that does not mean the actual performance is appreciably better.

The problem with my cat design in conjunction with the DTM was simply noise level. In every other respect, it is stunning, low down torque, throttle response, flexibility in urban traffic is just fabulous - taking me back to my 1984 Alfetta GTV6, to which I fitted CB cams.

I then went and bought a Ragazzon DTM rear silencer, having been told it offered very little "back - pressure". What a joke that was. It killed virtually all the gains I had made, but was fundamental in discovering the reasons why the Alfa OEM cats are so auful.

The Busso in my GTV has 18deg. valve overlap. 9deg. ATDC - inlet closes - 9deg. BTDC - exhaust opens. Although not excessive, this should definitely result in a sporty response, without being temperamental in urban traffic.

From new, the engine in my 916 always made a "Hissing" sound around the cylinder heads/exhaust ports, when ticking over or driving at low speeds. It also had a characteristic "Metallic Edge" to the sound. The garage that looked after my cars said they all sound like that - this was after I remarked upon it when picking the GTV up after the timing belts had been replaced.

Yet, when the cat I designed was fitted, in conjunction with the Phoenix and DTM Pipe, the engine was completely silent!!!!!! The only thing to indicate that the engine was running was the rev counter and the tail pipe.

So - What was going on? Well, cue X - flow, or lack of it with the Alfa cats which the restrictive Ragazzon Box mimicked, although not as aggressively as the Alfa cats.

The replacement cat I designed, with its straight through design, combines the two banks with Bank A generating a good "Pull" in Bank B's manifold, and vice versa. This ensures a healthy negative pressure exists during the valve overlap period.

Fundamentally, the cross - flow from inlet to exhaust ensures there is little suction pressure on the piston at and around TDC when a piston is about to draw in an air/fuel charge. It also ensures a healthy degree of torque is maintained at low revs.

With the Ragazzon Tail box, which mimics OEM Cats, it is so restrictive as the destroy any manifold depression which would stimulate cross - flow; promote "Scavenging".

So the Ragazzon Tail box has been a useful tool in identifying the problem with Alfas Twin - Cat design. Allegedly Alfas cats have 400 CPI, are ceramic  and are fitted into canisters designed to "Recirculate" exhaust gases - "to effectively catalyse exhaust pollutants" - perhaps because ceramic cats takes that much longer to heat up to catalysing temperature and it doesn't help when each are only doing 50% of the work of a single metallic cat.

The transit time through the capillaries of the cat should be circa 300ms., which is why the canisters have off - set inlets and outlets. So the back pressure from the cats inhibits exhaust gas, thus destroying the principal means of creating cross - flow, the very purpose that 18deg overlap was designed to generate.

The net audible consequence of this is the noise from the cylinder heads, which sounds "as if one is drawing air into ones lungs through clenched teeth".

With a free - flow exhaust system, greatly helped by combining banks before a single metallic cat canister, cross - flow is enhanced, and the difference in performance is startling. It is not arrogance when I say, no GTA engine sounds as good.Top speed, it won't match the GTA. But acceleration - I wouldn't mind betting on it against and unmodified GTA!!!!!

I needed to write this all down as age (79) plays havoc with the memory.

If there anything I can do to help - reference the 159, then by all means, just shout.

Kind regards,

Brian.