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Discussion Starter #1
When I purchased my '88 spider three years ago it came with a set of headers and replacement cat boxed up and ready to install. After removing the resonator and putting in a centerline stinger, and replacing the stock muffler with an Ansa, I decided to put in the headers.
I have run it that way for two years. Sounded good, felt like it had a little better low end torque. Quite a bit of decel burbling but generally enjoyable.

Fast forward to this week. California smog test coming up. So I pulled the headers and cat replacing them with the standard exhaust manifold and California cat. Results?
It may be perceived, but I feel like it runs better. Smoother exhaust note, with decel burbling gone. Low end torque seems fine.
I will need to drive it for a week or so, but it seems like a waste of time (and bruised knuckles) to put the headers back in.
 

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The factory 4 into 2 into 1 cast manifold is a good design, the 4 into 1 is not so good.

Unless racing and you have the 4-2-1 then yes the only advantage of a header is maybe less weight
Pete
 
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The 4 into 1 manifold was designed to meet the NO number in emissions testing. It does it by inhibiting the scavenging thereby diluting the incoming charge with spent gases. That knocks about 10 HP off the motor and will be felt more at higher rpm.
 
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The Vandenberg dyno tests at Hasselgren (Berkeley) in the 80s, printed in the Alfa Owner and reprinted in the AROC Spica manual, show the stock header as best by a few HP up to stock redline, like 5500. Tube headers were better above that. So for street driving stock will give best results on a basically stock engine.
Andrew
 

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The factory 4 into 2 into 1 cast manifold is a good design, the 4 into 1 is not so good.

Unless racing and you have the 4-2-1 then yes the only advantage of a header is maybe less weight
Pete
I guess that answers my question about early 2 piece verses late S2-S3 one piece manifolds.
 

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I put a Shankle header on my spider. I didn't like it because it was noisier than a cast iron manifold.
The late missed Merritt Carden told me the factory spent a lot designing their cars and it is "tough to improve on it."

Merritt won the "best mechanic" competition time after time from Alfa Romeo.
 

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I think from a flow perspective there wouldn't be much to choose between a pre-75 two-piece 4-2 cast header and a one-piece Bosch 4-2 cast header. Yes it's all in one piece but the passages inside don't know that.
BMW used basically the same thing on 2002 and 2002tii, one piece instead of two, but it's 4-2 internally.
Andrew
 

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This analogy may help understand what is going on.
The 4 exhaust pots are four roads that must merge into one, In the case of the 75 - 89 one piece manifold the four roads merge at one point. Light traffic merges freely and is not forced to slow down but as traffic builds up the cars on the four roads are forced to slow down. That is the equivalent of pressure build up in the pipes.
With a 4-2-1 system roads 1 &4 combine and roads 2&3 combine and the vehicles are controlled so that a car on road #1 arrives exactly midway between cars on road #4 and these two secondary roads may be wider than the four primaries. When the two secondaries merge then again the cars arriving are spaced so that they are exactly midway between each other and the final road may be wider than the secondaries. It is easy to see that this system will handle a larger volume of traffic before the cars on the four feeder roads are forced to slow down and the 4-2-1 system allows the engine to run at higher rpm before there is a pressure buildup at the exhaust ports.
The 4-2-1 factory system works fine for stock motors - it can handle the traffic.

Now if you improve the volumetric efficiency of the motor by installing cams with more lift and duration, modify the intakes to make them less restrictive, etc then you are putting more cars on the feeder roads and the factory 4-2-1 will start to limit power well before peak HP rpm. That is why you are unlikely to see one on a race car.

This analogy does not take into account acoustic waves in the exhaust system which are reflected at pipe merges and changes of diameter and which produce negative or positive pressures at the open exhaust valves at different rpms depending upon the configuration. A simple 4-1 header is not difficult to analyze but a 4-2-1 is almost impossible to model and professional race teams that use them (NASCAR) design them by trial and error with lots of dyno testing.

I tried changing the length of just the secondaries of Shankle headers and I made the overall performance worse. I gained a little at 6000 rpm and lost a lot more at 4500. I believe that Shankle put in a lot of dyno hours to design them. I posted details of my tests here Header configuration, AFR and Torque.
I believe too that Alfa Romeo put in a similar effort to design the GTA headers which are the basis for many aftermarket headers. Jim Steck's dyno headers are modified IAP headers which themselves a rough copy of GTA headers.
 

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Discussion Starter #9
Thank you all for the responses. I think I will stick with my stock 4 - 2 exhaust manifold.
Perhaps I can sell my headers/cat for enough to help replace the right rear taillight housing that I just broke!
 

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Here is the background to the exhaust manifold that AR fitted from 75 to 89.
When emissions standards were first imposed in the USA in the late 60's they focused on reducing CO and HC and left NOx limits fairly loose. Manufacturers were able to reduce CO and HC by more accurately controlling leaner mixtures but this resulted in higher combustion temperatures which in turn increased NOx. For the 75 model year NOx limits were reduced to 10% of the previous limit and manufacturers were forced to reduce combustion temperatures. In most cases this was achieved by diluting the combustible mixture with exhaust gas. Some used rather complex EGR systems but Alfa chose the simply install a restrictive exhaust manifold. This lowered the NOx emissions to meet the new standard but it reduced engine power by about 10 HP.
If you have a 4-1 manifold and you replace it with either a factory pre '75 4-2-1 system or good aftermarket tubular headers you should regain the lost 10 HP but you are unlikely to pass an emissions test.
 

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I think I will stick with my stock 4 - 2 exhaust manifold. Perhaps I can sell my headers/cat for enough to help replace the right rear taillight housing that I just broke!
You're confusing me when you say "my stock 4 - 2 exhaust manifold"; wouldn't the stock manifold for an '88 be a "four into one"? For your 2022 emissions test you might get away with adapting a pre-'75 manifold - a "four into two into one" - to your cat. Doubtful that the guy at the test center would know what sort of header was original to an '88 Alpha Romero.

Go ahead and sell the header, but if you sell the cat, how will you pass smog the next time around?
 

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I thought the 4-1 manifold was only 75-78/79 ish? didn't they go to a 1-piece 4-2 design in late S2 and Bosch models? Kinda replicating the early 2-piece design
 

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Discussion Starter #13
Alfajay...first of all, it's my Alfa Romeo. Not Alpha.
If you read my original post you would see I have two cats. One with the headers, and one California Cat
Finally. the 4 into 2 manifold is stock for an '88 spider
 

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I thought the 4-1 manifold was only 75-78/79 ish? didn't they go to a 1-piece 4-2 design in late S2 and Bosch models? Kinda replicating the early 2-piece design
The S3 cars that I have seen have had the one piece manifold. I think that the 2 piece returned with S4's which I think had cat converters that could reduce the NOx to acceptable levels.
 

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Correct, the 2-piece returned with the Motronic cars in 1990. I'm pretty sure all the S3 cars had the 4-1 manifold.
 

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Correct, the 2-piece returned with the Motronic cars in 1990. I'm pretty sure all the S3 cars had the 4-1 manifold.
I just verified that in my parts manual
 

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BTW: I have to add there is nothing wrong with a properly designed 4 into 1 exhaust manifold where the runners are equal length, but Alfa Romeo had other intentions with their 4 into 1, which is why the 4 into 2 into 1 is still the best.

If I was, for some weird reason, going to make an extractor system for my GTV, I would make a 4 into 1 ...
Pete
 

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My understanding is that 4-1 give you best top end power, 4-2-1 give you best torque. That assume right pipe size, length, collector design, etc.

The 4-1 was just a couple years, with air pump but no cat. Alfa was a late holdout for leaded gas along with Honda and BMW, hence no cat. Once they started using cats (1978?) the tailpipe emissions got a lot lower. As an aside, the 4-1 headers readily melt, especially with the too-effective hot-air stove. I've replaced a few.

Andrew
 
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