Motronic "small" port head - Alfa Romeo Bulletin Board & Forums
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post #1 of 19 (permalink) Old 08-06-2009, 08:37 AM Thread Starter
Richard Jemison
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Question Motronic "small" port head

I guess this is another of Alfa`s better ideas on the Motronic motors.

Look at the small (2mm smaller) exhaust port that is offset forward and barely into the exhaust system. You can see from both sides how ie is choked down and offset forward.

The inside port is 2mm smaller than the earlier Bosch and Spica
The 2nd pic is a stock Spica head.
I would put this on my flow bench but it would be wasting time. This has to be emission control / backpressure crap
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post #2 of 19 (permalink) Old 08-06-2009, 09:13 AM
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Richard,

Most interesting. Just for discussion, I think you're suggesting that Motronic style cams & pistons in conjunction with a SPICA head would produce more power. Correct?. Or, could they be playing with increasing velocity by using smaller ports. Or, or.... could you maybe do a quick comparison run on your flow bench?

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Originally Posted by Alfar7 View Post
I guess this is another of Alfa`s better ideas on the Motronic motors.

Look at the small (2mm smaller) exhaust port that is offset forward and barely into the exhaust system. You can see from both sides how ie is choked down and offset forward.

The inside port is 2mm smaller than the earlier Bosch and Spica
The 2nd pic is a stock Spica head.
I would put this on my flow bench but it would be wasting time. This has to be emission control / backpressure crap


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George Schweikle

1976 Spider (Dedicated Autocrosser, "SPICA, No Carbs")
1991 Spider Veloce (Retirement cruiser)
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post #3 of 19 (permalink) Old 08-06-2009, 05:26 PM
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Bigger is not always better!

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post #4 of 19 (permalink) Old 08-06-2009, 05:34 PM
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Keith,

I remember a factory "Small port" head used on GTA, GTAM, GTA Jr? or?

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Originally Posted by crashmctavish View Post
Bigger is not always better!


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post #5 of 19 (permalink) Old 08-06-2009, 05:56 PM
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Im not sure but as I say big is not always best. Also motronic twin sparks have flat pistons.Further the road cars have been mentioned as the valves being to big (GTA's smaller). Smooth non turbulent flow at sufficient volume to evacuate the cylinder is desirable, narrow bore creates a vacuum effect due to pressure drop. This would scavenge the cylinder better allowing more space for the next fresh charge.
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post #6 of 19 (permalink) Old 08-07-2009, 05:01 AM Thread Starter
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Geez...

You NitWits are the folks that look for flowers in scatological landfills!
And I`m not taking time to put this mess on the flow bench. 40 plus years of doing Alfa and other heads make this redundant unless someone want`s to pay for the time.

Look at the poor machining. This is just as I said Emissions. No extra performance from the design this head! What gains came from compression, FI improvements, cam timing events and intake lift. If they had left the exhaust port open you would had more.

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Last edited by Alfar7; 07-21-2010 at 01:40 PM.
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post #7 of 19 (permalink) Old 08-10-2009, 06:54 AM
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After seeing this thread Friday, I happened to be working on the heads for my 'new' V6 build and took the opportunity to put a gasket on the exhaust port. Not as bad as the worst one shown here, maybe half way between the good & bad.....but I'm taking the time to open them out to match the exhaust flange, as I'll be making new headers for this one. If I was sticking with the cast manifolds, I might not have bothered, as they kind of match the ports...although not particularly well it must be said. IIRC, the gaskets are usually 'blown out' around the top edge.

I recall seeing a thing ages ago about having a huge bore exhaust system with a step at the bottom of the port to supposedly give the best of both worlds. I think the idea was that at low volumes, the gas trickled over the step and didn't get sucked back in, but at high volumes, it whizzed round the top and didn't 'see' the step. Sadly, I can't recall the results of their experimentation. The fact we don't see these things in the wild might suggest it didn't work! Another variation was again a very large bore system, but with short inner smaller bore 'stubs' at the head. Again...no idea what the results were.
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post #8 of 19 (permalink) Old 08-10-2009, 08:43 AM
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What you describe are variations of the use of anti-reversionary baffles if you wanted to search up tech info on the how-why-for of them.




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post #9 of 19 (permalink) Old 08-10-2009, 08:54 AM
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That'll be it. A curiosity really - I don't think I'll be experimenting myself!
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post #10 of 19 (permalink) Old 08-10-2009, 11:47 PM
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Quote:
I'm taking the time to open them out to match the exhaust flange
It might be a good idea to match the gasket to the header flange but i'm not sure it's a good idea to match the port to the header flange. From what I have learned you should leave the port a few mm smaller in diameter for that anti-reversion step.
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post #11 of 19 (permalink) Old 08-10-2009, 11:55 PM
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Quote:
Originally Posted by CorseChris
I don't think I'll be experimenting myself!
Prolly for the best as it works most effectively within a very narrow RPM range based on harmonic pulses. (as in 'be prepared to do some math to dial one to the other')

It has some effect at other RPM ranges, but not really enough to bother farting around with. You'd be better served to have a well worked head first before even contemplating such things, then mabe when you're after that last teenie tiny bit give it a try. (but don't expect miracles)




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Last edited by Tifosi; 08-11-2009 at 12:01 AM.
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post #12 of 19 (permalink) Old 08-11-2009, 03:24 AM
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I should add - this engine I'm doing is for supercharging. If I have it right, that makes a lot of the niceties of exhaust design a bit redundant.
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post #13 of 19 (permalink) Old 08-11-2009, 03:53 AM
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1. If the port size/design is a solution to emission requirements, how can it be bad from a compliance standpoint?. After all, Alfa had to get the car approved for sale in the U.S. (Granted, it doesn't appear to be a good racing design).

2. If you didn't expect comments, why did you post in the first place?

3. "You NitWits"?... having a bad day are we?

Quote:
Originally Posted by Alfar7 View Post
You NitWits are the folks that look for flowers scatological landfills!
And I`m not taking time to put this mess on the flow bench. 40 plus years of doing Alfa and other heads make this redundant unless someone want`s to pay for the time.

Look at the poor machining. This is just as I said Emissions. No extra performance from the design this head! What gains came from compression, FI improvements, cam timing events and intake lift. If they had left the exhaust port open you would had more.


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George Schweikle

1976 Spider (Dedicated Autocrosser, "SPICA, No Carbs")
1991 Spider Veloce (Retirement cruiser)
Scuderia Non Originale
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post #14 of 19 (permalink) Old 08-11-2009, 08:31 AM Thread Starter
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Anit-reversion

Anti-reversion would be a lot more like pic # 2 (spica head) .

George, why should I be having a bad day because these NitWits?

It`s just another day on the BB

Richard Jemison
RJR Racing

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post #15 of 19 (permalink) Old 08-12-2009, 08:09 AM
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Nit wit ha ha

Alfa7
Whilst I agree wiith you that the matching of header to gasket & port is important. It does not always follow that the port needs to be the same size.
Years of playing with RG500's & other 2 strokes have proven that.
Tuners have opinions & certain combinations work. Whilst I have the greatest respect for your history with alfas it does not automatically make you right on all counts. The combination of RPM valve port header & baffles is key.
If you doubt my opinion then you are not open minded, check out Phil Manzano RIP ,Henk Vink RIP, Ton Pels & Jan Smit are in my list of associates.
Why did you post expecting no comment?

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