A112 head porting

Road-race engines and ancillaries - general discussion

A112 head porting

Post by FLY205 » March 8th, 2012, 6:39 pm

Hello everyone,
I'm are a frequent visitor of the site. I'm starting to prepare a head of an Autobianchi A112 that I'm going to mount on a road car.This particular head have the four intake pipes that converge in a central one and separate ones in the exaust.I woud like to comply with the regulations of the historic car rally, so I can't fit after market heads like PB8s or PLR with their 8 port design. Usually, in historic rally , these engines are prepared with:
Weber 36 carburetor ( twin choke) with specific manifold
Camshaft with 300 degrees
Intake valves 31.5-27.5 mm
Increased compression ratio.
Please,anyone can help for me the development of this particular head?
Thank you.

Std head ( the right one )Vs group 2 ( hystoric rally ) spec head:
note the enlargment of the intake port.
std_vs_group2.jpg (223.52 KiB) Viewed 9491 times

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Joined: December 9th, 2009, 6:24 pm

Re: Fiat ohv ( a112 abarth ) head porting

Post by fingers99 » March 8th, 2012, 8:24 pm

I don't know the regulations or that engine. But surely the Group 2 head is a completely different casting?

I mean, you couldn't enlarge the stock head to that extent without welding up the head.

Might be an idea to check if this is allowed!

Abarthnorway - Remi L
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Location: Oslo, Norway

Re: Fiat ohv ( a112 abarth ) head porting

Post by Abarthnorway - Remi L » March 9th, 2012, 12:21 am

FLY205 wrote:Weber 36 carburetor ( twin choke) with specific manifold
Camshaft with 300 degrees
Intake valves 31.5-27.5 mm
Increased compression ratio.


I have worked a little bit on these engines long ago (10+ years), and know of quite a few racing these engines today.

I did not catch what car this engine is destined for....

Carburation: The 36 DCD is a good choice for inline engines - and it has the advantage of removable venturies for various engine tunes. In an inline setup this carb much better than the 32 DMTR fitted originally. For a transverse unit the float bowl orientation is wrong, unless You mount it in paralell to the engine which is not an ideal flowpattern. The throttle butterflies also operate opposite of eachother so I suppose the inlet manifold better be "roomy" to get the best of this carb.

For transverse engines (and inline as well)I recommend an Alquati "swanneck" manifold for DCOEs (or similar), with separated runners and a splitter in the head intake opening. That way You effectively get a 2 port intake. This setup is always causing a bit of a discussion, but personally I have made it work, and it gave a really remarkable increase in mid and top end on an otherwise standard A112 engine - much better setup than the 36 DCD in my opinion. Somebody else will probably tell You another story - but thats my personal experience.

Regarding camshafts its a bit complicated as a 300` camshaft does not mean anything, unless You know valvelift, separation angle and acceleration in lift and closing phases. Scuderia Topolino in the US probably makes the best performance camshafts for these engines, and stock a huge variety of other performance parts as well. I really recommend their products and service. Its a lot of not so good products for this engine type around. Do not buy the wildest camshaft unless You want to upgade all other parts. The stock Abarth cam will very easily take You to 7000+ if the engine is prepared properly.... thats on the limit already.

I have no direct experience regarding increasing valve sizes, but the combustion chambers are quite close to the limit with 29 in and 26 ex (std for 903/1050 cc) Its a myth that the Abarth had bigger valves than the 127. Valveshrouding is a problem. Personally I would keep the std valves on a road/track car.

The compression of the Abarth a112 is quite high already 9,8:1 if You run the 1050 block. The normal procedure is to shave the head and block, and if thats Your gameplan take care of the spark plug hole as it is already very close to the head face. Forged pistons for this engine can be found easily and cheap, so it is worth considering.

The head entry hole can be enlarged almost as much as You would like - I am pretty sure the casting on Your two heads are the same. The ports leading to the valves is the same story, but I do not think runner size is an issue considering the valve sizes. Main thing is to match head opening to Your choice of manifold, and make a smooth curve from the opening and into the ports. This is a bit sharp edge original. The short side radius towards the valves is another area to look into, and of course valves, valve seats and valve guides.

Take care not to tune for the very high figures with this engines. They do break all the time!!!! The original design is 50`s tech and started with the 600 and some 20 hp. To push out 90+ hp and revving to 7500 and more just rips the engine apart, unless every part is uprated. Most racers in Norway using Fiat 850s or a112 convert to FIRE engines because of this. The design is just not suited to be highly tuned unless You spend a lot of money on racing parts.

I will see if I can find some pictures of the DCOE setup somewhere, but I doubt I have saved them - long time ago. For head comparison between Abarth and 903: http://guy-croft.com/viewtopic.php?f=13&t=1505

Best regards

Remi Lovhoiden

Guy Croft
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Re: Fiat ohv ( a112 abarth ) head porting

Post by Guy Croft » March 9th, 2012, 9:29 am

Flavio, greetings.

Please publish a series of good photos of the actual head you are going to work on. I assume it is cast-iron?

Views required (for me & to show others)
1. ex ports from manifold mounting face and chamber end
2. inlet port region at combustion chamber end
3. combustion chamber
4. Closeup of valve seat region (high def but not megabyte size please) showing standard angles and valve throat region
5. Inlet and ex short radius
6. Both valves showing head design
7. Inlet manifold - outside, carb entry section and exit region where it bolts to the head.

Also tell me:
a. valve diameters in/ex
b. does it have valve seat inserts or are the angles cut straight into the head?
c. Diameter of inlet and ex ports at entry/exit
d. Smallest diameter inside inlet and ex ports
e. Diameter of runners at smallest section in inlet manifold

This is roughly how you make an appraisal of a head before mods.

Then I will tell you exactly how to prep it. As it happens we have an engine of this type in prep at GCRE at the moment though it is on a US made PBS alloy head.



Re: Fiat ohv ( a112 abarth ) head porting

Post by FLY205 » March 9th, 2012, 3:28 pm

Hello everyone!
This is why I approach the Guy Croft site. Many other ones only speak in the internet forums, here, there is pure technique! Mr. Lovhoiden thank you very much for your speech very technical and very clear. Mr. Croft, the photos are made ​​on a mobile telephone.I' ll provide photos with everything You asked.
I do not know how to thank everyone for the suggestion already received.
The car is a junior A112, i 'll install an abarth 1050 engine. The engine is a bit tired, so I want to re fesh it and start to work on some heads that I have at home befor installing the engine in the car.
New photos coming soon.
Thank you very much
Section of the head, this is the inlet port
head_refit14.jpg (139.7 KiB) Viewed 9453 times
Inlet manifold "group 2 head "
head_refit8.jpg (91.39 KiB) Viewed 9453 times


Re: Fiat ohv ( a112 abarth ) head porting

Post by FLY205 » April 5th, 2012, 9:09 pm

Hy Guy,
I have some new photo as required, I hope this one will be better quality.
waiting for a reply.
Inlet port at combustion, also can see SSR
P4050029.JPG (482.51 KiB) Viewed 9347 times
Last edited by FLY205 on April 9th, 2012, 9:18 pm, edited 1 time in total.


Re: A112 head porting

Post by FLY205 » May 1st, 2012, 9:57 pm

I was very busy in the last months. I co-operate with a team in the organizzation of " rally campagnolo " here in Italy ( 2° run of italian historic championship) .
Starting list ( http://www.rallyclubisola.it/campagnolo ... agnolo.pdf).
Main website:http://www.rallyclubisola.it/notizie.html
Results:http://rally.ficr.it/tab_schedule.asp?p ... stazione=3
I try to send a P.M. to mr Guy explaining why I didn't post new photos.
So, sorry everyone. This is a new photo of the exaust . I hope it'll be fine.
Regard's Flavio
Exaust view from exaust gasket
P4150011.JPG (43.2 KiB) Viewed 9080 times
Inlet manifold carb entry
P4050037.JPG (133.36 KiB) Viewed 9080 times


Re: A112 head porting

Post by FLY205 » May 3rd, 2012, 11:30 am

Good morning,
further informations:
-Inlet valve diameter: 29 mm
-Exaust valve diameter: 26mm
Inlet port :
Oval shape : 75mm x 34mm
Throat : 26mm
Exaust Port:
Exaust manifold side: 30mm
runner throat minimun : 21mm x 35 mm
throat exaust :23mm
Regard' s

Guy Croft
Site Admin
Posts: 5022
Joined: June 18th, 2006, 9:31 am
Location: Lincoln, UK

Re: A112 head porting

Post by Guy Croft » May 3rd, 2012, 2:59 pm

Good-oh & thanks for the pm.

I will definitely get on this week commencing 14th May



Re: A112 head porting

Post by FLY205 » July 24th, 2012, 10:39 am

Dear Guy,
after a long time I start the porting in the a112 head.
Enlarge the runner , smoothing the area between the two inlet valve in the region near the head bolt. In this area the port it’s a bit sharp.
Smooth the little step in the throat port , without enlarge this area. The throat measure 26 mm almost 90% of the valve diameter ( 29mm valve diameter ),so don’t need to enlarge.
No 30-45-70 seat cut ( waiting for the cutter I order ). Std valve without back cut.
With my “ agricultural” flow bench ( 4” depression ) I didn’t find a lot of improvement after porting job, only 1;2% increase but better flow equalization between the two valves.

The inlet gallery maybe need to enlarge, at the moment the shape it ‘s a bit elliptical 26mm wide, 28 mm height. Following a rule I find reading some forum, a single port feeding two valves have to be at least 5% bigger than valve diameter. Maybe this area will be opened up to almost 30-32 mm so need to enlarge a bit more. Anyone can confirm?

Thank you in advance for your help.

Abarthnorway - Remi L
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Location: Oslo, Norway

Re: A112 head porting

Post by Abarthnorway - Remi L » July 28th, 2012, 12:33 am

FLY205 wrote: Following a rule I find reading some forum, a single port feeding two valves have to be at least 5% bigger than valve diameter. Anyone can confirm?
This is very interesting - Please Guy?....


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Re: A112 head porting

Post by WhizzMan » July 28th, 2012, 9:07 am

Abarthnorway - Remi L wrote:
FLY205 wrote: Following a rule I find reading some forum, a single port feeding two valves have to be at least 5% bigger than valve diameter. Anyone can confirm?
This is very interesting
Indeed. I'd say that would totally depend on how much overlap there would be between the ports timing. If no overlap, apart from resonance, I wouldn't know why you would want a bigger port. With single ports feeding multiples valves in the same cylinder, you will need double the surface area of the individual valve openings to be able to feed them both adequately. What exactly constitutes "5% bigger"? 5% more diameter? 5% more surface? 5% more circumference?
Book #348


Re: A112 head porting

Post by FLY205 » July 28th, 2012, 3:13 pm

having very little experience in head porting I'm triing to do the best I can do. Reading in iternet i find this rule "almost 3-5% more diameter than valve diameter". Any sort of suggestion t'll be welcome.In this particolar head I haven't so much alluminium in the casting and will be very difficult open the ports in separate runners.

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Re: A112 head porting

Post by SirYun » August 10th, 2012, 12:12 pm


a bit of an old thread but still.

I have no experience with this particular head. I do have some experience with other small heads (both 998 and 1275 cc mini's that are siamesed ported as well , but worse) and have started on a whole series of incremental tests (test, modify, test, modify,test, ect. ect.) mostly as a learning experience. As stated by a lot of experienced porters, the most critical area is the seat area and the 3 cm before and after. My testing confirms this yet again

First of all. There is a LOT of incorrect or incomplete information being given ( both by people on the net , but also in books other than hard core engineering textbooks). So if somebody says 3% extra or 10% I always think.. Why !? Is this for a head anywhere like the one i'm working on . or a chevy 8 liter head. You have to look carefully at the cam phasing of the siamese port to see if there is overlap and if you can expect this to be a problem. This is not really solvable anyway as this is a part of the design. For a mini they make scatter cams that try to reduce the problem but most newer race cams are normal again as get better results a lot of the time.

But as you have to have some starting point.

I would suggest starting with the cam you intend to run, as this dictates the compression (not much good to modify and enlarge the chambers, only to find out later, that the thickness of the deck of the cylinder will not be enough to insure a good gasket seal, after skimming the head to set compression). Furthermore gains above where to maximum cam lift are generally useless.

If you lift the valve very high the influence of the seat areas becomes smaller. However I have found that the influence of the seat area is up to a much larger lift than I thought possible. First measure the flow at you cam's maximum lift. It could well be that it will flow enough air as is indicating that the port does not need to be enlarged other than to increase the flow at lower lift (supply a better route for the air to reach the seat).

For some heads there is a lot of info so you can basically try copy an existing design, for others you are on your own and have to find it out yourself. I would see if you could get a few scrap heads (a bit broken or leaking does not matter) to test everything. Also get as many trash valves as you can to test different angles and withs of back cuts. In my application the back cut makes a lot of difference (~5%) and at lift range that is useful as well. It takes ten minutes to do. Without the back cut on the valve the 3 angle seat does nothing at all in my case.
I attached a few graphs from one of my heads that are somewhat alike in basic configuration, more to illustrate the point that the area around the valve and seat are very sensitive to modification. The amount of material removed was very very small

You do need a reliable/repeatable method of measuring flow as it is finding a lot of 3% gains, not one 10%. If you don't have that i would just stick to the basics. And be very careful with the seats and area directly before and after. I have lost and gained quite a bit of flow doing tiny modifications to the valve and seat.
these are the differences with a very minimal back cut on a new model valve (more like deburring than a proper cut) (Flow in CFM @ 28 inch depression).
break-vs-no-break-new-valve.gif (14.2 KiB) Viewed 8456 times

black is original new, red is deburred new , green old style valve with tiny back cut and blue is new valve with a ~2 mm backcut
break-vs--2mm-backcut-new-v.gif (13.4 KiB) Viewed 8447 times
Joost M. Riphagen


Re: A112 head porting

Post by FLY205 » September 14th, 2012, 7:53 am

Dear Guy,
I’ve finished the porting job in the Fiat push rod head.
All the work I made was to try to equalize the flow in the ports.
At the moment the maximum flow it’s almost the same in the 4 inlet ports.
Std valves with 30-45-70 seat and 30° valve back cut; throat 25 mm.
The average increase of flow it’s about 10% , and the peak flow it’s almost the same.
I think this can be an improvement in the efficiency and a bit power hike. The engine will be a 965 cc a112 engine with C& B camshaft in 296° ex 284° 105° 43°/73°-67°/37°.
Carburettor : weber 32 -32 twin choke ( from a112 abarth ) and 4-2-1 ( a112 abarth ) exhaust.
I hope it’ll work !!
Thanks for all the suggestions

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