Port sizes within the cylinder head

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Djaraceandrally

Daily Driver
Hi, just something thats been in the back of my mind having recently bought a modified head for my 1600 rally car build.

Having looked around at other heads, on the net Im wondering if the ports on my head are a little small (do I have little port syndrome??)

Its running 39.5 inlet valves and 33 exhausts. Porting has been done 30mm inlet / 31mm exhaust respectively, nicely done and reshaped chambers to accommodate flow around the increased valves within. Bowls again, nicely done, deep throats before the nicely radiused turn on the short side. Valve guides machined flat to face of the port.All to a great standard. But….

Now I know bigger is not always better, but looking at these heads coming out with 40/35mm valves, the port sizes at the face of the head are huge compaired to mine. I do wonder if they are all CFM but relatively slow airspeed?

Appreciate with different specs of components and intended use there may not be an optimum , but is there an average or reccomended size for ports

The fella who did this head builds grass trackers that tend to be smaller cc, short stroke engines, but they do big revolutions and produce pretty impressive power figures.

Ive got to port match to the inlet - taking it out of the head as opposed to the inlet which is 2mm all round bigger. So just want to make sure before I bust out the die grinder and flap wheels
 
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Look at the port information in this reference:

Ultimate SOHC engine.

Fiat SOHC Performance Tuning. 1. Introduction. Despite having very humble origins as a power-plant for the 1969 front drive revolution Fiat 128, in my opinion the SOHC is quite a spectacular engine and Aurelio Lampredi's finest mass produced work. Notwithstanding it's 'inferior' design non...
xwebforums.com

Some of it can be difficult to follow in places, the data may not all be completely relative to your application, and some pics are missing. But it offers some baseline porting info you may find helpful.
 
might be worthwhile chking out "speedtalk forum".

there is a rather useful inexpensive software program "pipemax".

darrian / erland cox / swr / and quite a few others delve into this exact question. take notes, and look closely at where in the port the min cross section is, and what is suggested for inner seat dia as % of valve dia. the taper also affects the tq curve. finally - take note of the posts on "pressure recovery" as this can greatly help effiecency (the chaps down in argentina / brazil weld chambers to make the little sohc into a copy of "LS" type chambers.)

im sure you are aware that the fiat sohc head has "2 pairs" of ports, and inbuilt swirl.
 
I have 39.5 inlet valves, half way in the ports are 31.5 mm dia. That's about the max you can safely go, I went to 32.5 on one and the casting was so thin at the top of the port at half way in coolant leaked through the porosity in the casting. Had to scrap that head.
 
I've found these heads are VERY porous castings. As a result I just do a light clean up of all the imperfections, flash, irregularities and port match them to the manifolds. But even that can get tricky because you will reveal more craters as you go.
 
Thanks all, yes read the Steve C posting several times. So its looking like Im near optimum on the port sizes with the head. On port matching, how far do you go into the port from the face as not to create a profound step? On the exhaust, a small step on the lower edge into the manifold is deemed productive, but on the inlet, thats quite a transition (about 1.5-2mm all round) different from the inlet, into the head. Clearly dont want to break through or go so thin, it gets porous as Greg mentioned
 
There is enough meat at the manifold mounting face of the head to enlarge the port to 33mm, or slightly more, for the first few mm, so there is no sharp step.
 
Taper from 33 to 31.5 over 15mm would be ok. Mine continue to taper to about 30mm vertical measurement near the valve guide, but 31.5 in width. With valve guides cut off you should be able to have 31.5 vertically as well.
IMG_20240405_093659_878~2.jpg
IMG_20240405_095654_427~2.jpg
Here's a couple of pics of my scrapped head, you can see the 6mm hole I drilled from the top to see how thick the casting was where it was leaking, less than 1mm in the 32.5mm port with too much removed from the roof. The other pic is looking into the water jacket with a screw plug removed.
 
I think it is more about the use of flow of the port than the size necessarily. Lord, why does that sound so bad?

Anyways, on my old 14:1 race motor, it was stock intake and exhaust valves and the head had work done to it, but it was about flow, not about how large of an intake area you can get. It was a monster to drive and made all types of power from a single 40 dcnf carb. When looking at the CFM per minute of the intake, the total CFM allowed through based on the valve size, and opening/duration and then all of that used together, then I think you will have a better idea of what it should be.

Back in the 90's, I remember trying to enlarge it as much as possible with the theory of "more air" but this was not really too good of an idea.

A flow bench is the best way to do it, and I am sure we have some others here who have spent quite a bit of time with one to determine what makes sense and what is just wasting time. Hopefully they will chime in on this conversation.
 
Yes, the engine is being built for a rally car, so outright HP is secondary to torque - so gas speed is key for that, ive not gone massive on the valves or too wild on the cam choice (or at least the requirements for the cam). But when it came to looking at port matching, thats when the questions arose
 
Djaraceandrally said:
Yes, the engine is being built for a rally car, so outright HP is secondary to torque - so gas speed is key for that, ive not gone massive on the valves or too wild on the cam choice (or at least the requirements for the cam). But when it came to looking at port matching, thats when the questions arose
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Exactly. As @fastx19 said, bigger is not necessarily better. It depends on the goal; big port/valve diameter tends to drop torque in favor of peak HP (assuming everything supports it). And while peak HP numbers may sound impressive, that's not necessarily what you want depending on the intended application.

As a side note, I was told by a very respected expert that back in the day one method for engine makers to try and cope with their underpowered vehicles was to use larger exhaust valves/ports (relative to the size of the intake side) and set everything up for higher a RPM target. That sacrificed torque in order to get a larger HP spec (that's what seems to sell cars). This makes the engine very "peaky" - needing to remain at redline to have any guts (which isn't practical in most cases). And his assessment of the stock SOHC was exactly that. Therefore he recommended NOT enlarging the exhaust valves or ports and concentrate on the intake side.
 
Djaraceandrally said:
Thanks Greg
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I said last week that 31.5 mm inlet ports would be as large as you could go, and that is probably correct. But this week I have to take the manifolds of my current head, so I measured the inlet ports on the replacement head. Seems I was more cautious than I remember, they are 30 mm about half way in. There was no obvious difference in the performance compared to the bigger ported head, although I didn't get it redynoed. It's giving about 135 hp at the flywheel, 10.7:1 CR, twin 40 DCOEs, Piper BP300 rally cam.
 
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