SOHC carburation and setting up

[Guy Croft]

Martin - either way - you're wrong.

You must not load up more fuel for cooling like that. The engine can only burn so much gasoline before the unburnt hydrocarbon level post-combustion becomes excessively high and when it does it contaminates the upper cylinder lubricant, damaging irreparably the rings and bores and wrecking the oil in the sump, the oil viscosity drops and bearing damage can follow as well. Worst case? No, I have seen this a lot.

If you want to argue any more, do it on another forum, NOT here.

GC

[Guy Croft]

As promised.

X19 1500 with standard size valves flowtest results standard/full spec compared with some other SOHC units. The ports on the Tempra 1600 and X19 are close to identical in final trim and I am pretty sure the difference between the X19 and Tempra (and big valve 42mm Uno is the valve size), at least a quick examination of the valve diameter ratio seems to point to that. I guess Fiat knew the SOHC head was 'undervalved' which is why they changed it from 36mm to 37.5mm.

I have indicated the port sizes, not massive and I don't think they should be - or need to be.

The differences look huge but as percentage increases - it's actually what you'd expect. The increase at any lift point will be greater by the ratio of valve (ie: throat) square of valve diameter.

GC

[James Bowen]

Guy,

Many thanks for publishing those flow test results. I am not experienced in interpreting the graphs but it appears to me that work on the the valve throat and seat area see's big gains in the zone of valve lift up to approx 6.5mm, and that the controlling factor above that lift, is the actual valve diameter (the throat getting bigger as well naturally) I presume that the port diameter figures do have a bearing on final flow, but really the idea is to size them appropiately to the diameter of the valve?

Have I got that right? If so, then simply opening my ports a bit (at the narrowest point only) will only have a benefit if the valve/throat area can flow more than it currently does at higher lifts.(which looking at your data, I'd say would be doubtful)

Regards, James

[Abarthnorway - Remi L]

Hi Guy, hi James!


Thanks first for very interesting graphs!

The way I read these graphs, both port size and valve size have to be increased to achieve higher flow ... its a lot of uncertainty here as I have never had the chance to compare the heads - might be some differences in port design/angles as well.

OK:

Up to approx 6 mm lift:

When comparing the dark blue line (1400 GC Tipo with 37.5 valves 27/25 port) to the light blue line (X1/9 1500 with 36 valve and 31.5/30 port) it clearly flows a lot less air even below 6mm lift.....
Actually the 36mm 1500 flows as well as any other up to around 5.5 mm lift including the 42mm big valve Uno (Purple line)

Only engines with port size of 30 mm + have a big flowgain at 6mm lift (regardless of valve size.)

Over 6 mm lift:

The bigger valved heads (Tempra - brown line + Uno purple line) seems to gain an advantage over the smaller valved heads, but only if port sizes are increased as well. The 27-25 port GC Tipo never even gets close to the smaller valved x1/9. This seems to me to be caused by an excessivly small port....
The 37.5 mm valved Tempra and 42 mm Uno (big ports) leads the way at higher lifts

So first of all increase port size within reason -
then if You want higher lift flow increase (6mm+) - valve size has to be increased as well!! :-)

Am I on to something here??


Best regards

Remi Lovhoiden

[Guy Croft]

Some good (valid) points by both, well done. If I get a moment I'll reply step by step, but certainly you're both thinking on the right lines.

How big a port needs to be is an interesting question to which I have devoted much thought in the 3 years since I got my flowbench. Of course it's true there will be some optimum port size based on airflow requirement and port velocity (borderline 0.5 Mach seems best) but it varies from head to head and there is no fixed rule. A steeply downdrafted head like an 4 valve F1 engine will have port diameter (per barrel) not far off valve throat diameter, allowing of course for the splitter between them, less well-downdrafted heads like Vauxhall XE will have say 26mm dia barrel/splitter region on valve 34mm an throat of about 31mm, sidedraft heads like the Fiat TC will have port diameter varying from 34mm to 36mm on valve types from 42mm - 46mm and the last two types mentioned, well, I have plenty of dyno results that say they're pretty well close to the optimum (if not right on it) for torque and power. I've examined port-valve ratios exhaustively but you don't learn that much from them really. But in my exp it seems that if the port diameter to valve ratio is 55% to 64% you're OK, once the ratio climbs over 67% - the X19 1500 shown is 69% you could likely reason the valve is too small.

Would you, under those circumstances enlarge the port and increase that ratio still further? to say, 31mm port and 77%? I think not. If anything has stimulated my interest on port sizes it's Dr Keith McMullan's 1600 SOHC, port size 32-31 on 42mm valve and 54% port-valve ratio and pushing out over 170 bhp because the port velocity is extremely high. Increase the valve size to get more power? Well, the GC modified Tempra at 64% seems to suggest that's right and Fiat probably thought the same. KM's ratio is 54%.

In terms of how big the port should be you have to think about the bend loss in the port.

The X19 1500 port I refer to in the graph has bpf of 74.9cfm @ 10". Take a look at the inboard manifold in the photo - it has been enlarged to more or less port size - 30.5mm at exit and look at the flow - 26% higher than the port. Or to put it another way, if the port had a straighter run-in to the valve (like a downdrafted head has) rather than an almost 90 deg turn, it would flow about 90 cfm as well. To an extent you can increase for higher valve throat exit flow (ie: BPF) by making the port bigger, after all, the percentage loss in the bend is so high you have to get as much air flowing down the port as you can, but sometimes, depending on the short side radius and valve throat diameter - if you go too big - you can end up with no gain at all, or worse - a loss due to some weird turbulence.

Some brush strokes to consider, hope it's interesting.

GC


GC

[James Bowen]

Guy and Remi,

Remi, good point about the 1400 Tipo plot pointing to the port size as the restrictor....though interestingly the standard X19 head flowed as well as the fully worked (valve/throat area) Tipo head till 5.5mm lift. So, yes the fully worked and larger port X19 head seems to alot better with the larger port, but it appears to have had a head start from the begining. Obviously we don't know the exact mod's made by Guy, and I wonder why the later head would seem to flow less well at the lower lifts in this graph.... Hmm

Based on the figures I have for my head, I have a 54% port / valve area ratio. (same as KM's !!) Assuming for a second that a bigger port in the region of the downturn toward the throat, (as this is the narrowest point anyway) would work better in terms of flow. Can I consider making the port more oval in this region? i.e. The distance between the SSR and roof is currently 25mm. (I just tidied this region up rather than dramatically reprofile) the sides are 28mm. Rather than muck about with the SSR and risk introducing flow robbing turbulance, could I remove material from the sides to increase them to the same as the port entry 31mm - 32mm? That would give a ratio of 60.5% - 62.5% approx.

I suspect not, as none of Guy's SOHC heads that are described on this site have a cross section of this type. I suppose in relation to my particular head, the question is a little academic as if and when I do make alterations, the head will get larger valves and ports to suit, but in isolation would an oval cross section work?

Regards, James

[Guy Croft]

James, hi

you should certainly not change a port that is essential circular in section to any other shape. A circular section gives the best velocity distribution.

With your motor I'd be thinking about the CR, the flow analysis of your particular cylinder head bpf and valve-in response - after all this is such a widely read thread most will be now wondering exactly how it does behave - and your cam type. In respect of the latter, for example, is it well-matched to the characteristics of the head?

Ultimately, with the right CR and a top performing cam (I know a few..), a well-prepped head, and given that we know Weber carbs work (!) we would then be left with only the optimisation of the header-inlet lengths. So ultimately what that engine gives, well, it won't give any more and then we stop and say 'that's as much as it's going to give..'.
Unless you decide to then fit FI, in which case, in the right hands, you will add on a decent improvement in the torque response and the top-end power (maybe as much as 10%).

GC

[James Bowen]

Guy,

Hello, OK understood about the port shape.

Yes I think your right. In terms of the development of the current setup, and by this I include the current cylinder head and cam configuration. There are no other gains to had by "fine tuning".

Compression ratio, you've alluded to. I worked it out thus:

CC'd volume of cylinder head (measured with syringe, oil, and plate of glass) = 28.5cc (yes I skimmed the head! - I know)
CC'd volume of piston cutouts = 5cc
Block decked 0.5 mm pistons pop up into gasket. Gasket 1.6mm compressed thickness = 6.4cc
Ring land in between piston and block = 0.5cc
Total = 40.4cc

CR = CC volume + Swept volume
CC Volume

CR = 10.3 :1


I would like to do exactly what you suggest, take it apart, and get the flow and valve response and cam all anaylised to see what is going on. But. Despite not producing the power or torque I would like, it is reliable and still fairly rapid, and gives me a lot of enjoyment. I shall in the next year, start to build another engine, employing what I have learnt since I built the current engine (mostly from this site through various posts by you and other knowledgable members). In the meantime, Ill put this threat to bed I hope that it will continue to provide some guidance to other members, if only how not to build an engine ! - Many thanks Guy

Kind regards,
James