SOHC carburation and setting up

[James Bowen]

Remi,

Thanks for your post. I have been watching your threads with interest already. Some of the best information gleaned, is usually as a result of someone elses question!

You look like your doing a very thorough job, not just on the engine, but the rest of the car as well.

Thankyou for your thoughts. The information on your build regarding ports sizes is interesting. I do not recall the exact throat diameter of the inlets on my engine, but I think it was somewhere in the region of 31mm. They are 3 angle 20/45/70. The head came with a car I bought sometime ago, so I'm not claiming knowledge of what was done and how... But I did some finishing myself. The combustion cambers had not been relieved to help with valve shrouding for example. I did notice, as you have pointed out, the bias to one side in the ports. (The side that is nearest an exhaust port) I did wonder at the time about taking away that "aerodynamic" curve, and getting a straight line from the manifold face, in. But, I had no idea what effect that would have on flow or turbulance, or whether there would be enough material to start chopping away.

I have e-mailed Guy some more data, so when he has time, I'm sure he will give his considered opinion. Jetting is definately better. The process described above surely points to the manifold primary runner length as being the main culprit.

Regards, James

[Guy Croft]

James, hi

you didn't publish it but I have it by mail, the first run showed 124.3 bhp (uncorrected) at 8010 rpm and you've told me that the rpm is out by 650, so peak power was actually at 7360, more or less where the final run ended up (7225). Since power is computed from torque and using P = T x rpm/5250, if we input 124.3bhp and 8010 we can see the torque at max power on the first run was 81lbf ft. Put that into the equation with the correct peak power rpm and we get a peak bhp of 113.6. This means (if I understand correctly) that the final run giving 124.1 (u/c) has yielded 10.5 bhp. And the peak torque has gone from 89.7lbf ft at true 5615rpm to 102.1lbf ft at 5605.

The standard X19 has (+/- from my factory figures):
Power - 85bhp at 6000rpm (yours 126.3 corrected at 7225, +41.3bhp)
Torque - 92lbf ft at 3200rpm (yours 102.1 at 5605, + 10.1lbf ft)

If you can get near 117lbf ft on a 1500 engine you are a 'made man' because that's close to the limit you'll ever get on an 8v unit. You're some way from that, but I've done it and it needs a lot more tuning than your motor has, notwithstanding that staement your torque isn't stunning for what it is. That's not a cricitism. But the gain over standard in top-end power over standard is demonstrable enough.

The fact that the engine is producing quite good top end power shows that - for what it is - it's 'breathing' quite well at the top end, thus it follows it should be doing so at peak torque. Yes, it might be possible to get more top end power from your unit by improving the breathing still more with head work (valves, ports) and yes there are limits to what that head can do. Remi's post is very good on some of the port/valve sizing issues. You have to be careful though, because small-ish ports are proven to yield excellent results from the high port velocity, so just plunging in and making them bigger/straighter might not be the answer. I'd certainly be interested in the valve-in response on the flowbench though and how much is being lost thru the valve throat compared with bpf. I'm just about to back-to-back flow test two heads (Steve Sharpe X19 1500 vs Gunter Pitzer Tempra 1.6 SOHC) to examine the effect of the bigger 37.4mm Tempra late 12 deg inlet valves vs X19 36mm early 'tulip' style on two GC prepped heads with similar port configurations and of course I'll publish this.

Now as far as the peak torque goes there is likely a wave problem affecting the filling (the head wants to ingest but cannot) and there it may well be possible to 'bump-up' the torque by lengthening the rampipes. Well worth trying given the dyno time to check it out. In saying that what I mean is - it's definitely possible to alter rampipes and get significant gains, because you're tuning the length to capture the 2nd harmonic inlet wave (an outgoing negative pulse). If the torque (at the rpm you're tuning for) is weak it invariably means the returning pressure wave (that bounces back from the rampipe end) is not getting to the cylinder before the valve closes. If the wave gets back after valve closure it's reflection off th evalve is positive, gets to the rmapipe end and comes back as a negative and sucks the air out (you get standoff). You can alter the inlet manifold length or the rampipe, it has the same effect. The position of the throttle plate, which is full open anyway, has no bearing on this. I've known one pick up 40bhp at peak torque by that method in a dyno session. The CR can be an issue too, it may a bit on the low side and contributing to the modest peak torque.

Jetting. I'm not impressed with the jetting methodology used in this case, rich everywhere (0.6 mid and top end 0.75-0.8) should always be dealt with by a change to a smaller main jet in the first instance. Rich at mid range means rich at peak torque - 5600. You always try to optimise peak torque first. Air correctors are NOT for that 5600rpm mid range jetting, main jets are. Top end correction with A/Cs is the range 6800-8000. The end result with 140 mains at peak torque was 0.76-0.8 Lambda which is way too rich - and will just wear out the rings and guides) and the real problem is cylinder filling. The tuning optimum is between 12-13 where 14.7 is stochiometric, that's 0.82-0.88 Lambda and I'd be looking for Lambda 0.86 at peak torque every time. Is it weak airflow per se? No, I alluded to that above. I'm talking here about pressure waves in the inlet tract that ram the air into the cylinder at high velocity, without which the engine won't develop any decent torque. A negative wave of -0.5bar is, just for an illustration, going to come back to the cylinder at 1.5 bar. These things matter.. Ultimately the mixture at peak torque ended up at 0.76-0.8 whereas from the outset that setting was considered too rich for the top end! Makes no sense to me. I learned jetting, as it happens, from world-class engine builder Tim Swadkin at Warrior, many years ago. We dynoed a lot of Fiat stuff there. The jetting was done the same way each and every time, over and over, always the same method - which is why I can write these posts from memory - and I still have all the dyno bsfc plots logging the changes. No-one can argue with my dyno results. There is a right and a wrong way and if you get an unexpected response you have to iron-out why before you proceed.

If the top end power is reasonable and the peak torque is weak you have to stop and consider what's happening. You can't make an engine more torquey by big main jets (and a 140 main is huge). I couldn't care less who reads this, sorry that's fact. I suppose it is always a dilemma - do I do the best with what I've got (by whatever means) or - do I fault find? The decision always come down to the big numbers and money. Yes, I've said that before.

GC

[James Bowen]

Guy,

Firstly thankyou for your response.

Just to clarify peak torque mixture was 0.86 at final settings.

We then finished 140 main 200 air. Mid range 0.76 - 0.8 Top end spot on at 0.86 from 5,000 rpm till 7,500 rpm

Though disregarding that for a second I hear what your saying about the main jet being too large. With the start settings being 130 main and 170 air, you would have come down on the jet size to 125 (?) and got the mid range correct before adjusting the air corrector. OK........, BUT having seen the operators carry out these runs on 3 occasions now, the problem seems to be that when putting in a more appropriate air corrector to modify that top end, the air corrector has a BIG effect lower down the range as well. I am not disagreeing with you Guy. I am certinaly not qualified to make ANY explaination or opinion, and will always take critisism offered by experts. I am reporting what I have witnessed.

Moving on from that though, I also see what you are saying about harmonics and pressure waves. Changing the exhaust primaries has the same effect? Looking at the power plots from previous runs and these ones, the change to longer primaries does not seem to have drasticaly changed the shape of the plots, but the "resonance" has definately moved down the rpm range, and the dip between 4,000 to 5,000 is more pronounced after lengthening the primaries. The dips occur at about 2,600 and 4,200. Would the next dip (maybe not seen as an actual dip in the plot but still there presumably) be at the same frequency ie 5,800 rpm and say 7,400 rpm? I understand what you are saying about different rampipe length maybe moving these waves to a different rpm.

Thanks for the detailed post,

Regards, James

[Acki]

Lambda 0,86 normally is too lean AFAIK?!

[Guy Croft]

James, Martin, hi

1st point, cross-purposes, James! Peak torque is the mid range! But yes, if I'd read your thorough email properly I would have seen the 5000+ remark, sorry. And I know what you mean about having to stand and watch and wishing you could try your own way.

Martin, L 0.86 or 12.6 A/F is good for peak torque- full power on a carburetted n/a engine. Study the plot below. I can show you others which are way richer and far less powerful and converse too. Note how much leaner fule injected engines can go. That engine is calibrated 10/10. If you run richer under working load (n/a or turbocharged) all you'll do is wear out the rings, bores and inlet guides and degrade the oil's viscosity with gasoline.

GC

[Acki]

Sure at this AFR you have the most power, but is this autobahn aproved? I think no.
I had a traineeship at a german tuner and he never made a care a AFR of 12.6 because the engine get damaged after some minutes a wide open throttle body.

[Guy Croft]

Martin,

You are off topic here. A/F in the range I have described does NOT necessarily mean the emissions will be illegally high. Good burn will convert the fuel in sensible proportions without generating excessive CO and HC. I have done hundreds of engines tuned that way (and many are owned by members here) and they have all passed UK emissions tests easily. Unless German regs require rolling-road tuning you could not in any case show that the emission levels were too high under load.
The critical issues in the thread and this site generally are what is best for the engine for high performance. I do permit discussion of road cars but kindly remember this is a race engine website.



GC

[Acki]

Sorry I don't mean emissions! You need the fuel to cool the engine. The chamber temp and the exhaust temp rises after some minutes.

[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