GC 8v TC & SOHC power/flowtest brain teaser!

[Guy Croft]

OK, try this!

I frequently have to make back-to-back comparisons of dyno and flowtest results to identify how much flow I need from a head and where it needs to be in the valve lift regime. Often I have to sit and think for a long time and then apply, er, 'educated GC intuition' (& a bit of guesswork too).

Just for interest, here is one flowtest overlay that 'foxes' me a bit. The more I see the less certain I become of anything except a dyno run!

The head I am interested in is no. GC.07.132 which is not yet fully developed in terms of flow yet - but is going to run on a 2 liter race Lancia Beta, with 12mm raec inlet, 11mm sprint-race ex cams on 45DCOE in circuit race in Colombia. It's the SOHC head flow & dyno result that has kind of 'thrown things' a bit, but yes of course I know power isn't just about flowbench head capability.

Take a very careful look at the graph and the legend and see what you make of it all! Does it have enough flow to get to 180 bhp? Note that the 130TC head has little by way of short side radius, on 07.132 - a modified 131 2 liter head - I have left it largely unaltered. The modified 130TC head is similar to the one in the dyno test below running 10.8/1CR on 45 Jenvey FI with pair GC 3A cams, true lift about 10.4mm.

GC

[Testament]

how does the displacement x rpm compare at maximum power and maximum torque compare, SOHC vs. DOHC? might give some indication of how "efficiently" the head/manifold/camshaft combination is working?

Tom

[sumplug]

I cannot understand why the SOHC engine makes so much power when the head only flows around 85-90CFM!!

Andy.

[Guy Croft]

Quite, Andy.

The point to note is that the flowbench is only a measure of how much extra flow you have managed to get from the head, obviously it's all about reducing losses and trying to make it a more efficient thing. There isn't any accurate way of saying how much power an engine will give from the results unless you've got dyno feedback. Since engine output (torque and power) is directly related to mass of air ingested per cycle, how is it that the 1600 SOHC with such low flow on the flow test rig turns out so powerful? 177bhp - And the peak torque is great too, 133.5 lbf ft. A similarly tuned 2 liter unit might give about 146lbf ft - only about 9% more. Sure the 2 liter is bigger so has better spread of torque but that's not the issue.

The big question when you start modifying is always 'how much flow does a head really need to have?'

I guess I could sit and figure it out - if I had time - which I don't at present - so I'm throwing it open to you clever people!

GC

[Julian]

I have a head for my race car that was engineered by one of the best in business but sadly he died a couple of years ago so I never had the chance to query the matter further. His own comment on the head is that it will flow as much as I need for my purposes but it will hit a flow optimum at around 14000 rpm. What exactly he meant by that comment is a bit of a mystery to me but it made me think about the issue.

From my perspective the first thing I discard are the power figures - I tend to work from the base reading which is torque. Making a like-for-like comparison between two engines if they make peak power at different rpm is somewhat pointless. I'm fairly sure more than a few people will tear me a strip for saying it but I'm just saying how I work. I just find it easier to look at two similarly scaled torque curves.

Part of the problem comes from actual engine displacement and compression ratio versus flow. I've done my thinking on the basis that to reach the peak figures I want then I need to get "this much" gas through the inlet ports. The nature of the camshaft profile will make a difference too as to do the same with a short duration would need a higher peak flow compared to one with a longer duration. Once again it is down to how much gas do I need.

The actual flow figures are a big help in this - they tell you what you boundaries are, in essence what could be achieved if your engine didn't have to breathe in gasps, it also tells you what cannot be done.

Ultimately the torque (or power) figures are down to how much air/fuel mix you can burn and dispose of in a given period, flowrates only play a part in this. Without comparing the flowrates against a matching dyno run it is very hard to see how the flow is affecting torque.

Guy has already summed it up: "How much flow does a head really need to have?"

[sumplug]

It seems to me its not so much the flow rate of the head, but how efficiently the combustion chamber burns the air/fuel mixture and how the air flows around the chamber, and how it scavenges the cylinders for the next charge. Compression ratio must also play an important part. After all, we are after a big bang!!

Andy

[Guy Croft]

Hey!

Even I know that! Don't dodge the million dollar question!

GC

[Julian]

So what sort of cam duration is that 1600 SOHC engine running compared with the twin cams presented in the graph? I would hazard a guess and say it took a fair bit of duration and overlap to get 177bhp.

Are all the engines running comparable compression ratios and cam profiles? How about peak torque and power for those that have been dyno'd? Better still the plotted curves?

Obviously the displacement is different but what about the bore and stroke? There are simply too many variables here to just point at the flowbench figures and say that the SOHC engine, which appears to flow less gas than even a stock 131, is breaking the laws of physics.

The flow graph is actually a bit more revealing as it shows that the maximum worthwhile lift is actually quite small compared with the twin cams. Peak flow first appears at about 10.5mm lift (ironically very similar to the stock 131) while the other heads show continued improvements upto peak measurement at 13mm lift. Sadly that is about as much as can be really determined from the graph. We don't have the stock 1600 SOHC flow to compare with.

We also have no idea about the induction and exhaust setup. For all I know the SOHC engine was tuned for that peak power as much through exhaust configuration as anything else and makes quite poor torque elsewhere as a result.

Under the circumstances I suspect Guy has been fair in his comparison (when selecting the plots to show on the graph) by picking comparable overall configuration in which case we are back at square one with no obvious explanation of why a smaller engine with poorer flow characteristics is making that much more power comparing with the MCHead - without dyno figures for the others it is extremely hard to say.

[sumplug]

To the question;-
"How much flow does a head really need to have"?
To answer this question, i have asked another question;-
"How does the head use the available air flow"?

Andy.

[Guy Croft]

Well, you are all doing very well.

The plot thickens: Here are the before/after flow graphs for both.

The SOHC (for info) was software tuned before dyno for 5000-9000, it doesn't have much torque below 4600. There is more about it in GC V/W under 'results'.


GC

[James Bowen]

Would we get a better idea by using standard SOHC and TC heads, (and thereby standard flow figures / characteristics) and optimising to get best torque / power. Then make comparisons, showing what the real gain in terms of torque is, due to improved flow. Both for each type of head, and in percentage terms, between types?

As has already been pointed out, whilst power headline figures are similar, the torque figures, and where that is produced in the rev band are very different.

Could a standard TC head be made to produce 175 bhp, if compression ratio is increased to 13:1 and torque is sacrificed somewhat, and moved up the rev range. ie. can you make it produce a similar result to the SOHC, but with standard flowing TC head?

If not, then outright flow would seem to be of less importance than other engine characterisics.........what ever they are.

Regards, James

[Testament]

Does anyone have data/dyno plot for a similarly powerful 1585cc twin cam? The displacement is the same and the bore to stroke ratio is much closer to the SOHC engine, very oversquare. The 2L is slightly undersquare which is going to have a significant effect on cylinder filling and head flow requirements.

[Guy Croft]

Only one or two of you are close to the issue. Remember that this topic is not about what spec we need to give X bhp, it's about this:

1. A head flows X cfm on preliminary flowtest
2. You're going to modify it - which in essence means reduce losses.
3. What should your flow target be and how do you work it out?

The SOHC 1600 head doesn't flow much but makes over 170bhp. A standard 131 TC 2 liter Fiat head (small ports but still outflowing the modified SOHC) on a 2 liter won't get anywhere near that in standard trim even with best cam, CR etc etc. Does porting matter/work? Yes it does, no question. Take the 2 liter XE Vauxhall. Without good porting and seats it won't top 230bhp in any spec. Same spec but well prepped head and it will top 270bhp.

Tip: think about volumetric efficiency - big engine - small engine..

GC

[sumplug]

Even though the SOHC engine flows a low CFM, what it has, it uses well.
I would say the ovesized ports as standard on a Fiat twin cam, maybe are slowing the airspeed down enough to keep power and torque down as standard due to the duration and overlap of cams.
The fact the SOHC engine has a high CR, will make the available energy work harder, and so up the efficiency.

Andy.

[lidox19]

Hi Guy,
If I read the graghs correctly the 130TC makes 165bhp @ 6k rpm and the SOHC makes 177bhp @ 7.9k rpm.
There for the Brake Mean Effective Pressure(BMEP) is higher for the SOHC, 182psi v 178psi for the TC. As I understand it higher is better, I'm no expert, I'm learning as I go.
thank you
Andy