K11 CG Cylinder Head Flow

Low Rider

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This is a long awaited topic I've wanted to start for some time but have been eagerly awaiting some cylinder head flow data in order to start it. As a starting point, I'll focus on what Nissan gave us from the factory.

There has been a LOT of myths floating around in the public domain regarding what is and what isn't possible from a CG engine.

Way back in the mid 90's, when Nissan Motorsport Europe was still a thing, there was a well known statement that, on a good day, the standard UKDM cylinder head was good for about 130bhp. So, unless you were looking to try and exceed that, there was little value in putting money into the head. Well, based on the time spent on the SuperFlow bench and dyno, I can confirm that Nissan Motorsport Europe were bang on the money.

Flow graph for a standard UKDM cylinder head below (all flow figures measured at 10" water).

Stock Head Flow.jpg


Translating the intake flow into theoretical power we need to do a bit of calculation. The caveat here is, the head has the capability of making these numbers providing the rest of the system can keep up, i.e. there aren't any restrictions elsewhere.

CFM x 0.43 x no. of cylinders = Theoretical power (bhp)

76 x 0.43 x 4 = 130.7bhp

Based on what we've seen on the dyno from various engines in the past, and what others within the competition arena have shown, this supports what the flowbench shows reasonably well. Granted, the flowbench isn't the be all and end all but it certainly provides a good guideline on how an engine can perform.

On a standard compression CG13DE with a set of Jenvey ITBs (effectively a de-restricted intake) a set of 264deg 9.1mm lift cams and a decent exhaust system, I personally saw 123bhp on a Dyno Dynamics rolling road. Between dynos and minor differences the average output range for this spec is typically around 125bhp average. Power wise, that's putting the head flow out to be 71-73CFM from a maximum potential of 76CFM.

Compare this to a stock intake manifold, which only typically musters around 100-105bhp depending on engine health with the same setup, this shows how much of a restriction the standard intake poses. Some quick calculations shows that, in order to make 100-105bhp the head would need to flow 58-61CFM equivalent peak. This is a significant drop on what the head is capable of so, the standard intake is helping to easily rob the engine of at least 10CFM right off the bat.

Given how much power the majority of people are seeing from the CG heads, both the dyno and flowbench shows that the standard exhaust valves and standard exhaust ports (although less than ideal on paper) are perfectly capable of flowing enough to be able to keep up with the intake.

Not that I am going to go into details on ported heads yet but, from what I've seen on a competition level and, again on the flowbench, a well ported head on standard valves also shows that the exhaust side of the head is man enough to support the intake flow. In fact, the exhaust side responds significantly better to porting than the intake side (typically by a factor of 2), which strongly suggests that another myth I've also heard (that it's not possible to port the exhaust side of the head effectively) is also false.

I've got plenty more data but it's going to take some time to go through, run some numbers. What I can say in advance is, the data shows why Nissan Motorsport Europe had a hard time reaching the golden 150bhp target that I see thrown around a lot.
 

SuperUno

Buy & Sell Member
Great work, makes me feel happy with the 126bhp, with standard inlet manifold and ported head (Fusion Motorsport / Surrey Rolling Road figures).
 

pollyp

Club Member
that is an amazing valuable bit of info there Dave, thank you (y)
guess I'll have to look into making a better intake & exhaust mani for my future turbo build :D
 
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Low Rider

Low Rider

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Great work, makes me feel happy with the 126bhp, with standard inlet manifold and ported head (Fusion Motorsport / Surrey Rolling Road figures).
A typical well ported head is worth around 10CFM@10" peak, (providing you lift the valve enough), so you're pretty much gaining back what the inlet manifold costs you in flow.
 
A typical well ported head is worth around 10CFM@10" peak, (providing you lift the valve enough), so you're pretty much gaining back what the inlet manifold costs you in flow.
I've been thinking about these 2 posts above and the original post and I'm not sure I understand.
I thought of engine flow tuning in terms of finding the bottleneck and improving that, so in this case the inlet manifold can only flow enough for about 100hp then all else being equal 100hp is all your going to get even if your head flows an extra 10cfm.
The explanation in my head is that all things aren't equal that SuperUno's engine might have for example increased compression or something else allowing it to make better more efficient use of the air it's able to get.
I guess flowbenching can't take into account the effect of inlet pulse tuning of longer runners or long duration cams, (in the same way I've read old 2 valve heads could use longer duration and lift than modern heads, the valve(s) were the bottleneck).
I don't know if I have the correct idea about how airflow responds to restrictive elements.
BTW are the power figures used above @ the wheels or flywheel? It would make sense that the cfm to hp calculation is @ the flywheel...
 
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Low Rider

Low Rider

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Good questions.

You're right in that a flowbench is only a guide and since it's a steady state flow, it cannot take into account any of the dynamics an engine sees nor the actual real world CFM flow that the engine actually sees. This is particularly so with the exhaust since that's under significantly higher pressure than the intake. This is why exhaust to intake flow ratios have always been such a hot topic. There really is no golden rule that fits everything. None the less, a flowbench is still a useful tool to help define a benchmark and improve on it.

With respect to bottlenecks, it really depends on how you define a bottleneck, as a restriction that reduces flow or an absolute wall that firmly limits it.

The intake manifold is a restriction but it's not a definitive absolute limit on maximum flow, it just reduces what the port and valve are capable of flowing. If it was an absolute limit, we wouldn't observe any benefit in flowing the cylinder head. You're also right in that we're trying to find these bottlenecks and improve on them. This is why we see such a significant gain with switching to a set of well proportioned individual throttlebodies, even on a standard cylinder head. We've essentially de-restricted the pre port intake as the throttlebodies are capable of supplying vastly more air that the head is capable of flowing. So, no more bottleneck.

With respect to power figures, since head flow is measured at the engine it's directly comparable with flywheel power.
 

SuperUno

Buy & Sell Member
Yep to be fair the porting is not the only mods, so it has Matt H spec rally cams, high compression ratio, Grp. A exhaust manifold, Grp. A airbox, 1.6 injectors and remapped by Fusion Motorsport.
 
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Low Rider

Low Rider

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Yep to be fair the porting is not the only mods, so it has Matt H spec rally cams, high compression ratio, Grp. A exhaust manifold, Grp. A airbox, 1.6 injectors and remapped by Fusion Motorsport.
I think 126bhp from that spec is perfectly respectable :)

Our engine was 773 cams, group A exhaust manifold, Jenveys, standalone ECU but standard compression. The Jenvey's mean that there's as good as no bottleneck pre-head, so the only way to do better is a mild compression bump. From then onwards a flowed head is the only way forward from there.

Compression ratio wise, you're looking at a theoretical increase as below for increases above the standard 9.5:1. Much like the flowbench, it's ideal scenarios, as you can't factor in for engine dynamics, so the real world gains are likely to be somewhat less but, it's a place to shoot at.

10.0:1 -- +1.7%
10.5:1 -- +3.3%
11.0:1 -- +4.8%
11.5:1 -- +6.2%
12.0:1 -- +7.5%
 
Yep to be fair the porting is not the only mods, so it has Matt H spec rally cams, high compression ratio, Grp. A exhaust manifold, Grp. A airbox, 1.6 injectors and remapped by Fusion Motorsport.
I suspected as much...
That said it's enough above the 100hp that it's not behaving the way I expected, or rather I wasn't taking into account the complications. You're spec is close to the engine I'm building for Spot my mini so 125hp in a 630kg car should be lively, I'm worried about traction though (165/r10's)
 
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Low Rider

Low Rider

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Right. My plan was to dump a whole load of data in here for a run of the mill gas flowed head to show what sort of gains can be expected. I'm still going to do that but there's a very nice interim step that will fit in nicely.

I have had some discussions with @Andy_S and have been given the opportunity to put his JDM cylinder head on the flowbench. There are a few rumours floating about that JDM cylinder heads are ported differently (better) than UKDM cylinder heads. So, considering we have all of the fixtures available to readily flow CG heads on the Superflow bench this is going to be the next step.

This will be more relevant to the small turbocharged community we have, but it should be a good opportunity to learn none the less. This is especially so since the car is shortly going to be re-dynod on a UK spec replacement engine. It'll be tuned by Ed @ Fusion Motorsport on the same dyno (Surrey Rolling Road) so we'll have both sets of power figures on the 2 setups. The figures will be as close to comparable as they can be without going back in time to dyno them on the same day.

Since we'll have a full set of figures and previous dynos to compare, I'll seek @Andy_S ' permission to post up some dyno comparisons, so we can have a go at seeing what difference the cylinder head made and where. Should these porting rumours turn out to be true, I will make a point of putting down as much data as possible which will include port dimensions, photos as well as a full set of flow data.

Seeing as there have been many casualties this year in terms of blown engines, it's about time I used some of the technology we have at our disposal in this day and age to further the knowledge this club has for the better.

WATCH THIS SPACE :cool:
 
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