Head Port Size, Velocity Vs Horsepower

[Fabiotorta]

Hy everyone, I am new to Sau but reader since an year.

I am starting some mods on my skyline gts-t and i want to start from the head.

I am willing to know if someone have ever tryed to reduce instead increase the size of the intake ports just before the valve or some other mods that can lead to this.

My assumptions are that an rb25det head can be good for more than 500 hp before the choke point, talking about port size (polishing around 60 grid and valve angle job needs to be done anyway to increase efficiency).

Does anyone have a dyno sheet comparison? Looks like to me standard heads don t show any choke point even at high hp so why increase the port area and loose velocity if not needed, considering that smaller ports would give better fuel atomisation and turbulence to fight detonation and low- end torque.

Thanks cheers!!!

[KiwiRS4T]

So how are you going to decrease the port diameters? Do a before and after dyno test!

Would be a lot easier to up the boost a couple of psi and make more power that way.

[GTRPSI]

You can playdoe the ports on a flow bench to test.

If happy weld away, ive seen people use JB Weld too.

Personally i wouldnt use JB weld, on the exhaust ports due to temps and on intakes, guess where it goes?

If your going to play after finding what you want with clay, get the ports welded.....

Edited August 21, 2015 by GTRPSI

[GTSBoy]

It's all about what the flow does as it comes past the valve. Everything that happens in the port is about conditioning the flow for that last bit of action. There's more to it than velocity (either too high or too low) in terms of things to think about when deciding what to do to a port.

[Fabiotorta]

I am looking to find some dyno differences between ported and stock head my point is from some dyno results i could find it doesn t look like to me there are any signs of power loss at the top end from running a non ported head, if a stock size port would show a choke situation it wouldn t be possible seen dyno charts with 500 hp from stock heads???!!! By fluidodynamic calculations of grams of air per intake phase at peak rpm and having a subsonic speed the size of the port should be if round around 22mm diameter.

On a flow bench you can see flow but that doesn t mean that on a running engine that flow is needed

Anyone tried to follow my ideas before???

[GTSBoy]

OK, here's the root question.

Your proposition is that stock ports are big enough for >500HP. (I will assume you mean at the flywheel).

You then propose to make the stock ports smaller so as to increase the velocity giving "better" characteristics at some lower flow limit.

My rebuttable is, Why would you want to do that instead of just making use of the available flow in order to make the possible power? After all, 500 engine HP is not that much power.

Further to that, the design goal of porting is not to approach the choke limit. There is an optimal velocity in the main port itself, and then you need to slow it down in the bowl region anyway, in order to prepare it to flow past the valve. So my original reply post was based around the statement that simple views of port velocity based on choke point are naive and ignore other important design considerations.

[KiwiRS4T]

I am looking to find some dyno differences between ported and stock head my point is from some dyno results i could find it doesn t look like to me there are any signs of power loss at the top end from running a non ported head, if a stock size port would show a choke situation it wouldn t be possible seen dyno charts with 500 hp from stock heads???!!! By fluidodynamic calculations of grams of air per intake phase at peak rpm and having a subsonic speed the size of the port should be if round around 22mm diameter.

On a flow bench you can see flow but that doesn t mean that on a running engine that flow is needed

Anyone tried to follow my ideas before???

There is a flaw in your observations - the fact that you can make 500hp with stock heads doesn't mean that the ports are too big for lower power. Head work is critical for n/a motors but less so for force fed engines. The engine that makes 500hp would make a little more with improved head flow or conversely need less boost to make the 500. You can be sure that Nissan has done a lot of work on head design.

[Fabiotorta]

I understand your point and i want to make an observation, if u port polish the head you usually remove all the imperfection so you make it smooth and this improve flow and remove friction from any portsize, so leaving this fact alone that is true for sure how can we say that is possible making more power from bigger ports or that the stock ones block flow at some point??

Leaving alone the flow bench i m talking about dyno results.

For example doesn t the neo engine have smaller head ports? I never seen one.

I was working with rally cars and i personally built a tu5jp4 engine, it s a 1.6 psa if someone knows it and i had the possibility to test it on dyno, i basicall made the intake port smaller raising the floor because and making it work theoretically at nearly the sound speed ( i did this because of the max intake area imposed from that class) so i couldn t improve the flow and i wanted to try to improve mixture speed.

The result was no loss in top power and quite a big difference on low- end.

Engine making around 190 hp

Now if it was turbo it probably would have worked the same since the air is denser and needs less sectional area to flow the same weight of air.

[Fabiotorta]

I mean we use around 100 m/s max port speed because after that is said we loose more power that what we gain, i say is this true, is this dyno proved or flow bench proved???

[discopotato03]

I think people sometimes try to get really involved in head port work when in fact the manufacturer had most of the smarts happening for the era of the engine - particularly a performance enhanced one . True they have emissions and some sane sort of consumption targets to meet but these RB heads are not dogs .

The limitations Nissan put on them were based around pretty tame performance numbers and fits in with quiet clean and acceptable consumption .

Once you improve the external inlet and exhaust tracts you can make pretty impressive power without major feats of internal reengineering .

Generally with production cylinder heads the greatest restriction to air flow is the inlet valves themselves , controlled by the standard valve train . If the ports were a major restriction higher performance cams wouldn't achieve much .

Also something to think about is that valves obviously aren't always open so flow will change with how far open they are in the lifting/closing phases .

In think in a road driven car the easiest way to improve internal air flow is a set of drop in aftermarket cams because they take the least effort to do , especially if its belt and tensioner/idler time .

Once you start modifying a head casting there is no going back if its botched . All I'd ever do to a street mild track RB head is to clean up any casting flash and smooth any machining marks in the throats .

Have a look at the dyno threads , plenty of examples of high outputs with little or no head porting .

As mention forced induction is a huge advantage in getting more air through inlet ports .

I would be very wary of big ports in an RB head because port velocity is important when it comes to mixing and carrying atomised fuel into the cylinders .

Making impressive power with turbocharging generally isn't difficult , keeping all the drive it round the street part throttle torque and flexibility is where the wheels fall off in too many cases .

Better to have the best of both with the fewest compromises .

Your call , cheers A .

[R31Nismoid]

I have heard of some workshops actually adding material to the head. I'm not sure where exactly, but the aim is for increasing response and from what I did hear - they said it did work.

Such places probably won't give out any info though I wouldn't imagine, it's that type of stuff that gives you an edge over others.

It's certainly something to consider and one would imagine that at a certain point it would actually become a problem (if chasing power). However if you are just aiming for a 300-330rwkw set-up and no more, then maybe you can get something out of it. Things do change over time as well, look at what E85 has done to performance.

[GTSBoy]

For example doesn t the neo engine have smaller head ports? I never seen one.

i basicall made the intake port smaller raising the floor because and making it work theoretically at nearly the sound speed ( i did this because of the max intake area imposed from that class) so i couldn t improve the flow and i wanted to try to improve mixture speed.

The result was no loss in top power and quite a big difference on low- end.

Engine making around 190 hp

Now if it was turbo it probably would have worked the same since the air is denser and needs less sectional area to flow the same weight of air.

Let's take these in order. Yes, the Neo ports are smaller. The valve angle is also completely different, the chamber shape is different and the cam profiles are different. Nissan did do all this to make it better than the old RB25 at the 490 HP point. They did it to make the thing make more torque and a little more power, but using significantly less fuel. Note, they did ALL those things. Not smaller ports in isolation

Secondly, your example of the Peugot engine is a perfect example of what I'm talking about. Adding material to the port floor, to improve the short turn, is about changing the characteristics of the air flow approaching the valve. It is not about making it faster. It is about redirecting the flow direction and distribution into the port bowl.

I have heard of some workshops actually adding material to the head. I'm not sure where exactly, but the aim is for increasing response and from what I did hear - they said it did work.

Such places probably won't give out any info though I wouldn't imagine, it's that type of stuff that gives you an edge over others.

Some workshops that people should be interested in do give away a hell of a lot of free information in this area. Doing so does not really cost them, because what they are actually doing is letting people know that there is a hell of a lot more to modifying ports than most people are capable of believing. I highly recommend searching for the dozens of excellent threads on Performance Forums by TK. Start with the oldest ones and keep reading.

[Fabiotorta]

First thank you all for this interesting answers.

Than what i m trying to do or say is not increase boost or put some cams on but i m trying to understand if if possible gain power using smaller ports, do something out of what most of the porting with flow bench is.

So if the flow restriction became the ntake valve it means the ports still big enought or too big, at what level of power/flow???

And when the port size became a bottlenech?? If valve is the biggest restriction change valve would increase port speed and performance and that means to me the port still not too small??? Does make sense???

Rb neo has got smaller port and smaller downdraft angle, lets say closer to f1 style, on this configuration (small angle) the port should be bigger from what david vizard says.

Ok neo has got other mods but the air needed to burn petrol and make power still the same, how can it draw in that amount of air with smaller ports and make more torque, power and fuel economy???

I believe same air drawn in but at higher speed for sure (smaller port) on a better angle but most important a good turbulence that mix more fuel and make better use of it, fighting detonation in a better way snce no drops of fuel gets in the chamber and better fuel economy and low-end torque without sacrifice top end.

Smaller port create greater velocity, greater mixture, greater combustion and more kinetic energy to help the charged air not go back on the overlap period but keep entering.

Am i crazy??

[GTSBoy]

Am i crazy??

First, I must correct a typo in my previous reply. Where I said "Nissan did do all this" I actually meant "Nissan did NOT do all this".

Secondly, to respond to your "am I crazy?" question......yes, you are a bit. I can make different amounts of air flow through the same minimum diameter hole just by changing the entry and exit geometry. This is exactly equivalent to a good port and a bad port.

Narrower valve angles are better because they permit steeper ports. Steeper ports are better, because they reduce the effect of the short turn and give the air a better shot at the back of the valve. But if the head is not tall enough to permit the ports to go higher up the head, then you can't have steeper ports and in that case narrower valve angles can be detrimental. This is because the narrower valve angle + lower port = worse short turn.

Nissan obviously did a reasonably good job of making the Neo's ports work. But we're not talking about the difference between a claggy old British tractor engine and an F1 here. We're talking about the difference between an engine designed in the 80's and a very lightly tweaked version of the same engine. So the change in performance characteristics in the Neo is not really wide enough to be able to point to the port design and say "Haha! That is responsible."

I will also go back to some of your earlier fascination with the sonic velocity. Calculate away to your heart's content. But do not expect your answers to be correct. Even on an NA engine, do you know for sure what the temperature and pressure of the air in the port are? Be honest. Do you? If you don't then you cannot calculate the sonic velocity and it could be quite different to what it is in normal atmospheric air. If you have a good reference used by SAE engineer types for those values, then that's good. If you're guessing or ignoring, then forget about it. In boosted engines it is even worse. The air temperature is all over the place and the pressure can be just about anything.

[Fabiotorta]

Ok absolutely right it is basically impossible understand what happen on an itake phase related to pressure and temperature, and derived velocity but one thing can be calculated, the fact that increase in pressure increase density (boost= density adjusting for the temperature increase) plus fuel vapour inside the port that cool down the air so increased density and frm what i know this means that on the same area you can draw more air ( in weight) that s why boost can increase flow even on n/a engines with small ports.

What i m trying to point is when really a stock intake port became a restriction in flow on a running engine under boost and when can u see this lack in power, choke, on the dyno or big boost increase with no power added at what power level? Even with bigger cams or valves when the smaller port area on a stock head become a restriction??? Considering that the non neo has got a say worse downdraft angle and so more prone to choke due to bad short turn and that means having a bad use of the area so restriction, a port like this should choke at say 700 hp just say a number or show a big lack of flow???

[GTSBoy]

You're not going to get a concencus on a magic number as to what power level is the limit of the RB25 port. That's because the testing environment is about 10000 engines all in different states of modification with different turbos, manifolds, fuels, intercoolers, etc etc blah blah. Because it is a turbo engine there is some possibility of just adding more boost (+ the necessary cooling and octane) to try to squeeze just a bit more air into the cylinder. Hit the limit with a stock exhaust manifold and you can switch that out for a freer flowing one and lift the limit further. Rinse and repeat until all the external factors are used up and you will find the answer to your question. But many people will break something or open the engine up to put in pistons and rods (and therefore probably fiddle the head too) before they use up all the freebies. And all at a different point in the mod cycle.

What you should do is spend some time looking at the 300kW and 400kW threads on this forum to see what I'm talking about.

[Fabiotorta]

Need some help to search, search button 400kw??? Doesn t work!!??

Anyway is there a thread you know about someone using n/a heads on turbo??? Like rb25de and rb30??? Would be glad to check it out..

Thanks!!!

[GTSBoy]

Here's a tip. When trying to search any forum, don't use the built in search function. They invariably suck dogsballs. Use google. Put the search terms you want into google and in this case, add site:sau.com.au onto the end. It will then search only this domain.

On top of that, the 300 and 400kW threads are never far from the first couple of pages of the forced induction forum.

There is also a whole NA+T thread in the NA forum.

[Fabiotorta]

Na+t sound good...i ll check everithing out

Thanks

[Ben C34]

Find Royce's result of 600kw or something stupid with an unopened neo.