Rb26 Head Flow Figures

[shanef]

Hi everyone.

Well recently i had an rb26 cylinder head flowed to see what it flowed intake verse exhaust. To say i was supprised how poorly the exhaust flowed compared to the intake would be an understatment.

Now the specs on the head in question are as follows, and theres a few pics of the ports/chambers too:

Tomei Cams - 270deg 10.8mm inlet 280deg 10.8mm exhaust

Tomei cam gears

Tomei cam cap studs

Supertech manganese guides

Supertech Valves - 1mm in 1mm ex

Supertech dual springs and titanium retainers

JHH labour - port & polish, clean & assemble

JHH custom shims (for smaller base circle cams)

JHH custom inner spring seat

The entire head work was performed by John Hill Engineering in brisbane.

The point of this thread is to have a discussion on what these figures mean in terms of i spose power delivery & boost response etc, to discuss how the head could be optimised to get more even flow between the 2 sides by either more/different port work, different valve combo etc, and how different cam profiles will affect the real life power delivery and boost response.

Also for those of you who have done some back to back flow testing with different head specs, if you could shed some light of your finidings that would be fantastic.

Shane

[Angus Smart]

I think we put my standard head on the flow bench a while ago it got somewhere near 160cfm across the board..

tho i guess different benches like dynos will give different readings...

[R31Nismoid]

^^ yup, i believe the conditions and test method can affect things.

Maybe you could find out more details on the testing itself?

[shanef]

Yeh i asked the guys about the different testing methods and they said it all boils down to what depth the test is done at. The std depth is 28". And you can see on the printouts my test was done ay 28" so should be comparable to other tests done at the same depth.

Some other figures i've seen of the std head are 148cfm inlet & 105cfm exhaust, but theres no details of how the test was performed

[Dale FZ1]

Well recently i had an rb26 cylinder head flowed to see what it flowed intake verse exhaust. To say i was supprised how poorly the exhaust flowed compared to the intake would be an understatment.

When I think back to Grade 10 high school chemistry, I recall that chemically combining compounds resulted in products with significantly changed physical properties. This includes their molecular weights and densities. The same concept applies to burning fuel in your engine via the 4-stroke process.

My recall is now not as good, but if you know the chemical composition of your petrol it is possible to work through a few chemical equations to get an idea of the difference in total combined molecular mass pre and post-combustion. High school chemistry will only get you so far though, and engineers are a quantum step beyond that.

Engineers know that when designing and sizing different aspects of your 26DE head they need to factor in what it takes to shift gases that will change their physical properties during combustion. Therefore you will always see inlet valves and ports bigger/better flowing than the exhaust.

Provided your flow bench has shown a measurable improvement from stock to modified, you have had some sort of a win.

[Angus Smart]

Ohh shii, well it looks like you've used a top of the line tester (compared to what we did anyway)

when i saw this machine it looked like something that was taken from the since lab at school about 20 years ago

it was simply a bot that made alot of noise with some nobs and some slides that could shut off the air and some red liquid gauge that went up and down...

[Keeper]

My head is ready for pickup from Gilberts I'm curious to my flow figures, paid $750 for the porting so I hope its good and I'm not just throwing my money around (the goal was less lag on the 2530s)

[R31Nismoid]

^^ well depending on how you look at it porting can increase lag...

When you open up an area, it takes longer to fill (exhaust side talking here).

Obviously there is more to it than just that, intake side lets more air in and so on

But its an interesting concept and very much a black art at times.

[shanef]

well i've had confirmation from another very experienced head guy and he said this head should be able make the 750bhp we're chasing, his opinion was that the way the exhaust currently flows that it'll take a bit longer to spool the turbo and it'll take some more boost to make the hp than it would if the exhaust flowed more

keep the discussion going

[Keeper]

^^ well depending on how you look at it porting can increase lag...

When you open up an area, it takes longer to fill (exhaust side talking here).

Obviously there is more to it than just that, intake side lets more air in and so on

But its an interesting concept and very much a black art at times.

shouldnt the engine having to do less work to get the air/fuel in and the exhaust out negate that?

[R31Nismoid]

well thats where the theory comes into play, but remember theory doesn't always workout in the real world and vice versa...

Would be good to hear from some builders on it all (that have tested etc) as i'm just theorising as you are

[BoostdR]

I can tell you as I have just had a pair cam doctored. The Gibson Motorsport Cams(as used in bathurst) run 25 degrees more duration on the exhaust and .25 more lift.

Seems they were all over this issue

[Angus Smart]

I have a Newish r34 gtr head. Before it gets started on (if its not already in pieces) I'll take it to my friends shop and throw it on his dinosaur flow bench. and then when its complete i will do the same. might take a photo of the dinosaur too.

would be ok to compare before and after results which is what my friend does with VW heads he works on. but i dont think comparing to other benches would be ok.

[juggernaut1]

I've done a bit of backyard head porting on my old 2 litre pinto powered rs2000 based on books from David Vizard on these motors (I made twice the rwhp with stock rejetted carby, inlet mani and aftermarket cam with same duration as stock).

Based on his testing its the shape of the ports that dictates flow and velocity rather than the shear size. From memory he said it was the quarter of an inch before and after the valve that was most critical.

Also wouldn't the larger exhaust valves and ports enable the exhaust gasses escape more quickly (one less bottleneck) and hence result in less exhaust gas residue in the cylinder....thereby reducing the possibility of detonation and/or being able to run more advance to assist in earlier spool.

My 2c

[bigmikespec]

Also wouldn't the larger exhaust valves and ports enable the exhaust gasses escape more quickly (one less bottleneck) and hence result in less exhaust gas residue in the cylinder....thereby reducing the possibility of detonation and/or being able to run more advance to assist in earlier spool.

My 2c

Negating the size of the valves etc, and without looking in my fluid dynamics books increasing the exhaust valve port (i.e. increasing cross sectional area) will decrease the velocity of the gases (assuming a constant mass flow rate of gases). It is just a question of what is more desirable in head design.

I would think larger inlet ports (to allow a larger volume of air) and a polished smooth exhaust port of smaller cross section (to allow the exhaust gases to flow at high velocity with the least amount of restriction possible) would be somewhere on the right track.

But "larger" and "smaller" are just words there would be a considerable amount of calculations involved to optimise the inlet and exhaust ports.

Very interesting topic.

Mike

[Beer Baron]

well bear in mind they can be flow tested a number of different ways. often they suck air through which is different to blowing air through. also, bear in mind things change quite a bit once boost is entered into the equation. head porting design is still a bit of a dark art an outright flow numbers don't really tell the whole story. the real proff of good head/bad head will be how it performs once it's on the engine. from here it's all guessing. it does look nice though...

[2rismo]

Shane, 28" is a pressure, not a depth.

TK gave you the hook-ups. Do what he said.

[Swiper the Fox]

bear in mind things change quite a bit once boost is entered into the equation.

precisely

with RB's under boost port shape is the key...not port size.

[juggernaut1]

Negating the size of the valves etc, and without looking in my fluid dynamics books increasing the exhaust valve port (i.e. increasing cross sectional area) will decrease the velocity of the gases (assuming a constant mass flow rate of gases). It is just a question of what is more desirable in head design.

I agree with what you've said on the basis you've assumed a constant mass flow rate. The issue is at what point do we need to improve the mass flow rate because it becomes a bottleneck in which case larger ex valves are required to ensure exhaust gas residue in the cylinder doesn't overly taint the fresh incoming charge. The answer to this question will be dictated by the intended application, rev range etc. I tend to think boost and intake manifolds will take care of the intake side of things to a point.

Returning back to my backyard porting I installed larger flat back and narrowed stem intake and ex valves in my Pinto motor with no loss in torque through the mid range and good hp gains throughout the rev range.

Incidently, I recently had my SR20 head also done by JHH and also went with 1mm oversize Supertech inconel ex valves. The port and chamber modifactions largely resemble the pics of RB head in this thread. My build is nearing completion so I can't comment on the affects of the head work at this time.

[shanef]

I spoke at length with Will from jhh earlier today and i'm quite confident this head is as near perfect.

From what he was saying, its not about getting the largest cfm rating possible, its about port design and taking meat out of the right areas.