Chris White

Its been a while...If I remember correctly a stock 951 head will flow right around 200cfm. The last 8v head I tested (head work done by a specialty shop I use) was improved to 249.6 @ .500”

Some interesting charts – both the 8v and 16v heads have been modified (more work on the 8v)….

Duke

Thanks for sharing Chris, now it's actually starting to feel like a technical board

333pg333

Dear God, we can't let that carry on for too long!

So am I reading those charts correctly where it suggests that the 8v flows a little better on exhaust at lower rpms?

What about this whole notion that 16v's lose a bunch of torque compared to 8v's? I don't really buy it. I think it's an assumption based on how a lot of twin cam n/a engines work in that they need to be revved out high to 'get on cam' and extract power.

ehall

Chris does that lower chart mean that the 8V head actually moves more air sooner, but less than the 16V as the RPM's climb?

Chris White

Yes, that 8v exhaust port does flow better at low lifts – keep in mind that the 8v head in that test has $3k worth of work done to it, its very optimized. I am not overly surprised at the 8v performing a little better at low lifts on the exhaust – at small openings its all about the flow right at valve seat area. As the valves open further the rest of the port comes into play.

The interesting thing I see in the graphs is that intake on the 16v flows 30% more that the 16v exhaust and in the 8v there is only a 20% difference at high lift….much different at low lift!

333pg333

If you're going to copy my questions at least make it more subtle than directly afterwards.

333pg333

So does that imply that there is an exhaust bottleneck at high rpms on the 16v? Interesting if so as we all assume that even a stock 16v is all about more flow, but perhaps it's all about more intake flow. Does this induce any backpressure in the system?

95ONE

Chris, sorry if I missed this.. Do you have flow numbers of the STOCK 16valve, or is the video of a stock 968 head? Oh yeah.. we ARE talking about a 968 head right? Not an S, or and S2 right?

Chris White

Keep in mind that if the different intake to exhaust balance in the 16v set up causes a restriction of some sort it won’t be back pressure in the exhaust manifold because the restriction would be in the exit of the combustion chamber. Back pressure in the exhaust manifold is caused by restrictions further down the pipe.

Also keep in mind that the 8v only flows about equal at low lift and then it is far outpaced as the valve lift increases.

Another issue to keep in mind is that the test system has slight variances in lift. The .100” reading may be .095” for one test and then .105” for the next. The quickie graphs and comparisons do not take this into account since its graphed against a presumed .100” lift. Once you get to the higher lifts the +/- .005” is irrelevant – but at .050” and .100” if makes a significant difference.

In the end the 16v intake out flows the 8v by 30% at high lift and the exhaust it is 20% better. At high lift is where the most actual flow takes place. A large advantage at very low lift does not increase the total mass flow by a lot.

It would be my guess (since it I have not measured it yet) that the 16v head is efficient enough that any back pressure or flow restriction is created by the turbo. If it turns out that the intake side is ‘too efficient’ there is not a real down side other than lower velocities at lower rpms.

Chris White

Take off less than 10% (roughly) to get stock 968 head flow numbers. Yes - its a 968 head. Ports are very similar, the intake valves are a little larger than the S2 valves.

You can tell a 968 head by the crosshatching in the casting.

95ONE

great info.. yay.. real data. thx Chris

gt37vgt

i did find some flow figures on the rennlist whale forum for 928 16v heads they were around 290cfm but i would guess the 928's only had the heads go as late as the S2.
and generally many people seem to believe 85% or more exhaust to inlet flow ratio is favorable for turbo set ups.
I guess part of the logic is that the turbo aids inlet flow and hinders exhaust flow I'm certainly giving my exhaust more work than my inlet perhaps going a bit over to try compensate for the tiny exhaust cam .

JDS968

Probably because NA engines usually develop better torque at low RPMs with smaller valve area [within limits, of course]; I think this is because they create higher intake charge velocity? Below your boost threshold, this probably still holds true, but as soon as you begin to develop boost, all of the same considerations about high-RPM breathing SHOULD apply, which SHOULD mean that the lower your boost threshold is, the sooner you'll benefit from a 16v head over an 8v head.

I have absolutely zero test data to back this up...

Chris White

Porsche was famous for ‘not caring’ about the intake side of their early turbo applications. The early 930 engines had awful manifolds – Porsche just thought that the air was being push through so it didn’t matter. The 951 manifold is actually not that bad, but it is still not optimized like NA manifolds.

As a very general rule if the ports / valves are set up for peak power you will give away power at low flow due to low velocity. It’s a lot worse in carbureted engines!

vette951s

What about the 300 cfm intake port 2.5L turbo head you mentioned in another thread? THAT I would like to see a video of on the flow bench!!

John