flow bench testing
#16
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
Thread Starter
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)….
Some interesting charts – both the 8v and 16v heads have been modified (more work on the 8v)….
#18
Rennlist Member
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.
#20
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
Thread Starter
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!
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!
#21
Rennlist Member
#22
Rennlist Member
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!
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!
#23
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?
#24
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
Thread Starter
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.
#25
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
Thread Starter
You can tell a 968 head by the crosshatching in the casting.
#26
great info.. yay.. real data. thx Chris
#27
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 .
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 .
#28
Bannana Shine
Rennlist Member
Rennlist Member
Join Date: Jul 2009
Location: Rochester Hills, MI
Posts: 21,055
Likes: 0
Received 334 Likes
on
219 Posts
I have absolutely zero test data to back this up...
#29
Addict
Rennlist Member
Rennlist Small
Business Sponsor
Rennlist Member
Rennlist Small
Business Sponsor
Thread Starter
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!
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!
#30
John