When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Do you guys have any theories about why the factory intake seems to hit a wall at 6000 RPM with 5L engine underneath?
Porsche has prioritized the best possible torque in the rpm range that is mostly used during normal driving, approximately between 1500 and 4500 rpm. I feel my cars (928S4) are not as quick above 5000 rpm as some other cars like Ford Mustang 5.0, BMW M models, Alpina B10 V8 and more. One of my friend recently acquired a new Mustang and the 5.0L NA engine spins easily to 7500 rpm inspite of a much longer stroke (93mm) than the 928 (78,9mm). One other thing, the S4 cams are very mild, one can almost say extremely mild.
Åke
Do you guys have any theories about why the factory intake seems to hit a wall at 6000 RPM with 5L engine underneath?
In my opinion, there are really two and half reasons.
First, the 75mm throttle body element and the two up chutes from it start becoming a bit of a penalty at something like 350 crank hp and pretty much hit a wall at 450 crank hp.
Second, the intake runners are long, too long to be ideal for over 6000 rpm either with the flappy open or flappy closed.
The remaining half reason is that every flow path is very tortured. Much more so than for example in the S3 intake manifold.
If one wants to make more naturally aspirated power with the stock S4 intake manifold, one needs to try to fix at least the first problem and try to improve the flow coefficients (the half problem) with some welding and grinding. Then, one needs camshafts with more intake duration, conservative exhaust duration, and a fairly large camshaft overlap, like with restrictor plate NASCAR engines. More compression needed as well, so much that ignition needs to be seriously retarded in the mid range rpms. The overlap requires headers in order to work at any rpm. The overlap also makes the part throttle behavior poor regardless of what is done to the exhaust side. Most people don’t do all these things and hit a wall. Even if you do all these things, the result isn’t that impressive compared to a generic 5.0L 4V V8.
First, Niklas and JDS figured out how to get EZK to run above 7100 rpm with a chip, a socket, and some "my dad's got a great set of TV repair tools from back in the day" soldering job. Now we’re at 8000 rpm and nowhere close to the new EZK limits.
Second, the base engine really wants to make peak torque below 6000 rpm and peak power slightly above 6000 rpm. This shows up in dyno graph produced with the flattish boost curve. It’s making that about 600 rwhp or better in the whole 6000-8000 rpm range but the power is not going up with rpms.
Is there some sort of a bug at the site that prevents edits?
I was going to add that “There is no oil out the breathers at all, cumulatively, from all the experiments with the new short block.” but for some reason the edit function doesn’t work anymore.
[edit: I am able to edit this post but not earlier ones.]
Additional observation on the 8000 rpm run: The simple formula predicts that it should make 388 lbf-ft at 8000 rpm and 27.13 psia. And it makes almost exactly that torque in practice. The S4 intake manifold (mostly the culprit) results in
the 625 lbf-ft torque at 6000 rpm dropping to about 390 lbf-ft at 8000 rpm.
Since you will need to replace at least the intake to meet your goals, how about trying 944 S heads? They have much larger inlet ports, which could be a problem on an NA S4 engine, but not so for what you want to do.
Since you will need to replace at least the intake to meet your goals, how about trying 944 S heads? They have much larger inlet ports, which could be a problem on an NA S4 engine, but not so for what you want to do.
Why would I need to replace the stock intake to meet my goals? I must have been communicating incredibly ineffectively if anything I wrote gave you that impression. I can boost the high rpm torque to whatever level I want, but I can’t boost the turbo to spool at low rpms.
Well, you have lost me there.
You have observed and documented rather accurately how that the intake did not "work" anymore above 6000 rpm... Don't tell me you are just going to turn up the boost? This is not going to fix the wrong pulse waves occurring at higher rpm and you will waste an awful lot of energy in heat, if that heat doesn't end up melting of breaking something. Interested to hear the EGT you will get at 30psi and 7500 rpm for example.
Well, you have lost me there. You have observed and documented rather accurately how that the intake did not "work" anymore above 6000 rpm... Don't tell me you are just going to turn up the boost? This is not going to fix the wrong pulse waves occurring at higher rpm and you will waste an awful lot of energy in heat, if that heat doesn't end up melting of breaking something. Interested to hear the EGT you will get at 30psi and 7500 rpm for example.
I’m telling you that I’m literally “just going to turn up the boost”.
It’s as simple as that. More people should try it. In fact, the belt-driven centrifugal supercharger people do it all day every days.
So, what about the heat and knock?
It's intercooled, so it will pull out a ton of heat from the charge. Then, importantly, the cooled charge will lose about 25% of the absolute pressure because of the intake manifold being out of tune. This will cool the gas that actually makes it in the cylinder, and that's what matter. The gain comes from the ability to run very hot charge thru the intercooler and have the inter cooler take out more heat from the whole intake system. Porsche is using this in all boxer turbo models now.
Only way this "backfires" is if the exhaust pressure gets too high and messes up the gas exchange. Good pint there. Right now we are at nowhere interesting on either exhaust back pressure or EGTs. This new engine is accepting race gas timing on pump gas without knock are the exhaust is cool as a cucumber...
Only way this "backfires" is if the exhaust pressure gets too high and messes up the gas exchange. Good pint there. Right now we are at nowhere interesting on either exhaust back pressure or EGTs. This new engine is accepting race gas timing on pump gas without knock are the exhaust is cool as a cucumber...
It sounds like the ghetto attitude is now accounted for by a reckless approach rather than adding a few wires to monitor crucial data...
You guys have created an engine which defies the laws of physics!
The exhaust gas is cool at this point because we are running low boost and we don't have to run a significantly retarded ignition yet. "Cool" being a relative term, of course. Let's say cooler than last year.
Our temperature sensors are slow, so we can't really get valid EGT measurements yet from these short dyno runs. Once the mapping process is further along, we can run the engine longer in a particular region of the map. Then we can also get valid EGT measurements. Until then, short dyno passes are safe for the same reason why the EGT sensor doesn't read correctly: Components have a lot of thermal inertia.
When enabling the "COR" feature, i.e., ramping up the boost starting at 6000 rpm, we also need to have the exhaust back pressure measurement to see where we are. Unlike with the EGTs, there isn't a set limit there. This is more just to understand how much we are "paying" for getting the turbos to spool early at low rpm in terms of having to force feed the engine at high rpms.
In terms of replacing the heads with bigger heads, I am not a believer. Basically, the 928 S4 heads are too large for performance use on the stock short block, turbo or no turbo. By my simplistic calculations, you need _minimum_ either 33% more rpms or 33% more displacement, or some combination of both that amounts to that about 1.33x factor, to make the S4 port sizes make any sense for performance use with bigger cams. We chose more rpms, which has the benefit of being easier on the transmission and the clutch.
The head also doesn't need larger intake valves, except if one wants to follow the high-tumble port strategy in which the valve and seat insert ring are made oversize and the roof of the port is shaped to be straight such that the port only flows near the roof (i.e.,"long side"). This high-tumble porting strategy makes sense when the heads have "excess" flow to start with for flow demand and when one is octane limited. We didn't do that, maybe in the next engine.
Putting any of the 944S / 944S2 / 968 heads on a 928 engine is huge amount of work and doesn't make any sense to me given the dimensions.
Agreed the standard s4 heads flow very well , my recent results support this , A local engine builder who a flow bench to test my heads was amazed at the testing results , he performs lots of porting to achieve this on the older 2 valve heads arrangements .
I only tidied up the casting dimples and flash , the next evolution which is a stroker will have the larger valves which in some case where delivered from the the factory will be interesting with the data log results .
08-15-2020, 12:38 PM
