Peckster

What's exhaust density? Back pressure?

Geo

Interestingly, there was just a letter to Racecar Engineering on this very subject. They have been doing a aerodynamics series using CFD with a NASCAR cup car as a model. Someone was looking at the the modeling of a splitter at the front and noted that with a splitter area at the base of the airdam where it meets the splitter is even higher pressure that at the base of the windshield.

Unfortunately the rules require me to source air either from under the hood or the stock location. The stock location fortunately allows me to use a CAI.

Not to big, not too small. But interestingly enough the Nissan SR20DE folks are running even larger primaries and have found even greater power. But that varies from engine to engine. But you're right, they are not huge. Keep in mind, our allowed modifications are very minor.

Yeah, that's an interesting idea I think I'll need to test. Certainly emptying into a partial vacuum should help scavenging. OTOH, the flow across the open pipe on the side should experience a partial vacuum as well. One of the problem with dyno testing is that it's static. Thanks for reminding us. Sometimes what works best on a dyno is not what works best in the real world.

Geo

Blessed? Nah. They just have a different mission from engineers for other manufacturers (Porsche has slightly different priorities, namely performance). It's funny how people get defensive about Porsche though.

An open airbox is just a noise maker.

Larger filter is not the problem. Cooler air is a very small part of the equation. But now I'm repeating myself.

GlenL

That's what I meant. And I believe I said: "it's pretty easy to size the intake plumbing to the needs of the engine"

Does add a nice throaty sound. A long-debated issue on the 928 is whether the 4' long intake tubes hinder flow. The answer is "no." At least letting in the hot air is worse.

Repeating how? Looking back at your first post I see an idea to change the intake tubing.

Less drop across the filter can be nothing but good. Perhaps the existing filter size good enough. I won't waste my dyno time on this one, but try a comparison between a new filter with an old dirty no-light-gettin-through filter. I know which will have more power. How about no filter? Anyone run that?

Air temp is an important part of the equation. Does the term "intercooler" ring a bell? Now the equation does use the absolute scale (Rankine or Kelvin) so a few degrees are not important. If you can start with 70F air instead of 180F air it does produce more power.

There are several parts to engine power level:

Getting fresh mixture into the cylinder. (This includes evacuationg the burned charge.)
The mechanics of the cylinder (CR, head shape, crank arm)
The fuel/air ratio
When the mess is ignited

If you're not going to change the cams or other engine mechanicals, there's just a few things to alter. Allowing the parts of the intake above the throttle to breathe better is a good approach.

Unfortunately for us, the blessed Porsche engineers have already got the N/A cars tuned fairly strongly. This makes it hard to get cheap easy extra ponies out of it.

Reminds me of another US car story:

Had this 1976 Olds Ninety-Eight coupe (RIP) with a 455 V8. Lots of torque off the line, but the small exhaust manifolds, cat and single exhaust held it back. Once I put headers and dual exhaust on it, the thing had a completely different top end. Bizarrely and unexpectedly quick acceleration. Added a switch in the ashtray to hold the kick-down switch on for high-revving stoplight action. Now that was a few hunderd well spent.

Manning

Believe me, at speed the underhood temp on the 944/924S is not that much higher than ambient. I know this to be a fact as I took measurements last summer and will repeat the experiement this summer to confirm my findings.

On the other hand, with the car sitting still (read no airflow) and at operating temperature the engine compartment temperature goes up quite a bit.

I have also posted before that on a dead cold engine I found idle performance and throttle response to be far better with the stock airbox and snorkle than with an MSDS cone filter setup. Heat in the engine bay was not a factor since, as I mentioned, the engine was cold when I did the comparison. Unfortunately no dyno info.

Geo

Perhaps you missed the part about resonance tuning.

What does a dirty filter have to do with a larger filter? If nothing else, a larger filter box will just slow down the intake charge. That will make matters worse. Air flow will stall. Not what we're looking for.

We aren't talking about forced induction here. Do you think you're going to see a 110* reduction in air temps on a NA? Hell no. In fact, I know three people who have conducted instrumented testing with thermocouples on two different cars and all three found that temps under the hood of a car moving at 50 mph or more was nearly ambient.

Where a properly designed CAI makes power is not through the cooler air, but through resonance tuning. The cooler air is a tiny part of the equation. Proper resonance tuning can greatly improve VE however.

Geo

Holy cats. Thanks for posting this Manning. You posted while I was typing.

OK, make that 4 people with 3 different cars have confirmed this!

Serge944

Performance =/= peak hp

Geo

True. But just because the most power may have been higher in the rev range doesn't mean power was lost anywhere else. That is not always the case. I've seen plenty of well engineered mods (intakes, headers, etc) that gain everywhere. They just didn't gain equally everywhere. But they still gained.

RedlineMan

Hey;

You lost HP because you lost volumetric efficiency. More air flow than the engine could use. Less back pressure than it needed to maintain that flow. To make more power, you first have to be able to process the air that's there. The filter/air box is likely not the limiting facter here. Ever measure the surface area of most cone filters?

Once you get more air in, you have to process it better. Throttle bodies, intakes, heads, and cams are what make power. That and associated RPM.

$$$$$$$$$$$$$$$$$$$ame old $$$$$$$$$$$ame old.

GlenL

It's an example to make a point. A dirty filter is more restrictive. A larger filter would be less restictive. (Gas near old fire: A new K&N will produce a lot more power than an type of dirty filter)

I my opinion, the air box is too far from the valves, with too many bends and restrictions in between, to affect system resonance. The runners from the plenum to the head are clearly designed to do produce resonance for each cylinder.

No, this is N/A land but the effect holds. Cooler = better.

The underhood temp issue depends on the situation. Standing start or straight-away? Those el cheapo cone filters sit directly behind the radiator. Can't be good. Removing the cold air duct leaves open that the air will be similarly heated.

Well, I'll resist repeating myself.

If you're going to make comparisons the base and new conditions have got to be defined. I'd like to see real numbers on what the changes upstream from the AFM are doing.

my84-944

I have been reading this post closely and have seen that you are all well educated on the subject. What I not seen is anyone in this thread or any post state how a na 944 can get substantial power gains in the lower RPM. I have also wondered how can I get more low end "take off" HP in my 84 NA?
Turbo? No, because of the $ to value, better to just by a factory 944 turbo.
CAI? Already has one of sorts from Porsche.
Rebuild? Maybe it would make like new HP, but is that enough?
I think so far the only real answer is to drive it on twisty roads and enjoy it as a sport touring car like it was made.
Just my two cents guys, but if you have a better idea and answers let me know.

Geo

The first thing I'd do is call Jon Milledge and talk with him about cams. See what he has. He concentrates on racing, so most will likely be oriented towards high rpm output, but he does so some street car stuff, so he might have a nice street cam available. $600 according to his web site.

The other thing you can do won't add hp per se, but will feel like you did... a low mass flywheel. Some people aren't impressed with them, but most people are. I love 'em. A low mass flywheel will help all the way through the rev range.

A good header should help. The MSDS and Bursch aren't so hot. The Milledge header is supposed to work well with an IT race engine. Don't know how it would do with a street engine. There is another commercially available header that is supposed to work well, but I cannot remember which one it is. It's a relatively common name though.

Skunk Workz

It isn't all in the size of parts...the shape of the parts (and what they do to the "system") are often doing more to the torque/hp than the shape and the flowbench CFM tells you it will.

60 hp per litre for a "sports car engine" isn't even close to "well tuned". A Ford 1.6-litre OHC "Pinto"-engine from the late Seventies had about 55 hp per litre being a dull daily driver. If it had pushrods,rockers and an insane valve spring pressure,then it might be considered ok. For an OHC design,it's not adequate. If it was a well tuned N/A engine the 944 should have 75+ hp per litre as a bone stock car...considering that motorcycle engines from the late Eighties had 110+ hp per litre as stock...and nowadays they have in excess of 200hp per litre at the crank,if they are "well tuned".

Skunk Workz

Get your intake port velocities up...then you'll get higher VE throughout the rpm range. That's what the 944 engine lacks.