onefastviking

I have a good bit of knowledge and understanding on restrictors, as well as quite a few dyno pulls with and without. It's not on a Cup car but the effects are the same, mine are on a 996 3.6L .
I don't think a carb car will respond quite the same tho.
What are you looking for ?

mark kibort

looking for the shape change of the curve and the amount of HP reduction for a20 to 50% restrictor.

If you have a bunch of them, lets see or hear the reductions of peak HP and torque, or post a dyno run comparison.

Ill do a wrap air filter test soon and post that.

carb should respond near the same. at least in theory.

Cory M

Thanks for all of the feedback.

Sounds like tuning the carbs and changing choke size might be the best way to optimize the power curve (I have 46mm PMO's). That said I will probably try the "quick and dirty" approach first with parts I have on hand including a couple of mufflers and a set of smaller diameter headers. If I plan the dyno test right it shouldn't be too bad to swap out the parts at the shop and get an accurate comparison. Although it sounds like the best idea I am hesitant to spend a bunch of money and time detuning the carbs because I don't know how much actual racing I'll be doing in the near future and I want to have the versatility to up the power again if I find myself doing more open track days or changing classes.

I don't know when I will get the dyno testing done but I'll try and remember to post up the resulting power changes.

mark kibort

air filter mods would be right up your ally then. no mods, no wrenching on headers to grab 10hp at the cost of more back pressure , and certainly much easier to reverse. all at the cost of shifting down the HP curve by 10-20hp with a pretty simple mod. fuel ratios will be left in tact as well, as their are controlled by mass flow through the carb's venturies. this is why the throttle plate can control flow and mixtures remain the same. less flow , less fuel.

how much hp do you need to lose to meet your "class" spec?

onefastviking

For what you are looking for the smaller headers and mufflers sound like the best for your application. When you dyno them watch what the hp/tq curves do down low and mid range, a good setup will gain you some more power down low at the cost of losing the higher end hp - which in this case is what you want.

Good luck and let us know how it all works out.

Cory M

I have to reweigh the car after the most recent round of mods so I don't have the exact number yet, I'm estimating I'll need to drop about 20hp. I could fit into a couple of other classes right now but I'm looking for the sweet spot- a class with several competitors that doesn't require me to run new slicks to keep up. If I can find that I'm willing to sacrifice some speed.

mark kibort

many times , those down low gains , are not usable exspecially with a closer ratio gear box. nice peak torque boosts are nice for chart boasting, but in actuality, most will never end up down in the mid range area anyway. 20hp is a lot, and thats going to be tough to get with headers and mufflers alone, plus what a pain for a marginal gain over a simple restrictor set up.
there might even be inserts you could put over or in the velocity stacks to make them increase their pressure drops and lose HP. the filter will be the easiest to test, and I would bet losing 20hp would be very easy.

Give it a shot. a $70 test on a dyno could save you a TON of work.

onefastviking

Just curious Mark, how much time do you have testing things like your covered air filter ?


Cory, the mufflers and smaller headers will work, it's easy to knock off 20 hp there and it really does best fit your goals. If it were me I think I would work with the headers first, then see what the mufflers will do. Keeping as much midrange power and torque as possible will keep it, or make it, an easier to drive car.

mark kibort

A lot of dyno runs. On all sorts of cars! Not so much covering the airfilter, but opening up the airboxes and adding other things. so, its kind of the same concept in reverse.

How many times have you removed carburators and replaced the choke tubes? (and then re-jeted, and redynoed) . just curious.
what would be the harm in a covered air fitler. its just a restrictor plate in effect. Its all about pressure drops. Like I said, my next dyno outing, ill test a couple of densities of covering materials and let you know how it goes. Ill also be doing the paper vs KN filter test too.

Just curious, how many times have you seen someone put on larger headers and remove mufflers to get 20hp? I think the most we have seen is 20hp for removing cats, and putting on headers and opening up the exhaust diameter overall. I dont know exactly what might happen here. you seemed to be confident that it will work, but when we did the exhaust mod with a 3.2L '84, it gained 10hp. Looking for 20, and ease of implentation, is worth looking at air filter or some kind of restrictor plate install. especially if he wants the ease of returning it to higher performance mode.

How does making more torque or keeping the torque with the mod, make it an easier to drive car? there are very few mods that can do this, besides cam changes. Do you have any dynos before and after that show more backpressure increasing torque. always, when we have opened exhaust with headers and exhaust diameter, we got gains everywhere. thats why I ask.

J richard

911s are very sensitive to exhaust tuning, the longer tube headders will provide more low to mid range power to the sacrifice of upper end. If you are running stock cams/heads the delta will be smaller. The issue of driveability is mostly due to the difference between how carbs and a metered air fuel injection react in part throttle. The injected car wont care it is injecting the right amount of fuel based on volume, the carbs are relying on both volume and velocity to get it right, hence the choke/emulsion tube selection...

As mark said you may not loose enough power just with the more restrictive exhaust, the question is how do you loose more without ruining throttle response.

A restrictor plate is good that up until you get to the rpm when you need the full volume of the orfice it will have a marginal effect...meaning it won't effect mid range power but limit the ultimate flow at high rpm. By placing a more restrictive filter on it seems (I've never tried this so I have no experience with it) you are restricting airflow at every rpm level which I would think effect power across the range.

I still like the smaller/longer tubes and smaller carbs...

Jim

onefastviking

I think the main differences between some of the suggestions here have been that some will restrict flow everywhere, while some have the option of tuning the curve some.

Yes Mark, I have lots of experience with carbs and countless hours and pulls dyno tuning. But you should know that since I have posted numerous dyno graphs of my own in the past, from you I always seem to see you posting World Challenge and other peoples dyno graphs, why not post your own ?
Mark, you do know I have been in the Porsche business since the 1980's,owned 3 of my own shops over the years, and also have a little bit of experience building, maintaining, and supporting a few race cars at various levels.

mark kibort

You must not be seeing all of my posts. Yes, Ive posted near all of my dyno runs here on the list. ( if they are relevant to a discussion).
Are you challenging me to a dyno quantity **** off?
usually shop owners dont get to the dyno that often. good that you are more involved than the usual porsche shop.

Look, I dont know what dyno runs Ive posted of yours, but if they are for a car in question, thats why I posted it. I do collect others dyno runs in a data base for reference. Now, if you want to see some of my own, Ill post a few tests.

Ive been testing and dynoing 928s and 911s since the mid 1990s, so what does that prove? oh, that I have some tests that might be of interest? if you have some dyno runs that make your point, lets see them!

Ill show you my exhaust change dyno run if you show me yours.

now, you seem to think an exhuast change will change the shape of the dyno. im sure this is possible, and would like to see it, on something lower reving than a cup car. In the 911 3.2 , 3.0 tests we have run, it seemed the exhaust just changed overall. the 928s, the same thing.
The point is, if you can keep the power levels down low, and only sacrafice the top, certainly that is the way to go. but, its a lot of work, as you know by owing porsche shops.

you have not tried a air fliter mod and dynoed it, have you. If you have, what did you see? Lets see it.

Anyway, I think it is worthy of the test, and I certainly will test it next time next month i go to the dyno for my beginning season dyno.

mark kibort

I agree Jim,

The intake filter restriction , will act much like a restrictor plate, as there is a pressure drop based on flow through it. in otherwords, the restricitve filter will look more free flow, at the lower rpm as well.

Its a hypothesis, and one that I will test soon.

Again, i think its a matter of the effort for one vs the other and the trade offs.
if you lose 20hp peak and and 20hp down low as well, it certainly wont be as good as losing 20hp peak and only 10 or no loss at post shift RPM. so, with no loss at post shift rpm, it might only be a weighted factor of less than averaging 20 with 0. (or 10), because you spend more time in the higher rpm range due to acceleration forces. so, that difference might only work out to be a 15 hp loss net. (quick area under the HP curve) however with the reduction of 20hp end to end, thats like giving up 5hp. (assuming an even spread curve).
is 5hp worth messing with entire header systems and exhaust, only to have to change it back for the open and DE days?. I mean, serious class racing, yes, but it sounds like convenience is a concern as well. This is why an air box mod might be looked at. Hey, its a good test. if you havent done it before, it would be good to have in your arsinal.



Mark

Matt Romanowski

Not what he said. His experience has been that a restrictor plate only makes a marginal difference until the restrictor orfice flow is maxed out. This has a small effect on mid range performance while reducing the ultimate power numbers.

You're way will cause restriction on everything and reduce all the numbers.

onefastviking

No, I don't want to enter any pissing contest online with you Mark, THAT I am sure you would win.
My reasoning on you not doing any dynos personally is because I haven't ever seen you post any of your own and you are a budget racer, by your own claims, and most budget guys don't see the cost of the dyno as an expense they can do. It wasn't a shot at you, just an observation.

I think the OP's has an idea what to do now.

BTW - I have tried many different air filters and know the differences just in air filters so I understand your concept completely which is why I suggest doing it another way. It's ok, it's just my opinion, you don't have to follow it.