Header primaries?
 

Curious, would there be any appreciable performance difference between 1 1/2" primaries vs 1 5/8" primaries?

I assume the smaller primaries would offer slightly faster exhaust gas velocity, aiding spool a tad, whereas the 1 5/8" primaries would offer better top end power, right?

Discuss please SmileWavy

Anyone?

in theory heat energy provides spool not velocity but the heat is gained with smaller primaries because of more restriction..

I guess nobody disagrees :)

OK, would there be any adverse effects using headers with 1 1/2" primaries on heads which have 1 5/8" exhaust ports?

Without having a smooth transition down to the smaller primaries, you'll have really poor exhaust flow since there's a lip that the gasses are slamming into, and the temp of your heads will also increase (this can't ever be a good thing in a Porsche).

If you bought or made some spigots and used them to make a transition down to the header primaries, they would work much better.

Also remember, most turbo experts want the least resistance as possible on the exhaust side. Typically, the scavanging ability of a faster exhaust flow rate (helping to fill the cylinder during the intake stroke) is not relevant in a turbo motor, as the compressor force feeds it. But too much exhaust volume slows low RPM flow, and it will increase lag - everything is a tradeoff of sorts.....

Pat K

I was always under the impression that turbos worked off of exhaust gas expansion. Merv, as far as your car is concerned, if you already have the ports inlarged for bigger headers, I would go for it. Your motor already has a boatload of mods that increase the flow on both intake and exhaust side, so the bigger primaries would more than likely be welcome. As Pat K. says it may lose a little bottom end, but mid-range and top end will be improved greatly. I think if your increasing the volume of exhaust gas to the turbo, then it gets squeezed in the turbine housing and released over the turbine, spool-up may not suffer too much.

Thanks guys.

As you all know, my car is a turbo'd Carrera and it has the standard 1 5/8" exhaust ports. I'm using GSF 930 turbo headers with 1 1/2" (38mm) primaries. Not the BEST match for my heads and certainly won't be giving optimal exhaust flow. Later on I may look into getting 1 5/8" (41mm) B&B's instead, but for now, she performs fine, albeit not at peak efficiency I'd say.

I say 1 5/8 primaries and a GT-42R- no make that 2! :D

Merve - in your situation I would find a way to transition the headers. Is the flange thick enough to machine a shallow transition?

To get big power out of these engines you need flow. Any bottleneck or mis-match will be less than optimal. You may not feel it but you are leaving something on the table.

If you are targetting throttle response or reduced lag then smaller primaries will promote greater velocity at the cost of top end power.

Do you have a copy of Corky Bell's book? Great explaination of how this works and how to design an optimal exhaust system for your needs.

hmmm, Carrera exhaust ports have a 1-5/8" flame ring, but the port id is 1.5", same as the id of the tube. 38.1mm.

930 Turbo exhaust ports have a 1.5" flame ring, but the port id is 1.375" or 35mm.

SO, by running 930 headers, I have a 3.1mm mismatch at the exhaust junction. NOT GOOD! I really need some 1 5/8" B&B's :mad:

Anyone wanna buy my low mileage 930 turbo headers for a REALLY good price? :D

I wonder if you could have just flared the headers a little to help smooth it all out ?

Quite a bit of metal that could be heated up and then tapered.

http://goingsuperfast.com/images/930...-heater-10.JPG

I guess 1 1/2" primaries make good torque, but would kill top end power. If you want best top end, then 1 5/8" is the way to go. If I sell my 930 headers, I'll be going with a set of 1 5/8" B&B's :cool:

Merv, do you think it will kill torque, or boost response by using larger manifolds? Im thinking that it would hinder boost resonse more than torque. I mean I know that on a N/A motor bigger primaries = less torque, but on a motor that is getting blown through, I think that more volume to the turbo the better.

When you say manifolds, do you mean exhaut manifold (i.e: headers), or intake manifold? Using bigger header primaries would yield more top end flow & therefore power. I guess if you hinder low-end boost response, then you are actually hindering low-end torque too, right?

Manifolds being headers; low end boost would be sacrificed and therefor low end torque, but what about mid range torque. Do you think torque may hit harder in the mid range or no?

Depends on a lot of things I'd say... type of CAMs being one thing. With some nice GT2-EVO CAMs, torque is at it's peak at between 4500-5500, so that would help a lot. Typically, the larger the primaries on the headers = more TOP END power.

Of course you can offset the low end responsiveness by doing many things, such as speed density, twin plugging, bigger throttle body etc...

There are many variables as you have noted.

I would shoot for the most free flowing intake and exhaust possible THEN build and tune for optimal boost threshold. You will have a high HP single turbo engine that has all the boost in before 2600rpm and lays down huge torque to redline. That translates to no perceptible lag.

Are 1 3/4" just way too big. I know theyre availible, but nobody seems to be using them. Would 1 3/4 be an improvment over 1 5/8 in a superchargered application?

Rod - you know the best exhaust for a supercharged engine is no exhaust at all. Turbos rely on the header configuration for optimal performance while superchargers just want to get the exhaust out. I would think a supercharged engine would only benefit from small primaries if it were a mild engine that needed to also be fairly quiet.