stolarzj

Part 1
https://rennlist.com/forums/928-forum/313283-tweaking-ez-f.html

PorKen

I had a muffler shop flare 2" pipe to make the sizes to fit perfectly, engine side end 2" ID, 2-3/16"ish OD on the other end to just fit into the 2.5" pipe.

In looking at the manifolds again, I noticed that the left side has a longer 'collector' than the right (30 vs. 55mm), so I made the right side 2" ID section longer to make them equal. I tack welded them into the X down pipes, with 3mm showing above the gasket.



This pic shows the construction of the manifolds. The holes are smaller than each branch. Maybe it's some kind of anti-reversion thing, but me thinks it's just a manufacturing shortcut.

This close to the engine, you can see where there is turbulence by color. A uniform carbon black would be ideal, but whereever there is a rough spot, the color turns to brown or white.

doktor-t

These manifolds (headers) are quite good flowing, for being stock units. Is it any idea switching to real tubular headers? I think the headers for 928´s is way too expensive. Perhaps it would be a good idea to purchase one of them DIY kits from Headman Hedders, lots of work though..?

http://www.hedman.com/products/?id=3524

http://www.hedman.com/images_products/4503.jpg

Tubular, especially longtubes is a great advantage when talking Camaro but their manifolds a really bad when it comes to flow. Then talking headers, the longer the tubing, the better the torque.

I´m tempted thou....

PorKen

I don't think I'd use full length headers on a street car, because of the space limitations.

I would rather see a 'shorty' header to bolt up to existing X-pipes. It doesn't even need to be equal length, just slightly larger primaries, and larger collector. On an engine with stock cams, I think that they would give near full length header like power increases, but with less hassle.

doktor-t

Groundclearance might even be an problem with longtubes. I also like the idea to retain the x-pipe and have all parts interchangeable individually with stock parts.


This might be a good Idea:

For primaries, perhaps 1,5/8" would be suitable and 3" collectors necking down with a coneshape to 2,5".

That wouldn´t be that hard to build. When I think about it, 1, 5/8" tubing along with 3" collectors is a common size for Chevy smallblock (5,0-5,7 liter) longtube headers and these are really cheap. If slaughtering a set of longtube headers there would be enough tubing to build a nice set of shorties. Fitting the flanges to the heads would probably just be a mather of cutting the flanges so they all become independent an maybe massaging the boltholes which are 3/8" on all Chevy smallblocks.

What do you think about the tubing diameter I mentioned above?


/T

PorKen

I don't know what the stock port sizes are, so I can't say what size they could/should be.

Recycling is a good idea, if a close match could be found. There's probably an existing shorty header which may be used, too.

doktor-t

I´m not in the position right now to get a manifold off to do any measurements but there must be someone in this forum who has done some measuring earlier. I we only could get a stock port-size it would be quite easy just looking up a header with a reasonable sizing.

Time is my problem, I´m stuck with my 15 month old son beeing completly of duty from my regular work just to take car of my kid for about a year from now. Needless to say there isn´t much time to either drive or work on my car. For now every saturday and every other sunday goes by working extra hours at a carpart-store. Anyway, it´s all for his best and then it´s time to send him to daycare center and I start working again I´ll atleast have a weekends to do some work on my car. Irritating getting almost nothing done! 15 month old kids isn´t very helpful when it comes to do work on the 928 Hopefulle these little arms and hands can in the future be able to reach bolts that is impossible for myself

PorKen

I could be fooling myself, but it seems that there's a bit more torque at lower rpms with the adapter mods.

Exhaust pipe temps at idle are within a degree side to side from the manifolds to the exhaust tips!

It is quieter at idle, and the idle is like buttah.

I did a second gear 5000 rpm run through a tunnel, and it was heaven. Exotic, 12 cylinder cool.

Oops! I recieved the Hedman 21120, and it's an adapter to delete the valve in the picture, doh!

I'm going to have to order the (Thorley from Summit), and wait.

doktor-t

Anything new about the exhaustvalve?

I´ve been thinking about piping diameter. If our 85-86 cars do have GT cams, wouldn´t they respond well to an full length dual 2,5" exhaust such as Mr Ott´s? For all I know he ran good numbers at the dyno with his GT using dual 2,5".

I felt it like my car lost it´s bottom-end torque when using 2,5" all the way. Could my "buttdyno" fooled me so bad?

PorKen

The valve is backordered. If it doesn't come in by next week, then I'm going to cancel the order.

Using the 2.5" pipe may have shifted the torque peak higher in rpms. You still had the stock chips at that time, too, right?

MaineShark

A faster-revving engine can often be mistaken for one with less torque.

slate blue

By Porken
I would whole heartedly agree with this, I thought to actually just increase the rear section or the stock manifold to 2.5" like the X pipe that follows. Instead of having to buy shorty headers although I like it that somebody has made tis product.

Glad to see others in on the bypass gig, also glad to see a lower price than what I paid years ago which was about $400 usd. Mine is made from alloy though. Is there a 3" version? I will have a look anyway. My car has great bottom end power even though it has a full header system with a three inch exhaust. My X id a 2.5" version which I hope gives good velocity.

BTW I am about to embark on my most ambitious exhaust project ever, it will also be the last one too.( I may have to pay somebody to do it for me as I don't know how my back and neck surgery will go) My new headers are made from inconel 625 and are 36" long and will weigh in a massive 18 pounds! or 7 kgs, target weight for the whole system is a beefy 22 kgs for a duel 3" system but exiting from the side of the car as apposed to the current lightweight system weighing in at 30 kgs. The weight of a stock S4 system is 50 kgs FYI.

Link to the old exhaust with the bypass valve.
https://rennlist.com/forums/showthre...weight+exhaust

Good luck with your results

Greg

doktor-t

Yep, stock chip is still there. Funny thing about the cams. I can really feel the cams waking up at around 5000 rpms so the definatly give some nice power way up in the powerband. The rear tires often brake loose at 5000 rpms too. Could this be a sign of the GT cams? If so, there might actually be a chance to get some increase in power with a full length 2,5" exhaust.

PorKen

I took the hex-pipe off. It didn't make any more HPs than the modd'ed muffler, and it didn't look so awesome after a week.

The valve finally arrived, > month after I ordered it.

To fit it, I had to install it on the driver's side, but that's actually better, because the passenger side piping is longer. It's hooked to manifold vacuum.

It didn't change the idle note, but part throttle is quieter, freeway too? It keeps the exhaust temps much higher through the system.

I think(?) it makes a difference at low rpms, but it's tough to say. Better throttle response? I need to dyno it, or put in a solenoid to turn it on and off while driving. I'll see after a few tanks if the mileage is better. I might put in a accumulator and/or a valve/restrictor to see if different delay times change the 'effect'.

I installed (an intake spacer), just after the valve install, and together they've made for some heavy duty torque at low speeds!





Check out the wastegate on my mid-turbo, brah!

PorKen

I updated the list with the size of the 'S' single side 'duals'. Only 52mm OD, and then they have that squashing at the rear x-member. '86.5 duals are 55mm OD, but the pumpkin inlet pipes are 52mm, go figure.

I think Carl shows a 28hp jump using his dual exhaust conversion + RMB. I reckon that's what a stock '86.5 + RMB has. The big '86.5 (aka GTS) resonators are straight through - heavy, but not restrictive.

I couldn't fit my rear H-pipe on the '85 without changing to GT resonators. The big resonators just don't fit. (The main reason the floor pan is different in '86.5.) It's a little burbly, but that's what I wanted on the 5-speed. It'll be a touch quieter when I get the HeX pipe valve hooked up.

The pre-86.5 x-member has unused holes on either side for putting in a strap. I'm using welding wire to hold it in the pic below.