gmgracing

Its hard to say at the moment if we are going to offer that.

This is a .2 GT3RS . As a little added update we will have some new sound footage with the headers and sound comparison videos from our track day this past weekend!

quick cell phone snap of the system while doing an inspection said testing at track day.


-Dom

Jimmy-D

^ To your ear; with the same set up with the OEM headers w/ your Bypass - are they louder and/or tone different ?

gmgracing

The high flow cats can let out a mean sound when you get up in the rev range. This video is a little crude, but its just a couple raw clips to hear the sound difference..

-Dom

erik_plus8

I would say it's the other way round. The primary pipes above are significantly longer than std. That is reflected in the gains in the low- to mid range rpms. With longer pipes the volumetric efficiency peaks (which give high torque) are pushed down in revs. It is also proven by what happens way up top, where the torque with the longer primaries actually dip below the shorter std pipes, already before 8500. So, in order to acheive gains on top revs, you would need to go with shorter primaries, shorter than the stds. You can't have both gains in the mid-range and on top, that's not how the physics work. (if we for the sake of discussion forget about 2nd and 3rd order pressure waves which come into play on really high rev engines, typically old school F1 engines with a max rev of 19500-20000. They will have ~3 volumetric efficiency (and hence torque) peaks over the rev range)

With all this said I think the gmg headers look really good and from the torque curve it's easy to understand how good they will be for driveabaility. I would definitely be interested for my car.

bogey1

any results for us poor 991.1RS guys.... I already have your center bypass on my car.

porscheflat6

These are great! Nice work

trompazo

Long tube headers make more power with higher revs. Short tube headers make more power from idle to mid range.

I have compared the increase in torque compared to oem on these headers and Dundon headers. Dundon are long tube headers. TThese are not. The difference is quite striking. These make way more torque below 5000rpm, whereas Dundon make way more at peak. These may be great for a street car. On a track, it would depend on where you need the power.

erik_plus8

I won't go into the complete theory behind gas exchange, I did my Master on the subject and it's a little too complex for this format. However: the purpose of gas exchange is to increase volumetric efficiency, i.e. maximize the amount of oxygen drawn (pushed) into the combustion chamber to maximize the amount of fuel burnt at given lambda. The trick is to utilize the high pressure waves that occur when the valves open, both on intake and exhaust side. Ideally, an over pressure pulse that forms when the exhaust valve opens (since pressure in the cylinder is higher than ambient when the valve opens) propagates along the primary pipe at the speed of sound. The pulse meets an area expansion (the collector) changes sign (becomes an under pressure pulse) and propagates back up towards the exhaust valve. Now - if you manage to time this correctly, the under pressure pulse will arrive when the exhaust valve is still open and by that assist in pulling the exhaust gases out from the cylinder. If you have really succeded in your tuning this happens during the overlap, when both intake and exhaust valves are open, and you will also pull fresh air from the intake into the cylinder => higher volumetric efficiency => higher torque/power.

Now - since the speed of sound is constant over the rev range, at HIGH revs, the primary pipes need to be SHORTER to get the under pressure pulse to arrive back at the exhaust valve before it closes. Conversely, at low/mid revs, the primaries have to be longer to time the arrival of the pulse to the exhaust valve opening and overlap.

The same mechanisms are in play on the intake side. But there, an under pressure pulse (since the pressure in the cylinder is lower than ambient when the valve opens) propagates up the primary when the intake valves opens, meets an area expansion (the intake plenum), changes sign and propagates back towards the intake valve as an over pressure pulse, to assist in the filling of the cylinder.

Also, pipe diameters can of course be used in the same way, with the theory being that larger diameters acts as shorter pipes, and smaller diameters acts as longer pipes.

The above is the very basic theory behind tuning the gas exchange system and in no way covers all the mechanisms behind. As I said, it is a complex subject. Also, above is valid for 1 cylinder, it becomes a lot more complex when more cylinders are in play, when pressure pulses from neighboring cylinders are travelling up and down the primaries, disturbing the gas exchange. However it's a good situation for our engines, when only 3 cylinder are connected to the same collector and the pulses are the separated by 240 crank degrees which means they should not "have time" to disturb each other. Some pulses might travel through the center exhaust though.

I'm fully aware that dyno results, own experiences, different brands etc might in some cases contradict the above. That might be because a std manifold is normally so packaging restricted (i.e too short) that it doesn't really support the gas exchange at all. A slightly longer pipe might in that case indicate better power on high revs, but that's just because it replaces something that was not working from the start. It doesn't however change how the underlying theory and the physics work, and I just wanted to highlight that. I am not at all surprised by the results GMG has got from its longer pipe mainfold - the practical result (the torque curve) matches the theory very well in my eyes

gmgracing

Going back to the sound comparison. Here some pieced together clips of a 991.2 GT3 with a GMG center section and our 991.2 GT3RS with Titanium center section + WC-Evo Header set.

GrantG

Thanks for taking the time to post that

Wlwaldock

Great explanation & well written thank you.

Jimmy-D

It sounds good but I think many here have a rear bypasses and would be interested in some data pulls from say 60 mph to 120 mph from your Rear Bypass w/OEM headers vs Rear Bypass with your NEW LT Headers. In other wards less interested in sound and more interested in functionality

I have a .2 with a Manual so I am partial to that data.

Thanks

Christian Plyler

I deleted the side mufflers but retained the OEM center. The result is not substantially louder in regular, casual street driving. I'd say maybe 20% tops. I think the center muffler is doing 80% of the actual muffling.

audiophilitis

Agree -- I have valved side deletes and Akra slip-on (not race) muffler and it is just a tad louder than stock and most importantly, drone-free.

gmgracing

Jimmy,

I was answering your question about the sound/tone differences . You'd be surprised how common this exact question is.

-Dom