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On other news, John's getting the new rear muffler sheet metal rolled next. That'll be interesting. And the whole exhaust can now be fabricated with the new manifold ready. It's another huge amount of work that should start after John delivers another white twin turbo to the owner on Wednesday.
The main pipes fused on both sides. One more wastegate circuit to go and the downpipes and the newly cast manifold can be installed for fabrication of the rest of the exhaust.
John's fabricating a big fat oval center pipe for the exhaust. This is where the pulses fully combine and the gas cools. It's going to need three or four vertical support rods to prevent and drumming, booming, or droning.
The big fat center pipe being test fitted. The real production piece will likely need to be made from heavier gauge steel. And as the photo shows, this is the biggest pipe that fits under the 928.
That thing is huge! It's hard to believe that such a large pipe is required, but that's the great thing about your project. John is sampling lots of data and whether something works or not will be demonstrated by the data.
John's fabricating a big fat oval center pipe for the exhaust. This is where the pulses fully combine and the gas cools. It's going to need three or four vertical support rods to prevent and drumming, booming, or droning.
Originally Posted by ptuomov
The big fat center pipe being test fitted. The real production piece will likely need to be made from heavier gauge steel. And as the photo shows, this is the biggest pipe that fits under the 928.
I'm interested in how you've calculated the cross sectional area of the oval. The sum of the area of the two down-pipes feeding it, or a different figure to allow for cooling or a desired flow characteristic?
I'm interested in how you've calculated the cross sectional area of the oval. The sum of the area of the two down-pipes feeding it, or a different figure to allow for cooling or a desired flow characteristic?
The big fat oval pipe is the maximum size that fits in that center section and ends up having a cross-sectional area very close to the dual 3.5" pipes that feed it.
The other option on the table is to cut a small segment off from two 3.5" pipes and weld them together to form a pipe with the infinity sign cross section. The cross-sectional area is only slightly smaller than dual 3.5 pipes, using the circle segment formula. That version is much stronger structurally and can therefore be made from lighter material and without support rods. The construction there isn't without issues either. If you cut open the whole pipe section side, it will spring open a bit and then that makes mating the ends more difficult.
From the velocity perspective, it would be ok the step down from 3.5" to 3" after the axle mufflers. Then after the rear muffler you could go to 4". That's how a car factory would (have) probably size(s) the exhaust for these power levels (in the 1980s), about constant velocity. I've got a cooling-density-velocity model in a spreadsheet workbook on that. I don't think we'll do that with this version, though. It's a lot easier to go down in size at the tail pipe than upsize the whole rear section, so we're going too large with dual 3.5" rear section and 4.5" tailpipe. Then one can tweak the tail pipe smaller and see when it starts hurting power.
kind of late now, but i've had success just using a hydraulic press, big spreader plates and some internal supports to crush round tube into more elliptical for vertical clearance.
kind of late now, but i've had success just using a hydraulic press, big spreader plates and some internal supports to crush round tube into more elliptical for vertical clearance.
To do that right (and in our case it needs to be done right since both ends connect to two round 3.5" pipes), I think one needs steel dies. Those are very expensive to have made.
If one just wants to flatten a single pipe for some section, press with plates is probably just fine.
09-04-2017, 01:42 PM
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