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Race exhaust system for Siamese ports

General engine tech -- Drag Racing to Circle Track

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oneaves2005
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Race exhaust system for Siamese ports

Post by oneaves2005 » Sat Jun 20, 2020 3:37 am

Hello all,

I wonder if you may be able to help with my next project.

We have a couple of BMC B-Series race engines for which we are looking to develop a race exhaust system for. The engine capacity is around 1850cc 4 cylinder. Max power is currently around 6,700rpm, approximately 165bhp. Cylinders 1-4 have there own ports, cylinders 2-3 share a port. Most off the shelf race systems use 1.625” pipes on 1-4 with a larger bore 1.75” pipe for the centre Siamese port... all a bit big I think. I’m interest you know thoughts on length and diameters, particularly for the shared centre pipe compared with the 1-4.

Thanks in advance

hoffman900
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Re: Race exhaust system for Siamese ports

Post by hoffman900 » Sat Jun 20, 2020 11:25 am

You're down about 20hp from a really good B engine here in the States. IMO, with modern cam and head development regime, I think 200bhp (so 20hp up on 1980s era development that most are still running) is possible on gasoline, SUs, and that can hold together.

I've thought a lot about how to approach the A-B series engines. AVP Engines (Aaron Kelly) and Joe Huffaker (Huffaker Engineering) are doing the best work on A-Series here.

Joe runs a LCB exhaust. The exhaust note on his car is very distinct and a bit bassy (sounds like a Subaru exhaust). Aaron Kelly uses a 3-1 header and the engine has a note like you would expect. Aaron runs a tube that matches the port off the head before stepping it from there. Huffaker uses a more traditional header sizing.

Sir Yun on here did a lot of sim work on A-Series exhausts. He found a short center branch manifold was the way to go. The thinking is you treat the center branch like secondary, it would require creative packaging.

My thoughts are to build a 3:1 and use Hytech A/R chambers. You can buy them from John (Hytech) for like $20 a pop. The center cylinder would be at like 4-6" off the head while the outer cylinders would be about 8-12" (for your rpm, go with 6 and 12). Carrying that secondary thought, the center branch would double step at the A/R chamber and carry that length to the collector. The outside ones would be a single step. Other than that, the diameters would be matching the port area off the head and diameters go from there. I haven't tried it or done a sim on it, but I would be interested to try. Aaron was going to try something like this, but his customers have been bringing him 8 port Datsun A-Series engines to develop, so he's been playing with them the last few years.


Here are two graphs from Aaron Kelly. The lower hp one is a SCCA FP engine that won the SCCA Runoffs a few years ago. .040 overbore, stock valve sizes, stock connecting rod length, SU carburetors, gasoline, no adding material to parts (but can remove), everything else is pretty much unlimited. The higher power engine is a SCCA GT-L engine: .040" overbore, stock head castings / block, everything else is pretty much unlimited. He converted to fuel injection and has a Mike Jones designed cam in it.
full-1055-3380-imag0403.jpg
full-1055-3379-imag0409.jpg

The Aaron Kelly built engine comes by a 0:10, Huffaker at 0:20

You can watch the race here: https://www.scca.com/videos/2027438
Another with Joe's Midget (wrecks with a Super 7): https://www.scca.com/videos/2029828



Calvin did an exhaust for a friend's TR4. 1 3/8" off the head and it's making about 1.75hp/ci. Put that into perspective vs what the MGB people offer...

in-car:
dyno:
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-Bob

oneaves2005
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Re: Race exhaust system for Siamese ports

Post by oneaves2005 » Wed Jun 24, 2020 7:28 am

Hi Bob,

thanks for your response above, most helpful. One thing I should have noted is that we are restricted to a 3-2-1 type system and it must visually be similar to those used in period. Only pump fuel is allowed in our series which I think differs slightly form what is permitted in the states. I have attached an image below of a typical system used. I'm more interested in fine tuning lengths and diameters, Once under the car I can probably use an A/R chamber after the final collector if of use but I can't do anything too crazy visually with the manifold.

Thanks
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hoffman900
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Re: Race exhaust system for Siamese ports

Post by hoffman900 » Wed Jun 24, 2020 9:23 am

I hate rules like that. We're at the point that those who were there in the day have died off or are very old. i watched a tech inspector, ~30 years younger than a friend of mine who raced and built Championship winning cars in the day, try to tell him what they did and did not do back then. Sure...

We're not restricted fuel wise. 110 octane leaded race fuel is common, hence higher compression ratios. R&D helps for sure. Billy Godbold / Comp was involved in an A-Series engine last summer that picked up 8% over Swiftune's best stuff and was more reliable (not pounding valve springs out of the head after every weekend).

Can you even build any stepped headers? They have been around since then. The Maniflow sucks, it makes a sharp 90* right off the head.
-Bob

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