Race exhaust system for Siamese ports

[oneaves2005]

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]

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:

[oneaves2005]

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

[hoffman900]

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.

[oneaves2005]

Hi All,

I'm bringing this thread back again in the hope that I can get some technical input in coming up with a specification for an exhaust manifold.

Would particularly welcome your thoughts on the latest design by one of the leading suppliers of off-the-shelf race headers, who use a step down on the centre branch, around 8-10" off the head face. I am aware of some testing and this design working favourably but cannot understand why, unless the centre primary is too big in the first place?

Kind regards

Ollie

[hoffman900]

Hi All,

I'm bringing this thread back again in the hope that I can get some technical input in coming up with a specification for an exhaust manifold.

Would particularly welcome your thoughts on the latest design by one of the leading suppliers of off-the-shelf race headers, who use a step down on the centre branch, around 8-10" off the head face. I am aware of some testing and this design working favourably but cannot understand why, unless the centre primary is too big in the first place?

Kind regards

Ollie

Probably this. There is a lot of wonky, head scratching stuff in the offbeat performance British sports car world.

[Walter R. Malik]

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

Some header designs incorporate a dead end "interference" tube for the center header pipe, some bifurcate numbers 1 & 4 together before joining the center tube and some just use a slightly larger tube for the center. AND, some use more than one of these "fixes" to get the level of exhaust scavenging they desire.
All seem to somewhat work for certain combinations.