Twin Carbs, manifold breathing advice

Post by **alanworland** » Sun Aug 24, 2008 9:53 pm

I am building for my sidevalve an inlet manifold to use 2 rebuilt HS1 carbs. Each carb has its own connection to the block to supply two cylinders and I am about to join the two together to make it 'one manifold assembly'
However all the manifolds I have seen (including the one my carbs came from - Austin Healey) have the two inlet ducts joined together? So they are not totally independant.
Any ideas why this is?

Post by **MoggyTech** » Sun Aug 24, 2008 11:53 pm

I think BMC is going to be your man for this one. Inlet manifold tuning is a bit of a black art, with things like airflow and air turbulence to consider.

Post by **alanworland** » Mon Aug 25, 2008 12:00 am

Thanks, I know BMC is a mine of info!
I understand the connection is called a balance pipe but its strange that Webers dont seem to use them!

Post by **MoggyTech** » Mon Aug 25, 2008 10:38 am

If the two inlet sections you have are a balanced pipe for the carbs you have, then that will be ideal in that they are matched for the carbs in question. It then depends which engine they were tuned for, as the design of the intlet valves and their ports, is all part of the tuning process.

I know in the standard OHV engines, the hotspot in the inlet manifold is the weakest part of the design, so anything that gets rid of that, will give a cooler intake charge, which means the air in the charge is more dense, which is a good thing.

If I haven't forgotten the basics, I think Webers are variable choke carbs as opposed to variable venturi, which might explain why they never used the balanced pipe. I could also be talking complete nonsense, as it's been way too many years since I tuned engines to any great degree, and even then, it was mostly bolts on bits I was told would be better.

Post by **bmcecosse** » Mon Aug 25, 2008 1:12 pm

The 'balance pipe' on the normal A series twin carb manifold is there to make balancing of the carbs less critical. Unfortunately -they way they did it rather spoils the flow of air from each carb into the port and makes the manifold less than ideal. Just make up flanges and tubes between each port and it's carb and provided the joins are 'stepless' you will have the ideal set-up. Yes - you will need to balance the carbs with a bit of care - but it's not difficult.

Post by **alanworland** » Mon Aug 25, 2008 2:07 pm

So, it sounds like it's best left off then.
I will probably just join the two intake tubes together so as I can assemble the carbs to the manifold.

Post by **bmcecosse** » Mon Aug 25, 2008 2:43 pm

H1s are desperately small - fitted to Mk 1 Sprite etc. Are you sure it wouldn't better to go to H2s ?

Post by **wanderinstar** » Mon Aug 25, 2008 2:54 pm

Alan,
Is the car in your signature yours. Reason I ask is that it looks different to normal Moggies. Think its the tyres. The whole car seems to be sat up in the air. Almost like those cars sat on 4x4 chassis.

Post by **bmcecosse** » Mon Aug 25, 2008 2:58 pm

Looks like it has the (now rare) early cut-away rear wings - and nice chunky wheels and tyres!

Post by **alanworland** » Mon Aug 25, 2008 5:50 pm

Hi BMC, yes the H1's are small but I understand that the smaller bore and hence high gas flow velocity is more suitable for these engines (and I already have them!)
Yes the car is standard apart from the wheels which are 175 x 14 and are mounted on the later 1000 axle and 8 inch front brakes. The front ride height is correct, but I have always thought the rear looks high (new 7 leaf early springs) I think this looks exagerated with the high arch rear wings.
The wheels are a lasting memory of my youth when I threw the originals away!

Post by **bmcecosse** » Tue Aug 26, 2008 10:54 am

Alan - I have a tubular steel inlet manifold for twin H1 carbs for an A series engine. Obviously it would need slight modification to fit to your side valve - but would be easier than making a new one ! PM me if interested!

Post by **tortron** » Tue Aug 26, 2008 8:18 pm

what sort of modifications would be needed out of interest?

Post by **bmcecosse** » Tue Aug 26, 2008 8:25 pm

Well - I doubt that the inlet port spacing will be the same, and my manifold has a small balance pipe fixed across the top - so likely it would need to be cut and either shortened or lengthened (or completely blanked off) to suit the spacing on the side-valve engine. Otherwise - it certainly fits the carbs, and the engine side flanges will very possibly be useable on the side valve with very little (if any) modification.

Post by **tortron** » Tue Aug 26, 2008 8:28 pm

The carb mounting angles would be different wouldnt they? (for h1 carbs anyway)

Post by **bmcecosse** » Tue Aug 26, 2008 8:40 pm

Well - the fancy steel manifold I have is designed to suit the H1 carbs I have - it was on a Sprite Mk1. Can't remember now - is the side valve engine downdraft into the engine - or side draft ?? The manifold is of course for a sidedraft installation.

Post by **alanworland** » Tue Aug 26, 2008 10:52 pm

Thanks for the offer BMC but I already have a Sprite inlet manifold for twin H1's (in aluminium) The port centres needed increasing and the engine end required modification (the mounting flange wasn't thick enough to match the exhaust) so I bit the bullet and have made the inlet ports, which are angled up at 30 deg.(sidedraft)
With a bit of work to the linkage I should be able to mount both float chambers between the carbs, which is needed because the rear chamber ended up very close to the downpipe (1/2 inch)!
I will put up some pics soon

Post by **bmcecosse** » Tue Aug 26, 2008 11:38 pm

Ok - I have both the original Sprite alloy manifold - and this long branch (about 4" from memory) tubular manifold that came with the car. It's probably made by some 'famous' - but can't find any ident marks on it at all! I'll dig it out tomorrow and send you a pic - in case it helps/inspires you at all! My first (3) cars were Morris 8E - with that sidevalve engine. I have some spare bits for H1 carbs if you need them.

Post by **alanworland** » Fri Aug 29, 2008 11:17 pm

Thanks BMC, sounds like your Sprite manifold is the the same one as mine, which I was going to try and use, but didn't!
I got the carbs and manifold from a guy in Portugal as at the time there didn't seem to be many/any about, they have not been messed about and will only require new jets, needles and seals etc , I have already rebushed the bodies and made the spindles fit (built the wear up with silver solder and turned to fit)
These are my inlets which are under going trial assembly at present
http://i248.photobucket.com/albums/gg17 ... ifolds.jpg
I shall bear in mind the offer of the carb spares!

Post by **bmcecosse** » Sat Aug 30, 2008 10:26 am

Ooooooooh - these look VERY fancy! Well done. Are your float chambers vertical when fitted to that angle manifold ?

Post by **Peetee** » Sat Aug 30, 2008 11:31 am

Are your float chambers vertical when fitted to that angle manifold

Very good point. My single manifold was so angled that it wouldn't allow me to use a HS4. I had to go to a HIF.