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MG Midget and Sprite Technical - HS2 inlet diameter

Random question I can't find an answer to whilst googling...

Can anyone tell me/measure the inlet diameter of a HS2 carb? Is it 1.25 inch? Or is it bigger and just the throttle/outlet is 1.25 inch?

Cheers,
Malc.
Malcolm

The effective diameter at the head, is 1.2 inches.

This manifold still has my twin hs2's mounted. To save me taking them off, I'll have a look in my loft later, for my spare bare manifold, and see what the diameter is at the carb face mount.


anamnesis

Sorry Anam, I meant the air filter side of the carb.

Cheers,
Malc.
Malcolm

Thought you might, but I was in the garage anyway, and these are on the shelf there.
anamnesis

No worries, another google with some different search terms and I think I found it. 35mm.

Thanks for looking for me Anam.




Malcolm

Does that help?


James Paul

Thanks James, it corroborates the above. Cheers.

Scheming new manifolds... the 1500 inlet ports are 1.25" so wondering if I can use HS2 stub stacks to smooth air from the plenum into the runners.

Malc.
Malcolm

Does the manifold diameter reduce in size at the head deliberately, to speed airflow?
anamnesis

Malcolm,

Do you have a lathe or access to one? It's not difficult to flare aluminium tubing with a suitable tool or an appropriate mallet and T stake to make it blend into your plenum.
David Billington

Malc---still not convinced 100% that the manifold is the whole issue--i'd be looking at the collector size/length of the exhaust headers first--what size primary pipes are you using and what are your collector sizes, dia/length---if there's an issue there the inlet can't work properly.
willy
William Revit

Hi Willy,

Exhaust is 1-1/8" (ID) by ~26" length primaries into a 1.75" 4 into 1 collector.

Cheers,
Malc.
Malcolm

Malcolm- Hi
There 'could' be an issue there-
Just to put you in the picture--If the collector diameter is too small, the exhaust doesn't get a chance to expand/blend and it just gets counted as an extension of the primaries which generally speaking results in possibly more low down power and less top end-
You being you have obviously worked out the primary diameter from valve seat throat size and that part is good--The length of the primaries is ok and that in itself isn't over critical--
I'm treating this as half a v8 because that's where I'm from,--- there's a formula for collectors that works- For a 4/1 header the collector should be (and I'm switching to outside dia. here just for easy measuring) 1.8 x primary tube size
So your primaries are 1.25"od x 1.8 = 2.25"od
Then the volume of the collector should be equal to 2 cylinders 'minimum'----which is ---um bear with me ---12" minimum
So--I'd suggest cutting your collector at a point where it's still large enough to be reshaped round at 2.25"od, which guessing, won't be all that far from the ends of the primaries add 12" of 2.25" tube and then stick the rest of the cone/reducer back on the end of that---
It's not a real big difference to what you have but just might be what's blocking your top end performance-
If you were going to pull the manifolds off for some reason it'd be worth a try or depending on access it could be done in situ.--?
willy

William Revit

Like this highly technical sketch?

(This is my manifold btw, before being welded up).

The collector is 1.875 OD outlet as it stands.

Cheers,
Malc.


Malcolm

Yeah, sort of but you need to cut the collector further forward so the 2.25" pipe butts up/matches to the collector directly at the 2.25" diameter point instead of reducing down then opening up
return sketchy sketch---

Is your exhaust system 1.75" or 1.875"
1.75" will flow 100-110hp
1.875"will flow about 125hp
If it is 1.75" you're getting close to the limit of your exhaust but in saying that, I'd imagine that would be more of a max. horsepower limit up high in the revs, not the sharpish dropoff that you're getting



William Revit

OK, gotcha.

Think 6" will be enough? Just last night I was doing a wee mod to my Alfa exhaust and have 6" of 2.25" pipe left off that!!

The more I ponder what you have suggested, the more I think I might have made an error... as there is also a 90 deg bend just after the collector. So everything is being squeezed together and being hurled round a bend at the same time!

Exhaust is 1.75" for info.

Thank you as ever for your sage advice Willy.

Cheers,
Malc.

Malcolm

BTW, whilst I have your attention Willy, did I ever share this?

A mates car, with a similar-ish spec engine but with an off the shelf exhaust system, twin DCOEs and dizzy ignition.

A smidge less overall, but revved cleanly to 6000.

Cheers,
Malc.


Malcolm

6" isn't enough, it needs to be equal to the capacity of 2 cylinders minimum- so 750cc in your case --12"+ by my calcs.----Straight is best-but-if your exhaust has to turn fairly close to the collector, maybe you might be better off getting a 2 1/4" gradual exhaust bend and makingthe collector curved to help straighten the line of the exhaust system a bit
William Revit

You'll laugh--looking at your mate's chart and I'm trying to remember where yours was hitting the wall, just about to ask then spotted the mepower-
Once you find whatever is stopping yours from peeking out you should have a fair old advantage over his spec.
Looks like you're up about 10% on him at 4500 then it starts dropping off.
Main thing with this sort of thing is to do one change at a time---If you do two changes, it might feel exactly the same and you think that didn't work---but one of them changes might have fixed it and the other made it worse and you'll probably not go back to the one that worked because you think it didn't-
It may well be the inlet manifold/plenum, but if there is a fault/restriction/mismatch with the exhaust it will directly effect how well your inlet works--the exhaust is easiest first fix and needs doing anyway--I've got my fingers crossed for you-

Random question what injectors are you running

willy
William Revit

Excuse the brevity, it's late, I'm away and I'm on my phone. These ones...

https://www.injectorplanet.com/collections/bosch-fuel-injectors/products/bosch-0280155787-land-rover-err6600

Malc.
Malcolm

All good--you've got plenty up your sleeve there so fuel supply isn't an issue--
William Revit

Malc/Willy

Although I don't really understand exhaust design in any detail, this thread still holds my attention. So, excuse my ignorance, but what is an exhaust "collector"? Is it where the branches of the manifold pipes join? In which case, why isn't it the rest of the exhaust pipe? What is it on a standard A series exhaust?

Every day is a learning day!
Bill Bretherton

Bill, these are collectors. It's where the branches merge into the single pipe.


anamnesis

Anam, thanks, pretty much what I thought. What I don't now understand is how the exhaust can be a smaller diameter than the collector but not restrict the flow further down the line,
Bill Bretherton

Bill
Is it that only one leg of the collector is in use at any one time as the engine strokes, but the outlet is pretty much always in use as it receives 4x as many strokes as an individual leg?
timmyk

Here's the answer.

https://www.enginelabs.com/engine-tech/exhaust/performance-exhaust-system-design-and-theory/
anamnesis

Timmy, yes that's probably it!

Anam, thanks for the interesting article.

Malc, sorry to divert your thread! Good luck with your ongoing project.
Bill Bretherton

No worries Bill. My threads aren't really threads, they are more just thinking out loud ramblings! :-)

Not had chance to really do or think about anything this weekend. Finished stripping all the rear suspension bits down for my axle/brakes/suspension rebuild and picked up some "new" half shafts.

Need to drain and strip off the fuel system, and sounds like the exhaust too now.

Car is going to have been totally rebuilt this year!

Malc.
Malcolm

This thread was discussed between 15/08/2023 and 21/08/2023

MG Midget and Sprite Technical index

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