8 replies to this topic

[ohsuchislife]

Ive built a 1330 engine with mid road cam LCB stage 3 head blah blah blah

like your thoughts on a pair of 1.5 su carbs or the current HIF44 set-up ... would love your thought on the pros and cons

[bmcecosse]

If it's a GOOD inlet manifold - the single HIF44 should be better - especially if 'Vizardised'. Each cylinder breathes independently - so each sees a 1.75" choke with the single - while each only sees 1.5" choke with the twins! The balance pipe does nothing - in fact - it spoils the air flow.

[Dan]

Whilst it appears that an engine can only intake through one cylinder at a time when looked at statically, dynamically it is very different. That's why port stealing occurs. Manifold balance pipes do work and do an important job, it's just not a job of supplying fuel to cylinders as I've said before.

Personally I like twins, they make for a more repsonsive engine because smaller chokes are more sensitive to small changes in throttle position and engine speed. Some people prefer a single, each to their own. In terms of power production there is little difference but for driveability most people who have actually owned a car using twins will say it is a superior setup. Would anyone argue that an injected car will be better with a single injector and plenum chamber setup than with individual throttle bodies? It's the same thing.

[bmcecosse]

Sorry - don't agree - have had twins and singles - the big single wins. BUT - if you want twins - use a 'weber' like inlet manifold with NO balance pipe to get the best from them. Twin HIF44 (or HS6) on such a manifold are superb!

[TimmyG]

Each cylinder breathes independently - so each sees a 1.75" choke with the single - while each only sees 1.5" choke with the twins! The balance pipe does nothing - in fact - it spoils the air flow.

I have to disagree, (with Vizard....) on the 1st point, as Dan says, dynamically it is not like that, unless of course you're talking record player engine speeds! Many pulses strung together in a row, at speed, becomes more of a smooth line, it's the same with electricity, sound, and of course our subject, multi cylinder engines. Can you hear or feel the individual beats of the cylinders in a four cylinder engine? no, not unless your nearly stalling it and nor will the (single) carb see it this way. It will still be dealing with some of the flow from the previous (other) cylinder's induction stroke, and therefore cannot offer it's full flow capability to the next cylinder.
However, atleast this helps keep things moving in the forward direction.

If this were not the case then a single carb'd 1000cc A-series would only need the same carb size as a 250cc single cylinder version!

What happens when you fit twin carbs, and the reason in my opinion, that doubling the carb area does not give double the power potential is that each carb is now seeing a much rougher set of induction pulses at any given speed as each is now only seeing 2 cylinders worth of pulses, made worse by the fact they are not happening evenly. When you look at the firing order at a different point it becomes 3-4-2-1 which shows better that each carb is seeing a sudden rush of air flow when it's pair of cylinder's fires, then a big gap (atleast without a balance pipe!) while the same happens on the other side. The engine simply isn't able to make full use of the extra throttle area due to it's 'random' demands upsetting continuous flow. Imagine driving down the motorway and someone brakes a bit ahead, it all piles up behind it, sometimes to a standstill and takes ages to get moving again- same principal.

A flow bench gives steady state airflow, a constant suck, similar to 4 cylinders breathing through one carb. A carb that flows a given amount under these conditions is unlikely to flow as much under more changable ones. A carb on a twin carb set-up without a balance pipe (or a choke of a weber etc) see's intake pulses about a quarter as smooth as a single set-up. Half as many pulses, grouped together in the same half of the total time period.
So the whole point of the balance pipe is to try maintain a flow through the opposite carb ( by stabilising manifold vacuum) while it's own pair of cylinders are not breathing, and therefore smooth things out a bit so the carb knows where it's at and there is less stagnation of flow. It obviously then must increase flow as the port is able to draw from the other carb also.... Something that Vizard exploited on his own design twin s.u. manifold.

The thing is, while you are reducing one problem (uneven pulses creating non linear flow) you create another ( tortuous and continuously reversing flow path through the balance pipe) so adversly affecting power producing capability despite having the theoretical flow capability of double the carb area.
If you don't have the balance pipe, you are stuck with the original problem.

Vizards quote of 'as a result, each cylinder sees only the cfm of the carb on the end of its port' is nonsense because that would mean that, using his figures, a single 1.5" HS4 and good manifold giving 114cfm's, would be capable of supplying the needs of about 55 hp per cylinder, or about 220 hp all together which of course would be more than adequate for any A-series!
It would also mean that you could fit one 1.5" SU to a 450hp v8 and it wouldn't restrict it in any way because only one cylinder draws at a time! I don't think so!

[bmcecosse]

I have seen twin manifolds with the balance pipe blocked off to give smooth flow to each port - the engine ran better. The balance pipe creates chaos at the port entry - it's only there to make 'carb balancing' less critical on road cars! And... the weber seems to manage just fine without a balance pipe........ Single HIF44 or HS6 - or twins on weber-like manifold for full race engine.

[Asphalt]

That has been done on some RACE cars. Fill the manifold with resin to make a good shape of the ports and REDRILL a SMAL balance pipe. The twin SU carburetters on a Weber manifold you are refering too are probably the Janspeed ones. Which do indeed have a rather large balance pipe!

And if you compare a Weber carburetter with an SU carburetter - sorry, that's just like comparing apples and oranges... It doesn't work. Read the post from TmmyG, imagine the SU's pistons under 'pulsed' vacuume conditions - and go on. A Weber simply has no pistons. If BMC got one thing right, it's the balance pipe on twin SU manifolds.

@TimmyG:
Thank you for that excellent post!

Cheers,
Jan

[TimmyG]

Cheers Asphalt! Exactly as you say about the weber vs SU!

[Saxo-Fiesta-Mini]

i have to agree with both side but the mini specialist ive spoken to mini sport, mini tech and an old guru i know every well told of the potential of a well set up hif 44

I listened waited 6 months and know i have a hiff44 with ram stack on the richest needle and set it on the gas analyser to 3 % CO2 and what a difference i havent however ever driven a 1275 a plus with same cam and spec but for myself id go for the hiff 44 all day