How can I tell if it's a HIF38 or HIF44 Carb.?

[Oddball67]

As you've been so helpfull answering another query I had, I thought I'd post another one!

In a whole box of bits I've unearthed, I have a Carb connected to a Heated Inlet manifold but, I'd like to know if it's a HIF38 or HIF44? Are there any noticeable differences?

Does anyone have any comments about what carb along with inlet manifold and exhaust system best suited to a MG 1275 engine? Was considering using a Multi-Branch exhaust system along with a 1 1/2" exhaust but don't want to unnecessarily spend my hard earned, and in short supply, money.

What advice can you give about inlet manifold?

Thanks again.

[chrisryder]

to tell if it's a '38 or a '44, take the carb off and measure the diameter accross the throttle butterfly. 38mm = HIF38, 44mm = HIF44. Simple!

HIF38 will do for a 1275 but a 44 will give more power apparently. If it's a standard 1275 (standard cam and head) you won't really gain much from a bigger carb, but the HIF38 would start to become a restriction on tuned engines. if you're on a budget, use what you've got. if it turns out you've got a 38, fit it and see how it goes, if you find yourself wanting for more oomph then keep an eye out for a 44!

Inlet manifold-wise, not all inlet manifolds clear all LCB's. I've found the 'Minispares' heated inlet to be just about ok, but it still took a few minutes with a file to get it to miss. I also have a 'Titan Motorsport' manifold that WILL NOT clear an LCB. if i machined away enough material to get it to clear, i'd have made a hole in the casting!

Also, being heated is not always a good thing. it makes the 'charge' warmer, so it expands, so you don't get as much in. with cold charge coming in, it doesn't expand so you can cram more into your cylinder. The up-side to heated inlets is that they keep inlet temperature fairly consistent which helps with mpg. And having a warm manifold in winter months reduces the risk of icing up when the carb is drawing in really cold air.

[Matt]

HIF44 on a MG metro (or equivalent) manifold with LBC and 1 3/4" inch system (maniflow make the best ones)

to decide whether its a 38 or 44 measure the size of the hole the air passes through at the back of the carb, it will either be 38 or 44mm

Dont connect the heated part ;)

[bmcecosse]

It certainly doesn't help prevent carb icing.....
An HIF 38 will be fine on a standard engine, and probably give the best economy. You can just use the standard exhaust manifold and system - just cut off/throw away the inlet section........ Slightly less power of course - but when cash is tight - one step at a time......

[MarkyB]

The difference between the two is only really going to be apparent at full throttle.
At anything much less than that then the smaller carb will be more efficient as the air speed through it will be higher leading to better atomisation of the petrol.

[bmcecosse]

Actually - the beauty of the variable choke of the SU carb is that the air speed remains high over the jet at all times......

[MarkyB]

You say "remains high", are you saying it's no higher in the smaller carb at a given speed?
I understood the smaller carb gives better torque for this very reason.

[bmcecosse]

The piston lifts as required by the air flow - to give an even air speed over the jet bridge, until of course it's at full lift. It's an extremely clever design. The amount of lift can be adjusted with the spring, and the rate-of-rise is adjusted with the grade of oil in the damper, and also by the amount of 'free play' in the little piston on the damper rod. So - an HIF38 can be adjusted to rise quickly, and an HIF 44 can be adjusted to rise slowly - with the 44 eventually able to pass more air than the 38 at full throttle.

[MarkyB]

Are you in politics at all Roy?
You are dancing round this question like a pro.
I've seen an SU carb, and understand how they work, and how they can be tuned in different ways.
None of that was what I asked.

[bmcecosse]

Well -what I am saying is - the HIF 44 will give just as good 'torque' from the engine as the HIF 38 - if set up correctly! If I was in 'politics' I would be able to claim expenses ------ sadly no expenses (for me anyway) on here.

[MarkyB]

if set up correctly!

You'd be great on the today program, steams gain, is politics loss.
How would they compare with the standard springs?
Unlike the today program, I have unlimited time .

[bmcecosse]

Probably just fine ! There is no 'standard spring' - different springs (and needles) were specified for these carbs for different engines. There is no 'one shot fits all'.

[MarkyB]

Well there are standard springs for the cars we get HIF44s from like a 1275 Mini for instance.