engine modifications

[Hopey66]

hi there, looking to get a little more power out of my 1098cc engine/ make it a bit faster, im not looking at major mods, and i dont want to spend 1000s. wandering if anyone has any ideas or suggestions, thanks

[bmcecosse]

Covered many many times in other threads......have you used the search function? It depends muchly on your budget - but a first mod is to fit a larger 1.5" carb - on an alloy inlet manifold for preference. Like this... http://www.ebay.co.uk/itm/Mini-HS4-Su-C ... 19d2a3e4d4 Just use the float etc from your old carb.

[Hopey66]

thanks, i have already got an alloy inlet manifold from mini spares, and it says that it will run with HS4/6 HIF38/4 carbs, but ive looked at them and they dont have the same square face plate as the inlet manifold, is there anyway around this?

[bmcecosse]

The HIF carbs DO have square flanges - but the others fit on the diagonal anyway. Don't pay big money for a carb... !

[IslipMinor]

Hopey,

The HIF 4/38 has the diagonal mounting (looking at a sample of 2 MGB's and a spare HIF 38 body). The mounting flange can be 'handed', hence on a single carb application the manifold will have all four holes, but are there also 4-bolt versions of the HIF 4/38? The bigger HIF6/44 has the 4-bolt square mounting.

The HIF 4/38 is the one to fit to a standard(ish) 1098. If the inlet manifold you have is marked as suitable for both, it could have the larger bore for the HIF6/44, and is not really good for the smaller carb. There would be a big step 'up' in size from the outlet of the carburettor to the manifold - better that than the bigger carb on the smaller manifold though!

What does the inlet diameter of your manifold measure? 1.5" or 1.75" - hopefully not something in between!

In the link you show '5' is the fuel inlet.

[bmcecosse]

Step UP is fine -no harm in that at all - especially as going into the two-way split......

[IslipMinor]

Steps cause a venturi effect, reducing flow, it will work but not at full flow.

[bmcecosse]

Not that step UP - the carb is effectively running into a 'chamber' that then splits both ways - it really is not harmful. There's always going to be chaotic flow in that area - especially with the unfortunate 21 then 34 firing order - two pulls on each leg of the manifold - and then two pulls on the other.........

[IslipMinor]

Not that step UP

Do not understand what you mean?

The step I am referring to is the sudden increase in cross-sectional area by fitting a 1.5" carburettor on to a 1.75" diamter inlet manifold. The sudden increase in cross-sectional area (36%) will cause significant fuelling problems, especially at low revs and during the warm-up phase, when there will be fuel droplet fall out caused by the sudden reduction in air flow speed.

The basic design principle of any intake system is for a gradual tapering reduction in port area to increase the air flow speed as it progresses towards the valve throat, and any significant sudden increase in port size from the carburettor intake side to valve throat works against this basic principle.

There are any number of views from well respected A and B Series engine developers on how the air/fuel mixture flows through the split siamesed ports in the head, from seeing a need to 'split' the flow with a sharp edge in the centre of each port to treating each inlet tract separately.

I would definitely not do it, but as always there will be other opinions.

[bmcecosse]

Hahaha - but even Vizard suggests it's ok to step UP into effectively a chamber where the ports diverge left and right. Yes - after that - it wants a nice smooth flow. Mine is like that - and works a dream! Can't see it would be any different if I fitted an HIF 44 - the same air will be flowing in the same path...... I suppose if really worried about this - the carb exit (after the throttle) could be tapered out from 38 to 44mm. I'm sure the effect if minimal.

[IslipMinor]

but even Vizard suggests it's ok to step UP

Yes, like everything - in moderation! The diagram in his book shows a very small increase in port diameter between manifold and head, and he says it can be between 1/32" and 1/16" in diameter difference, that is just 6% on average, and he also suggests combining it with a flowed entry into the larger diameter.

The whole emphasis in his book is about smooth flow, not a cross-sectional step change in area of more than 35%! I very much doubt he would subscribe to that.

[chrisryder]

Does the step change not blend itself out anyway? Ok you'd get 'eddies' around the edge, but would that not just help with fuel suspension?

[bmcecosse]

The point is - it's a fragmented flow anyway - with the stream diverting from side to side every rev of the engine.... the carb is just running into a chamber at this point - and the stream starts there to left and right. But if I ever get the time - I'll try filing out my HIF 38 to 44 mm on the exit side - but I don't really expect it to make any detectable change to the power. On the exhaust side - some tuning firms use purposely stepped joints - the idea being to obstruct the reflected pulses in the stream without harming the positive going pulses. Very possibly the same idea will work in the inlet - although I hardly think my mildly modified engine is anywhere near 'pulse tuned'.........