just a random any brand/car type of question
if i was to bolt a 1400 bottom end in place of a 1300 ....is it likely the standard carb could handle the approx 7% extra capacity?
just need to know if its highly likely or very unlikely?
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Unless the carb was already restrictive at max air-flow; it should be OK-ish. Most carbs operate well under their full capacity.
The fuelling wouldn't be optimal; but probably workable. Re-jetting/re-choking would be needed for exact matching.
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It would work reasonably well;the only time any restriction would be noticed is at full load and high revs. and you won't be at that setting very often.
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it would be the same as it is the conrods that are different lengths rather than the throw of the crank ;-o
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bell boy im not understanding what your getting at
the fact the engine is 100cc bigger would demand more air and fuel regardless of how the extra capacity is achived
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If you are achieving the increase just by bolting on another standard bottom end, that implies that the larger model already exists as an option. The easy thing would just be to swap the appropriate carb at the same time, or get a new needle or jet per the spec of the larger variant.
There might be other minor variations too - timing, distibutor advance, etc. Or the head or inlet valve size might be different.
Often over time all these bits commonly get mixed and matched, as people just pull bits from scrap yards that appear to be from the same basic model. It usually doesn't seem to matter much.
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steveo a lot of engines get their different capacities from different length conrods or smaller/bigger pistons,i therefore gave you the conundrum that your crank change may well make no difference
see.......
clear as mud ;-)
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fwiw the later 1400 injection models bottom end would bolt up to my head carb etc...the later head wouldnt have a fuel pump drive and i cant be bothered bolting on inj gear and the wiring
the head surface is dead flat with no chambers so it should be a genuine increase
but anyway....so theres good chance the carb may work well?
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Steveo - if you need any SU carb info I live 20o yards from a specialist in Su and Stromberg carbs, assuming that is what you have got. E mail me if you need any help or info!
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Sorry to disagree bb but conrod length doesn't alter cubic capacity or 'swept volume' as it used to be called... conrods can be 50 ft long but swept volume will be the same with the same crankshaft. Of course bigger bore pistons will also increase capacity... but longer conrods, no.
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but but lud if the conrods are shorter then the pistons dont go up as far and you have a low compression engine instead of a high compression one
im thinking ford here by the way fiesta mk 2 comes to mind
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Not sure of the car either but can I assume its a single point injection system rather than a carb (I know they are virtually the same).
If engine variant is available in 1300 and 1400 then chances are its the same spi anyway and likely to be swept area/piston area for difference in cc and so would likely be a straight forward swap.
But more details would deliver more accuracy.
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I was thinking maybe CVH.
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Capacity (cc) is a function of bore and stroke. Theoretically speaking, if a longer throw crank is fitted, capacity increases. However, if there's not enough clearance, the pistons will hit the head.
Perhaps this is where the confusion's arising?
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Perhaps this is where the confusion's arising?
No. The important thing, the only thing in cubic capacity bb, is 'swept volume' - the volume of the cylinders swept by the pistons between top dead centre and bottom. The space in the cylinder head not swept by the pistons isn't counted.
By fitting slightly longer conrods you move the pistons further up the bores and increase the compression ratio, by effectively making the space in the cylinder head at top dead centre smaller. But the pistons move up and down the same distance provided the crankshaft throw remains the same, so the capacity remains the same.
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As Lud points out, putting a 1300 head on a 1400 block would increase compression ratio, so that would cause the motor to pink.
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Unless it was an engine with the combustion chamber in the piston,of course.
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On the other hand:
Many years ago, I fitted a 250cc cylinder and piston to a 200cc Villiers engine.
I was too young to figure out the bore and stroke, but it didn't matter.
The piston hit the head.
The head was modified to correct this.
The vibration loosened the mixture screw, which entered the inlet port and buffered (one letter out) the piston.
This was repaired, and the bottom end gave out!
The lesson learned:
An awful lot of R & D goes into engines, YOU (PROBABLY) CAN'T IMPROVE ON IT.
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The basic confusion here, as Lud has pointed out, is between cubic capacity (swept volume) and compression ratio.
The two are inter-related however. eg if you increase the stroke, but alter the piston design so that it stops at the same point at TDC, then by compressing a larger volume of gas into the same space, you are increasing the CR.
That change might well occasion a need for other alterations other than simply a larger carburettor needle. But in the example given, the increase in volume is not very great, and we don't know what changes are being made to the pistons. The longer crank throw might for example be accompanied by shorter dished pistons.
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Indeed Cliff,
Compression ration = 1 + (swept volume / clearance volume)
swept volume = pi*(bore^2)*stroke/4
cubic capacity = number of cylinders * swept volume
Number_Cruncher
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Unless it was an engine with the combustion chamber in the piston of course.
Correct, I misread the op's last post, he did say the head was flat, so the chambers will be in the pistons.
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If it's a single barrel carb, it'll be a compromise between power and torque/economy anyway.
When a single barrel carb is replaced by a differential twin choke card, eg Weber, the primary barrel is usually much smaller to acheive better torque/economy with the secondary barrel only being used to provide extra air flow at wide throttle openings.
Given how imprecise a carb is anyway, compared to multi-point fuel injection, I doubt you'd notice any difference.
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