Do bigger carbs make more power???
Discussion
More air and more fuel into the engine often means more power, so bigger carbs can theoretically be helpful. Obviously "more fuel" means "higher fuel consumption" if the additional power is used.
Often just as much can be achieved by polishing inlet tracts etc to improve performance of the existing setup.
Often just as much can be achieved by polishing inlet tracts etc to improve performance of the existing setup.
No. In this instance you'd over-carb the engine as it's a small capacity unit. You'd be better off performance spec'ing the the existing carbs, upgrade to HIF4's or, if with deeper pockets, some carefully spec'd Webers. In all cases you'd need to flow head, head valve work and upgrade exhaust manifold to see any real performance increase. If not upgraded as a complete package you'll be throwing money away. You may like to read Des Hamill's book 'How to Build & Power Tune SU Carburettors', it gives a thorough overview of the brilliance that is the SU carburettor.
as stated, no it won't
best way to get performance is to deal with the basics first -
. full and proper regular servicing
. keeping the whole car (not just engine and carbs) in good running condition with parts and components in good working order
. have the whole car set up properly
. regularly using the car (year round) on reasonable length journeys
. driver taking some sort of additional driver training
best way to get performance is to deal with the basics first -
. full and proper regular servicing
. keeping the whole car (not just engine and carbs) in good running condition with parts and components in good working order
. have the whole car set up properly
. regularly using the car (year round) on reasonable length journeys
. driver taking some sort of additional driver training
He's right though. Did a track day with a pal in his Alfa Spider a good few years ago and our lap times were consistently seven seconds apart. Can you imagine how much you'd need to spend on your engine to pick up seven seconds over a one hundred second lap?
The only difference was that one of us had done a lot of that sort of thing, the other hadn't.
The only difference was that one of us had done a lot of that sort of thing, the other hadn't.
The carbs job is just to atomise the fuel so tiny drops of it are susspended and carried along by the air, and get that fuel/air mix in to the cylinders so there's something to squeeze and make go bang.
In general a smaller carb will more air faster, leading to better atimisation of the fuel and more of it making it in to the cylinder. It will start to struggle with higher RPM though and reach a point where it can't flow enough fuel/air to fill the cyclinder. So good general driving performance getting to a track day but clipping ultimate power out on track.
An over-sized carb will have no problems at the top end, easilly able to flow enough fuel/air to fill the cylinders even at max RPM but will struggle at part throttle/low RPM. Here the engine doesn't need much fuel/air to run and in the over-sized carb the air's actually moving quite slowly. This in turn means it doesn't pick the fuel up and atomise it very well and a lot of the fuel suspended in the air will 'fall out' and just create puddles of fuel in the inlet manifold. This means you're good for ultimate power on the track but it will run like a dog and waste loads of fuel getting there.
A correctly-sized carb will be in the middle so for general driving the air's moving quick-enough to pick up the fuel and get it in to the engine while also being big-enough to fill the cylinder at max RPM.
For the Triumph 1500 if you're talking about a road car then it's hard to beat a pair of HS4s, with a rolling road session to get the needles spot-on. These will be fine even up to the 'stage 3' level of tune (free-flow filters/hotter cam/tubular manifold/free-flow exhaust). If it's an out-and-out race car twin Webbers will be slightly better but will usually work out less-drivable on the road (unless you're willing to spends hours and £££ on a rolling road getting the jets spot-on) and drink a little more fuel.
Of if you want more torque and more power ditch the carbs and fit mappable electronic fuel injection.
In general a smaller carb will more air faster, leading to better atimisation of the fuel and more of it making it in to the cylinder. It will start to struggle with higher RPM though and reach a point where it can't flow enough fuel/air to fill the cyclinder. So good general driving performance getting to a track day but clipping ultimate power out on track.
An over-sized carb will have no problems at the top end, easilly able to flow enough fuel/air to fill the cylinders even at max RPM but will struggle at part throttle/low RPM. Here the engine doesn't need much fuel/air to run and in the over-sized carb the air's actually moving quite slowly. This in turn means it doesn't pick the fuel up and atomise it very well and a lot of the fuel suspended in the air will 'fall out' and just create puddles of fuel in the inlet manifold. This means you're good for ultimate power on the track but it will run like a dog and waste loads of fuel getting there.
A correctly-sized carb will be in the middle so for general driving the air's moving quick-enough to pick up the fuel and get it in to the engine while also being big-enough to fill the cylinder at max RPM.
For the Triumph 1500 if you're talking about a road car then it's hard to beat a pair of HS4s, with a rolling road session to get the needles spot-on. These will be fine even up to the 'stage 3' level of tune (free-flow filters/hotter cam/tubular manifold/free-flow exhaust). If it's an out-and-out race car twin Webbers will be slightly better but will usually work out less-drivable on the road (unless you're willing to spends hours and £££ on a rolling road getting the jets spot-on) and drink a little more fuel.
Of if you want more torque and more power ditch the carbs and fit mappable electronic fuel injection.
When I bought my Sunbeam Lotus it was running 48DHLA and hot cams, the rest of the engine was largely standard and badly assembled, it was giving just under 180bhp. When I sold it I had put the standard and smaller 45DHLA back on but had built a proper engine for it - higher CR, bigger valves, good balance etc - and it was now giving 210bhp, the new owner has subsequently found 6 more horses.
A carb is basically a venturi that meters fuel into it's stream, if you make the venturi bigger and keep the engine pulling the air through the carb the same it has to work harder to get the same effect. This usually manifests itself as a flat spot while the engine load increases until the carb starts to work properly, then suddenly the power arrives. This can be so sudden you think it's more powerful but it rarely is.
Money would be better spent getting the existing gas out of the engine, a more efficient exhaust manifold and system would be a better starting point.
eta. Beaten to it by a minute!
A carb is basically a venturi that meters fuel into it's stream, if you make the venturi bigger and keep the engine pulling the air through the carb the same it has to work harder to get the same effect. This usually manifests itself as a flat spot while the engine load increases until the carb starts to work properly, then suddenly the power arrives. This can be so sudden you think it's more powerful but it rarely is.
Money would be better spent getting the existing gas out of the engine, a more efficient exhaust manifold and system would be a better starting point.
eta. Beaten to it by a minute!
Edited by 72twink on Friday 6th February 09:52
thegreenhell said:
MaxRothery said:
Hi, I've got a Triumph 1500 TC with twin HS4 carbs, would swapping to HS6 carbs make it more powerful?
nta16 said:
best way to get performance is to deal with the basics first -
. driver taking some sort of additional driver training
lol. driver taking some sort of additional driver training
personally I'm not over keen on track driving for improvements but they do help, I'm more for road driving improvements as that's where most of us use our vehicles
most of us don't recognise how poor our driving techniques are and how much improvement there are in them
I've been on one course but am still a poor driver, luckily I have a slow car but it's still sometimes beyond my capabilities
nta16 said:
thegreenhell said:
MaxRothery said:
Hi, I've got a Triumph 1500 TC with twin HS4 carbs, would swapping to HS6 carbs make it more powerful?
nta16 said:
best way to get performance is to deal with the basics first -
. driver taking some sort of additional driver training
lol. driver taking some sort of additional driver training
personally I'm not over keen on track driving for improvements but they do help, I'm more for road driving improvements as that's where most of us use our vehicles
"Of course it would help most if he just lost a few pounds himself first"
yeap 
I used to put a good reduction to the power-to-weight ratio of the lighter cars I used to own
then of course there's the extra weight of carrying a spare wheel, jack and brace, all the spares that might ever be or have been required by every classic ever, the tools and so on ...
but better not get into that debate, again
I used to put a good reduction to the power-to-weight ratio of the lighter cars I used to own
then of course there's the extra weight of carrying a spare wheel, jack and brace, all the spares that might ever be or have been required by every classic ever, the tools and so on ...
but better not get into that debate, again
nta16 said:
as stated, no it won't
best way to get performance is to deal with the basics first -
. full and proper regular servicing
. keeping the whole car (not just engine and carbs) in good running condition with parts and components in good working order
. have the whole car set up properly
. regularly using the car (year round) on reasonable length journeys
. driver taking some sort of additional driver training
???????????????????????????????????????????????????????????????????????????????????????????????best way to get performance is to deal with the basics first -
. full and proper regular servicing
. keeping the whole car (not just engine and carbs) in good running condition with parts and components in good working order
. have the whole car set up properly
. regularly using the car (year round) on reasonable length journeys
. driver taking some sort of additional driver training
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