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Useable vs peak power?

Mike R

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Dear Triumphants,

Like many I like the idea of a little more oomph in my GT6, but then I rarely go over 3000rpm and never over 4000rpm. So this is a little improvement I made a few years ago to hopefully increase power in the lower rev range.

The concept was to fit the weaker stromberg dashpot springs combined with richer needles. Before anyone rushes to condemn, read on, and see what you think.

The thinking is that at peak power 5500 to 6000 rpm, 105bhp the carb air valve will be right at the top of its stroke and give little or no obstruction to induction air flow. However, at 3000rpm, power is more like 60bhp. With power around 60% of peak, induction air flow will also be significantly lower and so the air valve will sit significantly lower in the carb. The air valve therefore becomes a restriction when at full throttle at lower revs. See sketch below.

The air valve is lifted by suction from the air as it flows over the carb bridge, which is balanced by downward force provided by the weight of the valve plus the force of the spring. So weaker spring reduces the downward force, and so the valve sits higher.

Now to change the needles and get it right across the rev range needs ability to test under load. This I achieved by installing an exhaust lambda sensor with a gauge in the cabin so that I can measure air fuel ratio as I drive.  (See Buckeye triumphs website for excellent description of how to do this )

So I bought and fitted the weaker springs. This made the mixture so weak that the engine would not run. This is good because it shows it’s had the desired effect of the air valve sitting higher.

I’ve heard it said that Strombergs have few needles available, but that wasn’t my experience. Burlen produce a little booklet for a few quid that lists about 30 odd needle profiles for CD150 carbs and detail the precise needle diameter profile for each. It took me a couple of trial and error attempts to get the right needles utilising the Lambda sensor and testing on the road, but I got there. It may be a bit of wishful thinking, but she did feel a little more responsive.

To sum up ....

* The air valve at lower rpm will be a restriction in the induction system to some extent (although not sure how much)

* Weaker carb springs has definitely made the air valve sit higher for any given engine load and therefore be less of a restriction (how much less is more debatable)

* Changing the needles has increased the fuel flow so that the mixture is correct for the new air valve position across the rev range.

* Because the air valve sits higher, the carbs will max out at lower revs that a standard GT6, so my improvement in usable power is at the expense of peak power.

As most power improvements are focussed on peak power, I’ve never seen anything like this suggested. I’d be interested in anyones thoughts.



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Novel idea!

It would be interesting to have a 'before and after' rolling road comparison. Is the engine really sucking hard enough at 3-4,000 rpm for the air valve piston to be a significant obstruction?

I would be concerned about the consequences of running the engine at full throttle above the self-imposed rev limit of 4,000 rpm. At higher revs with the engine drawing more air, the metering needle would already be at the top of its travel with no possibility to increase fuel flow. I expect the mixture would go very weak and combustion temp could increase enough to damage the pistons or exhaust valves.

Just my thoughts...


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Well, interesting idea and I follow your chain of logic.  However, I question your assertion that the dashpot piston really causes a significant obstruction to airflow.  Point being that the airflow rate through the carbs is related mostly to rpm.

At wide open throttle (WOT) a 2L engine "pumps" 1L of air every revolution.  At 6000 rpm that is 6000 l/min or 100l/s.  We know that the carb is capable of that much in standard calibration and that the piston is pretty much at the top of it's travel at that point.  The original factory calibration is good (as you would hope) and the CD150 is a good size match for the Triumph 2L .  At 3000 rpm you halve the airflow requirement to 3000 l/min (50 l/s), so the piston needs to be no more than half way up.  In the same way, the last "half" of the throttle opening brings no benefit as the engine can already draw all the air it wants.  The other side to this is that inlet flow velocity matters.  On some engines at lower rpms in a fairly high gear, as you add throttle you will feel an increase in urge up to say (1/3rd throttle), but when more throttle is added without waiting for the rpms to rise not only do you get no extra urge (though maybe some more noise) you may actually get a little less.

I agree with Nigel that a back to back rolling road comparison is the only sure way to confirm.  Unlike Nigel I think it will go rich once you've maxed out the piston travel.  Certainly this is what happens when you "run out of carb" on a tuned engine.  The thing to bear in mind is that way that a CD carb works (including SUs) is to balance the jet size  - that is the size of the hole left by the fit of the needle in the jet - with the vacuum above the jet, which between them determine the flow of fuel through it.  By weakening the spring you reduce the vacuum above the jet reducing fuel flow - so have to use a needle with a smaller cross section to compensate.

So, you may have sightly reduced pumping losses.  Any minor gains here could well be offset by velocity losses.  It probably does feel different - more induction noise, possibly responding to initial throttle more eagerly.  Congratulations are due for getting the carb back in calibration having altered it's operating range - not a particularly easy thing to do, however, I doubt steady state torque is much affected - but hard to demonstrate without a RR session.  How about fuel economy?  Do you have enough mileage to compare?

One thing I have to question though......

A good Triumph 2L 6 should be sweet and free-revving.  Keeping below 4000 rpm, let alone 3000 rpm should be chore.  I know the later Mk3 engines with their domed pistons and "emissions" cam are said to be less keen on revs, but certainly all the mk2 based ones (and tuned beyond) have been just getting into their stride by 3000 rpm.  Best way to get a bit more urge is to use more of the rev range - or, if low down grunt is really your thing -  you need to go 2.5.😊


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The Stromberg - or SU  - carburettors are "constant depression" devices, in which the flow through the throat is 'sensed' by the piston, in the way you describe, so that the needle attached to the piston opens the jet to the requred amount to meter in the fuel so that for any flow the result is a stoichiometric mixture.     The piston is not a restriction to the air flow, it is the (fixed) throat that is the restriction and as flow changes the piston moves to cause the same reduction in pressure (depression, by Bernouille's law) at any flow.     

A far greater restriction to the air flow is the throttle butterfly - that's it's job!    Alternatives usually seek to minimise this at Wide Open Throttle, by smoothing the profile of the butterfly and spindle, or using slide or barrel throttles.     When not at WOT, any restriction to flow caused b y the throat and piston will be less than that of the butterfly.

It may be that you can flatten the power curve in this way, giving more power at lower revs at the expence of top end.   But I doubt it and you will have to produce some dyno work to prove it!

If you really want more torque at lower revs, the easiest route is a 2.5L engine, that can't reliably rev over 6K anyway!



PS Edited later!  Wow!  Nick and I must have been in quantum entanglement without knowing it!

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Thanks all, interesting discussion.

Totally agree the only way to really know is the rolling road.

The only thing that doesn’t quite make sense to me is why would the throttle be less restriction than the air valve? At full throttle the throttle is a thin disc parallel to flow obscuring maybe 5% of the bore, whereas the air valve is a fat piece of metal slap bang in the middle of the air flow obscuring maybe 40% of the bore. The only thing is the air valve has smooth transition in and out so you get pressure recovery (as per Bernoulli) which you don’t get with the throttle butterfly.

So, yes it may give very little, but it can only help, and was more about me tinkering than anything else!



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as its the air flow and depression over the jet bridge and under the air piston  that draws the fuel and atomizes , then  raising the air piston reduces the fuel supply and fitting a revised needle or even the jet surely brings you back to where you started yes the   area of piston has reduced as you planned  but its a lot of work to gain .....what in real terms is  very little 

and from my factory days  on emissions    my  dyno went to china when the emmision dept closed down in 93  otherwise we could have had a good play 


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I get the feeling all you have done is upset the balance of the carb (between the piston and the butterfly) so probably hasn't helped and may have made things worse. put simply, piston higher, more air so more fuel needed, so you now need less throttle. 

The reality is that you want good AFR at any revs and throttle position (maybe 15 at light cruise, 12.5 foot to the floor, that sort of thing) and therefore if the AFR is correct that is as good as you can adjust the carbs. The job of the carbs is purely to supply the right amount of fuel for the amount of air (and that does vary according to throttle position/load)

Sounds like you are trying to achieve more mid range torque. That would be best done by adjustments to timing curves (mappable ignition ideal and accurate), and also camshaft choice.

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The fundamental problem is that you think you have a limitation (OK, you say "restriction" but limit is what you're thinking) that simply isn't there. The carb can flow enough air for 6000 RPM full chat, as it is, without any meddling. At 3000 RPM the maximum air flow needed is much less than this. The carb will happily flow it.

Ah yes, you say, but it doesn't flow it without a pressure drop!

Well, no. The laws of physics demand that there is a pressure drop, otherwise there's no flow. The design of a carburettor requires a pressure drop otherwise there's no fuel flow. The drop in question is minimal - too small for you to notice and probably too small for a dyno to reliably measure the effect. As Clive says, there are much easier ways to gain 1%

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