2" carbs on a Mk9 engine

Is this worth doing, as a winter project in the future?
A somewhat better acceleration at take off would not hurt, in this heavy car.
Calls for the intake manifold from the 420, or open the existing holes. The immense air filter canister is not to my liking either. Black flex tube down to the front right corner of the engine “room” space, andvabthick ITG or similar filter seems better…

Two inches will only increase top end not low end grunt.

You need to increase the velocity or speed of the incoming congestion charge.

I’d change out to 1 5/8 or 1 3/4 units with a transition adapter.

I’ve had two inch triples on my IX, those are getting replaced with a smaller set.

That will get you low end grunt.

New filter assembly will mean a jetting change likely.

I agree on the improvement at the higher rpm range, esp with Webers, which I donˋ t want. But I read about a long time owner of a Mk9 who changed from the stock 1,75" carbs, to the larger 2" SUs, and that gave him much better acc at lower revs.
I know about the need for different needles with foam filters.

I think the word on that would be anecdotal!

Peruse ANY book dealing with intakes and carburetion, as the science is the same.

Low end will suffer with a drop in velocity or the speed of the charge going to the combustion chamber.

The 2" carbs, will most definitely LOWER the velocity or the speed of that charge.

You can’t change physics, but it’s your car.

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And don’t forget there is something about the viscosity of the oil in the carb dampeners that has an impact on initial acceleration. I think it is thinner oil that allows the piston and needle to jump up faster aiding acceleration. .

Quite right John,
And last week I read a good thread here in the archives about finding an oil which corresponds to the SAE20 of the 50s and 60s.
Itś important to get the right Viscosity Index at low temperatures, say 10-40 deg C. The forum member listed some alternatives from Castrol, Shell and Mobil, but they are odd industrial oils sold in 20 litre “bottles”.
I took a picture of the page, but lost the name of the author and the title but can be found here somewhere. 20-50 engine oil has too high viscosity, and might be what I have in the SUs on the Mk9. Easily changed though.
Peder

3 in 1 oil is about equivalent to sae 20, its what I use in the ser 11

Thicker oil slows piston/needle llilft, thereby causing the vacuum to suck proportionately more fuel through the jet. This gives a slight ‘accelerator pump’ effect.

SAE 20Wxx multigrade engine oil is fine.

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A long time ago, a Triumph racer detailed some changes to help acceleration:
Use “Marvel Mystery oil” in the Carb, which is much thinner and allows the Piston to react quicker.
Shorten the Carb Piston spring- I did that by tying up several turns with very thin wire so not to bind up in the travel. The change will lesson pressure needed to actuate th e Piston.

He also recommended “shaving” the needles (with Sandpaper in a Drill) so the maximum fuel delivery comes faster. I did it on my V12 E Type and it seemed to work, but unsure about two carb engines.

Greetings All,

“Shaving needles” on the SU will be interesting and possibly counterproductive as the SU’s needles are divided into segments. The taper of the needle corresponds to the throttle opening position. The segments make up the throttle opening range.

There is a reason SU’s have over 300 different needles.

If you were to “shave” the needle uniformly, you would flow more fuel through the entire range. Doing that would mean if you had a lean area in one range, you improved that range while making the rest of the range overly rich.

I use multiple O2 sensors to measure carb mixture and select needles. I’ve a set of manifolds with ports that were brazed in place to allow the sensors to be run and gather needed data.

While I’ve made some changes, none have been radically different from the original factory offering.

Agree that’s true for non-CD type carbies. But with SUs, how does the engine (at low speed) know that it’s running on, say, three 2 inch SUs with the pistons down versus two 1.5 (or 1.75) inch SUs with the pistons up?

At low speed the pistons in both manifold set-ups are down, you are running through the idle circuit, why do you ask?

Interesting question…are you implying that because SU are constant velocity that will “smooth out” any differences?

Both Hammill, and another Jag engine builder/author clearly state that “overcarbing” will only be beneficial at high rpm, AND at the cost of low-end tractability and idle stability

That would be based on real experience, so is likely to be correct

I can double-check Hammills books, but I thought it was to do with (simplify) turbulence, which probably would be affected by the opening cross-section, mixture density etc etc, which would be different for each carb setup regards piston rise,

afaic they are substantially raised up @ 1500-2000rpm

all genuine interest questions

Hi Tony! I agree that what Hammill says, particularly about Jaguars and OE Jaguar carbs is correct. I was speaking more theoretically (as I have a wont to do) about the relative influence of carb size on non-flat-out performance compared between conventional and CV carbs.

IMHO, with ordinary carbs, size is very important and to get more high end performance requires progressive linkage of some sort; otherwise low end performance would suffer. I had thought that this is far less an issue with CV. For example, Jaguar itself utilized as little as a pair of 1 3/4 inch carbs and as much as three 2 inch carbs on 4.2 liter engines. That’s a factor of two in choke area.

Ordinarily, the venturi area is a fixed fraction of the choke area, and opening the throttle creates more depression, which sucks more petrol through a fixed jet. With CV, the venturi area changes and the depression remains constant while the jet area increases. The venturi area is not directly related to choke area (carb “size”). In principle it would seem that one could run a lawnmower on an HD8, but Hammil’s empirical point is that this wouldn’t be optimum.

I agree that turbulence and similar factors are important in sizing the entire intake system–including carb chokes and intake manifold. That influences carb size choice as well.

Agreed, an interesting topic…I’d like to hear from others.

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How would progressive linkage work?

Wouldn’t you would be starving one bank of cylinders as the three or two cylinders, closest to the carb that first opens would get most of the combustion charge because of the distance?

The others would starve, either way, it would not be equally distributed.

Either way, velocity of the combustion charge would be affected.

Interesting comments, I wonder what was the reasoning behind 2" SU’s on the S1 XJ6 2.8? A 2.8 needs all the low down grunt it can get although it does have a 6K redline.

It’s not ideal for the reason you state. Often, the primary and secondary carbs are part of the same casting. Particularly with downdraft “log” designs (or V8s) the unit simply replaces a single carb already there. If the carb castings are separate, the primary (or more than one) is in the best location and the secondary(s) somewhere else. On flat six engines, there is an obvious choice on each bank due to the reversal of intake flow that occurs during the firing order. On inline sixes, “log” manifolds with runners can accommodate a primary-secondary pair by design.

Anyway, the idea is you have a small carb at part throttle and a big one at full throttle. You can design the secondary so it opens slowly to prevent stalling when you goose it at low revs.

BTW, because of the variable venturi principle, a pair of 2" SUs seems OK for a 2.8 (as does 3 for a 3.8) but, according to owners, the factory fitment of twin 2" SUs on the Rover 2.0 is a little too much.

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I’ve often thought the same: had I easy access to s pair of 1.75” SUs, Id give’em a whirl on Margaret, and see what/if any improvement there’d be.

My only data point is having briefly owned an SC, which had a single HS6, on s log manifold, and though more sedate, it seemed to have a bit more “zizz” at lower rpms.

Hahaha. Certainly glad you agree, Paul, since you’re the one who told me.

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Hehehe: I could be fulla…merde!

The Rover engine is * not* a turner, more of a torquer. The pistons/rods are heavier, by a LOT (Heren chamber, flat head) than a Jag: the bottom end could do well as a diesel engine!

It really struggles to reach 5500 rpm, and that is where…I presume…any bennies of 2 2-inch carbs would appear.

It wouldnt take a huge amount of work to whip up a set-up, with something like HIF6s.