Carbies on a 4.6 V8

[garrycol]

I building up a 4.6 from a 4.0 block, 4.6 crank and rods and 4.0 low compression pistons to replace a 3.5 I have. Initially I intent to put the 3.5 manifold and CD175 carbies on the engine until I work out what induction system I want to use in the longer term.

So does anyone know what size jets and needles I need to go to for the 4.6. As each carby has to run 2.3litres which is not much less than a 6 cylinder Series engine I thought the needles and jets that go into the 6 cylinder carby might be the right size to use in the V8 carbies.

Thanks

Garry

[greenmeanie]

WHen I redid the engine (Stock 3.5 block, post 76 heads for bigger valves, new cam and RR pistons for a bump in compression) in my 101 I looked at fueling. What I dound is that the neeedle choice for the ZS 175 carbs is pretty poor and the other options avaiable were just as bad as the existing 2AY but maybe in other directions. I modified the dash pots to use SU needles shortened by 1/4" which have a huge range allowing for the tuning you need. Incidently using the SU needle means they are far more tolerant of misalignment which tends to create sticky needles in the Zeniths.

Having done that I borrowed a wide band O2 sensor and took the truck for a few runs to go through the rev and load range for the engine and using the lambda values could see where I needed more or less fuel. I matched this to the table of needles in the Haynes book on ZS/SU carbs and bracketed my needs from there.

All this is to point out that if you want to do it properly you can't just drop a needle in and hope. Its a bit of a science experiment and you'd be better getting over to the SU needle range to give the options you need to get it right.

[garrycol]

Thanks for that information - a great response - I was hoping for first hand experiences.

Cheers

Garry

[mickeyw]

Not trying to discourage you Gary, I am just curious as to the availability of carb needles these days.

I have messed around with such carb related matters in the past, but not to the level of science that greenmeanie mentions.

I am also glad to see the first response was not one rubbishing carbs for newer tech. I hope you find a carb based solution.

Another thought - didn't old Jag V12s use big SUs not dissimilar to the ones on a V8, but perhaps with different needles and bigger diameter maybe?

[garrycol]

Not trying to discourage you Gary, I am just curious as to the availability of carb needles these days.

I am also glad to see the first response was not one rubbishing carbs for newer tech. I hope you find a carb based solution.

Another thought - didn't old Jag V12s use big SUs not dissimilar to the ones on a V8, but perhaps with different needles and bigger diameter maybe?

I have no idea about availability of needles etc. As suggested there are alternatives though and the "experts" can do wonders - or so I am told.

After I have the engine up and running and run in - I will be looking at injection but I prefer to do one thing at a time - I know carbs and I know the engine will run with them straight off - if I went the injection route now it would delay having the engine up and running as there would be more to sort.

The Jag V12 used the same Zenith CD175 carbs as the Rover V8 used. I have a Jag v12 with them and they have been untouched for over 30 years and at times the car has been laid up for up to 8 years in one stretch. It is testiment to these carbs that I was able to start after 8 years and the engine settled to a smooth idle with full rev range and no missing or lumpiness. Once set up and dashpots full they tend to stay in tune and stay working.

Garry

[Soren Frimodt]

Theoretically, you shouldn't have to do anything to your carbs. The placement which sucks the air in doesn't affect the mixture. The mixture is relative to the air that can pass through the carbs venturi (or airbox), and since you are not changing this, you should be good to go. But of course sometimes theory doesn't wanna dance with reality, and a number of things plays in on the mixture needed. I would build the engine and fit them, run it and see how she does. If you are afraid of running too lean, fit a lamdba sensor and sleep comfortably

[Tanuki]

Theoretically, you shouldn't have to do anything to your carbs. The placement which sucks the air in doesn't affect the mixture. The mixture is relative to the air that can pass through the carbs venturi (or airbox), and since you are not changing this, you should be good to go. But of course sometimes theory doesn't wanna dance with reality, and a number of things plays in on the mixture needed. I would build the engine and fit them, run it and see how she does. If you are afraid of running too lean, fit a lamdba sensor and sleep comfortably

There are 2 basic things that affect how a carb works: the velocity of the air through the carb, and the degree to which the air is flowing as a continuous stream or whether it has peaks and troughs and pulsations.

Usually the needles of a "CD" carb [stromberg or SU] are specified so that at small/medium throttle openings and low gas-flows the mixture is on the lean side (for cruise economy) but at wider throttle openings/flow-rates the mixture should go richer (to provide increased power and also so that the extra fuel can reduce the combustion temperatures somewhat).

The "pulsation" issue is that air in the intake-tract can accelerate and decellerate more rapidly than fuel in the jet-mechanism: so when each cylinder 'sucks' initially it will start getting a weak mixture then the fuel will catvch up, and as the cylinder stops sucking the air-flow will slow down but the fuel-flow will 'overshoot' a bit. The precise effect of this is difficult to predict - it's generally more an issue with one-carb=barrel-per-cylinder engines.

Going from a 3.5 to a 4.6 will suck more and so push the fuelling and air-flow more towards what you'd have got on the 3.5 at fairly large air-flows/throttle-openings. You may find that at smaller throttle openings it runs lean if you've set it for correct mixture at wider throttle openings.

The other thing to consider is total fuel-flow capability: if the throttles are wide-open and the diaphragms/needles have been sucked to the top of their travel so the needles are as far out of the jets as possible, this sets an absolute limit on the fuel-flow. But if the engine is now sucking a 4.6-litre lungful of air rather than a 3.5-litre one the available fuel will be distributed through a larger volume of air - i.e. the mixture will be lean just at the worst possible time! It'll burn hot and this can easily lead to burned pistns/valves/cremated spark-plugs if operated like this for a significant time.

The Germans have a lovely word - Vollgasfest - to describe an engine that can be run sustainedly at full-throttle/peak-power-RPM without harm. It's under these conditions that any fuelling issues will be brought to the fore. I'd suggest that if you do run 3.5 carbs on a 4.6 you check that the mixture is correct at the upper end of the revs/throttle-opening/power scale if you want the engine to have a long and happy life.

[garrycol]

Thats for those last lot of comments - the points about cruise on the 4.6 being the same as the 3.5 at high power is also back on my mind - hence if I set up the carbs the same way as a Landrover Series 3 6 cylinder carb I should be right for an initial setup. The single CD175 on the 6 cylinder is dealing with 2.6 litres (with each suck being 433cc) - each carb on the 4.6 V8 is dealing with 2.3litres (but each suck is 575cc). So while each suck is greater with the V8, overall air volume going if is less for equivalent revs - though the V8 has about 600 more revs at peak power.

So I would have thought that if the carbs are setup the same as the single 2.6litre carb that should be a good start.

Any views on this hypothesis?

Thanks

Garry

[Soren Frimodt]

I respect what you guys say, but as the flow of the 3,5 head, manifold and airbox is not improved in any way, it will not alter the mixture needed! The thing being that these restrictions are so massive that the cylinder simply will not get filled with neither air nor fuel, it will not use all its capacity because the flow is not big enough to let it fill on its travel.

Several years I've helped people tuning small 2- and 4- strokes, and you do not have to alter the mixture because you increase the bore, UNLESS you alter something on the inlet or exhaust side. But if yor previous cylinder/ piston was worn or you now have higher compression an adjustment can of course be necessary.

Just talking from my own experience, I could of course be wrong, but I would still just bolt it together and see how it goes. If you change too many things right off the gate, fine tuning can be a real pain

[Chicken Drumstick]

Not sure how much help I can be. But curious as to why you'd opt for low CR pistons in a 4.6? And on a similar note wouldn't twin carbs kinda strangle a 4.6? They are a fairly good limiting point on a 3.5

[mikec]

Why are you wanting to use the zeniths? On my 3.5 I changed the original zeniths to su's and noticed a difference straight away. Maybe they were just no good, dunno. I'd be interested how you get on with carbs on a 4.6

[greenmeanie]

For what its worth with SU here are a couple of topics from ove ron the V8 forum

http://www.v8forum.co.uk/forum/viewtopic.php?t=11201

http://www.v8forum.co.uk/forum/viewtopic.php?t=11140

They seem to imply that the 1.75 su carbs are not a big restriction on deveoping power from which I'd make the wild deduction that the ZS are not too bad either for a low tune engine like described here.

[garrycol]

Not sure how much help I can be. But curious as to why you'd opt for low CR pistons in a 4.6? And on a similar note wouldn't twin carbs kinda strangle a 4.6? They are a fairly good limiting point on a 3.5

Well I have 4.0 low compression pistons and 4.6 low compression pistons - if you put the 4.6 low compression pistons in you have a low compression engine but if you put 4,0 low compression pistons in a 4.6 you have a moderately hi compression engine - not as high as a high compression 4.6 which would require 98 octane fuel that may not be available in remote areas in Australia. The 4.6 with the 4.0 low compression pistons gives a compression ratio of about 9.1:1 which can use 95 octane which is more commonalty available. The bowl in the 4.0 pistons is much smaller than the 4.6 pistons.

The purpose of the thread is to try and determine to see how not to strangle the engine - one carb works on a 2.6 litre 6 so two should be Ok on a 4.6.

Why are you wanting to use the zeniths? On my 3.5 I changed the original zeniths to su's and noticed a difference straight away. Maybe they were just no good, dunno. I'd be interested how you get on with carbs on a 4.6

I have a fully serviceable 3.5 in my 101 so what to use the manifold and carbs to run in the engine and get it all running. Ultimately I want the engine to be petrol and LPG injected with dual ignition maps but I haven't sorted what I want for this as yet and I do not want to delay installing the engine until I sort all this which can be done later with the engine in place. Of course SU were installed on 3.5s before the Strombergs and there is not a lot of difference between the two carbs when properly set up and the Stombergs stay in tune where the SUs wonder.

For what its worth with SU here are a couple of topics from ove ron the V8 forum

http://www.v8forum.co.uk/forum/viewtopic.php?t=11201

http://www.v8forum.co.uk/forum/viewtopic.php?t=11140

They seem to imply that the 1.75 su carbs are not a big restriction on deveoping power from which I'd make the wild deduction that the ZS are not too bad either for a low tune engine like described here.

Certainly in Jags the SUs when set up correct are not producing much more power than correctly setup Strombergs. Sometimes the Strombergs get bagged but they were introduced when pollution gear was being fitted and poor performance issues were blamed on the stromies when it was the pollution gear.

Garry

[Bowie69]

I think the bigger restriction may be the inlet manifold.

There must be a reason why a 3.5 on carbs is 135BHP, but on injection it is 155BHP.

[mikec]

I think the bigger restriction may be the inlet manifold.

There must be a reason why a 3.5 on carbs is 135BHP, but on injection it is 155BHP.

One of the books say the standard carbs are good for 200bhp

[Chicken Drumstick]

I think it depends on what carbs and the rest of the setup. But remember a twin carb setup is nothing more than 1 carb per 4 cylinders. Even a little Mini Cooper runs better with 1 carb per 2 cylinders and most performance orientated Jags where 3 carbs to 6 cylinders.

When the RV8 was raced they used to use a 4 SU setup to get the power and performance from it. Not saying you can't have a nice running twin carb setup, but I'd have thought EFI or a 4 brl carb like an Edlebrock would just be a better bet.

[Chicken Drumstick]

One of the books say the standard carbs are good for 200bhp

The only thing here is what kind of 200bhp? (and how old a book) Not all bhp is equal. For instance the Buick 215 was rated by Buick @ 155hp GROSS. Which in today's ratings would likely have meant 110-120bhp SAE Net/DIN.

Somehow Rover changed almost nothing on the actual engine spec yet it amazingly made 160hp in the P5b and P6. I'm willing to wager this not a realistic figure, and that is is likely no better than the Buick GROSS hp claim, so 120 or under in today's ratings.

[mikec]

The only thing here is what kind of 200bhp? (and how old a book) Not all bhp is equal. For instance the Buick 215 was rated by Buick @ 155hp GROSS. Which in today's ratings would likely have meant 110-120bhp SAE Net/DIN.

Somehow Rover changed almost nothing on the actual engine spec yet it amazingly made 160hp in the P5b and P6. I'm willing to wager this not a realistic figure, and that is is likely no better than the Buick GROSS hp claim, so 120 or under in today's ratings.

I think it's one of the tuning rover v8 books. Though it could be the how to power tune rover v8 book ill check tonight

[FridgeFreezer]

Before everyone gets too far along the argument about the best bucket of clockwork to bolt to the engine, bear in mine the OP said it was only a stop-gap before he fits EFI later on. Suitably jetted twin carbs should be adequate, more than that is potentially a waste of time & effort & money.

[garrycol]

Before everyone gets too far along the argument about the best bucket of clockwork to bolt to the engine, bear in mine the OP said it was only a stop-gap before he fits EFI later on. Suitably jetted twin carbs should be adequate, more than that is potentially a waste of time & effort & money.

Thanks - a;also I might ad that I know a 4.6 will run fine on a 3.5 inlet manifold and carbies - it has been done many times - just trying to understand what they did to the carbs jet wise - and I thought someone on this forum may have already done it and provide the answer. Hasn't come yet but I welcome the discussion as it helps understand the issues better.

Also I may be not totally correct here but I thought the basic heads were the same for all the standard Landrover V8 engines irrespective of size (number of head bolts has changed) and as such I would have thought any restriction would have been in the inlet system in the head.

[greenmeanie]

The heads changed in '76 to have bigger valves so your 101, 3.5L has the smaller valves with more restriction and at some point they skimmed the heads or at least reduced the combustion chamber volume to account for the move to composite gaskets. After that you're about right.

If you go over to that V8 forum you'll find lots and lots of discussion on running 4.6L engines on carbs.

[FridgeFreezer]

What he said - the very early ones were different, by the mid-1980's it was kinda same-same with most stuff being EFI Range Rover, then with the 4.0 & 4.6 they skimmed the heads, deleted some head bolts, fitted better (composite) gaskets, cross-bolted the block, etc.

You can run with old heads, it doesn't make a huge difference, although that sentence has been known to kick off holy wars round here... obviously running the latest heads is preferable.

[garrycol]

The heads changed in '76 to have bigger valves so your 101, 3.5L has the smaller valves with more restriction and at some point they skimmed the heads or at least reduced the combustion chamber volume to account for the move to composite gaskets. After that you're about right.

If you go over to that V8 forum you'll find lots and lots of discussion on running 4.6L engines on carbs.

My 101 is a later model - early 78 so will have later heads - however using the standard 4.0/4.6 heads on the 4.6. I only looked at the links provided and have not had a wider look at the forum.

[deep]

Thanks - a;also I might ad that I know a 4.6 will run fine on a 3.5 inlet manifold and carbies - it has been done many times - just trying to understand what they did to the carbs jet wise - and I thought someone on this forum may have already done it and provide the answer. Hasn't come yet but I welcome the discussion as it helps understand the issues better.

Also I may be not totally correct here but I thought the basic heads were the same for all the standard Landrover V8 engines irrespective of size (number of head bolts has changed) and as such I would have thought any restriction would have been in the inlet system in the head.

I've read this discussion with mild interest as I've just put a carburettor setup on a 3.9 block (and can confirm, for that motor at least, it's a bolt-off/bolt-on job, with not too many hassles on the mixture front in terms of initial set-up). Before I went forward with the project, I did look at a friends 4.6 block which had the manifold removed and it is different in that there seems to be cut-aways in the inlet ports, I believe for the injectors (the 3.9 doesn't have the cut-aways). Clearly, the heads are different and the inlet tracts are different but I can't remember seeing anything that would actually stop the conversion.

As a starting point with my project, I enrichened the carburettors by one turn. It was a good guess - in the absence of better information I'd suggest you go out one and a half turns and work back from there. Mind you, as someone has pointed out, that won't work at high revs and full throttle when the jet will be restricting but, if you keep away from that scenario, you should have a workable short-term solution.

Don

[greenmeanie]

My 101 is a later model - early 78 so will have later heads - however using the standard 4.0/4.6 heads on the 4.6. I only looked at the links provided and have not had a wider look at the forum.

Other than a few prototypes and preprod trucks all 101s were built from 72 to 78 with major production occuring in 75-77. All the vehicles used the same pre76 design engine with the only difference being 12V vs 24V electrics. It may only have entered service in Australia in 78 but it was but the design and the engine specified for it was created back before 1972.