The V8 Owners Forum -

Ever since we moved to a 4.6 V8 and running it with twin SU's we have been having lots of problems with the engine and think it might be under fueling. We have changed the needles to some 'special' ones and still have the same problems.

Basically there is a lot of power a low speeds and grunt when needed but at mid and high speeds there's nothing. The engine is kind of gagging like when nearly out of fuel.

What do people on here think could be the problem(s)?

Should we alter anything else on the SU's???

Also, we are running a red top Facet pump (7 PSI) and have been playing around running it through a regulator and not. Nothing seems to alter. What PSI should the regulator be set at?

Cheers

Just a thought but guys take SU's of 3.5 because they think it holds the engine back so it could be they just arent big enough. Personally I would say they are borderline. They will make a low revving high torque engine but if you havent changed the cam for that then you could have a bit of a miss-match.

Pull the choke out and see if it tuns better. If it does your arent getting enough fuel if its worse your getting enough or too much. I would think about getting a wideband or something to tune with that way you will know exactly where you are.

Hi,

Pulling the Choke down the straights whilst racing makes no difference to the engine. We do have a nice CAM in the engine for extra torque and it seems to be working.

What would the wideband tell us and where would i get one from?

Cheers

Wideband will tell you what your mixture is at very accurately so you will know if its too much fuel or not enough.

Just out of interest what is this engine in and what racing is it. Racing and straights generally don't go together with SU's on a 4.6. What RPM are you meaning when you say low mid and high? Last but not least what cam is it?

Right.......

The 4.6 is in a Land Rover 88" Comp Safari Racer. Belting around forests, etc. but the odd straight / field.

When in 1st and 2nd mid range (of the 3 speed Jag Borg Warner 66 Auto Box) it seems fine. It seems to be when we finally get the chance of a nice field, nothing there. Not got a rev counter in the car, sorry.

It's a DJE V8 cam that would 'pull trees' and it does. So much grunt when pulling away and out of corners.

Cheers

Hi
got any timing specs for the cam? can you post them? been on the dje website and can't find anything but the price (now there is a suprise ).
Is it a short duration high lift jobby (sounds like it)? If it is a very short duration cam it could be all over by 4500 to 5000 revs.
What are the pistons on the SUs doing, when do they reach the top of their travel? because if it is fully raised at 3500 revs then you will not alter the mixture with the needle after that and it will just be metered on the main jet size. Have you changed the springs when you put the carbs on the 4.6?
The carbs and manifold should see you through to 5000 revs, though they will be restricting things a bit after about 4000
I would get a wideband lambda as Markie says and a revcounter then you should have a good chance of setting it up.
sorry lots of questions and thin on answers.
best regards
Mike

LC1 can log the RPM anyway, if i recall. Might need someone to cushion the laptop though.

What's an LC1 and where would I get a wideband lambda from?

It's a high lift cam, but I'll have to find out more details about it.

I'll have a look at the pistons on the SU's, but that does sound like where the problem could be. We've not changed the springs, just the needles. What effect would changing the springs have? What do we change them for/to?

Cheers

The springs control the venturi size and therefore the velocity of the air through the carb with revs (trying to keep air velocity constant). They also control the metering of the fuel by regulating which part of the needle is in the top top jet at a given revs. When the needle is as far out the jet as it can be then the air fuel ratio is dependent of air velocity (which has to rise as the engine demands more fuel, at this point the the higher velocity causes the mixture to richen with the more demand for air.
What you aim for with spring set up on an SU is for the piston to reach the top of its travel about 500 revs before max rpm and let it richen the last bit on the main jet, (dash pot out) this in theory has the maximum venturi size at max power rpm and then it richens as the engine goes "over the top" so no risk of a lean mixture. You then choose a needle that will ajust the mixture to match the AFR curve on full throttle as the revs rise (and fine tweek it with needle height ajustment). In theory an SU can be adapted to a wide range of engine sizes and set ups.
The limmitation is that on a bigger engine the stiffer spring restricts the air flow all the way through the rev range, but in mid range and part throttle they can be set up to match the engine very well.
With your set up I suspect the carbs you have, with the spring you have, are maxing out at about 3500 to 400 revs, giving you good controle up to the top of the mid range then giving you a very rich mixture after that hence it goes like a rocket up to about 500 revs beyond the point of max lift them the powre dies as the mixture becomes richer.
You have two choices, change the springs and you should be able to set it up most of the way through the rev range, though it will restrict a little more, but you will be able to get it right at or close to max power rpm.
Or go for a bigger SU, which can be done on that manifold, the 2" of a jag can be fitted, you have tobore out the manifold a bit and make up a linkage (or adapt the one off the 1.75"s you have). The bigger carb I think has a smaller range of needles off the shelf but will probably give you better range of controle through the rev range and certainly much more controle at max power rpm with much less restriction.
Either way you will need an LC1 ( I think Phill ringwood who markets megasquirt extra does them) or this guy sell them websales@diyautotune.com I have bought stuff off him in the past, he is in the states, he takes paypal payments and dispatches very quickly and was a really good bloke to deal with.
I hope this epic is of some use
Best regards
Mike