The V8 Owners Forum -

Hi everyone, this might sound a little boring in comparison to some subjects but here goes,

I have a landrover 3.5 V8 with twin SU's and i'm wondering if its better/ easier to stick with them or change to a 4 barrel holley carb, what are the pro's and con's???? It will be used on & offroad!

All advise will be a massive help.

People are quick to dismiss SU carbs but they can make good power- I think it's the inlet manifold that is the biggest downfall with them, with 4bbl there is a better choice of good inlet manifolds which tbh I think is where the difference is.

SU's may produce good torque, 4bbl's are generally better for higher revs and hp. You can put nice long air feeds on SU's in a 4x4 which will help torque, I'd bet the fuel economy is much better with SU's as well and they are much cheaper!

well thats a good enough reason for me to keep the SU's then (i like the caost effective option). I will just give them a good strip/clean and carry on using them.

Hi
I like the su aswell set up properly and in good condition they are a very tunable carb, you have far more accurate metering with one. Tom is right that the manifold is not what it could be, however it is just a 2 barrel US manifold with an adaptor cast on top so what do you expect!
Anyway back to the SU it will easily support a rover up to 210 (real) BHP as is, with a little tweeking to the carb and tidy up the manifold you can add another 15 to that, then you can bore out the manifold for 2" SUs off the jag and it will support about 240 to 250, but the manifold is really limmiting things here not the carb. You could probably adapt a modern 4 barrel design to SUs to good advantage .
You will get better fuel economy and smoother part throttle to full throttle transition with SUs and as tom says you can add tubing to boost bottom end torque.
Best regards
Mike

Wow, thanks for your input. i had thought that SU's were a little crap until i made this post but i think they will do me just fine. when you say tidying the inlet manifold is there a certain area i should be looking at ?? and when tom and yourself mention the tubing to improve torque is there a certain length or are we just talking of fitting a snorkel?? what are filter would you guys recommend, i was going to fit K&N cone filters ????


Really appreciate all this info

Hi
with regard cleaning up the manifold the best approach is to start by looking back into the ports for any casting flash and joint linesleft from manufacture and remove them with a file/ emery cloth, don't try and change the shape, just clean them up to a finnish given by 80 to 100 grit emery. From the top of the manifold look into the top bit where the carbs bolt on, there is a flat base for the carb, this is fiddly but can be done with care you want to stick a radius on the inside face of the 1 3/4 hole that leads into the manifold, about half to 3/4 the thickness of the metal, get this right it will give you 5-8% more flow into the manifold finally you want to match the manifold to the ports in the heads, the best way to do this is to make a template for the head ports using cornflake packet, locate the cornflake packet to the heads using a couple of studs and mark out the ports into the card using light taps with a ball pein hammer (round end) and cut it out with a craft knife. Mark up on the manifold with the template held acuratly relative to where the head will bolt to the heads and scribe this onto the manifold, use a file and emery to match the ports to the scribed lines. If you find the manifold is bigger than the ports in the heads, then match them in the same way with a card pattern. Either way take off the minimum to match everything up. don't be tempted to match everything using the gaskets, the ports end up too big and the flow suffers.on the carbs at the front of the piston mark a line 1/4" from the bottom edge. Now with the piston of the carb lift the piston so the line lines up with the top edge of the ventury and scribe a line around the front of the ventury onto the piston. on the bottom of the piston scribe a straight line across the piston to form a chord across the bottom betweeen these lines radius the front edge of the piston as evenly as you can using a file, you want a 1/4" radius at the cantre going to nothing at the sides. Keep checking as you do this that the guide peg at the front of the piston wont drop out of it's slot when the piston is fully lifted, you mau have to settle for slightly less than 1/4" at the front. don't be tempted to do anything else to the carb in the way of smoothing the internals, you really need to know alot about SUs before you touch anything in there!
Finally with the piston out of the carb and the carbs mounted with all their gaskets and the throttle plates open check there are steps from the back face of the carb into the manifold top, if there are smoothe them out with emery. If you have carbs with poppet valves on the throttle plates remove them and solder thin brass covers over the holes (or replace with plates without the valves). The final thing you can do is streamline the throttle shafts but be carefull take off too much and it will weeken the shafts and they will fail and knife edge the throttle plates to about half their original thickness, google or any book on cark tuning should give you pictures on what to do (same principles work on 4 barrels and webbers so hunt out articles for pictures.
K&N conicle filters are worse than the original set up with the u bends as they take in hot air from anove the exhaust and they retain the worst bit of the original set up, the adaptor into the carb, what you want is a short (about 3-5") tapered trumpet (have a look at the jag engined dragster engine here http://the-wild-bunch.co.uk/gallery.aspx?page=9 for an idea of what you are looking for) that is fitted into a closed box that is fed with cold air sron outside, if you cant get trumpets then a simple 3/8 ally plate with a 3/8 radius is better than the original and have that as bhe back of the air box, remember to drill be ballancing holes on either side or the carb won't work. as for length, trial and error, use 2" ally pipe from the box and see what gives you a boost where you want is. personally to start with I would use the standard set up with a clean new paper element and feed it with cold air.
Best regards
Mike

4 barrels are not very good off-road. The fuel sloshes around and floods the engine. A well maintained SU will be fine (if you must use carbs.. )