SU carbs on 4.6 RV8?

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ratwing
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SU carbs on 4.6 RV8?

Post by ratwing »

I've been looking for a 3.9 engine on ebay and noticed quite a few 4.6's at reasonable prices but they look too 'modern'.
I would keep the older/standard look by fitting the timing cover, sump, distributor etc off my p6 engine but what about the carbs - bearing in mind I'd be looking for more low and midrange power rather than more top end, would the 3.5 ones be ok or too small?



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topcatcustom
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Post by topcatcustom »

Heads, covers etc are all the same size, though if you fit an older front cover (I wouldn't) you will need an older style cam as the noses are different. A pair of SU carbs could easily be tuned to suit a 4.6, not sure about Strombergs, I think a single 2" SU carb can feed up to 200bhp and they work very well so don't get caught up on sizes too much!!
TC

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ChrisJC
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Post by ChrisJC »

You can definitely build up a 4.6 to look 'old', with carbs and a distributor. You need a spacer for the end of the crank, and that's about it!

I ran 1.75" SU's for a bit on my 4.6, but they seemed to strangle it at the top end. I switched to 2" SU's, which run fine right around to the red line. Manifold needed porting though.

Chris.
--
Series IIA 4.6 V8
R/R P38 4.6 V8
R/R L405 4.4 SDV8

ratwing
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Post by ratwing »

Thanks for the replies guys, I'll try the 1¾" carbs and see what happens.

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Post by kiwicar »

Hi
there ia a lot you can do to improve the flow through the 1.75" su all very cheaply, radius the leading edge of the piston, put the right spring in it for the 4.6, put some good trumpets on the front of the carb and match them to the of the carbs clean up the throttle plates and streamline the spindle, some people use only half the spindle and flatten the section, knife edge the throttle plates (get rid of any over run gubbins on the plates) then match the manifold to the carbs, radius the exit into the penta roof section of the manifold and clean up the casting flash.
Best regards
Mike
poppet valves rule!

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Post by ratwing »

Thanks for that, I'd already reckoned on having to change the needles but hadn't thought about the springs :oops:
The mods sound pretty straightforward but what size radius should I be aiming for on the pistons and do you radius the entire edge showing when the piston is down?

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Post by unstable load »

Somewhere in the P6 Rover forum is the skinny on how to do the mod to the SU's, maybe you will get luckier than I did with a search.
Cheers,
John

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Post by ratwing »

I'm on that forum too, I couldn't find anything either so tried here. I'll post a question and if anyone replies I'll add it here.

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Post by unstable load »

I think it was Chris York who posted the thread.
Cheers,
John

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Post by kiwicar »

Hi
the radius is normally about 1/4 inch in the center but that is determined by the tag thing that stops the piston rotating. on the bottom face of the piston you rule a chord across the face about 1/4 inch back at the center and hold the piston near the top of it's travel so again the bottom face is 1/4" down from the top and scribe another line around the throat of the carb and join the lines with a file to give a radius in one plane but do not let the tag thingy come out the bottom of the groove!
The carb mods are in "how to modify your mini" by David Vizard and how to set them up.
best regards
Mike
poppet valves rule!

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Post by unstable load »

Here is an extract from a document I have that details some changes that can be made to the carbs and intake..
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 venturi and scribe a line around the front of the venturi onto the piston. On the bottom of the piston scribe a straight line across the piston to form a chord across the bottom between 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 centre 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 may 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 a lot about SU’s 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 smooth 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 careful (take off too much and it will weaken the shafts and they will fail) and knife edge the throttle plates to about half their original thickness. Google or any book on carb tuning should give you pictures on what to do (same principles work on 4 barrels and Webers so hunt out articles for pictures.
K&N conical filters are worse than the original set up with the
U-bends, as they take in hot air from above the exhaust and they retain the worst bit of the original set up, the adaptor into the carb.
Cheers,
John

ratwing
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Post by ratwing »

Thanks for the replies guys, I'll give it a go - file carefully, go slowly and lots of checking should get it done :D

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