I'm running an V8 from an SD1 with SU's.
I'm going to remove the poppet valve from the buttefly and blank the holes, while I'm in there, I was going to slim down the spindle, but then I thought, why not move the butterfly fixing screws to the very edges and cut out the centre section of the spindle, leaving it rotating on two stubs.
Of course it will be weaker, but as there is nothing on one end of the stub, it has no force to deal with.
Anybody tried this, or tell me I'm missing something big!
steveshaw wrote:I'm running an V8 from an SD1 with SU's.
I'm going to remove the poppet valve from the buttefly and blank the holes, while I'm in there, I was going to slim down the spindle, but then I thought, why not move the butterfly fixing screws to the very edges and cut out the centre section of the spindle, leaving it rotating on two stubs.
Of course it will be weaker, but as there is nothing on one end of the stub, it has no force to deal with.
Anybody tried this, or tell me I'm missing something big!
I've not done this but there are forces involved that you have not considered such as, the vacuum at idle and even more at overrun conditions and forces from a backfire. I would think the throttle will deform and jam after a backfire given the minimal support.
Cheers denis
1950 A40 Devon Hotrod with 5.0 twin turbo RV8.
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 57 years of driving. Same Car, 5 engine upgrades !!!
Hi
Definatly dump the poppet valves they are knackered by 30K miles anyway and are a nightmare!
The general routine for "gas flowing" SUs is to slim down the throttle spindle by cutting the front half off altogether, usunr counter sunk screws to attach the butterfly (polishing them flat to the face of the butterfly. and reducing the thickness of the remaining back side of the spindle by about 30%. the rearsoning being that the front half of the spindle is in the airflow while the rear half is in the shadow of the butterfly. the butterfly is also generally feathered on the leading and trailing edges to about 30% original thickness Topside of the leading edge and underside trailing edge. the final bit of simple low effect high gain modification is to radius the leading edge of the piston, by about 1/4 to 3/8 inch radius at the centre. The way to do this is to lift the piston to the top of it's travel and mark around the front of the venturi, take the piston out and place a 1/4 or 3/8 strip of metal across the leading edge of the piston on the under side and mark across the underside of the piston. Using these marks as guides radius the leading edge, to not take off so much that the guide slot and pin that keep the piston pointing in the right direction no longer works properly (or move the guide pin).
Note the last modification will effect the metering slightly, it will make the mixture slightly leaner at and just off idle and and slightly richer at full piston lift, however to get this to work properly it becomes more important to get everything set up right, particulally the spring selection, you want the piston to hit the top of travel at 500 to 600 revs before max used revs, then the mixture will go gradually rich (about 5% richer over the 500 rev range) due to the modification above and the needle will control the rest. The metering will be a bit more sensative to changes once you have done this but it will not become unstable.
The rest of the mods above do not effect metering.
A good set of bell mouthes will also help both flow and stability of the carb, you want ones about 5" long with about 7 degrees of taper over that distance and a nice generous radius on the front.
These mods should give you 25% to 30% more flow if done well and the carb will still be easy to set up (probably easier)
Do not go further gasflowing without really understanding what you are doing and having access to a dyno as you do it, you can get the flow up to about 75% extra but by then the rev range over which it operates is very narrow and it is not for a road car (mini 7 racing modifications here where you had to use a 1 1/4 SU and were screeming the engine to silly revs.
David Vizard did a series of articles for Cars and Car conversions years ago on how to modify the SU and what to do with the manifolds to get them to work when you had done the (more extream) modifications, if you sre interested in tuning the SU I would have a hunt around 'tinternet and see what you can find.
Best regards
Mike
Thanks for all the input, given me me plenty to think about.
One other thing I had not considered, is that the flow capabilities of the carbs, may already exeed that of the standard head, making any of my efforts futile, but I'll try anyway.
Hi
Standard SUs on a standard rover manifold will support about 210 BHP without being a restriction, clean up the manifold and match the manifold to the ports in the head and to the carb probably another 5 to 7 before it is restricting things again. By this stage your heads are really struggling and are the biggest restriction. The carb mods above will help, but don't expect big gains, they will help mostly by allowing you to make the most of everything else and may make the engine more drivable but doing everything described above. On a set of fully ported heads and matched manifold on a 3.5 or 3.9 you may only see 5 bhp extra, rover heads are really the big restriction (have I said that already )
Best regards
Mike
Thanks for the information Mike, its the "free" horsepower I'm after, so I think I'll start to re-build the spare engine and do some porting and polishing.
Cheers!
Steve
As mike said the biggest resriction on the RV8 is the heads, but some simple mods to the valve seats and throat areas provide the biggest gains for time spent if you have the tools and skill to do it.
Have a look at Peter Burgess site for Econotune heads for the Rover, don't be put off by the word Econotune as these simple mods achieve 30-35 BHP with standard size (waisted stem) valves and stock cam, a further cheap mod is to use a 3.9 cam ( approx £60) for a few extra BHP.
Quote from Kiwicar.. " David Vizard did a series of articles for Cars and Car conversions years ago on how to modify the SU and what to do with the manifolds to get them to work when you had done the (more extream) modifications, if you sre interested in tuning the SU I would have a hunt around 'tinternet and see what you can find.
Best regards
Mike "
Not sure about the David Vizard articles you mention, I have his books on Cylinder head modifications, and saw him a few years ago lecturing about camshafts.
But there was a Clive Trickey (Unfortunately no longer with us) who wrote many articles for 'Triple-C' (Cars & Car Conversions) mainly centring on tuning the Mini. He also published a couple of books under the Triple-C banner :- "Tuning The Mini" (cost 17/6d), and "More Mini Tuning" cost 12/6d). He goes into a bit more detail with SU mods in the second book but generally it's as mentioned in Kiwicar's post above.
Just thought I'd mention the titles in case it makes searching easier.
I'd post a copy if I knew how to attach a good quality jpeg, there again there's the copyright thingy.
DaveEFI wrote:Mr Burgess's site mentions MGB heads. Does the same apply to the Vitesse ones which are said to be an improvement on the carb ones?
Dave,
The vitesse engine was i believe rated at 190 BHP, the heads had waisted stem valves and the engine was higher compression at 9.75/1 + the EFI was adding efficiency, but I have heard that a little additional work was done behind the valves to achieve the quoted 190 BHP.
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