The V8 Owners Forum -

Hi Guys

I have a 3.5 RV8 which I am looking to supercharge. I am looking at using SU's from a Jag 6 on the standard Rover manifold (albiet modified for the bigger carbs).

I don't want to run the carbs in front of the charger as I don't believe it will handle having fuel going through it. So I need to modify the carbs to handle positive pressure. Has anyone done this/knows how to do this/can point me to a guide?

I would prefer to modify the carbs rather than mount them in a sealed box.

I realise that an EFI setup would be better but I just don't have the money to source one. They are not as widely available here in NZ as elsewhere.

Cheers
Cam

Hi
Not done it myself, but what you need to get done is to add a pair of lip seals on the throttle shafts and hold them in place. If you can fins a thin enough seal it is just a case of counter boreing the hole the shaft passes through. A simple D bit made of silver steel with a pilot to hold it central in the bore is all that is needed, and a dab of glue when you put the seal in will do it.
Best regards
Mike

cammmy wrote:Hi Guys

I have a 3.5 RV8 which I am looking to supercharge. I am looking at using SU's from a Jag 6 on the standard Rover manifold (albiet modified for the bigger carbs).

I don't want to run the carbs in front of the charger as I don't believe it will handle having fuel going through it. So I need to modify the carbs to handle positive pressure. Has anyone done this/knows how to do this/can point me to a guide?

I would prefer to modify the carbs rather than mount them in a sealed box.

I realise that an EFI setup would be better but I just don't have the money to source one. They are not as widely available here in NZ as elsewhere.

Cheers
Cam

Exactly which carbs do you have ?

I would say it would be very very difficult if not impossible to seal the old style with the side fuel bowl, mainly because of that fuel bowl.

You would need to be using the more modern HIF series.

For those, check out the turbo minis forum

http://www.turbominis.co.uk/forums/index.php

You'll find all you need there.

For a low boost setup, a base HIF series will work with minimal modification. For higher boost, you may need to seal up then dashpot etc better.

And if those Jag carbs are 2", I wouldnt even consider using them.

1.75" ( x 2 of course ) will be more than adequate.

And of equal importance, if not more is the makeup of the plenum directing air into the carbs. The OE plenum is designed with a small restrictor at the intake which is critical to causing the carb to lift extra fuel when on boost. This is essential. Again you'll get dimensions of this on the Mini forum above.

You'll also need to source a Malpassi or similar boost referenced fuel pressure regulator and have a fuel pump capable of supplying adequate pressure too ( boost + base fuel pressure )

I run 2" SU's on my 4.6 V8 because I found the 1.75" SU's to be inadequate at high engine speed / loads.

But I have no idea how you would modify them to run under positive pressure. As stated above, I think you would have a challenge around the jet and float bowl assemblies. Ought to be doable though if time is free.

It's also a bit of a challenge getting them to fit the standard manifold and clear the rocker covers.

Chris.

Thanks for the replies guys

This is the motor/carbs. They are 2"



I will need to give it a freshen up. I have now sourced a couple of manual g/boxes which came with a duplex timing chain and adjustable pulley + electronic ignition module with matching dizzy amongst other things.

I thought the advantage of SU's is that it is very hard to over carb something due to the venturi's not being fixed?

I got my carbs set up on a rolling road by Peter Baldwin
http://www.wilshers-garages.com/
Not cheap, but he knows his SU's (and has racks of needles etc)

Pics of my install here (partway down):
http://www.cowdery.org.uk/v8rebuild-08.php

Chris.

cammmy wrote:Thanks for the replies guys

This is the motor/carbs. They are 2"



I will need to give it a freshen up. I have now sourced a couple of manual g/boxes which came with a duplex timing chain and adjustable pulley + electronic ignition module with matching dizzy amongst other things.

I thought the advantage of SU's is that it is very hard to over carb something due to the venturi's not being fixed?

The carb can still be too big, which then causes problems at lower throttle openings etc or throttle transients. Although the SU may be more forgiving, with boost a little to small is always easier and safer than too big.
But no way will a pair of 1.75" turbo SU's be too small for any Rover V8.
One SU can handle in excess of 200bhp quite easily.

But again, I think you will struggle to get those old SU's to seal and be suitable for boost.

The float bowls and line to bottom of the jet would need to be able to handle positive pressure. The float bowls themselves need a boost reference.

And generally getting the jet unit itself to seal will be difficult. The floats would also need to be plastic, although I think most SU's are ?

And then there is the dashpot sealing too and spring etc.

In reality...you'e maybe be better looking to a single Weber 38DGAS or similar style carburettor and working with that

If I have to sort out another way to fuel the car i'll be going EFi. This would set the build back a looooooooooooooooooooooooooooooong time though so would definitely prefer not to.

I have a pair of trump 1 - 3/4" SU's too. These definitely have plastic floats.

Labour is free or very cheap so i'm only really need to account for the parts cost.

Just had a though.

If I'm going to make a pipe from the superchager that splits and goes to the two carbies. What if I just made those bolt directly to the manifold and have injector bosses put on them. Twin point injection and use megasquirt with MAP sensor/TPS to run it?

Not ideal but possibly a simpler, lower cost solution?

I think my supercharger came with a throttle body.

What sort of supercharger is it ?


You could run 2 fuel injectors on a very crude system. But they would need to be fairly large. Although that in itself shouldnt be much of a problem.

But I cant see any easy, or difficult way to modify an old SU for boost without sealing it in a box.

The only one that will easily adapt for boost is an HIF series with the integral fuel bowl.

It's an SC14 from a Toyota. Twin lobe roots.

Not the ideal charger but they are cheap, plentiful and come with an electric clutch and bypass

I may have a lead on a manifold and injectors/rail from a Disco. Don't know price or whether they are willing to let it go though so not getting my hopes up.

Could add more bosses and use more injectors. Yes it would be crude but at absolute best I'm looking for no more than 75HP/L (and not even expecting that) so not exactly a highly tuned race build

Hi
Twin throttle body injection and Megasquirt should work quite well as you described. The throttle body off any 1.6 to 2 litre engine with a turbo should be fine, you just need a bolt pattern adaptor plate.
Best regards
Mike

Brill, thanks!

The TB I think I have is from the 2.8 V6 the charger came from.

I'm still waiting to hear about this Disco motor. If the guys is in need of cash I might be able to get the whole thing. That would give me a 4L and an EFI manifold with 8 injectors and a matching throttle body as well as saving me from having to re-build mine. Would throw away the standard management and megasquirt it using MAP or possibly twin MAPs for barometric correction.

The 4L motors are more prone to cracking cylinder walls due to core shift when casting? I'll have to have a re-read of my RV8 tuning book to see what the go is with these. Would be super awesome if it had cross bolted mains or the bosses that can be drilled out to cross-bolt them. Probably won't though.

Without wanting to jinx anything. I think I have the 4L EFI motor sorted

Hi,

Just a idea, and not the most common option over your side of the world if even poss, but how about 1 3/4" HIF carb fitted to MG Montego's ( please dont laugh !! ) these were a blow through design and compared to the rest of the car were reliable.

Cheap too there a complete engine and trans on Fleabay for £ 50, unbolt the carb and weigh the rest in!

http://cgi.ebay.co.uk/MG-Montego-Turbo- ... 43a6c475d8

Cheers John