SU Carb sizes.

[Pete]

Hey guys,

I'm hoping someone will be able to help me.

I've started a small project that requires me to obtain an SU carby from any 2 litre motor. When i do quick searches around the net, I get different throat sizes - inch and a half, inch and an eighth etc.

Can anyone point me in the direction of what I should be looking for please? I just keep finding variant opinions. Basically, I'm doing a suck through turbo job on a 1.3l carb.



Thanks
Pete.

[TurboHead]

"Performance" 1600cc motors back in the day would typically run 2 x 1 1/2" SUs and 1800-2000cc motors 2x 1 3/4" SUs. Probably what you want is an SU off a Jag or similar... They would be 1 3/4 or even 2". Good thing with SUs is that (within reason) you can't go too big as you want as they are a variable Venturi design so a big carb will only open as much as it needs. You should look for a good condition HS6 or HIF6 SU carb.

My old man is probably Perthstreetpissers leading SU expert so if you have any specific questions, let me know and I'll run them past him. I can also see if he has a big SU he wants to sell (rebuilt or not)

[Pete]

Thanks, muchly appreciated. Just read a metric shit ton of sites, and came across this graph.

http://www.sw-em.com/SU_vs_disp.jpg

so roughly, the mighty 1.3 with about 5 psi through it, I'm hoping to get around 70ish bhp. Going by that chart, a 1 3/4 could be ok. I think from stock we're talking about 40 hp.

Pete.

[TurboHead]

The MG Metro turbo ran a 1.3 A series engine (basically a mini engine) and used a blow through HIF6 carb and made 90bhp (on only 4.5psi boost). If you go suck through, a bigger carb may be required.

[TurboHead]

Yeah, 1 3/4" should be ok if 70hp is your aim. A single Carby of that size can flow enough air for a N/A 1310cc mini engine to make about 110hp

[Pete]

Mine will be a 1300 Suzuki Sierra motor with a td04h-l, so pretty small turbo. I wanted blow through, but seems a bit harder to set up, i've worked one the 2.6 Mitsi motors running suck through before in a Scorpion and a L200 ute, so thought that way might be a bit easier.

[TurboHead]

Either way will work. Sounds like fun regardless. Maybe if there are quality inlet manifolds for SUs to fit the Suzuki motor then consider blow through, but if you have to make the lot, then suck through.

[Pete]

Not that I have seen, So between a mate ( or few ), we should be able to get a full suck through setup, with possibly a small low pressure electric fuel pump to match, and remove the stock mechanical pump. Will also probably need to mod the stock fuel tank somehow and involve an extra line or 2 for the return system.

We'll work it out when it gets to it.

[TurboHead]

Just looking through an old tuning book and the HIF6 flows about 15% better than the older HS6.

[S85FI]

Jusy beware when you do get the right carbie for cfm flow the fuel jet may jave been changed. Be prepared to play with that.

Also why SU and not CD? Or what I reckon are better, side draft webbers.

The Webber has a good accelerator pump for cold engine starts amd works well for off road applications.
uI used to run twin side draft webbers on a datto L18 motor going up and down sand dudes in a off road racing buggy.
I

[Pete]

I chose SU because it's easier for me to make a small flanged pipe to sit the SU in front of the turbo, and then just another pipe with a few bends and 2 flanges to go from the turbo to the inlet.

[TurboHead]

The Spitfire runs SU carbies.... Nuff said.

[TurboHead]

Jusy beware when you do get the right carbie for cfm flow the fuel jet may jave been changed. Be prepared to play with.

Jet, probably not unless worn. Needle, quite possibly.

[S85FI]

Jet, probably not unless worn. Needle, quite possibly.

SU are a pain in the ass to tune. To many variables when fuckers fuck with them.

Jets..... fuckers always go too big.
Needles fuckers don't understand the difference between a biased and unbiased needle
The spring rate (they are coloured for tension to deliver fuel) pingpingpingpings just swap them out and not understanding the colours are tension matched
Oil for the chamber - you need to fuck with the viscosity to get it right
Balance meter you need one so you know what the cfm is for jet size and needle, spring rate and viscosity rate.
Another tool I can't remember wjats its called but it screws into the plug hole sonyoubcan see what the combustion is doing for rich/lean


It's getting matched to a turbo... ive never done a turbo depression style carbie but with boost and lag I imagine the above will be critical to have no flat spot under acceleration or lean out under load.

[TurboHead]

Nah, they are easy to tune. Dash pot oil, just use a 30 weight or even a 20w50 will work. Piston spring, go stronger if piston rises to fast, checking mixture, hold at revs and lightly lift piston with screwdriver, if revs go up its to rich, if revs drop straight away, too lean, if revs rise then drop within a second or 2, mixture is fine. Making note of the section of needle at each rpm checked will allow a more appropriate needle profile to be selected. My old man has books that list all the needle profiles.

That other tool you are thinking about is a see through spark plug but they really aren't required.

[S85FI]

I used to work in a carbie shop rebuilding tuning these things. Brings back memories.
I remember one customer had a red motor 186 with triple SU. Took a while to tune this thing but fuck me for an antiquated fibre wheel cam shaft set up this thing in the end was quick.

Had to put Auto Trans fluid Dextron II in that one to make it work.

Ford Carter plastic Thermaquad carbies.... plastic carbies in the 70s.... who would of thought
Holden/Chev with the Rochester Carbie - now that thing was a monster. 4 barrel (non square bore) was my Favourite. We got those to flow at 780cfm. And still complied with emissions.

Then there were all the jap stuff. Fuck me those things had ball bearings and springs everywhere.

Just went into my library and still got all those books. A quick flick and its a bit of a laugh with the people in the photos and their fashion set ups....

Fuck me...just reminds me how time can really escape you :)

Looking forward to seeing this thing cruising the streets :)