Performance of SU Carbs w/ 530L 306D Cam

[JSM]

I picked up this car recently. It has an E31 Head w/ 530 Lift and 306 Duration Cam on a F54 Block bored 30 over w/ Flat tops. Block and Head were just decked not shaved. The car was intended to run on race gas only because of the supossed 11.5 to 1 compression ratio. How they got this calculation I have no idea?

 

Anyway, it currently has Tripple SK carbs on there now but they need to be tuned and maybe rebuilt. WHat kind of performance loss would I be getting if I switch to SUs? I have a set that runs great on my smashed car and it has a mild cam in it now. Would the carbs handle this Cam?

 

Thanks

 

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Claimed Head work:

New Isky Springs and Retainers

Intake:New Stailess Steel hollow Sodium filled valves 1 mm bigger than 280z valves swirl polished

Exhaust:Reduced shanke 280z valves

New Followers, Lash Pads, Stellite(sp) Seats, rocker arms

Mild porting and polishing

[JMortensen]

You could make it work, but I'd guess you'd give up 40-50 hp in the end. Not worth it in my opinion. Just get the SKs running. Might ask Dan Baldwin what his jets are, just as a baseline. His should be pretty close to what you need.

[JSM]

Any idea on where they came up with 11.5 to 1 ratio?

[JMortensen]

Don't know that. I know my flat top L28 with E31 requires 95 octane to run advance in the mid 30's with a smaller cam than what you've got. I opened my chambers up a bit and shaved the head a bit, so chambers are probably similar in size to what you have there.

[Tony D]

That cam spec is pretty stout, although this goes back to the 'what do the cam numbers really mean' question. I would think the SU's would drop off quite a bit of top end horsepower. Sure they might work but with all the hassle of getting a WBO2 hooked up and screwing with the SU's to tune them, the effort is just as much as getting the SK's running. If you can tuen the SU's, tuning the SK's is nothing different.

 

Synch at idle, Synch off idle, optimize idle mixture, start chacking progression.

IMO, getting the jetting right (if it's even off by much...) is easier on SK's due to having segregated jets for each section of the operation curve: idle/progression, main run over 3K, air bleeds and emulsion tubes to determine how those mains come on and where they start really adding fuel.

 

I believe they use standard Weber jets, so getting parts---again IF you need them, shouldn't be that hard.

 

As for 'rebuilding' them, do they even run now? I see people take stuff apart all the time---and I swear it's because they just wanted to take it apart, or think that taking something apart will miraculously solve something without expending any effort on the front-end towards diagnosis. I don't know how many times people have bought a car, driven it home and because there was a cugh or sputter the FIRST thing they do is take the carbs all apart and perform augury. In the end bad plug wires....but a month down the road we've got a bunch of new parts on the engine and a frustrated owner that spent a lot of money doing not much of anything.

 

I'd see if they run first, and go from there. Usually the jetting has pretty wide lattitude on what will 'run'---using a WBO2 will get you closer, faster. Doing power pulls some secluded place will probably get you dialed in pretty well, and a couple of pulls on the dyno afterwards should dial them in for the duration.

 

After that you check idle synch, idle mixture...but really aren't changing anything once they're set. My Mikuinis on the 73 were synched and set sometime in 1991, and well over 75K mile ago. Idle mixture screws have had a lot of 1/4 turn this way, 1/4 turn that way....but nothing's changed since then. Triple carbs, once set up, are just as 'trouble free' as anything else. It's people who have apenchant for taking everything loose and starting over every tune up (or every time they hear a cough) that give them the 'maintenance intensive' reputation.

 

I'd stick with the bigger carbs, they will give you just as good, if not better mileage than SU's anyway....as well as positively more horsepower on the top end.

[JSM]

Thanks Tony!

 

The car ran horribly when I got it home. The previous owner told me they needed to be rebuilt/cleaned based on the sitting. You can really smell the old gas and see the gumming of old gas.

 

From searching for SK's here on the forum i've found little. It appears SK is out of business. Any links for rebuilding them?

[JMortensen]

I know they use a combination of Weber and Mikuni jets. Search for Dan Baldwin's old threads. He had some older ones where we got into a little detail, he might have more info if you PM him. Haven't seen him around as of late, but he was getting ~255 whp from his 45 SKs on a stroker motor.

[JSM]

Thanks Jon!

 

I figured the member search function out just now! Appreciate the info.

[JSM]

If I decide to dump the 45 SKs for simplicity of finding a parts source and go w/ Webers, do I want 40's or 45's. As far as performance goes what kind of differences am I looking at?

[JMortensen]

I'd definitely go 45s. Even on an L24 I'd go with 45s. This comes after watching a friend run his L16 on 40s and then 44s and seeing the difference it made.