Big bore SU experience?

[gogriz91]

I'm finishing a replacement N42 head plus a retune of my existing SUs. Got a great deal on another set of SUs plus intake and am debating having them bored out and dropped on my refreshed top end motor. I'm really debating breaking down and getting a set of triple Mikunis but the bored out SUs seem like a better deal in terms of cost, reliability and ease of tunability. Engine is an L-28 with an N42 head, a Schneider street cam and a header. It's a street car with an occasional run to the track for HPDE/autocross.

 

Anyone have big bore SUs? How do you like them? Any tricks to tuning them? Should I have the machine work done then buy the Ztherapy rebuild kit or vice-versa? Any needle recommendations? I've searched and found alot that talks around them but not necessarily to them or what was done to mod them.

[Zmanco]

With SUs the mixture is determined by the profile (taper) of the needles. With a more aggressive cam the profile of the stock needles is no longer going to match the mixture requirements across the rev range. Tuning that would be done by replacing jets on Mikunis or Webers is done instead by grinding the needles. I found that once I had a good mixture at WOT at high revs the mixture at idle became too rich. This was the main reason I gave up on the SUs - it was too hard to tune across various engine conditions for a non-stock cam.

 

That said, I suspect the stock SUs would flow enough air for just about any reasonable L28 built for street use. There's a famous story of Norm who used to run 12's with SUs... I'm sure he was a master at grinding needles.

[gnosez]

I had a set of ZTherapy 2-inch SUs on my 3.0L and they were just about set and forget. By talking with ZTherapy before hand they matched the needles to my application and location.

 

When I went up to a 3.2L stroker I added 44mm Mikunis as the SUs just couldn't let in enough air. They sound and operate dam near perfect and are pretty close to set and forget as is possible.

 

That said the SUs were better at doing an autocross course due to the side-to-side movement of fuel, which unless you add a baffle to the Mikunis causes you to go lean on hard left hand turns. I have yet to replicate that problem on the street, doing The Tail of the Dragon or on a roadcourse set-up.

[JMortensen]

That said the SUs were better at doing an autocross course due to the side-to-side movement of fuel, which unless you add a baffle to the Mikunis causes you to go lean on hard left hand turns. I have yet to replicate that problem on the street, doing The Tail of the Dragon or on a roadcourse set-up.

I've had that problem on a road course. Tom Holt posted a really easy fix on classiczcars.com a while back: http://www.classiczcars.com/forums/showthread.php?t=18948

 

If the choice is SU's or 44 Mikunis, I'd go 44 Mikunis. Don't bother with 40s.

[gnosez]

Thanks. I have no idea how I missed that thread but it will be added to this year's list of things to do.

 

I had been re-positioning the float before and after AX events and that seemed to help a bit but on real tight technical courses (read Miata) it would stumble no matter what float level it was set at.

 

Rebello had my engine back last year and now I wonder why this wasn't part of the rebuild. Oh, well.

[gogriz91]

I'll start looking for a set of 44s, then weight the cost which always seems to be the long pole in the tent. From just about everything I've read a set of triples is like bolting on between 20-40hp, I was curious if the big bores would get me half of that. 20-40 would put me right around 200hp which is my goal with not too much else in engine mods.

[JMortensen]

I just saw your post on classiczcars and then I noticed that you have a "street cam". If you want to run a small cam, the SU's will probably be the better choice. I think the triples will ultimately make more power, but you'll need a bigger cam to take full advantage. That said I'd be inclined to get a bigger cam AND 44's.

[gogriz91]

I just saw your post on classiczcars and then I noticed that you have a "street cam". If you want to run a small cam, the SU's will probably be the better choice. I think the triples will ultimately make more power, but you'll need a bigger cam to take full advantage. That said I'd be inclined to get a bigger cam AND 44's.

 

I don't know the lift and duration of the cam, I'll have to do some homework on that. The dream 44 setup I'm looking at is the comp one on Wolf Creek Racing, it'll be 6 months to scrounge the money together though.

[wishihadaz]

I had a set of ZTherapy 2-inch SUs on my 3.0L and they were just about set and forget. By talking with ZTherapy before hand they matched the needles to my application and location.

 

Now that my build is done, I know for sure that the SUs aren't right.

I have a 2.8 with a mildly ported N47 and Isky 290/490.

Under full throttle it pulls like an athlete with one of those parachutes dragging behind.

I'm still breaking it in and haven't attempted anything over 4500rpm, but in third at that point the bogging increases.

 

I'm hoping I can get them re-tuned without having to step up to Webers or Mikunis.

 

Reccomendations?

[Austin240Z]

Now that my build is done, I know for sure that the SUs aren't right.

I have a 2.8 with a mildly ported N47 and Isky 290/490.

Under full throttle it pulls like an athlete with one of those parachutes dragging behind.

I'm still breaking it in and haven't attempted anything over 4500rpm, but in third at that point the bogging increases.

 

I'm hoping I can get them re-tuned without having to step up to Webers or Mikunis.

 

Reccomendations?

 

If i remeber what I read in my SU book a while back (dont have it with me) so long as the needles are right you shouldnt be having a problem.

 

This is a dumb question, but did you Sync the carbs?

 

With my old stock 2.4L I had a bad front carb (previous owner did some serious damage with his "rebuild") and when I attemted to sync the carbs, I just couldnt get them right. Because of the way I wound up setting them, I had amazing torque, but the carbs started fighting above 4K power completely died off at 5,200, and the tach stumbled at 5,600. didnt get it above 92whp... But the torque at the begining would throw you back in your seat.

 

I have a feeling you already know what to do, but try synchronizing them again and run it. Sounds to me like they might not be quite in sync.

[Tony D]

Similar to Austin's Experience I had similar symptoms that I swore were ignition related save forthe way the car would pull if I slightly lifted my foot off WOT. Turned out one of the damn jets was in the 'start' (almost all the way down) position. I went round and round for quite a while under the logic I had just gong through the carbs and the jets were working fine. Something got in there and made it stick, and viola...no running over 5600rpms! Surged, bucked, acted like there was a rev-limiter. Stuck that jet back 'full up' retwisted the carbs to proper mixture and it zings right up there like it did years ago!!!

 

Not having time to do much more, I have to start the car without the starter system, kind of a pain when it's less than 50 degrees out...not doing my starter any favors, but I got other things to do than go through those SU's again.... I'll get to it when it's nearing fall I suppose...through some necessity or anger over having burned up a perfectly servicable starter due to sloth alone!