× 1-800-946-2642 Home My Account Social / Forum Articles Contact My Cart
Shop Now
Select Your Car Type Sale Items Clearance Items New Items

 Details on my 16% HP gain by switching from single 1-3/4" SU to 45DCOE

 Created by: jeffm5150
Orig. Posting Date User Name Edit Date
Aug 27, 2014 11:53AM jchealey  
Aug 27, 2014 11:36AM jeffm5150 Edited: Aug 28, 2014 05:58AM 
   Forum Width:     Forum Type: 

Found 22 Messages

Previous Set of Pages 1 | 2

 Posted: Aug 27, 2014 11:53AM
Total posts: 358
Last post: Feb 20, 2020
Member since:Jul 20, 2013
Cars in Garage: 0
Photos: 0
WorkBench Posts: 0
Very good on the documentation of the change in carbs. I found a dyno about 13 miles away, but they want $100 for a run. Maybe I will stick with the Butt Dyno for awhile. What idle RPM did you end up with?


 Posted: Aug 27, 2014 11:36AM
 Edited:  Aug 28, 2014 05:58AM
Total posts: 193
Last post: May 19, 2015
Member since:Feb 5, 2013
Cars in Garage: 0
Photos: 0
WorkBench Posts: 0

Hey all - I just wanted to post my findings of my adventure from replacing my SU carb with a Weber.  I have dyno sheets (see attachment) from before and after the swap to show the HP and torque changes.  Only the carb, intake, and air filter was changed and no other engine changes were made during the swap.  So far I'm very happy with the result.  I still have some slight tweaking to do in the transition circuit but it's now very street drivable and had quite a bit more power at WOT.

Engine specs:

  • 1310cc block (or 1330, I can never remember)
  • lightened flywheel
  • 123 distributor
  • GT7 longman head
  • 1.5 ratio roller rockers
  • Kent Megadyne 266 fast street cam

Old intake/carb setup:

  • single 1-3/4" HIF-6 carb with custom needle profile
  • C-AHT770 cast aluminum intake manifold
  • K&N cone air filter

New intake/carb setup:

  • Weber 45DCOE9 carb
  • C-AHT777 3.5" Maniflow steel intake manifold (ceramic coated inside and out)
  • ITG 15JC25 Megaflow foam sausage filter
  • 36mm chokes
  • 16mm air horns

SU dyno results were 70 HP @ 6000 RPM, 72 lb-ft torque @ 3400 RPM.  45DCOE dyno results were 81 HP @ 6100 RPM, 76 lb-ft torque @ 4800 RPM.  One thing to note is that I was able to retain effectively all of the mid-range torque of the vacuum-driven SU with the 45DCOE.  All dyno tuning was done with 91 octane pump gas.

Some of my thoughts now that I'm virtually done with my swap:

1) I'm no longer using the vacuum advance on my 123 dizzy since the Weber doesn't really have any decent solution for providing vacuum.  I haven't (and probably won't) collect any MPG information, although the Weber setup certainly has the potential for consuming much more fuel at WOT.

2) Although the Maniflow 3.5" intake is marketed by some with "Allows for Weber carb installation without modifying your Mini’s firewall." - good luck with that.  Using the Weber o-ring carb to intake plates caused the carb to fit without firewall modification, but there was no way (that I could see) to mount air horns let alone an air filter setup.  I didn't want to cut up my firewall but eventually decided it was worth it.  I was only able to retain the 3-clock gauge binnacle in it's stock location by using a right-angle speedometer cable adapter.

3) I was surprised to find that I gained over 1 HP (not much I know, but it is power) simply by closing off the firewall with the "Weber box".  I initially noticed that the air/fuel ratio went very lean (2 whole numbers) at cruise once I closed off the firewall.  We confirmed this on the dyno by doing a run with the hood off, then a run with the hood off and a carpet drier pointing at the firewall held just above the valve cover - it resulted in more than 1HP power gain.  The cabin engine noise and smell is also virtually eliminated by closing off the firewall hole.  I'm very happy with that result.

4) This swap was a lot more expensive and involved that I thought it would be.  I had to re-route the wiring harness where it went through the firewall.  I had to move the brake and clutch lines (LHD car).  I had to buy a lot of [costly] Weber calibration parts to get my initial setup going.  The air filter options are limited and I had mine hand made by ITG in the U.K.  Similar with the air horns as the short 16mm (5/8") air horns are not readily available in the U.S. - I ultimately bought a pair of spun aluminum horns from KAD in the U.K.  I had to pay for dyno time for some power tuning once I got the new carb street drivable.  The cost of the carb paled in comparison to the cost of the entire project.


So I'm including a scan of the chassis dyno comparison between the SU and Weber power curves.  The SU dyno pull was done in June and the Weber dyno pull was done in August.  Both on the same dynomometer - an environmentally calibrated Dynojet single wheel dyno.  Graph info: Dark blue is Weber HP, light blue is Weber torque.  Dark red is SU HP, light red is SU torque.

(You can view a full-size image of the dyno comparison image here: //desertwave.com/gallery2/main.php?g2_itemId=18864&g2_imageViewsIndex=1 )

SU 1-3/4" setup (prior to the swap):


Weber 45DCOE9 swap complete (except for painting the firewall enclosure):

Found 22 Messages

Previous Set of Pages 1 | 2