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Modifying a Stromberg 175 Carb

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Old Aug 20th, 2021, 08:53   #1
Burdekin
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Default Modifying a Stromberg 175 Carb

Has anyone done the SU mods to a Stromberg 175 as outlined by David Vizard in his book; "Tuning the A-Series Engine"? If so what were the results? I have a dyno session next Friday to see what my car is running at currently and would like to do the mods to measure the effects. Not much time to get them done though.
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Old Aug 20th, 2021, 09:26   #2
Laird Scooby
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For those of us who haven't read the book, what mods are you talking about specifically? The Stromberg is a different animal to the SU in many ways although they work and perform in a similar way.
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Old Aug 20th, 2021, 18:27   #3
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Squaring off the internal casting post bridge, reducing butterfly shaft, knife edge butterfly, dome head butterfly screws and taper piston leading edge and back edge at full up position. Want to see if the additional flow will increase or hurt low to mid torque or just give a bit of extra high end HP. Not sure if there is a rich enough standard needle though so that might also need modifying, currently running one of the richest needles and that is just with a stub stack and K&N filter fitted which gave a good increase in flow.
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Old Aug 20th, 2021, 18:53   #4
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Squaring off the internal casting post bridge, reducing butterfly shaft, knife edge butterfly, dome head butterfly screws and taper piston leading edge and back edge at full up position. Want to see if the additional flow will increase or hurt low to mid torque or just give a bit of extra high end HP. Not sure if there is a rich enough standard needle though so that might also need modifying, currently running one of the richest needles and that is just with a stub stack and K&N filter fitted which gave a good increase in flow.
On an A-series with a relatively short inlet manifold that would help, not so sure you'd even notice the difference on something twice the size in terms of capacity and with a relatively larger inlet manifold.

What oil are you using in the damper?

Also if you want to really increase power/flow capability, have you considered an SU HS8?

Gut feeling says you'll lose driveability by modding your Stromberg that way but i've not tried it so can't be certain.
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Old Aug 20th, 2021, 19:47   #5
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On an A-series with a relatively short inlet manifold that would help, not so sure you'd even notice the difference on something twice the size in terms of capacity and with a relatively larger inlet manifold.

What oil are you using in the damper?

Also if you want to really increase power/flow capability, have you considered an SU HS8?

Gut feeling says you'll lose driveability by modding your Stromberg that way but i've not tried it so can't be certain.
80 90 gear oil, works a treat but will also try 75 80.

The mods to a SU6 will result in it flowing similar to a HS8.

The stromberg is a good cheap carb and personally think it is better than the HS SU carbs. Mine works really well. I just want to see if the extra flow will give increased performance and if I can get performance close to the B20B. Just having a play.
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Old Aug 20th, 2021, 20:48   #6
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80 90 gear oil, works a treat but will also try 75 80.

The mods to a SU6 will result in it flowing similar to a HS8.

The stromberg is a good cheap carb and personally think it is better than the HS SU carbs. Mine works really well. I just want to see if the extra flow will give increased performance and if I can get performance close to the B20B. Just having a play.
I've always used Dexron II-D in Strombergs and SUs as well. Given that you're running a bit lean at the top end, 80W90 would work well too.

The limiting factor on an HS6 will always be the 1 3/4" diameter, the HS8 will give that vital extra 1/4" diameter or you could opt for twin carbs of a Weber 38DGV conversion which is in effect twin HS4s in terms of flow area but are easier to alter the jetting.

What other mods have you done to the engine?
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Old Aug 20th, 2021, 21:03   #7
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I've always used Dexron II-D in Strombergs and SUs as well. Given that you're running a bit lean at the top end, 80W90 would work well too.

The limiting factor on an HS6 will always be the 1 3/4" diameter, the HS8 will give that vital extra 1/4" diameter or you could opt for twin carbs of a Weber 38DGV conversion which is in effect twin HS4s in terms of flow area but are easier to alter the jetting.

What other mods have you done to the engine?
Not looking to change the carb! I have the current Stromberg carb in the car setup really well and it goes very well. I’m just wanting to experiment with the strombergs and see if they are capable of more power/torque, just tinkering for my own education. I have a few spare strombergs. I’m just wanting to know if anyone else has done the mods or anything similar.
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Old Aug 20th, 2021, 22:32   #8
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Ah, i see - i misunderstood and thought the object of the exercise was more power.
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Old Aug 30th, 2021, 13:57   #9
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On paper the Stromberg apparently has a higher flow rate but for performance it is usual to swap them out with SU's. Emissions was the main reason for the change to Stromberg's.
Knife edging etc is for top end and probably only measurable on a dyno. Having a set of short medium and long trumpets might confirm some ideas but needle selection probably the key issue. Are Stromberg needles readily available at a tuners? Filing is a black art and you never really know what you end up with. Dyno time is expensive unless it's being done on a friendly basis!
Don't forget to fiddle with the timing.
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Old Aug 30th, 2021, 14:13   #10
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On paper the Stromberg apparently has a higher flow rate but for performance it is usual to swap them out with SU's. Emissions was the main reason for the change to Stromberg's.
Knife edging etc is for top end and probably only measurable on a dyno. Having a set of short medium and long trumpets might confirm some ideas but needle selection probably the key issue. Are Stromberg needles readily available at a tuners? Filing is a black art and you never really know what you end up with. Dyno time is expensive unless it's being done on a friendly basis!
Don't forget to fiddle with the timing.
Thanks Derek. The Stromberg has a smaller stepped area directly after the bridge so there is potential for extra flow but the casting is pretty thin.

I have a stub stack with a K&N filter fitted inside the original housing, it definitely flowed more and became too lean with the B1CC needle. I did a spreadsheet with all of the needles available and did a graph and showed three best choices with the best of those being the B1AN needle. It is one of the richest needles. I test ran it with my O2 wideband sensor and it was spot on. The dyno guy also said it was spot on, a little rich at max revs was all. The dyno came in at 90Hp (76.4hp at the wheels) so a pretty conservative conversion. That could be a increase of 8 or more Hp compared to stock so if correct a good result. The car goes lovely. Not sure when I will get round to modifying a carb, I would like to just see what difference it makes. There aren't many richer needles to chose from so won't be straight forward.
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