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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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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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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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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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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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? :thinking: |
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Ah, i see - i misunderstood and thought the object of the exercise was more power.
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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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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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Your dyno guy may be suffering from 'digital EFI think' where it would be easy to tweak specific cells in a fuel map to push that AFR from 13.4 up to 13.7 around 5000 RPM. I suspect the Stromberg doesn't have that ability to adjust fuel delivery at 5000 RPM wide open throttle without having some knock on effects at lower RPM. Your AFR does seem to be excessive at around 2200 RPM; but, if part throttle AFR at 2200 RPM is good then wide open throttle AFR at 2200 RPM may not be an issue. On the matter of potential modifications to the Stromberg to improve flow, you might want to consider mapping the manifold air pressure during a dyno run. New MPX 4250AP MAP sensors are fairly inexpensive (about $35 Cdn); but, a salvaged MAP sensor from just about any car will likely fit the bill (our local pick and pull salvage yard had a flat $5 charge for any sensor pulled from a wreck). The sensors almost universally have a 0-5 volt linear output and you can usually find the slope calibrations on line. With a 5 volt power supply, an analog input data logger and the sensor you can record your actual static manifold pressure during engine operation. If your manifold pressure is running just 3 or 4 kPa below atmospheric pressure during peak RPM wide open throttle conditions there is probably nothing to be had by beating your self up fiddling around with the Strombergs. |
The richness at about 2.2k showing badly on the AFR graph is mirrored slightly on the torque curve. The needles might like a bit of extra meat there but probably makes the car feel lively too. Good set of curves. Smoothing the bridge would I think reduce the atomisation at lower revs and that isn't good.
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The idea of the porting post bridge is to make the air flow easier the same as at the front of the carb with stub stack at the entrance. It's opening up the area at the outboard side of the piston. Tapering the lower front and rear edges of the piston is suppose to improve throttle response. There definitely is extra Hp to be had with more carb. |
Years ago I rounded the front of the pistons on my B16B. Didn't notice any difference. My comment about keeping the bridge sharp was because at that point the fuel is being sucked from the jet so having the smooth airflow changed to "rough" helps the atomised air in the inlet manifold. It doesn't want to drop out of atomisation onto the walls of the manifold. This can happen with the alloy manifold as it runs cooler than the cast iron one. The GT one piece is the best of all of the various one and 2 piece setups. The alloy inlet looks cool but it may be too cool at times!
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