Modifying SU carbs

[30 ounce]

In the SU carburettor high performance manual by Des Hammill he recommends using the lightest springs (1-2oz/inch) and little or no dashpot oil (or the lightest oil possible) to get the piston assembly to react quickly with engine vacuum and throttle input. He then recommends thinning the needles to get the proper fuel mixture. I have read in these forums that some have used the oil in the dashpots to get their mixtures correct under load by using thicker (as much as 20wt) oil to slow the piston's rise (thus choking the engine) and enrichening the mixture. This seams to me to be the wrong way of enrichening the mixture. I have tried heavier weight oils in my SU's (5,10,15,20wt motorcycle fork oil) with poor results. The heavier the oil the slower the engine accelerated. My question is weather anyone has tried these mods and what were their results?

[Tony D]

SU's are SU's, and the balance is one that won't be covered in a short post.

 

Someone who used a re-tapered needle and heavier damper oil (which is intertwined with spring weight, as well as supplemental weight on the piston which also affects how it opens) may have a na engine which will fall on it's face from mix dropout if the piston opens a velocity drops too fast.

 

The SU is first and foremost a CONSTANT VELOCITY carburettor. The airflow across the metering bridge should be the same at idle as it is at WOT and maximum RPMS. In between there are many different ways to make the carb go, but generally whatever you do towards keeping velocity accross the bridge similar under all operating conditions and throttle/rpm ranges.

 

Many of the 'performance' mods will make the carbs difficult to drive on the street, or get terrible fuel economy (which is the reason they are on there in the first place.)

[rayaapp2]

In the SU carburettor high performance manual by Des Hammill he recommends using the lightest springs (1-2oz/inch) and little or no dashpot oil (or the lightest oil possible) to get the piston assembly to react quickly with engine vacuum and throttle input. He then recommends thinning the needles to get the proper fuel mixture. I have read in these forums that some have used the oil in the dashpots to get their mixtures correct under load by using thicker (as much as 20wt) oil to slow the piston's rise (thus choking the engine) and enrichening the mixture. This seams to me to be the wrong way of enrichening the mixture. I have tried heavier weight oils in my SU's (5,10,15,20wt motorcycle fork oil) with poor results. The heavier the oil the slower the engine accelerated. My question is weather anyone has tried these mods and what were their results?

 

I have posted a few times about slide dampener oil. The oil ONLY affects 1 thing. "Transitional Period" Specifically this is anytime you are dealing with 'acceleration'(Physics term here) across the bridge. To break it down simply, any time the throttle plate is MOVING or engine LOAD changes you are going to get 'acceleration'. Apply this to what Tony said above.

 

The effect of the spring also plays some part in how fast the slide will open but it isnt its main purpose. I am not fully aware of all its affects to be quite honest. My speculation is that it plays a part in dampener harmonics much like the small valve springs and not so much as a limit for slide movement.

 

After years of fooling around tuning SU's by ear and butt dyno I can say that you will NEVER get the mixture correct this way. You can get close, but with the inherit flaws with stock SU carbs, close isnt really close enough in my opinion. Using a 4 or 5 gas analyzer on a dyno or if need be on the street is going to be the BEST solution. Using a wideband is #2. The Gas analyzer was the tune of choice for many years with these cars and with the loss of carbureted vehicles it is becoming a lost art to tune this way. Of course if you tune this way your going to actually see the differences with each change and actually make some REAL progress. You will also see how each part affects the mixture at what event so it can become a learning lesson as well to become more intimate with the car, unless of course you are in a monogamous relationship already in which case I cannot condone your behavior.

 

Cheers

[madkaw]

I agree that you will only guessing without a wideband. I thought my carbs were dialed in pretty good until I got on a dyno and they showed running way lean at WOT in 3rd gear. I could change the oil in the SU's, but without a wideband I wouldn't know if the change was good or not--unless my butt dyno indicated otherwise. Hence, the wideband is installed, just no real world testing yet.

 

I have read both arguments of thicker and thinner oil, but have opted for close to stock for now. Can't wait to drive around with the wideband to see what is really happening.

 

Probably the best mod is a Z therapy rebuild:)

[Tony D]

I would say the best mod is from Patton Machine...

 

Clickety Click

 

 

Carbs? Whassa Carbs man? You means like Pasta and Breads and stuff, man?

[WEBEZEEed]

Madkaw, please do post what you observe with your wideband. I'm interested to see if you see the same things as I do with mine.

[johnc]

I know of a few racers that have run what's recommended by Des Hammill and its generally for a setup where the throttle is either fully open or closed. The chassis needs to be setup for that and the the driver needs to be willing to drive the car in that way. Its normally used when the rules dictate a stock cam and flywheel which tend to dampen or slow down the engine response.

[30 ounce]

I could see if a guy was running a stock cam and stock flywheel that the piston would (should?) be set up to rise only as fast as that engine would allow. Rising faster than the needed would cause the velocity to slow accross the bridge resulting in a lean mixture. In a hot engine: 3.0 liter, 15 lb flywheel, hot cam, head ported properly, that could rev faster than the stock spring and damper oil could respond is where I'm thinking Hammill's recommendations make sense.

 

I would love to go FI but want to get the most out of these SU's first. Nostalgia you know.

[Racer Z]

At this point we're beating a dead horse, but, here is my 2 cents worth.

 

First off, I agree with what I have read. I would have to say that they have a better understanding of the SU than I.

 

The oil damper only effects the response time from when you press the gas pedal to the carburettor reacting. The oil will not effect the fuel/air ratio, the idle speed or how far the carb opens at WOT. We have experimented with oil weight with no noticeable results. We also have a track car that is seldom driven on the street. Around town a thinner oil should offer a crisper response to throttle opening.

 

Please keep in mind that until the carbs are properly cleaned, adjusted and synchronized, you can't compare oils. The rest of the engine also needs to be in good repair and tuned properly as well.

 

For street use, I'm not so sure you would see much, if any, improvements by actually tweaking the carbs. A mild cam would offer better results and you can still use the unmodified SU's The SU (Hitachi built with a license from SU) can be found on many cars including the Jaguar. It really is a good design and doesn't need any tweaking.

[Tony D]

I know of a few racers that have run what's recommended by Des Hammill and its generally for a setup where the throttle is either fully open or closed. The chassis needs to be setup for that and the the driver needs to be willing to drive the car in that way. Its normally used when the rules dictate a stock cam and flywheel which tend to dampen or slow down the engine response.

 

Yep

 

"Many of the 'performance' mods will make the carbs difficult to drive on the street, or get terrible fuel economy (which is the reason they are on there in the first place.) "

 

Patton Machine, the sensible alternative...or as one person put it mildly: "Modify your SU's for better performance? You're about to enter a world of pain my friend!"

 

Been there, done that, never again. Embrace technology, Patton's setup lets you keep 'the look' without the hairy edges.

Edited January 16, 2011 by Tony D