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djwarner

Just how much can one get out of DGVs

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I was stimulated to investigate my Carbs when I continued to accumulate soot around the exhaust after my engine overhaul while the plugs showed the classic tan.

 

I was also not particularly happy with 0-60 times of my 186K mile AT car. Even after a fresh overhaul, I was able to post a 10.9 second 0-60 without over-revving the engine prior to launch. An August 1971 Road & Track review of an AT 240Z said the best 0-60 time they could achieve was 10.4 seconds. This serves as my benchmark.

 

My engine is otherwise completely stock and wouldn't benefit from going to triple DCOEs. 

 

Researching Webers, in Pat Braden's  HP Book on WEBER CARBURETORS, the author states the magic of Webers is that the can be tuned to fit your needs whether it economy driving or for the race track. It turns out DGVs have seen track service on Escorts and other smaller cars. In addition, Theory of operation for the DGV is almost identical to a 40 DCOE.

 

So I began to wonder just how many people install DGVs accepting the jetting that came with their kits. And just how much more performance could be attained by proper tuning.

 

For those unfamiliar with DGVs, they are two barrel progressive carbs with 32mm and 36mm barrels. Common opinion is that they are sedan carbs with decent fuel economy but definitely non-sporting performance.

 

So I contacted Redline, the US distributor for Genuine Weber (as opposed to Asian knock-offs) for any additional information they might have about proper tuning on out 240Z's. I received fantastic support from them and was able to make substantial improvements.

 

One of their first comments is that modern gasoline formulations burn much leaner than even 5 years ago and that any carb that was tuned more than five years ago is likely to under perform. Second, they stated that they have changed the specs for the plastic floats,  lowering the closed level and reducing the range of travel.

 

They now measure plastic floats from the top of the float as opposed from the previously specified bottom. 17mm closed and 21-22mm fully open.

 

Redline also suggested a 10-15% performance improvement could be made by converting the linkage from progressive to synchronous operation. With a progressive linkage, the throttle travel operates only the first barrel for the first half of travel and starts opening the second barrel after that. With synchronous operation, both barrel open together. While this doesn't affect wide open throttle performance, it provides the equivalent of a 49mm single barrel carb compared to the 48mm of our SUs. The standard 32/36 DGV operates solely on the primary about 90% of the time.

 

First, I documented the jetting that came with the car, these turned out to be standard jets for a generic 32/36 DGV 5A carb and were not at all tuned for a 240Z. The standard idle speed/mixture setting procedure indicated that the primary idle jet was in fact too rich. There is a half size jet difference between the primary and secondary idle jets. swapping the jets so the primary had the leaner jet made an improvement, but it now indicated the main jets were too lean.

 

This was enough to confirm my need for a pair of jetting kits. I order a set of synchronous linkages at the same time.

 

I have installed the linkages and richened the main mixtures by three steps as well as leaning the idle mixtures to the proper sizes.

 

Mid throttle response, where we live 90% of the time, has dramatically improved since I'm using the full 49mm equivalent rather than the 36mm of the original setup. It makes driving the car a much more enjoyable experience.

 

I haven't had time to do a 0-60 run, but if there is any wide open throttle performance improvement, it will be from the richer jetting and not the synchronous operation.

 

The original progressive linkage

post-26587-0-96146600-1439577840_thumb.j

 

The new synchronous linkage

post-26587-0-82871700-1439577913_thumb.j

 

All in all, for less than the cost of one new carb, I've gotten a significant boost in performance.

 

 

 

 

​

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I finally got around to doing some 0-60 runs after re-jetting the mains and converting the carbs to dual barrel synchronous operation.

As I mentioned above, I didn't expect the conversion to synchronous operation to improve 0-60 times since this is done at full throttle. Where it does make a very significant difference is in the 40-70 mph driving range. At 50 mph the Z is loafing along with the throttle slightly opened. In this state, the car accelerates to 60 surprisingly quick without a hint of strain. I suspect that the light throttle position at 50 leaves lots of accelerator pump action in reserve.

Now for the best part, after re-jetting I dropped a full second off the 0-60 time, 9.9 seconds versus the 10.9 secs previously. This is actually a half second faster than the writers for Road and Track were able to do back in 1971.@ 10.4 seconds(Remember this is for a car with an AT.)

Other than the DGVs my engine is completely stock. I did a bottom end overhaul this year. The head was rebuilt about 40k miles ago and tested good so I left it alone save replacing the cam with another stock cam.

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