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631 (6 cylinders, 3 carbs, 1 turbo)


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The on-boost enrichment can be accomplished conventionally with larger jets...but the use of 'modulator rings' at the carb inlet between the box and the carb will allow you to tailor when you get enrichment, and not really need larger main jets.

 

The pressure differential that exists in the venturi sucks (some would say pushes) fuel from the float bowl through emulsion tube, past the corrector jet and into booster venturi. Your mixture depends on the float measurement holding the fuel level in the jet well consistent.

 

If the float bowl isn't vented, you can literally push air back into the fuel bowl!

 

Using Modulator rings AFTER YOU HAVE A GOOD JETTING BASELINE AS AN N/A (say you have a 44 mm carb, with 38 mm main venturi) you start with a ring maybe 5% larger than your main venturi... so with this instance, 40mm and put it between your surge box and the carb inlet. Running at lower airflows there is no disruption of airflow. Everything functions fine.

 

But as your rpms increase and boost flows more air...you get a differential across the modulator ring as it enters the main throat of the carb. The Main venturi still functions as normally, but now what you have is this LITTLE BITTY difference in air pressure (faaaaaaaar under even 1/4 psi!) between the pressure that exists in the plenum (and therefore the float bowl) and the throat of the carb. What effect this has is the fuel level is depressed slightly in the main fuel bowl, and raises higher in the jet well. It sounds counterintuitive but it acts like a higher float level setting. You are basically using what was before atmospheric pressure now using boost to push the fuel into the  venturi at a greater rate. This makes the fuel easier to get into the booster/main venturi and thereby richens the mix under boost. No larger jet needed that affects cruise and even in some cases low boost operations. But once the flow really starts going, the enrichment comes on...and the more flow, the  more enrichment you get.

 

The smaller the ring you have, the earlier it comes on, and the higher the enrichment level will be as flow increases. You will never use a ring smaller than booster diameter. The larger the ring, the later the enrichment occurs, and the less of a slope the enrichment takes.

 

You have to experiment with the size of the ring to get the diameter you need for the correct on-boost enrichment.

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Really, if you have a good running (dyno pulled) N/A jetting setup, using modulator rings will get most people where they need to be. Higher fuel levels in the jet well SIGNIFICANTLY increase AFR under boost. Many times you wont have to increase main jet size at all.

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Alright guys, I have a quick question. on the BACK of the weber in between the runners. I have a small vent hole.  do these need to be plugged/ blocked. I am at work right now and can not upload a picture.     http://i35.photobucket.com/albums/d184/KJR111/DCOE_1.jpg

the carb on the left, there is a square plate in between the two throttle plates.  that is the hole  in question. 

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That plate is nothing, don't worry about it. It allows access to some linkages and pins for the throttle shaft. The one circled in red is what equalizes your float bowls. Some people didn't use that hole, and instead put a longer bolt on the jet cover, and a spacer with a hose to the surge tank and pressurised the float bowl that way.

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Nice to see another owner venturing down this path.. thread subscribed.

 

I've been working 'quietly' on my setup as well, don't want to thread jack so I'll keep it short for now. 

 

Running the dreaded cartech set up with 44 Mikunis (hopefully Steve won't get overly excited seeing this :)), as per Tony's advice I was moments away from cutting out an aluminum panel to alter the airflow via modulator rings, but the leaning issue I had was actually attributed to boost spikes in the wastegate which has been addressed.

 

At just a hair fatter than my NA jettings, currently on 150 main / 190 air / 62.5 pilots, I'm holding steady 11 AFR in 4th. I'll actually need to drop down to 145 main so I can see something closer to 11.5  ~ 12 AFR. 

 

Wastegate is set at 9 lb, current status is pretty close to being 'done' as far as tuning goes, I'll post a comprehensive setup info thread soon enough, 'ts definitely been a learning experience.

 

20140630_115350_1.jpg

 

20140630_115432.jpg

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Just stupid and not being attentive on my part - I've been staring at the wideband/rev limiter too long (located on the right side of the steering), and the boost gauge sits on the left. 

What was funny was it still held about 13 afr for a while at 16+ lbs with the efi pump + mallory 3 port regulator, felt like a million bucks, finally glanced to my left on one of the runs and the boost gauge was pegged ;p

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As I said, past torque peak we pulled fuel like crazy.

 

AFR is not an indication of Horsepower. The  reason it felt like a million bucks was likely it was  running right where it was supposed to be running!

 

We were anywhere from 12.8 to 13.8 AFR above 4,500 to 7,400 where JP just decided to stop testing....mainly because the turbo was out of air but also because he was frightened knowing L-Engines catastrophically detonate and kill everybody within a 200m kill radius if they hit 7,500 ... or at least that's what I read on the internet.

 

Since then, it's become apparent there was exaggeration and the end of the world does NOT come at 7,500 rpms on an L28 stroker, nor does it come at 8,500...

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the car runs great under 3 pounds of boost, and then it lean spikes. I have a 5 lbs spring in the waste gate. 
I always have my eye on the wide band, around normal cruising, it is any where from 15.2- 16.4 on the afrs.
when I do give it some gas and it spools to 3 psi, I can see the fuel enrichment come on, but then it just goes 
way to lean and wants to go 19.0 + afrs.  I do have a fucked up jetting in it.  I am curious about your setup and 
at work tonight I will change the jets and e tubes at work to a similar setup.  

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the car runs great under 3 pounds of boost, and then it lean spikes. I have a 5 lbs spring in the waste gate. 

I always have my eye on the wide band, around normal cruising, it is any where from 15.2- 16.4 on the afrs.

when I do give it some gas and it spools to 3 psi, I can see the fuel enrichment come on, but then it just goes 

way to lean and wants to go 19.0 + afrs.  I do have a weird jetting in it.  I am curious about your setup and 

at work tonight I will change the jets and e tubes at work to a similar setup.  

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What's the current jetting specs?

 

IMO 3 to 5 psi shouldn't spike anything to 19+afr unless you're really far off on jetting, I'm suspecting fuel starvation of some sort? Didn't really audit your specs but how's your fuel pump + regulator set up? (Hopefully boost referenced regulator with 1:1 rising per psi)

 

I think the biggest challenge I had to address on this whole thing; 1. keeping the float bowls full, 2. keeping the carbs properly sealed under pressure

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my current jets stand as following    f11 emulsion tubes.  145 mains, .189 air correctors.

I have a holley feeling the 2l swirl pot which goes to a walbro, that then goes to a boost reference 1:1 rising rate regulator, which then goes to a 3/8 line to a fuel block to the carbs.   I have not upgraded the needle and seat in the webers.

Edited by japscrap
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