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| DCOE Jetting https://www.slantsix.org/forum/viewtopic.php?t=62629 |
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| Author: | Mike Andreas [ Mon Jul 23, 2018 8:19 am ] |
| Post subject: | DCOE Jetting |
I ran at North Wilksboro last weekend for the first time with my WEBER 40 DCOE's. The car ran pretty consistently but I think my jetting is way off. Upon doing more research in HPBooks WEBER Carburetors book it seems that my WEBERS have the same jetting as a LOTUS (something). They don't list anything that is very close to the 225. My car is about 600 CC per Cyl. So, 2 DCOE's on a 2.4L engine might be close. I'm sure I'm going to end up buying a bunch of jets. I"d just like to start with a responsible guess. This is a google doc with my current jets and what I think I should have based on the info I got in "Speedpro Series How to Build & Powertune WEBER & Dellorto Carbs. https://docs.google.com/spreadsheets/d/ ... sp=sharing Let me know if anyone can see this. |
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| Author: | Joshie225 [ Mon Jul 23, 2018 10:09 am ] |
| Post subject: | Re: DCOE Jetting |
You cannot put a 38-40 mm choke in a 40DCOE. If you really need a choke bigger than a 32mm then you need 45DCOE carburetors or larger. Yes, 36mm chokes are available, but there is an optimum ratio between the throttle bore (40mm in this case) and the main venturi diameter. This is about 80% which is how 32mm ends up being the largest consistent with a good functioning carburetor. How did you come up with that choke size? What induction did you have before and at what RPM do you shift at the strip? |
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| Author: | Mike Andreas [ Mon Jul 23, 2018 12:01 pm ] |
| Post subject: | Re: DCOE Jetting |
Yeah. I know there aren't chokes that big for a 40 DCOE. I should have clarified. I got those number from the Speed Pro book It says that for a 3.7L 6 cyl with 617 CC's per cyl the optimal choke is 38-40. That's not considering all the parameters that apply to a slant six. Long stroke. low redline, etc. I have a 26 and a 30 choke. I don't think I'd go any larger than 32. The jetting I'm running is what was in the carbs when I got them. I think I'm really lean on the Idle jets because I have to turn the adjuster out about 4 turns to get it to idle and the Idle speed screws are a couple of turns in as well. Should be about 1 turn on them both from what I've read. Main jets seem small too as at steady state 2500rpm I get intermittent sputtering back through the carbs. Also Air corrector might be wrong too as the motor really doesn't pull hard past 4000. I'm in the beginning of the learning curve here so there's lots to explore I was running a 9400 AFB on an Offy intake before. Life was simpler then. |
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| Author: | Mike Andreas [ Mon Jul 23, 2018 5:34 pm ] |
| Post subject: | Re: DCOE Jetting |
I'm launching at about 3500 and shifting to 2nd and 3rd at 4k. That seemed to work best last weekend at Wilksboro. With the AFB I used to shift at 5k. I think my 26 chokes are too small to have much top end. I'll try the 30's but I think I'll need larger idle and main jets |
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| Author: | Dart270 [ Mon Jul 23, 2018 9:23 pm ] |
| Post subject: | Re: DCOE Jetting |
AFR gauge... Huge time saver. Lou |
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| Author: | Joshie225 [ Mon Jul 23, 2018 9:37 pm ] |
| Post subject: | Re: DCOE Jetting |
I saw that the car was quicker and faster shifting at a lower RPM. Without knowing if the A/F ratio was going lean it's hard to say for certain if it's just fuel or also air, but I do think more air would help. The Haynes book I have on Webers says for a 600cc cylinder with a maximum power RPM of 6,000 RPM a 40mm choke is appropriate, but it also says that a smaller choke may be desirable for increased flexibility. If the engine has a well prepared cylinder head, 270° or longer duration cam and proper exhaust then I would probably go for 36mm chokes in a 45 DCOE. But if you have 30mm chokes and the tool to change them then I would do that. I agree that the idle jet is likely too small. I second Lou's suggestion of an AFR meter. The time saved is well worth the expense. |
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| Author: | Mike Andreas [ Tue Jul 24, 2018 7:03 am ] |
| Post subject: | Re: DCOE Jetting |
Ported head. Headers. Big exhaust. Dont remember he cam specs put it’s pretty agressive. I have DCOE 40’s so 45’s aren't happening. I can change out the chokes fron 26 to 30’s. May eventually source 32’s. I think for now I’ll install the 30’s and rejet accordingly. While i had it apart last month I had an O2 sensor bung welded in. Ran out of time before wilksboro to get it installed. Also, I’m running the mechanical fuel pump. I have everything to install an electric pump. Just ran out of time. Should be all sorted out by Clay City. I appriciate the advice. |
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| Author: | Dart270 [ Tue Jul 24, 2018 10:55 am ] |
| Post subject: | Re: DCOE Jetting |
Here are good gauges. https://www.summitracing.com/parts/avm-30-4110 https://www.summitracing.com/parts/avm-30-0300 Lou |
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| Author: | Exner Geek [ Wed Jul 25, 2018 7:38 pm ] |
| Post subject: | Re: DCOE Jetting |
I had a set of 45s that came with both 38 and 40MM chokes. I was reluctant to use the 40s because of what Joshie said about the ratio of the choke to the throttle bore, but when I did, it did make more power. This was a race only car and maybe it would have affected it's street manors but I wouldn't be afraid to use a 36 choke in a 40 DOCE. |
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| Author: | Mike Andreas [ Thu Jul 26, 2018 1:10 pm ] |
| Post subject: | Re: DCOE Jetting |
Yeah I think the trick for me is to get the jetting right, whatever choke I decide on. I have 30's. I'm going to swap them in and start there. I should get better breathing above 4000. I'm looking for drivability too. I don't want this to be a "Race Car". |
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