General Category > Quadrajet Carb Talk and Tips
Little help here, please (part 2 of 2)
novadude:
Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum. I am running 12 deg initial with the vacuum advance on ported. Nice broad powerband with a ton of midrange torque (i shift it around 5500-5800 rpm). I can leave it in 4th gear (Muncie) going around corners in the neighborhood and can easily accelerate from ~1000 rpm without downshifting with no lugging, bucking, etc. I think the short duration is key.
I definitely agree with you on shrinking the lower idle air bleed on these late 70s Chevy carbs. Seems to make a nice improvement, and as you said, you can run much smaller idle tubes & DCRs.
King Scooter:
Novadude, uncanny how similar our builds are. Makes me think there is more vacuum to be had. Thanks for the input.
Cliff, duly noted and I agree with everything you say. Hindsight being 20/20 I would do a few things different. Now I'm just working with what I have, to get what I can.
You sent me some good distributor/timing info and I will be doing some work on that once the car is back on the road. Maybe with some distributor and carb tuning, I can find some more vacuum.
I'll resize my lower IABs to .063-.067 and then follow recipe 2. I can always drill bigger, but it's hard to drill smaller!
Thanks
Cliff Ruggles:
"Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum."
For sure short seat timing saves the day here. Head flow and ICL are big players here as well, plus a few other things like combustion chamber shape and quench distance.
You could have but another full point of compression in that build, 15-20 degrees more seat timing, wider LSA and it would have idled equally as well and made more power at every RPM and not needed any more octane to be happy......
novadude:
--- Quote from: Cliff Ruggles on April 30, 2024, 01:22:17 AM ---"Cliff... My 9.6:1 355 with a 270/278, 217/225 @ 0.050", 108 LSA Hyd roller cam idles at 750 rpm with ~15.5" vacuum."
For sure short seat timing saves the day here. Head flow and ICL are big players here as well, plus a few other things like combustion chamber shape and quench distance.
You could have but another full point of compression in that build, 15-20 degrees more seat timing, wider LSA and it would have idled equally as well and made more power at every RPM and not needed any more octane to be happy......
--- End quote ---
I have a Muncie M20 and 3.36 gears. I was interested in maximum torque in the 3000-4500 range, since that is where the engine spends most of it's time in a 1/4 mile pass. Tight LSA cams seem to be great for pumping up the midrange torque.
It works very well (12.9x @ 108 mph with a poor 2.0 60 ft in a Chevy II), gets great gas mileage, and with the lower compression it even tolerates lower octane pump gas (tight quench). All-in-all, I am very happy with the combination.
Cliff Ruggles:
I'm sure it works well.
Tight LSA narrows the power curve and pushes peak torque higher and earlier. Both recipes for detonation so for sure you have to keep static compression down to be safe.
You can take the same build, add another full point of compression, another ten degrees of duration, wider LSA, a little later intake closing and it will idle equally as good if not better, and make more power at every RPM with more upper mid-range and top end power.
I'll attach overlapping dyno sheets of how this works. The short version of the long story is that my customer and his engine builder weren't overly happy with the power of the "new" 455 build they had on the dyno. The were using a Comp XR276HR cam, 224/230/110LSA. They said it had a noticeable idle, but quit early and only around 425HP. With the professionally ported head they were expecting around 480-490hp.
Of course I got the call because EVERYONE at the dyno shop right down to the guy taking out the trash was blaming the Q-jet. Instead of talking about carbs I supplied cam specs for a better cam instead. 289/308 @ .006" and 236/246 @ .050" on a 114LSA.
Reluctant at first but the engine builder called Comp and order the cam and installed it. I got a call back and they sent the dyno charts below......
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