Carb Sizing/vacuum or Mechanical

General engine tech -- Drag Racing to Circle Track

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Horder
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Re: Carb Sizing/vacuum or Mechanical

Post by Horder »

I think the 650 or 750 would be great. I have never seen these before. The Satin style finish is a bit less bling and the carbs seem to be loaded with adjustability
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Re: Carb Sizing/vacuum or Mechanical

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Horder wrote: Tue Dec 27, 2022 7:54 pm I think the 650 or 750 would be great. I have never seen these before. The Satin style finish is a bit less bling and the carbs seem to be loaded with adjustability
Have you watched the vid of how much adjustability?

https://youtu.be/qW1essuPVAI

I look forward to seeing them on fb marketplace in the future when people go nuts trying to integrate the 4 circuits
Last edited by HQM383 on Tue Dec 27, 2022 8:07 pm, edited 1 time in total.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
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Re: Carb Sizing/vacuum or Mechanical

Post by Horder »

HQM383 wrote: Tue Dec 27, 2022 8:00 pm
Horder wrote: Tue Dec 27, 2022 7:54 pm I think the 650 or 750 would be great. I have never seen these before. The Satin style finish is a bit less bling and the carbs seem to be loaded with adjustability
Have you watched the vid of how much adjustability?
You mean all the circuits? All the adjustable bleeds etc? Yes. Have you used one? Any feedback? Or just not required?
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Re: Carb Sizing/vacuum or Mechanical

Post by HQM383 »

Horder wrote: Tue Dec 27, 2022 8:06 pm
HQM383 wrote: Tue Dec 27, 2022 8:00 pm
Horder wrote: Tue Dec 27, 2022 7:54 pm I think the 650 or 750 would be great. I have never seen these before. The Satin style finish is a bit less bling and the carbs seem to be loaded with adjustability
Have you watched the vid of how much adjustability?
You mean all the circuits? All the adjustable bleeds etc? Yes. Have you used one? Any feedback? Or just not required?
No I haven't tried one but would mind doing so. Prohibitively expensive to buy one to give a go here in Aus. Personal opinion, a two circuit 4150 will more than handle what you want to do with less headaches.
Last edited by HQM383 on Tue Dec 27, 2022 8:25 pm, edited 1 time in total.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
Horder
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Re: Carb Sizing/vacuum or Mechanical

Post by Horder »

HQM383 wrote: Tue Dec 27, 2022 8:18 pm
Horder wrote: Tue Dec 27, 2022 8:06 pm
HQM383 wrote: Tue Dec 27, 2022 8:00 pm

Have you watched the vid of how much adjustability?
You mean all the circuits? All the adjustable bleeds etc? Yes. Have you used one? Any feedback? Or just not required?
No I haven't tried one but would mind doing so. Prohibitively expensive to buy one to give a go here in Aus. Personal opinion, a two circuit 4150 will mie than handle what you want to do with less headaches.
I KNOW you are right…lol

Too much time in my hands today looking at carbs and reading. Those carbs appeal to me, I like the look and the engineering. But…really not required for what I am doing.
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Re: Carb Sizing/vacuum or Mechanical

Post by rgalajda »

Coloradoracer wrote: Tue Dec 27, 2022 12:36 pm
Tom68 wrote: Mon Dec 26, 2022 8:00 pm Don't forget the smaller Quadrajets were something like 750cfm.
Early ones were. Later versions were 800 cfm and used on all sizes of engines when equipped with a four barrel carb.
More accurate would be 750 and 800 cfm capable.
Quadrajet carbs were used on engines ranging fro 305 to 454 and 455 cu in
Not all quadrajets flowed to 750 or 800 cfm . For example when used on a 305 or 350 cu in low performance engine the secondary air valve did not open fully. Limiting the cfm this way was how the factory made these carbs so versatile. Some quadrajets were limited to approximately 600 cfm.
This was not adjustable as from the factory either. These carbs used different shafts on the secondary air valve to limit the cfm for the engine application.
I have restored a lot of these OE carbs and I keep a record of the air valve opening measurements .
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Re: Carb Sizing/vacuum or Mechanical

Post by rfoll »

All of my regular street vehicles have Quadrajets. I like the large primary carbs better. I have never tried one with a big cam. For that I use a 3310/780, 4780, or 4781.
So much to do, so little time...
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Re: Carb Sizing/vacuum or Mechanical

Post by rgalajda »

"I have access to a 750 VS that I could borrow and tune."

If you don't mind me asking , do you know the list no. of the 750 VS ?
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Re: Carb Sizing/vacuum or Mechanical

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Its a 3310 750 CFM
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Re: Carb Sizing/vacuum or Mechanical

Post by FC-Pilot »

Horder wrote: Wed Dec 28, 2022 10:19 am Its a 3310 750 CFM
If it’s an older one they are great carbs. The newer ones are decent, but can be darn good with some of the mods to make them like the old ones.

Paul
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Re: Carb Sizing/vacuum or Mechanical

Post by skinny z »

Horder wrote: Mon Dec 26, 2022 11:20 am ... 10 deg vac adv
Ported or manifold vacuum? (I didn't see the subject of ignition timing come up in the conversation but that doesn't mean it didn't!)
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Re: Carb Sizing/vacuum or Mechanical

Post by Horder »

I run manifold vacuum
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Re: Carb Sizing/vacuum or Mechanical

Post by skinny z »

Horder wrote: Wed Dec 28, 2022 8:14 pm I run manifold vacuum
Interesting.
Did I see you reported that there was a sag when you released the clutch?
Somewhere buried in my tech papers is an article on vacuum advance and the full manifold vs ported vacuum advance debate.
One takeaway from that was about the use of full manifold vacuum and a manual transmission. It seems that, as you can imagine, at the point just before clutch engagement, RPMs and manifold vacuum are high enough to keep the vacuum advance curve in the advanced range. Torque is up due to that advance.
However, when the clutch engages, RPMs and the subsequent manifold vacuum both drop. So does the timing. If the vacuum advance curve isn't tuned to counter this then that tech paper describes the phenomena as "torque droop". And there goes that snappy throttle response as a consequence.
( lord knows I've spent hours getting my automatic to respond favourably to full manifold vacuum as the benefits for me are very evident).
Just throwing that thought out there.
The testing and tuning with a mechanical distributor, adjustable advance cans, limiter plates and the rest is just tedious. But the results paid dividends for me. I can imagine it would even be more pronounced with a manual transmission. (A high stall converter can crutch a lot of this).
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Re: Carb Sizing/vacuum or Mechanical

Post by Horder »

Yes, exactly as you describe it. Its almost as though it wants to droop unless you give it enough pedal and then because of the excellent low speed power it makes, it wants to chirp the tires. This is below 2000 RPM

Very interesting. Something to experiment with. I appreciate that info.

I have had this vehicle for a while. Its my third 327. This distributor however is new to me. I enjoy tuning the carb and distributor. This gives me another option. I have a few different vacuum cans that I ran in my old distributor and possibly a B28 can that stays deployed at a lower vacuum would help a bit. The can thats in there is fully advanced at idle so there would be a large drop in advance on engagement. Especially since my springs aren't allowing any mechanical advance until about 1400 RPM. If I engage the clutch closer to 1600-1800 it takes off too well. Lol

I think you are on to something

My other thought was that my .028 nozzle might be a bit small for this engine
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Re: Carb Sizing/vacuum or Mechanical

Post by skinny z »

I found it a pain in the ass to tune.
Locked out the mechanical (zip ties on the weights).
Vacuum pumped the advance to see onset point, rate and total.
Measured engine vacuum at various RPMs (no load).
Built a curve from there with an adjustable can for onset and a limiter plate for total.
Also worked out a high advance lean cruise to claw back some highway fuel economy.

Next up is Progression Ignition's fully programmable distributor (or similar).
Kevin
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