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#1 |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Freeway driving and carb secondaries
Hey there, I had a question about my carb and the secondaries during freeway driving. The truck is a 1970 c10 with a th400 and 3000 rpm stall. The rearend is a GM 12 bolt with 3.73 gears. When i'm on the freeway doing 65 mph my tachometer says 3200 RPMs and, now I know that's a little bit wound up but what I'm wondering is MI into the secondaries on my carburetor or not? Because I would make a huge difference in miles per gallon on the freeway.
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#2 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Freeway driving and carb secondaries
You would not normally be, no. Solid state cruise on a highway should be all on primaries, no power valve involved, no secondaries. Passing and climbing hills would be different.
You might not be aware, but you're allowed to use the terms "mileage" and "double pumper" on the same forum. :-)
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#3 | |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
Quote:
Gear vendor is my "one day" dream haha |
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#4 | |
Registered User
Join Date: Aug 2016
Location: Lake Forest, CA
Posts: 2,177
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Re: Freeway driving and carb secondaries
Quote:
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"Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC |
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#5 |
Registered User
Join Date: Jun 2013
Location: Somewhere
Posts: 3,081
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Re: Freeway driving and carb secondaries
The only way to know for sure is to take the air cleaner off and mount a camera over the carb...
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#6 |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
A 700r4 wouldn't hold up or serve the purpose of the truck unfortunately. It would be great, but it's not for this truck. I think a GV is going to be around 2700 bucks all said and done, maybe it's just time to start saving.
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#7 |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
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#8 |
Registered User
Join Date: May 2016
Location: Henderson NC
Posts: 975
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Re: Freeway driving and carb secondaries
If you have an engine that needs a 3000 stall converter you shouldn't be concerned with gas mileage.
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#9 | |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
Quote:
Alot of guys build 4x4's with exo-cages, but they don't go rolling down rocks on purpose. |
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#10 |
Registered User
Join Date: May 2013
Location: Oregon
Posts: 328
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Re: Freeway driving and carb secondaries
If your engine is turning at 3200 at 65 mph, what's it turning at 55 and less? I'd be worried about having the stall speed so close to the cruising speed. You might be getting your atf pretty hot if you cruise much at speed below 50-55.
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#11 |
Registered User
Join Date: Aug 2013
Location: Fort Worth, TX
Posts: 309
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Re: Freeway driving and carb secondaries
Which Holley is the carb? Is it a double pumper? If so, the mechanical secondaries might allow you to feel where the secondaries are being opened in the pedal movement. You could judge at what point they open with someone looking at the carb while you push the pedal. Then correlate that to where the pedal is at going down the highway. It is not exact, but you could get an idea.
The GoPro is a more fun idea! Besides, you get sound. Post it when you do it. |
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#12 |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
I have an external trans cooler that seems to work pretty well. Typically the temp is around 160, if I drive it hard it spikes to 180 and then comes down. I was a little worried about that myself, but it seems to stay pretty cool.
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#13 |
Registered User
Join Date: Apr 2012
Location: Ocala,Fl
Posts: 733
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Re: Freeway driving and carb secondaries
Double pumper or vacuum secondaries?
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#14 | |
Registered User
Join Date: May 2016
Location: Henderson NC
Posts: 975
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Re: Freeway driving and carb secondaries
Quote:
Also things have changed since the sixties, if you want a truck with a lot of hp there are better ways to get it than with a big ol' cam. If you have vacuum secondary's you can change the spring in the little vacuum canister or what ever it's called to adjust when the four barrel opens. |
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#15 | |
Registered User
Join Date: Nov 2015
Location: Colfax California
Posts: 1,644
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Re: Freeway driving and carb secondaries
Quote:
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#16 | |
Registered User
Join Date: May 2016
Location: Henderson NC
Posts: 975
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Re: Freeway driving and carb secondaries
Quote:
I understand wanting the sound of the big cam but unless you are running at the track and using racing fuel it's not really beneficial. As you said different strokes for different folks. I have a GM Vortec crate engine with a 770 Holley (vacuum secondary's) and 2 1/2 exhaust in my 1970 C10. I could change the cam and get 400 hp easily but I'm happy with the measly 300+ hp I have now. One day I do plan to put my 383 back in my S10. It will be strictly a short distance hotrod though. |
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#17 |
Registered User
Join Date: May 2016
Location: Henderson NC
Posts: 975
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Re: Freeway driving and carb secondaries
I forgot to say in my above post that if you have a 750 dp you should have a progressive linkage. Your secondary's don't come in until the primary's are half open. They shouldn't come in during highway driving unless you have something really wrong with your set up.
Last edited by garyd1961; 10-09-2016 at 09:29 AM. |
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