Freeway driving and carb secondaries

[Mrturner1]

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.

[davepl]

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. :-)

[Mrturner1]

Originally Posted by davepl

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. :-)

Funny you caught that lol I wasn't gonna tell unless I had to. I downloaded a GPS speedometer on my phone this morn and was testing it out on the freeway. I don't really like holding it at 3000 rpms but I need to at least no impede the flow of traffic.

Gear vendor is my "one day" dream haha

[demian5]

Originally Posted by Mrturner1

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.

You could swap out to a 700r4 or go with 3.23 or 3.55 rear end gears. A 700r4 AND a rear gear change is still wayyyyyyyyyyyyyyy cheaper than a GV (even used).

[franken]

The only way to know for sure is to take the air cleaner off and mount a camera over the carb...

[Mrturner1]

Originally Posted by demian5

You could swap out to a 700r4 or go with 3.23 or 3.55 rear end gears. A 700r4 AND a rear gear change is still wayyyyyyyyyyyyyyy cheaper than a GV (even used).

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.

[Mrturner1]

Originally Posted by franken

The only way to know for sure is to take the air cleaner off and mount a camera over the carb...

I have a GoPro with one of those flexible arms and a clamp on the end, I think I'll give this a try tomorrow.

[garyd1961]

If you have an engine that needs a 3000 stall converter you shouldn't be concerned with gas mileage.

[Mrturner1]

Originally Posted by garyd1961

If you have an engine that needs a 3000 stall converter you shouldn't be concerned with gas mileage.

That's a little silly, being concerned about fuel mileage and wanting my truck to run tip top is two separate things. My current carb is my first Holley, and I'm still learning to tune it so I was curious if it's normal to be into the secondaries on the freeway with a setup like mine. Hopefully some guys will see this who have similar setups or experience with it, and we'll see what they think on the matter. I probably have enough cam for 2 trucks, but if I can better tune my engine/carb and make the thing as efficient as possible I certainly will.

Alot of guys build 4x4's with exo-cages, but they don't go rolling down rocks on purpose.

[CastIron]

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.

[rcazwillis]

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.

[Mrturner1]

Originally Posted by CastIron

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.

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.

[Shinysideup]

Double pumper or vacuum secondaries?

[garyd1961]

Originally Posted by Mrturner1

That's a little silly, being concerned about fuel mileage and wanting my truck to run tip top is two separate things. My current carb is my first Holley, and I'm still learning to tune it so I was curious if it's normal to be into the secondaries on the freeway with a setup like mine. Hopefully some guys will see this who have similar setups or experience with it, and we'll see what they think on the matter. I probably have enough cam for 2 trucks, but if I can better tune my engine/carb and make the thing as efficient as possible I certainly will.

Alot of guys build 4x4's with exo-cages, but they don't go rolling down rocks on purpose.

For a short while I had a 383 with a big hydraulic roller cam and 3600 stall in my truck. It sounded awesome but I hated it. It had two speeds, idle and wide open. I have always heard if you put a race car motor in a truck you will have a half azz race car and a half azz truck. I had to find out the hard way though.
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.

[Mrturner1]

Originally Posted by garyd1961

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.

Well, "better" is in the eye of the beholder I suppose. I have a 99 Silverado with 5.3 and 4L65e sittin around in perfect shape, I could pull it all out and throw a cheap turbo on it and about 8lbs of boost and have a 450hp DD that's reliable and quiet too. But I like the first gen SBC and I love havin a 750 DP that I'm getting good at tuning. And I especially love that big ole cam I put in there different strokes I guess

[garyd1961]

Originally Posted by Mrturner1

Well, "better" is in the eye of the beholder I suppose. I have a 99 Silverado with 5.3 and 4L65e sittin around in perfect shape, I could pull it all out and throw a cheap turbo on it and about 8lbs of boost and have a 450hp DD that's reliable and quiet too. But I like the first gen SBC and I love havin a 750 DP that I'm getting good at tuning. And I especially love that big ole cam I put in there different strokes I guess

There's nothing wrong with having a hotrod, just most hotrods spend more time in the shop than on the highway. You don't need a turbo either, good flowing aluminum heads and good flowing exhaust is the best way to make hp these days. A good cam helps also but a big lumpy cam usually brings more problems than hp.
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.

[garyd1961]

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.