Author Topic: Top Speed of 70?  (Read 6728 times)

Offline teamcurtis

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Top Speed of 70?
« on: April 04, 2017, 10:48:41 AM »
I have a pretty stock 78 c10. 350
I just replaced the intake manifold with a more performance oriented one and gave it a general tune.
The truck idles and runs great. Off the line there is good power and smooth acceleration.
On the freeway the other day i was going about 70 (speedo jumps around). When i punched it to get around someone the truck did nothing. There just wasnt anything left.
Is this normal for the truck? Do I have a kickdown issue? Some other issue?
1978 C10 Short Bed
350 smogged for CA

Offline fitz

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Re: Top Speed of 70?
« Reply #1 on: April 04, 2017, 04:14:46 PM »
How was the truck before the manifold swap?
What do you have for a trans?
I'm gonna guess it's a turbo 350, in which case I wouldn't think it should downshift into 2nd gear going 70 mph.

Offline teamcurtis

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Re: Top Speed of 70?
« Reply #2 on: April 04, 2017, 05:02:26 PM »
performance overall is defn better with new manifold.
turbo 350 trans
honestly i don't think i ever really tired hitting the gas over 60 before. Ive only had the truck a year
1978 C10 Short Bed
350 smogged for CA

Offline Rapid Roy

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Re: Top Speed of 70?
« Reply #3 on: April 04, 2017, 05:16:37 PM »
My stock 350/TH350 will only kick down till about 62 or 63mph. After that have normal acceleration. Not sure if this is correct, but that's how it is working now. I don't have a problem with it though.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline teamcurtis

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Re: Top Speed of 70?
« Reply #4 on: April 04, 2017, 05:34:25 PM »
My stock 350/TH350 will only kick down till about 62 or 63mph. After that have normal acceleration. Not sure if this is correct, but that's how it is working now. I don't have a problem with it though.

what would you say your trucks top speed is?
1978 C10 Short Bed
350 smogged for CA

Offline Rapid Roy

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Re: Top Speed of 70?
« Reply #5 on: April 04, 2017, 06:08:49 PM »
My stock 350/TH350 will only kick down till about 62 or 63mph. After that have normal acceleration. Not sure if this is correct, but that's how it is working now. I don't have a problem with it though.

what would you say your trucks top speed is?

I have had it to 90 twice. Stopped there, did not push it. Not sure how high it would go. A little afraid to find out. LOL.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline septantrionalis

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Re: Top Speed of 70?
« Reply #6 on: April 05, 2017, 12:03:37 PM »
I have a 74 K10 with the original 350 motor, TH350 transmission, NP203 tcase, and 3.73 rear end.  I added a 4" lift with 35" tires.  Max speed on my truck is around 70 on a good day.  I can probably hit 75 when going downhill.  Going uphill is brutal.  Semi's pass me up.

Offline teamcurtis

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Re: Top Speed of 70?
« Reply #7 on: April 05, 2017, 02:49:53 PM »
thanks....  doesnt sound like im TOO far out of line
at least not something to worry about for now
1978 C10 Short Bed
350 smogged for CA

Offline Rapid Roy

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Re: Top Speed of 70?
« Reply #8 on: April 06, 2017, 06:39:56 PM »
teamcurtis, do you have a 4bbl carb? If you do, the secondary should engage and give more power for acceleration. Not like kickdown into 2nd gear, but more acceleration power to gain more speed. Mine does with 4 bbl carb. quadrajet.

Have thought about this possibly an issue. I think you should be able to go faster than 70mph!.....if you want.
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline teamcurtis

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Re: Top Speed of 70?
« Reply #9 on: April 07, 2017, 01:05:00 PM »
I do have the 4 barrel...
I'll have too to be sure the secondaries are opening up..
I would the the truck COULD do 80-90... not that it wouldnt be horrifying

ps love the jim croce reference.
1978 C10 Short Bed
350 smogged for CA

Offline 79ScottsdaleGuy

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Top Speed of 70?
« Reply #10 on: August 13, 2017, 11:58:04 PM »
I would look at possible adding in somemore timing.  Granted, im running vortec heads, a 310 duration cam, 11:1 compression, with good ole hyperutetics, 2500 rpm stall,  and a weiand speed warrior air gap intake, carb spacer, quick fuel 650 double bumper, roller tip rockers, bigger valves, stronger pushrods, stronger springs, but it also unfortunatley has the factory 2.76 rear end.  I had to add alot more timing than what i first had speculated.  Now, i can cruise doing 60-70, punch the throttle, the th350 with stage 2 shift kit, automatically downshifts to second, and the rear end gets squirley as i pass my intended target.  I would look into the timing.  Also, what gears you do you have in your differential?


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1979 Chevrolet Scottsdale C10 Auto 3 speed Non-A.C.

Offline blazer74

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Top Speed of 70?
« Reply #11 on: August 14, 2017, 12:50:34 AM »
You both should be able to run 70 plus. Even with or without 35's and 3:73 gears.
How many miles on your engines?
Do you know what rpm's you top out at?
Engine specs, initial and mechanical timing, vac advance hooked up and working?
Exhaust condition?
Compression test?

For ref: 74 K5 Blazer with my rebuilt original 1974 350 engine (not stock) with vortec heads, vortec intake, Quadrajet, stock HEI, Headers with duals, 268H comp cam 218 duration. Compression ratio is approx 8.8:1.
10 degrees initial, 24 mech and another 12-15 vac advance.

has no issues reaching and surpassing 70 mph. Off hand I don't recall at what rpm/speed  I don't kick down with the TH350.

I don't like to run that fast tho.

I also run 35 tires with 4:10 gears and 2000 stall.

Even before I installed the vortec heads/stall and removing the original 882 heads I had no issues with 70.
« Last Edit: August 14, 2017, 12:59:02 AM by blazer74 »

Offline 75gmck25

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  • 1975 GMC K25 Camper Special, 350/TH350/NP203
Re: Top Speed of 70?
« Reply #12 on: August 16, 2017, 09:22:58 PM »
I have a fairly stock GMC K25 (stock 16" tires and 4.10 gears, 350 crate engine, TH350, etc.), and I have had it up to about 85 MPH, and it was still going. 

- Make sure your TH350 kickdown cable is adjusted correctly.  You must readjust it any time you make any manifold or carb changes.  I'm not sure how fast I can go before I don't get a full throttle kickdown. 
- Make sure your throttle cable is adjusted so that at full throttle it pulls the secondaries open.  I had mine set up wrong, and didn't realize the secondaries were not opening until I found couldn't get the engine to go rich at full throttle (I have a wide-band AFR).  Once I got it adjusted, I could see the AFR at about 12 at full throttle as the secondaries kicked in.  Without secondaries working properly, at full throttle it just stayed lean (14-15) and the power flattened out.

Bruce