Register or Log In To remove these advertisements. |
|
|
|
Thread Tools | Display Modes |
11-23-2019, 12:02 PM | #1 |
Registered User
Join Date: Jan 2008
Location: Riegelsville P.A
Posts: 38
|
78 K20 towing with 454
Hello I have a 1978 Chevy K20 that I plan on swapping a big block into with a TH400 transmission. I would like to hear from anyone with some experience on what final drive ratio to go with as I would like it to be able to tow some weight and not be a dog, but have a final drive ratio that isn't screaming too bad a 75 MPH on the highway. The truck has a 2" lift with 265/75R16 tires. I realize that I'm not going to get good gas mileage no matter what, but that's why the truck has 2 fuel tanks right ?
Truck is currently set up with a 350 SB with th350 trans and 4.10 gears and it is really buzzing that 350 at highway speeds. |
11-23-2019, 12:37 PM | #2 |
Post Whore
Join Date: May 2015
Location: Alabama
Posts: 14,670
|
Re: 78 K20 towing with 454
With your current setup (4.10ratio,,31.6tall tire,,1:1trans,,75mph)..your turning around 3270rpm at 75mph
Gear swap sounds like it's in order....or get a 4l80 trans and have the best of both worlds...low ratios and od....the od would get you down to 2289rpm at 75mph Bad part of gear swap is cost x2 for a k series.. Heres a rpm calculator thatll help you....http://www.csgnetwork.com/multirpmcalc.html
__________________
Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver |
11-23-2019, 12:43 PM | #3 |
Registered User
Join Date: Jan 2008
Location: Riegelsville P.A
Posts: 38
|
Re: 78 K20 towing with 454
I currently have a TH400/208 that I got for a song and dance. Does anyone here have experience with fitting the 208 to a 4L80E? Seems to be lots of questionable information on the web regarding this swap.
|
11-23-2019, 01:00 PM | #4 |
Post Whore
Join Date: May 2015
Location: Alabama
Posts: 14,670
|
Re: 78 K20 towing with 454
If... and I mean if...you can find a 90-91 3/4 suburban 4x4...it will have a 4l80/np241 pass side drop..
The 208/241 is a light casting transfer case...I would look for a 205 if it were me.....sometimes cheap ain't the best way
__________________
Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver |
11-23-2019, 01:46 PM | #5 |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
|
Re: 78 K20 towing with 454
Are you towing at 75 mph ? (Gooseneck?)
Can one tow with a 4L80 in overdrive? (not supposed to with 4L60/700r4)
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
11-23-2019, 03:30 PM | #6 |
Registered User
Join Date: Dec 2012
Location: Temple City
Posts: 3,628
|
Re: 78 K20 towing with 454
https://www.richmondgear.com/wp-cont...alculator.html
This will help you choose a ratio that you feel is the best of both worlds. |
11-23-2019, 08:24 PM | #7 |
Registered User
Join Date: Jun 2008
Location: Moss Point,Ms.
Posts: 1,439
|
Re: 78 K20 towing with 454
I have a 82,k-10 with a 454,t-400,373 rear gears. On 12.50x33x15 B.F. Goodrich tires.
I towed a 28' travel trailer all over the country,including the Smoky Mountains, east side of the Rockies and over the continental divide. Got about 8 miles to the gallon average. And,yes I towed at 70 mph. Mike. |
11-24-2019, 01:54 AM | #8 |
Registered User
Join Date: Aug 2009
Location: Sunray,TX
Posts: 251
|
Re: 78 K20 towing with 454
I tow with an 8.1/ 80e in my k30 with 4:10 gears, and i'm wanting to go deeper (have an extra set of 4.56 CUCV axles), but I am running 315/75 tires.
with your tire and no OD, 4:10 will pull awesome, but pull at 65mph... at least that's my advice. Even though I can pull 80-100 with my setup, it's stupid to do so. My experience is that trailer tires stay together better when ran under 70mph. For optimum pulling with a big block, it probably needs to be in the range of 2500-2800rpm in cruising gear while at pulling speed. 3:73 gears would be about perfect for your setup if you plan to stay on those smaller tires. If you go up to a 285 or bigger (rubber overdrive), then the 4:10's you have will be about perfect. |
11-24-2019, 01:48 PM | #9 |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
|
Re: 78 K20 towing with 454
From my experience and observation, 70 mph with a bumper pull is really up there at the ragged edge. With a gooseneck or fifth wheel it's really just how fast can your truck go
There seems to be a lot more trailer wrecks in the past few years. I attribute it to trucks that can actually pull a trailer 80 mph. In the past you might be lucky to get up to 65. Passing a semi going 65, when that bow wave hits the tail end of your trailer … well, you better have a really steady hand and tap the trailer brakes to straighten out or you're going to end up in the ditch with a big pile of splinters. Lots of truck dash cams on youtube if you don't believe me. We were passing a pickup and camper trailer in our little car down by Crow Agency last summer, when he blew a trailer tire. It scared the sh1t out of me! Fortunately, it was his right rear tandem so it didn't swing over into our lane, and he managed to slow it down without going into driver induced yaw. We were going the posted 80 so he must have been doing 70-75. Too hot, too fast, and maybe too old of a tire …?
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
11-25-2019, 09:13 AM | #10 |
Registered User
Join Date: Jan 2008
Location: Riegelsville P.A
Posts: 38
|
Re: 78 K20 towing with 454
The truck will be used for hauling small construction equipment ( Kubota L47TLB ) around town to different job sites, sometimes on the highway but no long hauls. After reading the input on this post I will probably do the engine and trans swap and I have a GM14 bolt 11.5 rear with 3.73's ill throw in to see if that works a little better on the highway
|
11-25-2019, 11:09 AM | #11 | |
Registered User
Join Date: Jun 2008
Location: Moss Point,Ms.
Posts: 1,439
|
Re: 78 K20 towing with 454
Quote:
BUT,you must have sway control and weight stabilizers installed and adjusted correctly! Tires with stiff side walls are required.Mine weren't till later after the BFG's wore out. I also had a helper spring on the front axle to help with the weight. Many times big trucks would fly past,trying to blow me off the road and did it too! I have a C.B. in the truck and could listen to them laughing at me! Manually over-ride the trailer brakes! Mike. |
|
11-25-2019, 11:40 AM | #12 |
Post Whore
Join Date: May 2015
Location: Alabama
Posts: 14,670
|
Re: 78 K20 towing with 454
Matt...are you just gonna try the 3.73s to see how you like it? ....if so, just remember to not use 4wd until the front ratio matches..
Towing at 70mph is not a big deal...if your equipped correctly..I regularly pull a 9500lb gross loaded trailer..its a bumper hitch and I pull it with my C30..and yes I also run 65-70...never had any issues.. Towing around town like you'll be doing you should be fine...
__________________
Mongo...aka Greg RIP Dad RIP Jesse 1981 C30 LQ9 NV4500..http://67-72chevytrucks.com/vboard/s...d.php?t=753598 Mongos AD- LS3 TR6060...http://67-72chevytrucks.com/vboard/s...34#post8522334 Columbus..the 1957 IH 4x4...http://67-72chevytrucks.com/vboard/s...63#post8082563 2023 Chevy Z71..daily driver Last edited by mongocanfly; 11-25-2019 at 11:46 AM. |
11-25-2019, 12:18 PM | #13 |
Registered User
Join Date: Jan 2008
Location: Riegelsville P.A
Posts: 38
|
Re: 78 K20 towing with 454
Mongocanfly, yes haha. I understand the front axle will need to be re-geared which is no problem. It currently has a 14bolt 9.5 rear which is a semi-float light duty axle.
I got the 454 TH400 and 14bolt 11.5 with 3.73 out of a C20 suburban for free. The engine has less than 4k miles on it. Guy put a bunch of money into the burb and then a tree fell on it during the Sandy hurricane. "get it off my lawn and you can take it " deal. |
11-25-2019, 02:00 PM | #14 | ||
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
|
Re: 78 K20 towing with 454
Quote:
Quote:
I used to purposely whip my trailer back and forth now and then just to see if there was someone hiding directly behind me in my blind spot. I've pulled everything up to a 10x50 mobile home, but that was with the old V-6 GMC and couldn't get it going that fast, and it was still 55 limit back then. Once I got used to the size, that was about the easiest trailer to pull.
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
||
11-27-2019, 11:09 AM | #15 |
Registered User
Join Date: Dec 2013
Location: Columbus, Oh
Posts: 367
|
Re: 78 K20 towing with 454
|
11-30-2019, 06:29 PM | #16 |
Registered User
Join Date: Oct 2015
Location: Ottawa, Ontario, Canada
Posts: 766
|
Re: 78 K20 towing with 454
The big thing with any A/T trans overdrive is to keep the speed/RPM over the up shift or down shift points. If your going drive just at the speed/RPM where it down shifting every few seconds it will kill any A/t Trans in time. So you should not drive in over drive un till you can keep the over drive speed/RPM in the gear. get a good cooler!
__________________
1977 REG cab Long Bed with a 1984 Front Clip, 82 305 .60 over 9.0.1 flat top pistons, 4bbl, TH350, w/84 SS-intake & cam , w/ 1977 big block rad, 1975 gear box, 1984 master w/ metric fittings top & SAE fitting @ portion block, 1975 3.42 gear open diff. Duel 77-87 Factory air cleaner, duel inlet from rad support . Down sized 9' to new 8' Steel flat bed Was a 77 454 w/308 gear. Taken out 550 lbs. up front with motor clip change. |
11-30-2019, 07:10 PM | #17 | |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
|
Re: 78 K20 towing with 454
Quote:
Pulling a trailer over McDonald Pass, I found that it would stay in Drive/high until it slowed down to about 30 mpg then finally shift into 2nd and roar back up towards 50 then shift back into drive and start slowing down again. Enough with that, I would just manually drop into 2nd about 45 mph and hold it there all the way up with no extra throttle. Even if it wasn't verboten, I can't imagine towing in OD with my transmission. I wish I had a 2500 with the better automatic ...
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
|
12-04-2019, 01:01 PM | #18 |
Registered User
Join Date: Dec 2013
Location: Columbus, Oh
Posts: 367
|
Re: 78 K20 towing with 454
My 85 Suburban C20 454/400 turbo updated to a 4L80E netted about 25% improvement in Fuel mileage towing at highway speed of 65 MPH. Rpm went from 2750 to 1950 with 3.73 and 235/85/16 tires.
|
12-04-2019, 01:41 PM | #19 |
Senior Member
Join Date: Mar 2013
Location: Ann Arbor, MI
Posts: 1,029
|
Re: 78 K20 towing with 454
My old tow rig. 454, TH400, 3.73’s, rear helper bags, and 265 tires.
I towed with this combo for years with similar or heavier loads shown at 65-70MPH. Never checked mileage but the motor was in the sweet spot at those speeds. |
12-04-2019, 04:50 PM | #20 |
Registered User
Join Date: May 2015
Location: Montana
Posts: 3,696
|
Re: 78 K20 towing with 454
We had a 1977 C-10 Suburban with a small block and SM465. No idea the exact axle ratio, but it was fairly low. Still we towed a camper and a big u-haul trailer up to 65mph on the interstate. As I recall it got about 13 mpg towing and about 18 mpg solo, which seems pretty reasonable.
Now I'm almost sorry we traded it off because it always ran so good and pulled so strong. But it was a rugged brute: manual steering, no headliner or insulation, and the rust that had been painted over before we got it was coming through pretty bad. The 2wd did fine on snow and ice but not so well on muddy Montana back roads.
__________________
Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
Bookmarks |
|
|