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Old 09-26-2021, 07:20 PM   #1
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1970 L47 400 BB Torque and HP chart

My google-fu is broken. I'm looking for the complete Torque and HP curves for the stock engine, not just peak numbers. I've seen it here somewhere, but don't seem to be able to relocate it. They aren't in the repair manual I have, and any search on the internet comes up with SB 400 and BB 427s and 454s.
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Old 09-26-2021, 07:47 PM   #2
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Re: 1970 L47 400 BB Torque and HP chart

https://www.google.com/search?channe...e+and+HP+chart

Scroll down 4 or 5 and you'll find some info from this site.

Last edited by MySons68C20; 09-26-2021 at 07:49 PM. Reason: missing info
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Old 09-26-2021, 08:43 PM   #3
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Re: 1970 L47 400 BB Torque and HP chart

Thanks, but that looks almost exactly like my search results. I didn't find the chart in that group of searches, either. I KNOW that I've seen the chart somewhere here, but I just can't find it.
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Old 09-26-2021, 09:54 PM   #4
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Re: 1970 L47 400 BB Torque and HP chart

This the one.


http://67-72chevytrucks.com/vboard/s...d.php?t=512343
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Old 09-26-2021, 10:06 PM   #5
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Re: 1970 L47 400 BB Torque and HP chart

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Originally Posted by geezer#99 View Post
I saw that one earlier, in my searching. I'd really prefer to see the whole torque and HP curves, and I know I've seen them in the repair manuals for some of the engines, but the BB 400 is missing. There is also no way that my stock L47 produces 460 HP at 2000 RPM.
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Old 09-26-2021, 10:21 PM   #6
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Re: 1970 L47 400 BB Torque and HP chart

You read the chart wrong.
The red line is hp.
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Old 09-26-2021, 11:14 PM   #7
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Re: 1970 L47 400 BB Torque and HP chart

OK, convince me that my engine puts out 460 Lb-Ft of torque at that RPM. The legends weren't clear to me, either. Desk top Dyno isn't the factory spec.
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Old 09-27-2021, 07:29 AM   #8
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Re: 1970 L47 400 BB Torque and HP chart

Are you looking for numbers for a big block 400 chevy? Ur gonna look till ur eyes bleed cause Chevy didn't make a BB400. They made a 396 and 402, which are the same motor, just a 402 is a .030 over 396. I wanna say it had to do with a car having a bigger motor than in a truck.
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Old 09-27-2021, 10:00 AM   #9
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Re: 1970 L47 400 BB Torque and HP chart

I know that. I just posted what the fender says, but I also specified L47. There are lots of stories abut the extra 6 cubes in the 402, but I don't remember which one is true.
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Old 09-27-2021, 12:57 PM   #10
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Re: 1970 L47 400 BB Torque and HP chart

This dyno sheet is the best I can find. I know you have run across it here but haven't seen it on this thread.

Thread is here http://67-72chevytrucks.com/vboard/s...d.php?t=694281
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Old 09-27-2021, 01:16 PM   #11
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Re: 1970 L47 400 BB Torque and HP chart

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This dyno sheet is the best I can find. I know you have run across it here but haven't seen it on this thread.

Thread is here http://67-72chevytrucks.com/vboard/s...d.php?t=694281
Thanks! I'm looking to see if the engine I have will pull hills better if I swap from 3.54 to say, 4.11, which I expect that it will. It really drags on 6% grades, second gear all the way up, at about 40- 45 MPH. The engine speed would go from ~2100 RPM to ~2490 RPM at 55 MPH if I put in the 4.11 gears (it would require a pumpkin, I know) which is towing speed here in California. That's below the engine speed shown on the chart. I'm running over 12,000 pounds with the truck and trailer combined, which is above the 10,000 pound specification. It's that or drop in a 454. The 454 will pull that load, looking at the towing specs for '73 and up. I have a 1T frame, as it is a Longhorn, so I'm not abusing any of that. I guess I could extrapolate the curve, but I don't like doing that if I can help it. Some of the engines in the FSM do have the charts.
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Old 09-27-2021, 02:44 PM   #12
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Re: 1970 L47 400 BB Torque and HP chart

Pulling that much weight you’d want to be turning closer to 3000 rpm which would require a change to a 4.88 gear.
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Old 09-27-2021, 03:58 PM   #13
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Re: 1970 L47 400 BB Torque and HP chart

That's mighty deep, so I'd want overdrive when I'm not towing. Though I don't generally "commute" anywhere with the truck. I'm trying to figure this out so that I only do a thing (diff, engine) once. If I could see how the torque climbs before 3000 RPM, I might be able to get away with a taller gear. That engine is going to be singing, going down the road...
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Old 09-27-2021, 04:21 PM   #14
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Re: 1970 L47 400 BB Torque and HP chart

My buddy built a towing rig for his 5th wheel holiday trailer.
71 gmc with L88 spec 427. 12.5 to 1 compression. ZL1 cam. Idled rough at 1100 rpm. Hooked to a turbo 400 and 4.56 gear. Pulled at 65mph at 3200 rpm all day. Surprised a lot of people when he’d pull out and pass.
Rpm won’t hurt your motor.
I’ve run lots of big blocks from 396’s to LS6’s. They all liked 3000 rpm or more.


Looking at that chart, torque peak at 3000 is right where you want to run.
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Old 09-27-2021, 04:31 PM   #15
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Re: 1970 L47 400 BB Torque and HP chart

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Rpm wonít hurt your motor.
They hurt your ears, and your sense of vibrations, and your fuel tank. Revving high is annoying as all hell with a 4 wheeled vehicle.
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Old 09-27-2021, 05:00 PM   #16
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Re: 1970 L47 400 BB Torque and HP chart

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They hurt your ears, and your sense of vibrations, and your fuel tank. Revving high is annoying as all hell with a 4 wheeled vehicle.
You Prius drivers are all the same!
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Old 09-27-2021, 08:29 PM   #17
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Re: 1970 L47 400 BB Torque and HP chart

Quote:
Originally Posted by geezer#99 View Post
My buddy built a towing rig for his 5th wheel holiday trailer.
71 gmc with L88 spec 427. 12.5 to 1 compression. ZL1 cam. Idled rough at 1100 rpm. Hooked to a turbo 400 and 4.56 gear. Pulled at 65mph at 3200 rpm all day. Surprised a lot of people when he’d pull out and pass.
Rpm won’t hurt your motor.
I’ve run lots of big blocks from 396’s to LS6’s. They all liked 3000 rpm or more.


Looking at that chart, torque peak at 3000 is right where you want to run.
Agreed. My (aftermarket) tach's busted for some reason. I have a new tach to put in it. I'll have to put that in and take the truck out and see if the wife can stand the noise. We're limited to speed because of her ear balance issues; she gets weird/dizzy over 55 MPH already, so even if we were to go to a state that allows speeds higher than 55 MPH, we won't be doing that. Guess I'll have to drive it in second gear for that noise test.

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They hurt your ears, and your sense of vibrations, and your fuel tank. Revving high is annoying as all hell with a 4 wheeled vehicle.
I already only get 7 MPG when towing. I'm hoping that this doesn't kill it more. My wife only said that we'd better make sure that the saddle tanks are fixed! Given the fact that going up hills sends the vacuum gauge to basically zero, I'm sort of thinking that not having to go so deep into the secondaries in second gear might actually help mileage. As for noise, we'll have to wear noise-cancelling headphones, I guess.

My wife was complaining about how her dad had a trailer before that he towed with no problem. I know from my own experience with it that it ran hotter than a pistol pulling that trailer, and didn't pull hills all that well, and that was a much smaller trailer.

Whatever happens, I'm sure that the expense of a new vehicle will be way over what I'm thinking of doing. She's talking about selling one of the econoboxes and getting a new Suburban, but those new things are crazy expensive and don't get THAT much better mileage, anyway. I'm retired, she wants to retire. A new vehicle payment is way more.
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Old 09-27-2021, 08:43 PM   #18
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Re: 1970 L47 400 BB Torque and HP chart

What about a 8.1/4l80 swap?
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Old 09-27-2021, 09:28 PM   #19
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Re: 1970 L47 400 BB Torque and HP chart

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What about a 8.1/4l80 swap?
Toomuchamunny! And a lot of work that I am too old to do, sad to say. Even 10 years ago, that would have been a walk in the park.
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Old 09-28-2021, 02:53 AM   #20
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Re: 1970 L47 400 BB Torque and HP chart

Beef up with what you have (small supercharger) or make a cam change and spin it tight in 2nd. I don't exactly know what I'd do as it's late but we are thinking
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Old 09-28-2021, 08:34 AM   #21
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Re: 1970 L47 400 BB Torque and HP chart

I dont think that gear swap is going to help you unfortunately. I just hauled gravel with my 57 GMC D250 and with 3 tons on the back it was right at 12k pounds. It was all it could do to pull a grade, even configured with 5.14s, granny 4spd, and a SBC. My milage was around 5 or 6. The motor is a stock 305HO. Not the best but not shabby for a stocker. Truck is spinning about 3k rpm at 55mph.

My thought for the wieght is deep gears, 454, and an OD trans. A cummins diesel would actually fit the bill better but you didn't ask and it would be a ton of work. I towed a 12k camper with a 5spd and 3.55 gears comfortably.
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Old 09-28-2021, 09:29 AM   #22
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Re: 1970 L47 400 BB Torque and HP chart

The general must be really optimistic, because they claim that a '74 with a 454 and a 3.73 axle can max out at 13.5k pounds. The 454 can't have that much more torque. Here are the charts, one for '69, one for '74.
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Old 09-28-2021, 11:47 AM   #23
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Re: 1970 L47 400 BB Torque and HP chart

I’m not sure what to make of that chart. A C10 with 4.57 has 1000# more towing capacity than a big block C20?

A gear change I think would help you a bunch. 10% more torque at any speed makes a noticeable difference. Like going from 350 ci to 396. Or from 396 to 454 ci.

**EDIT** since you have 3.54 gears, it would be 15.8 % increase in effective torque at same highway speed. Remember that torque is multiplied by the gears and hp is mathematical function of torque. So if you are turning 2500 rpm with the 3.54 gears and the 4.10 gears put you at 2900 rpm, you get to now multiply the higher 4.10 number by the higher torque output of the engine. Since we can’t see below 3000 rpm on the charts posted we can guesstimate. I found a 325 hp 396 chart (which I think is same as the truck 310) that showed torque to be 375 at 2500 rpm and 410 at 3000. So let’s pretend same at 2900… 375x3.54=1327.5 effective lb ft versus 410x4.10=1681. So 1681-1328 (I rounded) is 353 ft/lb. 353/1328=25% increase in available torque to do work. Here’s the graph I found:

https://www.automobile-catalog.com/c...e_4-speed.html

It is certainly the easiest way to get more effective power. Torque is a function of displacement and compression ratio almost completely. And it’s hard to bump the compression on a big block that has 100cc heads without changing pistons.

A cam swap can move the power around but mostly trades RPM for HP by shifting the scale up the chart.

For a low performance application, towing is one place where small, long tube headers can really pay off by increasing torque without moving the scale.
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Last edited by Mike C; 09-28-2021 at 12:03 PM.
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Old 09-28-2021, 12:08 PM   #24
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Re: 1970 L47 400 BB Torque and HP chart

Considering your wife’s need for max 55 mph and quiet running then a swap to 4.10 gear isn’t worth it.
In second uphill you’re turning 3300 now. Switch to 4.10 and you’ll be up to 3900 rpm.
You might not get into 3rd. If you do it might be louder. Drones usually occur at lower rpm.
Best thing to do might be swapping to a lighter trailer.
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Old 09-28-2021, 12:27 PM   #25
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Re: 1970 L47 400 BB Torque and HP chart

Quote:
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Iím not sure what to make of that chart. A C10 with 4.57 has 1000# more towing capacity than a big block C20?

A gear change I think would help you a bunch. 10% more torque at any speed makes a noticeable difference. Like going from 350 ci to 396. Or from 396 to 454 ci.

**EDIT** since you have 3.54 gears, it would be 15.8 % increase in effective torque at same highway speed. Remember that torque is multiplied by the gears and hp is mathematical function of torque. So if you are turning 2500 rpm with the 3.54 gears and the 4.10 gears put you at 2900 rpm, you get to now multiply the higher 4.10 number by the higher torque output of the engine. Since we canít see below 3000 rpm on the charts posted we can guesstimate. I found a 325 hp 396 chart (which I think is same as the truck 310) that showed torque to be 375 at 2500 rpm and 410 at 3000. So letís pretend same at 2900Ö 375x3.54=1327.5 effective lb ft versus 410x4.10=1681. So 1681-1328 (I rounded) is 353 ft/lb. 353/1328=25% increase in available torque to do work. Hereís the graph I found:

https://www.automobile-catalog.com/c...e_4-speed.html

It is certainly the easiest way to get more effective power. Torque is a function of displacement and compression ratio almost completely. And itís hard to bump the compression on a big block that has 100cc heads without changing pistons.

A cam swap can move the power around but mostly trades RPM for HP by shifting the scale up the chart.

For a low performance application, towing is one place where small, long tube headers can really pay off by increasing torque without moving the scale.
Yeah, that "Iím not sure what to make of that chart. A C10 with 4.57 has 1000# more towing capacity than a big block C20?" doesn't make sense to me, either.

Thanks for finding those charts!

The engine is up for rebuilding, as it has about 100k since the first rebuild, and it's using oil (not to the ground). If I could find a 454 (I have a wtb ad posted) I would rebuild that for practically no more money than the 402.

Maybe I can find some better flowing cast manifolds. I don't like the ringing of headers, though. Guess I could wrap them.
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