How much can HP make up for gear ratio?

[howdiy]

As it is right now I'll be running 42" TSL's, np205 and 3.73 gears.

If I build a big block with say 400-450hp and 500 or so lb's torque, would it make up for the lack of gear ratio?

I'm not really that familiar with crawling and stuff, I had 3.08's with 39's and a mild 350 and it seemed to do fine.

The engine would be something like this, pretty mild, maybe a little more compression and cam.

http://www.currymotors.gmmotors.com...4HOGMPerformancePartsCrateEngineAssembly.aspx

 

[drivelineman]

I noticed a big difference with my swap- Mainly from the torque of the big block -I run 4:56's though- Jess

 

[boz42]

Torque can make up for high gear ratios. After all the gears only mutiple your mechanical advantage. If you have more torque you need less advantage.

On the other side you would be giving up control. I am running a 547 stroker, doubler, 5.13's & 42's. I can spin all 4 pretty easy in 4wd high. But the truck gets moving pretty fast in a hurry. You would not want to be bouncing all over some trails like that. If I go to 4:1 I can crawl & on some intricate trails it is nice to be able to finese it up & over instead of just powering through it.

 

[80' 427]

With that high a gear it better be making tq right off idle. I went from 3.08s and 33 to 4.10s with a 305 and it made a bigger difference than when I went to the 350. I have 37s and 4.10s and think its too high geared.

 

[howdiy]

Torque can make up for high gear ratios. After all the gears only mutiple your mechanical advantage. If you have more torque you need less advantage.

On the other side you would be giving up control. I am running a 547 stroker, doubler, 5.13's & 42's. I can spin all 4 pretty easy in 4wd high. But the truck gets moving pretty fast in a hurry. You would not want to be bouncing all over some trails like that. If I go to 4:1 I can crawl & on some intricate trails it is nice to be able to finese it up & over instead of just powering through it.

I want to eventually get a doubler but the Big block route would be cheaper and easier for me at the moment.

I love your truck by the way

 

[Avery4jc]

Your tranny is going to hate you...

 

[Slapperbar]

Your tranny is going to hate you...

That being said... watcha runnin?

 

[Greg72]

Depends a lot WHERE in the RPM range you get your peak torque number also....

It won't do you much good to have a BBC producing 500 Lb/Ft at 4000RPM if it only has 200Lb/Ft at the 1500RPM you need for crawling.

Most of the crate motors I see appear to be setup for lighter vehicles, and the peak HP and peak TQ come in higher than you'd want in a heavy truck.

Here's an example of a motor built specifically for a truck application: (It happens to be a 496BBC that's sitting in my garage now)

The torque is "ALL IN" by 2500RPM.......and there's 620Lb/Ft of it to get things moving. You'll notice that I traded off an impressive "Max HP" number for more TQ. In a heavy truck, torque is all that really matters.

(unless you are a mudder)

 

[boz42]

I want to eventually get a doubler but the Big block route would be cheaper and easier for me at the moment.

I love your truck by the way

Thanks, I am very proud of my rig. It took me a lonnnngggg time to build it.

Well I love big engines. The gearing does make it easier to crawl. Unless you have a huge low end torque figure you will not be able to crawl. You will have to go on full throttle assaults. I built my rig to trail ride here in the south. Most of our terrain is dirt & wet slick rocks(not slickrock). It takes a lot of horsepower cause you can't crawl a lot of this stuff. It takes tire speed. I only use the doubler for steep downhill descents to keep from having to use a lot of brake. I hope to eventually go riding west of the Mississippi & actually need to use the 4:1.

 

[howdiy]

Your tranny is going to hate you...

That being said... watcha runnin?

Th350 at the moment . I think I may build it up and put on a BIG cooler, like from a mack dump truck

I'll see what I can do about getting a 400 first though, I have a 2wd th400 with a shift kit at the moment. I just would have to try and find the th400 adapter for my np205, and the th400 4x4 output.

Depends a lot WHERE in the RPM range you get your peak torque number also....

It won't do you much good to have a BBC producing 500 Lb/Ft at 4000RPM if it only has 200Lb/Ft at the 1500RPM you need for crawling.

Most of the crate motors I see appear to be setup for lighter vehicles, and the peak HP and peak TQ come in higher than you'd want in a heavy truck.

Here's an example of a motor built specifically for a truck application: (It happens to be a 496BBC that's sitting in my garage now)

The torque is "ALL IN" by 2500RPM.......and there's 620Lb/Ft of it to get things moving. You'll notice that I traded off an impressive "Max HP" number for more TQ. In a heavy truck, torque is all that really matters.

(unless you are a mudder)

Yeah I think I'll probably end with one of these cams

http://store.summitracing.com/egnsearch.asp?N=700+400114+309523+115&Nao=0&autoview=sku#rstop

 

[boz42]

The torque is "ALL IN" by 2500RPM.......and there's 620Lb/Ft of it to get things moving. You'll notice that I traded off an impressive "Max HP" number for more TQ. In a heavy truck, torque is all that really matters.

My engine is built very similiar to yours.

cam specs
duration at .050 int=230 exh=236
lobe seperation = 112
gross lift .562 & .562

rockers are full roller shaft type 1.7 ratio

bore = 4.340
stroke = 4.600
545 cubic inches

pistons are Ross racing forged w/ small dish & valve reliefs & 1/16" rings. Ross did about 1/2 of their wieght reducton tricks to them. static compression is 9.5:1. rods are full float "I" beam bbc 6.800 long w/ "H" series bearings. bob wieght is 2347 grams


heads are no smog cast # "950". they have 2.19 int & 1.90 exh. they flow 245 cfm on intake & 165 on exh. at .500 lift.

dual plane edelbrock performer intake. long tube 1 7/8" headers w/ 3" collectors, Single 4" mandrel exhaust. 800 cfm Jet quadra-jet. D.U.I. HEI distrubutor.


I have never dynoed it, but the old desktop dyno showed it was 550 ft lbs at 2000 rpm. I also know it will eat a T-400. I am on my third now. I am proving who can & cannot build transmissions around here.

 

[Greg72]

Just to clarify my previous post a bit more:


I'm not an advocate of building a TQ motor as a replacement for deep gearing.

The issue is still going to come down to the crawl ratio and the number of "inches traveled per engine RPM". 1 inch per engine RPM is the de-facto number that most of us try achieve for low-speed crawling. With a LOT of low-end torque, you might be able to run a low enough RPM to get the same travel speed as a properly geared truck. However, as soon as you blip the throttle you are going to build wheel speed at a MUCH faster rate than a properly geared truck....and that's where the strategy will fail.

Throttle modulation will be a lot easier on a properly geared truck, on a poorly-geared,high TQ setup the throttle will be unnecessarily sensitive to inputs and will make it nearly impossible to use "finesse" on obstacles. Depending on the type of terrain (or personal driving style) this might still be acceptable.....but I'd guess that for slow, technical rock crawling it's going to be an issue.

Tranny heat is going to be the other big factor. Poor gearing is dumping a bunch of extra heat into the transmission, so if you run a slushbox....it's going to be a constant battle to keep from destroying them.

 

[blazin_blazer]

from what i've read, you can't just put a short output shaft in a th400 trans and use a th400/np205 adapter and bolt it together, you have to swap the transfer case input to a 32 spline, and that involves having your 205 case being machined to be the right size for the 400's input bearing which is bigger.

 

[6.2Blazer]

Big power can make up for gearing but only to a certain extent, and more so in certain conditions as compared to others. A good example is comparing a friends truck to mine.

My K5 has a mostly stock 6.2 diesel (around 150 HP, 250 torque or so), but a 34:1 crawl ratio (auto trans) with 39.5" tires. His is a pickup with a 300+hp, 500+torque 460 (built for low-end grunt) but only has a 19:1 crawl ratio with larger 42" tires.

Crunching the numbers the max torque output to the rear wheels is almost identical.

Obviously there is no contest when you are in a situation that wheel speed is required as he can hit 5,000 rpm on demand, and with a 19:1 ratio and 42s produces a LOT of wheelspeed. Even with substantially more gearing and slightly smaller tires I can get maybe 3,600 rpm (governed speed of the 6.2), can't pull the next gear up.

Very technical crawling I probably have a slight advantage but it's not because he has a lack of power, but as mentioned above he doesn't have quite the control. He does a lot more spinning and bouncing.

As far as making an auto survive, he's had pretty good luck so far with a C6. When he first bought the truck ('78 or '79) it had an obviously swapped in an unknown condtion C6 installed. He did need to have it rebuilt several years ago, after a couple years on the trail, but there was signs of water intrusion in the trans. It was rebuilt to stock with the exception of a mild shift kit and he runs a big cooler.

 

[howdiy]

I never knew an sm475 had a 6.55:1 first gear, I knew it was taller but not that tall

Maybe I should look into swapping in a manual tranny, seems like the best bang for the buck.

 

[boz42]

Sm465 or 420 = Poor mans doubler!

Then you can add a doubler! But you would need the uber rare 465/203 for that.

 

[Fabrimacator21]

I never knew an sm475 had a 6.55:1 first gear, I knew it was taller but not that tall

Maybe I should look into swapping in a manual tranny, seems like the best bang for the buck.

I think you mean short.

Tall= high gear

Short= Low gear.

The manuals are bulletproof but they will slow your rig down in a drag race. Long throws. I haven't drag raced my k5 yet so you should be good.

 

[howdiy]

I need practice with a manual anyways

Now I am off to research on the swap

I already found someone that wants to trade the sm465/205 combo for an auto/205 combo

Edit: I get tall and short mixed up lol

 

[Greg72]

I never knew an sm475 had a 6.55:1 first gear, I knew it was taller but not that tall

Maybe I should look into swapping in a manual tranny, seems like the best bang for the buck.

Manual trannies do have deeper gearing, but remember it's also not like that 6.55:1 is twice as good as the 1st gear of a TH400.... at crawl speeds (below stall) the auto will behave like it's 2.48 X 2.... or about 4.96:1

Not bad.


For crawl ratios > 100:1 there really isn't a simple way to do it without adding an additional reduction box somewhere.

 

[howdiy]

Do you mean from the converter slipping?