Low speed torque and driveability ?

[273]

We generally speak like Low speed torque and driveability is synonyms with each other, I have myself but the more I think of it there seems to be a flaw with this line of thought. A lot of times when we see an upgrade that loses low speed torque the consensus seems driveability will suffer, but were comparing a full throttle dyno run to part throttle driving, which an normal driven engine is obviously only making a fraction of it's full throttle capability anyways.

So at part throttle normal driving the engine needs only to make the power required to do normal driving which is generally a lot less than it's full throttle capability, even a /6 doesn't need to be driven at full throttle to get around town so even a /6 has a lot of excess torque available. And at full throttle were gonna spend very little time at lower rpms which leads me to believe that a lot of low speed torque isn't overly needed.

Which makes me wonder what we mean by driveability and what factors are we really talking about?

I would guess it's how the car handles normal driving and goes from normal to more aggressive to full throttle driving and obviously people's personal compromises come into play.

I have some ideas, but like to see what others think, what are the main factors that causes poor driveability ? And I guess what are we meaning by driveability ? Pretty sure it varies a lot person to person.

 

[Mopar Tim]

To little vacuum caused by to much cam. Number one IMO. It only takes about 45 HP to move our cars down the road at 70MPH. Give or take some...

 

[Newbomb Turk]

Drivability is pure speculation at best. It is wholly subjective.

It means very little when someone says such and such is undrivable. That is THEIR opinion and nothing more, although some treat their own words as written in stone.

 

[66jim]

Dyno tests and engine output data needs to include average power not only peak (area under the curve). As well as engine weight, physical engine size, average fuel usage
under different loads, octane requirements and other data. Too much weight is given to peak numbers and output per/CID. Example: A small CID turbo engine using 93 Octance putting out lots of peak power but has little low end takes up a lot of space weights quite a lots (plumbing included) and get crap fuel real world fuel mileage

 

[273]

To little vacuum caused by to much cam. Number one IMO.

That's one of the major one's I was thinking of.

It only takes about 45 HP to move our cars down the road at 70MPH. Give or take some...

Agreed.

 

[273]

Dyno tests and engine output data needs to include average power not only peak (area under the curve).

That's what the torque curve and tq per cid does, gives an idea how well the under curve hp is. But I'm basically talking idle-3000rpms or so.

 

[Phreakish]

Drivability is pure speculation at best. It is wholly subjective.

It's means very little when someone says such and such is undrivable. That is THEIR opinion and nothing more, although some treat their own words as written in stone.

Exactly. My 4 banger econobox cruises in 5th at 4500-4800rpm, doesn't bother me a bit. My busted *** old suv feels like I'm taxing it if I cruise over 2800rpm. My Dart annoys me over 3500 rpm and when I honestly assess this "driveability" metric it really only comes down to noise and harshness.
The econobox damps some of the unpleasantness of turning so many revs where the suv is likely designed to make you avoid higher speeds/revs (3k in OD is about 85).
My Dart is only annoying because of the flow/drone masters. With the efi, I programmed in a bit more idle valve open when at cruise, this helps reduce engine braking when easing off the throttle and completely changes how "taxed" the engine feels at 4200rpm on the highway (3.91 gears and 25" tires..).
Getting the tuneup right also helps a ton. A lack of timing will make the engine run hot and take a ton more pedal to hold speed up a hill and make a ton more noise. With proper load/no-load timing it just drives like any other car.

 

[273]

I was driving my Avenger RT today and paying attention to the rpms, granny driving, taking off at a light with slowest acceleration for me to get up to speed I was in the 1800-2100 rpm band at that rate any engine makes more than enough power for that. More normal driving I'm fairly heavy footed 2000-3000 rpm, hovered at 2500 rpm average and any more aggressive part throttle (1/2 throttle) like passing, merging or just having fun it dropped down a gear and was 3000-4000 rpm and up to 5000 rpms if I stayed in it.

What I get for this car torque under 2500 rpm really don't matter much for driveability performance wise as long the car likes to run down there cause that's where it cruises. Above that any real acceleration it drops a gear and is in the 3000 + rpms and has decent pull there.

When I get my Valiant on the road this week I look at how performs at these rpms, being a 380hp crate with a smallish 2800 stall and highway gears 2.xx I know driveability is alright could be better but far from being a slug.

 

[Phreakish]

I think the whole "driveability" deal is a function of the fact that most hotrodders for the past several decades have built with an automatic transmission and for a long time it was much more difficult to get a converter that worked well with anything but a mild cam, and especially if it was intended to be driven more than 30 minutes at a time.
Times have changed, and lots of folks "daily drive" some pretty wild stuff anymore. At least when compared to street cars of yore. There's also a lot more tech and more builders who understand it than before and so the value of low rpm torque isn't what it used to be. But for the guy wanting a nostalgic build with simpler components, it can be an important factor.

 

[Phreakish]

I was driving my Avenger RT today and paying attention to the rpms, granny driving, taking off at a light with slowest acceleration for me to get up to speed I was in the 1800-2100 rpm band at that rate any engine makes more than enough power for that. More normal driving I'm fairly heavy footed 2000-3000 rpm, hovered at 2500 rpm average and any more aggressive part throttle (1/2 throttle) like passing, merging or just having fun it dropped down a gear and was 3000-4000 rpm and up to 5000 rpms if I stayed in it.

What I get for this car torque under 2500 rpm really don't matter much for driveability performance wise as long the car likes to run down there cause that's where it cruises. Above that any real acceleration it drops a gear and is in the 3000 + rpms and has decent pull there.

When I get my Valiant on the road this week I look at how performs at these rpms, being a 380hp crate with a smallish 2800 stall and highway gears 2.xx I know driveability is alright could be better but far from being a slug.

My 408 has an advertised duration right around 300 and doesn't like to live under 1600rpm, but will happily cruise the back roads at 1850+. A moderate hill and I'm more comfortable at 2200. It's a 4spd, so it's a little different animal than if it was an auto though.
Back when I had the 318/904 the car came with, it had a 9" converter that would flash to 4800 but on the highway it still only turned 4k at 75. It turned 3k almost all the time at neighborhood speeds too. It felt "racy" but in all honesty it was perfectly driveable. Still is, but I know folks who have ridden in it who think it's way too much for the street. Just reinforces to me the notion that drivable is super subjective because I still don't consider it fast, let alone a racecar.

 

[273]

I think the whole "driveability" deal is a function of the fact that most hotrodders for the past several decades have built with an automatic transmission and for a long time it was much more difficult to get a converter that worked well with anything but a mild cam, and especially if it was intended to be driven more than 30 minutes at a time.
Times have changed, and lots of folks "daily drive" some pretty wild stuff anymore. At least when compared to street cars of yore. There's also a lot more tech and more builders who understand it than before and so the value of low rpm torque isn't what it used to be. But for the guy wanting a nostalgic build with simpler components, it can be an important factor.

I think improper torque converter is a big part, overlap and cruising rpm, throttle response, how it goes from regular driving to aggressive driving, I think even the power difference cause as you build more and more power, down low doesn't gain much and could even lose some making a big difference how you engine feels at low rpms compared to higher rpms.

 

[Phreakish]

I think improper torque converter is a big part, overlap and cruising rpm, throttle response, how it goes from regular driving to aggressive driving, I think even the power difference cause as you build more and more power, down low doesn't gain much and could even lose some making a big difference how you engine feels at low rpms compared to higher rpms.

Exactly. If enough lown is lost, it just means the build needs to be setup to take advantage of where the new "low end" actually is.
A car that idles at 1500 probably won't cruise at 2k real well, LOL.
But with proper setup, no reason it couldn't go merrily down the highway at 3 or 4k. It's just our notion of what "high rpm" is and our tolerance for noise and vibration that make it off putting.

 

[fishmens67]

Drivability is pure speculation at best. It is wholly subjective.

My daily driver had 4.57 gears, my wife learned how to drive in it. My first born was a rear passenger for several years.

 

[TrailBeast]

To me, good drivability is how the car does what you want and when you want.
It should just flow smoothly from the moment you turn the key to braking to park.
No matter how much power it has.

I have driven some pretty fast street cars that were generally horrible in the drivability aspect, but they sure were fun.

I’ve driven some that were really nice drivability wise, but sucked.

 

[OldmanRick]

Imo, big cube long stroke engines with mild cams deliver the best compromise. Anyone that's driven the older stock GM 455's knows what i'm talking about. Since we're talkin A bodies, i'd build a 9 to 1 low deck BB with a 4'' plus crank, iron heads, 220ish cam and the regulation bolt ons with a 2300 stall speed, 3.23 gear and have a absolute blast.......Now if mileage comes into play, that's a whole nother issue.

 

[Dan the man]

To little vacuum caused by to much cam. Number one IMO. It only takes about 45 HP to move our cars down the road at 70MPH. Give or take some...

Don't forget that you lose about 25% of your power between the flywheel and rear tires.

 

[Dan the man]

Exactly. My 4 banger econobox cruises in 5th at 4500-4800rpm, doesn't bother me a bit. My busted *** old suv feels like I'm taxing it if I cruise over 2800rpm. My Dart annoys me over 3500 rpm and when I honestly assess this "driveability" metric it really only comes down to noise and harshness.
The econobox damps some of the unpleasantness of turning so many revs where the suv is likely designed to make you avoid higher speeds/revs (3k in OD is about 85).
My Dart is only annoying because of the flow/drone masters. With the efi, I programmed in a bit more idle valve open when at cruise, this helps reduce engine braking when easing off the throttle and completely changes how "taxed" the engine feels at 4200rpm on the highway (3.91 gears and 25" tires..).
Getting the tuneup right also helps a ton. A lack of timing will make the engine run hot and take a ton more pedal to hold speed up a hill and make a ton more noise. With proper load/no-load timing it just drives like any other car.

4500 - 4800 rpm's at what speed? You must not have a overdrive transmission

 

[Newbomb Turk]

Don't forget that you lose about 25% of your power between the flywheel and rear tires.

How did you come up with 25%?

 

[a-bodyguy]

25% lol
I swear Dan is 14 yrs old and loves mopars

 

[DFX 340 Duster]

I will get this in before this thread goes off the rails.
My low speed driveability preferences used to include great throttle response, excellent handling and cornering, and power steering and power brakes.
Now that I’m 68 years old my low speed driveability preferences include lumbar support, arm rests, no blind spots, and green lights.

 

[fishmens67]

How did you come up with 25%?

His torqueflite needs rebuilt or his clutch is fried

 

[Dan the man]

How did you come up with 25%?

I think that the actual formula is .80 for a at and .85 for a mt. I've read this in several magazines, I may have the numbers crossed. If it's right say that you have 325 flywheel horsepower and you have to at you would divide 325 by. 80 equals rear wheel horsepower. I've heard the 25% thing for years

 

[fishmens67]

I've heard the 25% thing for years

You're spot on that generic number is what was thrown around. it's closer to 15%-20 loss

 

[Newbomb Turk]

Don't forget that you lose about 25% of your power between the flywheel and rear tires.

I think that the actual formula is .80 for a at and .85 for a mt. I've read this in several magazines, I may have the numbers crossed. If it's right say that you have 325 flywheel horsepower and you have to at you would divide 325 by. 80 equals rear wheel horsepower. I've heard the 25% thing for years

The fact is I’ve never see a 25% driveline loss.

The only way to KNOW what the drive train losses are is to do a coast down. That will tell you exactly what the loss is.

Claiming a standard 25% means that the numbers will always be higher than they should be.

 

[Dan the man]

The fact is I’ve never see a 25% driveline loss.

The only way to KNOW what the drive train losses are is to do a coast down. That will tell you exactly what the loss is.

Claiming a standard 25% means that the numbers will always be higher than they should be.

Check out the formula in post #22. That formula is mentioned in every article on how to calculate rear wheel horsepower. I may have them crossed.