Low speed torque and driveability ?

[Dale Davies]

Drivability. HMMM. Yes, well that is a keg of worms discussion and involves a number of topics.
Idle quality to many people is the first to come to mind. Generally a fairly smooth idle is desired and is partly the trend to wide LSA's on modern engines. This is compounded by the high port flows found in these engines. Then there are many tha find a bit of lope to the idle acceptable, while a top fuel idle is not street friendly. This all comes down to intended end use, driving to church or for groceries, towing a trailer, weekend track days or an all in race car.
Will the car or truck pull away from a stop smoothly without fuss and bother?
Required rear gearing and converter stall RPM. If a manual trans, do you need to apply throttle just to get the clutch engaged. I am referring to more throttle than a factory combination. These all relate to driveability. A high stall and a low final drive are great for rapid acceleration but get real tiring on the highway. Not only that but fuel economy. There is a local fellow with a poked and stroked and built Big Block Buick that can not make 200km which is 120 miles on a full tank of 91 gasoline. Like a Saturn 5 that liftoff kick in the backside is thrilling but the cost to play is expensive. Listening to the engine ticking over at 4,000RPM at 65 to 70 MPH wears thin quickly.
How much tinkering has to go into keeping the engine running properly is part of driveability. The more power per cube built into the engine, the more high strung they become. Atmospheric changes tend to affect it more than the OEM offerings, dictating tinkering.

 

[Dale Davies]

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...

Definately correct. Automotive engines are low duty cycle designs. As you state, the power requirement is low compared to the capability. This is how OEM engines get away with small fillet radius crankshafts and narrow webs between the main and rod journals.
Aircraft engines are designed for 75% duty cycle. For takeoff they are wide open throttle and full power for 5 minutes. After that the power is reduced to 75% of maximum for the rest of the flight and possibly less for economy cruise. Running at 75% power for hours puts a lot of stress on the con rods and crankshafts, so many certified engines are required to be overhauled at 1,500 to 2,000 hours run time. Not just magnaflux rods and cranks like automotive, but x-ray to check for subsurface cracks developing. Anything that is not 100%good gets replaced. Some have put auto engines in "Experimental" or kit aircraft, but these engines generally have to be replaced at 750 to 1,000 hours as the stresses are fatigueing components.
Aircraft engines have to be light but also durable. They have wider cylinder centwr spacing to allow wider crankshaft webs and larger radius fillets for strength. Heavy duty and industrial engines are similar for strength but without the requirement to keep weight down.

 

[273]

Drivability. HMMM. Yes, well that is a keg of worms discussion and involves a number of topics.
Idle quality to many people is the first to come to mind. Generally a fairly smooth idle is desired and is partly the trend to wide LSA's on modern engines. This is compounded by the high port flows found in these engines. Then there are many tha find a bit of lope to the idle acceptable, while a top fuel idle is not street friendly. This all comes down to intended end use, driving to church or for groceries, towing a trailer, weekend track days or an all in race car.
Will the car or truck pull away from a stop smoothly without fuss and bother?
Required rear gearing and converter stall RPM. If a manual trans, do you need to apply throttle just to get the clutch engaged. I am referring to more throttle than a factory combination. These all relate to driveability. A high stall and a low final drive are great for rapid acceleration but get real tiring on the highway. Not only that but fuel economy. There is a local fellow with a poked and stroked and built Big Block Buick that can not make 200km which is 120 miles on a full tank of 91 gasoline. Like a Saturn 5 that liftoff kick in the backside is thrilling but the cost to play is expensive. Listening to the engine ticking over at 4,000RPM at 65 to 70 MPH wears thin quickly.
How much tinkering has to go into keeping the engine running properly is part of driveability. The more power per cube built into the engine, the more high strung they become. Atmospheric changes tend to affect it more than the OEM offerings, dictating tinkering.

I kept driveability fairly open ended.

What I mean by driveability in this case is how well the car sit at a light then pulls away and also how it cruises at low rpm, basically reasonably functional, what I find especially with cam choice people automatically assume a loss of low speed is gonna kill this basic low speed functionality especially with stock stall and high gears. And also some like AJ with CR & V/P among other things.

Obviously losing torque is not good but at this point I don't think it's the culprit for low rpm driveability as I described earlier.

 

[273]

Plus I question in relative terms are we really losing that much in the under 3000 rpms to be that concerned with especially when we spend fairly little time at full throttle under 3000 rpms?

 

[Dale Davies]

Plus I'm questioning that big cam have poor low speed torque, now obviously we've all seen bigger cams give up torque to smaller cams but when put in perspective and especially vs a stock low powered 2bbl version of the engine bigger cams still seem to do decently down low, obviously we generally don't know what's going on under 2000 rpm.

Here's a dyno of a stock 2bbl cam to xe262h cam among other mods, as you can see all doing decent down to 2000 rpms, the one below doing well under 3000 rpms would have to look the cams up but are 390-430 hp 360, cams must be fairly decent and Richard Holdener dyno of a stock 2bbl 360 engine dyno'd 330 lbs-ft @ 2000-3000 rpm and the 380hp crate from my other post doing better than all at 381 tq @ 2500 rpm.

And the bunch of 318/340 dyno's with various cam all making decent 2000-3000 rpm torque numbers from what we can tell.

I'm just not seeing big torque drops were you could say that's why driveability suck and gonna need stall and gears to get her to be able to pull away from a stop sign, especially when there making way more torque than a stock /6 273 318 even 2bbl 360.

Torque
RPM	1	2	3	4	5	6
2000	357.3	376.9	346.7	356.8		349.6
2200	350.7	370.9	347.9	351.3		345.8
2400	349.8	367.5	355.9	353.1		350.4
2600	351.5	367.1	359.5	352.6	361.0	353.8
2800	358.8	374.1	367.1	357.7	359.7	361.0
3000	370.1	388.0	373.6	363.2	364.1	371.9
3100	371.7	390.6	375.9	366.0	369.3	375.6
3200	371.2	390.7	376.0	368.7	375.7	381.6
3300	371.3	390.3	377.1	368.8	381.0	389.6
3400	367.8	388.9	374.1	368.7	386.3	396.1
3600	361.2	381.7	369.2	367.0	390.8	400.1
3700	357.7	375.0	367.1	363.3	391.8	400.6
3800	349.9	370.4	363.6	358.7	386.9	398.0
4000	342.8	357.8	354.9	348.3	380.3	394.0
4200	331.0	346.3	344.9	337.6	383.1	389.5
4400	316.4	329.1	329.0	325.1	378.1	385.5
4600	295.7	311.0	313.3	310.5	368.2	375.6
4800	286.5	293.8	299.8	290.7	355.1	363.3
5000	270.9	278.7	282.0	272.4	342.7	351.9
5200		330.3	337.8
5400	311.8	321.7

360 build

Dyno Test
Horsepower				Torque
RPM	Test 1	Test 2	Test 3	RPM	Test 1	Test 2	Test 3
2600	172.7	N/A	166.1	2600	348.9	N/A	335.6
2800	190.6	196.6	183.9	2800	357.7	365.8	345.1
3000	204.3	207.5	197	3000	357.7	363.3	345
3200	190.1	226	216.2	3200	359.6	370.7	354.9
3400	237.6	242.4	235.3	3400	367	374.5	363.5
3600	258.3	267.7	253.1	3600	376.8	390.6	369.3
3800	281.4	281.8	274.9	3800	388.9	389.4	379.9
4000	299.6	310.1	297.1	4000	393.5	407.2	390
4200	319.1	313.8	314.4	4200	399.1	392.3	393
4400	320.8	329.4	328.5	4400	383.8	393.1	392
4600	354.5	352.7	352	4600	404.8	402.7	401.6
4800	359.5	379.4	375.4	4700	397.2	420.2	408.4
5000	377.4	392.3	389.4	4800	393.4	415.2	410.7
5200	378.8	396.5	401.6	4900	399.8	410.6	412.2
5400	390.3	401	411.7	5000	396.4	412	409.1
5600	384	399.1	420	5200	382.6	400.4	405.7
5800	387	392.3	426.5	5400	379.5	390	400.5
6000	369.2	391.8	430.2	5600	360.2	374.3	394
6200	370.2	377	423.3	5800	350.5	355.3	386.1

Now there was the qualifier, "among other mods". To properly compare and assert something you need as direct a comparison as possible.
About 25 to 30 yarns ago, PHR magazine did a series of tests to compare 383 Chev builds. They rebuilt a 350 and dyno'd it. Then they put a stroker crank in with 400 rods. Now you could use the 350 pistons, but compression would be up a bit so they had pistons to keep the compression as close as possible installed. The cam was the same, and they tested it this way.
Then they pulled it apart to install 5.7" rods instead of the 400 engine 5.565" rods. Of course the piston pin location had to be changed, so new pistons with the crowns identical to the second set. It was dyno'd again.
The increase in displacement provided a modest increase in torque and power, but not what you would expect. Changing rod length had virtually no effect on torque or power.
What did make a big improvement was to change the cam to one more suited to the 383. This mainly involved LSA and valve lift with a very small duration increase. The main increase was the LSA change to match the cylinder displacement and intake valve diameter and the bit more lift. In a 350 engine the LSA would not have been correct, so any benefit from the minor increase in duration and lift would have been less than what would be expected.
The one thing they did note was that the longer 5.7" rods made the 383 quieter due to less piston slap.

 

[273]

Now there was the qualifier, "among other mods". To properly compare and assert something you need as direct a comparison as possible.

What I was trying to show with all the dyno is that even with various cams in 318/340/360 from stock to fairly wild all made decent torque at 3000 rpm for sure, decent evidence that at 2500 rpm most if not all still doing ok, less evidence but even at 2000 rpm all seems good especially is compared to stock low powered /6,273,318 & even 360. I know the there's a lot of flaws and some guesstimation going on, not designed to eliminate all variables, I wish I had a dyno facility to do that but working with what we got.

There's a lot of assumption in the peoples general premise too bigger cam generally lose torque and driveability that the former must be the cause.

 

[Hysteric]

And yet how many times have you heard its like a light switch at 3K......

 

[273]

And yet how many times have you heard its like a light switch at 3K......

I’d say cause an engine has a relatively flat torque curve not as flat as an electric motor more of a frown curve.

On a Street engines torque peak is generally in the 3000 rpm’s range and hp as a percentage torque starts to be a decent percentage, 57% at 3000 rpm, 76% by 4000 rpm and climbing.

Basically 3000 rpm or so is where the powerband starts for most street engines.

 

[273]

Plus if talking more street strip engine they generally then to make more torque per cid so even if there powerband starts higher there still making decent above 3000 rpm numbers.

 

[Dale Davies]

Plus I question in relative terms are we really losing that much in the under 3000 rpms to be that concerned with especially when we spend fairly little time at full throttle under 3000 rpms?

And you spend little time at full throttle in the top 1500 RPM range. Probably spend more time in the off idle to 3000 RPM range at full throttle with aggressive driving. Even more important is part throttle like say 50% in that lower RPM range where most drive. That is where torque is a factor.

 

[273]

And you spend little time at full throttle in the top 1500 RPM range. Probably spend more time in the off idle to 3000 RPM range at full throttle with aggressive driving. Even more important is part throttle like say 50% in that lower RPM range where most drive. That is where torque is a factor.

Question is at part throttle your not using full amount of torque anyways so does it matter if your down 20-30 tq at full when your probably below both at part throttle most of the time?

 

[Dale Davies]

Question is at part throttle your not using full amount of torque anyways so does it matter if your down 20-30 tq at full when your probably below both at part throttle most of the time?

You are the one that posted about driveability. I replied that there is a lot involved. Now you want to talk WOT at low RPM and high RPM. Like I stated and others have mentioned, driveability is a complex topic and subjective. Give your head a shake and get out of that narrow space you are in.

 

[Newbomb Turk]

You are the one that posted about driveability. I replied that there is a lot involved. Now you want to talk WOT at low RPM and high RPM. Like I stated and others have mentioned, driveability is a complex topic and subjective. Give your head a shake and get out of that narrow space you are in.

lol. Red X.

 

[273]

You are the one that posted about driveability.

I did, but mainly on low speed torque effect on driveability/functionality.

I replied that there is a lot involved.

I understand that, might not be able to walk away with any solid conclusions but don't mean can't come up with plausible explanations with what data we have.

Now you want to talk WOT at low RPM

Have since origin post

and high RPM.

Haven't been talking about high rpm

Like I stated and others have mentioned, driveability is a complex topic and subjective.

It is but I narrow down fairly well what part of driveability im talking about.

Give your head a shake and get out of that narrow space you are in.

It's my thread you don't want to discuss it that's fine.

 

[273]

Obviously this ain't an everyman build but does shows higher hp per cid engines built with high torque to cid can carry a lot of torque at low rpms too. But I bet even with all that low end torque this engine wouldn't be happy with stock stall and 2.xx gears especially with it's 106 lsa.


https://www.motortrend.com/articles/1208phr-371ci-mopar-small-block/
Here's an 371 with ported EQ 246/244 hydraulic roller making 515 @ 5900/6000 rpm
495 @ 4800 and 401 @ 2500 rpm, this engine putting out similar hp & torque at similar rpms as some 408.

RPM	TQ	HP
2,500	401.5	191.1
2,700	408.4	202.1
2,900	426.2	235.3
3,100	429.6	253.6
3,300	430.8	270.7
3,500	425.3	283.4
3,700	431.3	303.8
3,900	444.9	330.4
4,100	461.2	360.0
4,300	473.9	388.0
4,500	486.1	416.5
4,700	493.7	441.8
4,800	495.0	452.4
4,900	494.3	461.1
5,100	491.9	477.7
5,300	485.9	490.4
5,500	480.7	503.4
5,700	470.9	511.1
5,900	458.4	515.0
6,000	450.9	515.1
6,100	441.5	512.7
6,300	424.1	508.7
6,500	402.9	498.6

 

[ir3333]

That's a lot of power for a street driver....most guys i know can only afford gas for 1 cruz nite per week at those power levels. That engine will out rev most 408's and seems more like a race prepped engine to me.I don't know why the 372 isn't more popular? As for drivability i think most guys want strong acceleration at lo rpm when driving at any speed. I remember reading an extensive article on designing the 383 roadrunner. I think they tried 4 camshafts before they found one that wasn't lazy and would squeal the tires from idle.
...and a roadrunner has 17,550 total parts according to the article.