A flat torque curve motor has a unique feel, it has instant power, but then it never really gets much better! It doesn't have the typical power output that doubles with RPM of a typical performance motor. How strong it is strictly depends on the power to weight ratio. They do not require as much shifting and are kind of tolerant of being in the wrong gear at the wrong rpm, or having a sharp tune, etc.

My point is one 400 hp motor is as fast as any other 400 hp motor, (when tuned correctly), but the one with more torque feels like it is loafing while it's doing it, while the hi rpm low tq engine feels all wound up and straining at the bit. One engine is for a laid-back driving style, the other is for a very driver involved & aggressive driving style. (imagine a 500 CI Caddy vs 400 hp 283?)

After all the original Hi Perf 302 DZ Z28's were 15 second slugs in stock form! (3.73) But they felt great doing it. But they were slow, (stock). The 350 LT-1 version spanked it rather badly with it's extra 50 lb tq but not one more hp. (14 flat also stock with 3.73) Could the 302 be as fast as the 350? With enough gear; yes. But which one is the easier to drive street car?

The OP has a 400 HP SBC and does not feel the need for any more HP, much less 500, 600 or 700. But he would like more TQ.
Cubes is where he'll get it.

It'll never feel like, or be as fast as, a 600 hp motor, and he doesn't want it to.

 

I also agree, all this talk about a 390 hp 427 being a torque monster are completely unfounded. They were slugs. The 390 hp was a gross output and those numbers pale when viewed as net numbers.

 

This thread has gone on long enough, and I did learn that a 427 short block is pushing the extremes on a short block, as I suspected, and which was my original question to be answered. What I did not learn is why big torque numbers from 1000 to 4000 on the dyno graphs posted do not indicate strong low end acceleration i.e. pushing you back into seat power, and it seems most of you cannot explain it. Most of you imply that this a weak engine, because it does not produce large numbers at the top end. I guess there is only one correct answer. I guess a performance engine can only be an engine that makes power at 5000 plus and you just have to drive it that way to have fun.

Thanks for your input. And thanks to Leigh for the supporting documentation.

 

F=MA therefore A=F/M
F=force
M=mass
A=acceleration

HP at wheels (F) is torque * RPM with a transmission gear multiplier. Without RPM there is no F. Without F there is no A. Cruising around on the street there's very little change in RPM, depending on gearing nd how heavy your foot is you may never see more than 3,000 rpm. Again, without RPM there is no F.

My Lexus is a classic example of this, in 'ECO' mode its in 8th gear by 40mph and acceleration sucks.

 

A 406 can be made to make 560 horsepower easily and 480-490 ft/lbs. of torque.......this is more power than most can even put to the ground in a C3 without a lot of mods and careful planning.......
If the tires are spinning anyway, why bother with a bigger engine? The 406 has a happy rod ratio, and uses shelf parts.......
My own 406 is right at the limit of what my chassis can handle now and it makes 500 horsepower.......it should run high 11's.......on slicks. You have to ask yourself what you are really hoping to achieve......is this a boulevard tire toaster or are you racing? An 11 second car is pretty fast......do you need more? A girl I went to high school with has a 69' Camaro with a Shaffiroff 434 in it, she says it's a hoot....and posted a vid smoking tires on Woodward during the dream cruise......but she also has an extra $5000 in the chassis on top of the $10,000 engine and a $3000 trans to handle it........that is a lot of coin to me.
I built the 406 on a budget and it smokes em with the best of them......I have $7000 total in the setup. It all depends on what you want to do and expendable funds. An 11 second car is as much as I want on a public street.

Jebby

 

Point taken Jebby, and I understand what you are saying. This is why I think a 427 is just too much. I have been talking to Chris Straub, and he said the same thing......the car, i.e. driveline, half shafts, etc. all have to be able to handle huge torque. So, my only point from the beginning was that I don't want to have to go to 5000 RPM to get the torque I want. I want is down lower. My 350 kicks *** once I get it wound up, I just don't enjoy driving it when I have to push it to 5000 RPM to push me back in the seat. But, I concede......and hope a 406 will give me what I want.

 

All this sounds good, but here is the difference If I can have 450 ft lbs of torque at 2500 RPM with one engine, is this not different than 450 ft lbs of torque at 4000 RPM? I want it at 2500 lbs so I don't have to push it to 4000 to push me exactly the same amount back in the seat. Maybe I am missing something, or just stupid, but 450 ft lbs of torque is 450 ft lbs of torque. The car, nor me, cares about these numbers,......its just about when the power occurs.

The best arguement suggested so far is by Jebby, who talked about have a strong torque curve from down low, the whole way, and that is better than a peak torque number that is down low, but drops off as RPM increases. That does make sense to me.

 

There is a 130ish horsepower difference between 450 ft lbs at 2500 rpm and 450 ft lbs at 4000 rpm.

Torque without rpm is useless in the context of driving a car. You apply torque when you park on a hill - with no motion. When you add motion you are talking about power: the rate at which work can be done.

What you want is low RPM power. I’m just laughing to myself because what you want is essentially an Oldsmobile big block. They don’t like turning more than 5500 RPM and they make gobs of low RPM power. Look there for inspiration (hint: 180 cc intake ports are only slightly smaller than the Olds heads).

 

i guess I will just remain stupid. What you say makes no sense to me and I have read many car publications that routinely talk about the lack of understanding that torque is what moves a car. Hp is a mathematical equation. I really am losing interest in this back and forth. I will build a 406 and cam it based on Chris Straub recommendation and hope it drives like I want it to and runs better than my wife’s Toyota 4 cylinder Camry..... my 350 does not as neither does **** without flooring it which is not my definition of performance or fun. My Harley, on the other hand does have torque and moves forward with every stroke of the piston.

 

Imagine you have a CVT that will vary the gear ratio to make you always run one RPM. You take your 450 ftlbs at 2500 rpm in one car and 450 ftlbs at 4000 rpm in a different car. The 4000 rpm version will out accelerate the 2500 rpm version every time. It can do the work (applying force over a distance) faster than the other car. That is what power is. Yes, it’s a simple mathematical formula, but torque can exist without movement - power can’t (in this context). When you add movement, power is what actually matters. Less power = slower.

What you are looking for is low RPM power. Since RPM is fixed, you gain power with torque.

It sounds nit-picky, but your example of 450 ftlbs being the same kick in the pants regardless of RPM is wrong. You need significantly more torque at 2500 RPM to get the feeling of the same torque at 4000 rpm. If you like how your current engine feels at 6000 RPM, grab that power number, do the math, and see how much torque you need at lower RPM to equal it. Then build to that goal.

 

Your Harley is a perfect example of torque …. Inline 4 Kawasaki bike would walk the dog on you in an acceleration contest any day of the week , Now put a 400 lbs fat girl on back of both bikes along with a 750 pound trailer and now all of a sudden the Harley is the fast bike

 

OP, I'm guessing you'll be happy with a tq band that has 500lbft from 1800-4500, it'll pull hard. Great for Drag racing probably not, but if you cruise at 2500 and push the pedal it's going to pull pretty good to 4000. I think a key point of this debate is that when you talk Dyno numbers it's WOT, the OP is talking part throttle and I'm thinking the step up in cubes and hopefully a well chosen intake tract/cam should have good throttle response and that's something that isn't typically part of a Dyno session. Raw Dyno data doesn't show how an Engine will perform for various driving scenarios. Time covered by a flat torque curve shows acceleration potential.

 

I got Flotek 220 heads on my 406 I'm building.. Yours Should be a torque monster as well. No replacement for displacement !

 

HP is what determines the acceleration of a vehicle torque does not, without knowing the RPM it is being generated at because that gives you HP. (I know this is not what some uninformed peope and magizines say but it is true) Hp tells you how much work can be done and that number is determined by taking torque times RPM and dividing it by 5252. The higher the HP the more work that can be done in the area where you are using the engine. High torque at low RPM still results is higher HP when compared to low torque at low RPM. The fastest 0 to 60 MPH times will be generated by the highest HP over the area where the engine is being used. Do the math (Tq x RPM)/5252 = HP The higher the Tq at any given RPM the higher the HP. The key point here is Hp generated over the area where the engine is being used. If your engine makes great HP at 6000 RPM but you always shift at 5000 RPM or before you will never use the full potential of that combination since you are not using the engine at the point where it makes that HP. This is just a math problem and I am going to try and make it understandable. (By the way I know what you want which is highest HP at low RPM which also means highest torque at low RPM)

Lets use the example you listed above. 450 ftlbs of torque at 2500 RPM vs 450 ftlbs at 4000 RPM. The max acceleration at any given time is the same in 1st gear but the higher RPM (and HP) engine can apply that acceleration over a much higher MPH range which means you will accelerate faster over time. For this discussion we will just use 2500 and 4000 RPM but in the real world there would be a curve for both of these engines that would be similer in nature.

I will show acceleration force in each gear by MPH, so you can see what I am talling about.

(450 x 2500)/5252 = 214 HP (450 x 4000)/5252 = 342 HP of the two combinations, same torque

Torque to rear axle with each combination

4 speed gear ratio's
1st = 2.64
2nd = 1.75
3rd = 1.34
4th = 1

450tq x 2.64 1st gear ratio x 3.55 rear gear = 4217 ftlbs torque with both combinations in first gear to rear axle
450tq x 1.75 2nd gear ratio x 3.55 rear gear = 2795 ftlbs torque with both combinations in 2nd gear to rear axle
450tq x 1.34 3rd gear ratio x 3.55 rear gear = 2140 ftlbs torque with both combinations in 3rd gear to rear axle
450tq x 1.0 4th gear ratio x 3.55 rear gear = 1597 ftlbs torque with both combinations in 4th gear to rear axle

2500 Low RPM engine mean 1st gear is good to 0 to 20 MPH, 4000 high RPM engine mean 1st gear is good to 0 to 33 MPH
2500 Low RPM engine mean 2nd gear is good to 20 to 31 MPH, 4000 high RPM engine mean 2nd gear is good to 33 to 50 MPH
2500 Low RPM engine mean 3rd gear is good to 31 to 41 MPH, 4000 high RPM engine mean 3rd gear is good to 50 to 65 MPH
2500 Low RPM engine mean 4th gear is good to 41 to 54 MPH, 4000 high RPM engine mean 4th gear is good to 87 to MPH

I know this is simplified and I don't need people complaining about the RPM ranges, what this shows is why HP is important and two engines with the same torque but different HP will not accelerate the same. The HP will always win if the engine is used in the correct RPM range where the HP is genereted.

As you can see 1st gear acceleration will be the same with both cars to 20 MPH but after that the low RPM engine must shift and the high RPM engine can keep going in 1st which puts more torque and HP to the rear tires.After 20 MPH the higher HP car with the same torque keeps pulling farter and farther away.

Build whatever engine you want but understand that by keeping the RPM low and grabbing another gear you will loose acceleration. To me building a 406 or 427 is not the problem either will work, the 427 will have more torque and HP at the same low RPM but the difference is minor.

 

All true. As long as you are willing to downshift, the higher rev'd up engine will be faster.

If you don't downshift, the engine with the most torque at that rpm wins.

I thought the OP was trying to get rid of the soggy bottom where he put his foot in it and nothing much happened?
Downshifting would solve that, but he wanted immediate punch.

He specifically stated he wanted as much power as possible UNDER 4000. And his current engine peaks well over that. And he never got it up "there" to enjoy it.

 

I agree, and am answering his question why the same torque on a 2500 rpm engine vs 4000 rpm engine is not the same acceleration. 4000 is still a low shift point and will drive great.

 

No, the more powerful engine will be faster. A 7000 rpm engine that makes 150 ftlbs makes less power than a 3500 rpm engine making 400 ftlbs. The 7000 rpm engine will be slower.

But focusing on torque has apparently led some to believe a 400 ftlb engine turning 2000 rpm accelerates the same as a 400 ftlb engine turning 3500 rpm. It doesn’t. There’s a significant power difference (114 hp to be exact) and that translates to a slower car.

He wants low RPM power. By focusing on torque without relating it to RPM he runs the risk of not getting what he wants. 500 ftlbs at 2000 rpm sounds impressive, but it’s only 190 hp. That isn’t going to put you back in your seat much. Not unless it stays at 500 ftlbs at higher rpm where it can make more power

 

All these numbers mean nothing to me, but what is clear is that most of you seem to believe the only way to push me back in the seat is with 5000 rpm. So what does it take to push me back in the seat at 2000 rpm because it appears you believe that is not possible.....but it is. Call it power, torque or Cookie Monster, but I want better than my wife’s 4 cylinder Camry that will accelerate well if I stand in the pedal. What engine will push me I.e. accelerate the car without resorting to 5000 rpm? Does it exist?

What I think the real story is, that you just simply like making a bunch of noise and running RPM up to 6000, 7000 RPM cause it makes you feel good. That is your definition of power. Anything else, to you, is simply nothing but a weak motor. The constant reference to a "fast" car, means fast speed. All cars today are fast.

 

Again, yes. As I said in my first response to you, look up how big block Oldsmobile’s are built for inspiration. They are fundamentally rpm limited and have to resort to low rpm power. Look at the ports, the intakes, the valves, the cams. Find Chevy parts that are similar for your build.

Chevrolet engines aren’t nearly as limited in the RPM department, and it’s easier to build power at high rpm...so that is how they are typically built. The BOP guys don’t have that luxury.

 

Olds 455 was rarely done in that it was under square. 4.126 bore, 4.250 stroke. Had over a 10.6 deck height block, 6.73 length rods. oiling system was not very good. Friend of mine spun a rod bearing in his W30 455.