Flat Torque Curve

[William Quantrel]

I will never forget the day I went for a ride in a late model Viper SRT10 (500hp/500tq) and immediately went for a ride in a Dodge Neon SRT4 right after.

Dodge had an open house day and we could sample vehicles.

Both rides in the Viper and the Neon (I was a passenger) were down the drag strip with rolling starts.

Viper was first. Acceleration was brutal - constant - and pushed me back in the seat.

SRT4 was next. The engine has been staged a little and I suspect close to 350 hp out of a 2.4L turbo. Acceleration was *SURPRISINGLY* near as good. Might have been the same. It weighs about 800 lbs less than the Viper.

Which was more fun? The SRT4. The acceleration was more exciting or how the engine delivered the power. I felt that Viper's acceleration was "flat" and the SRT4's acceleration built up or increased the faster we went.

This power delivery - of the SRT4 kind - is what I desire from an engine to go into a GT40.

I would like to hear your comments on this "acceleration affect" or difference between the two engines. I am concerned that a stroked 427 will leave me with a "flat" or "boring" acceleration curve similar to the Viper.

Would a 363 (500 hp/450tq) fulfill my needs in a similar fashion to the SRT4 style of acceleration?

I am equating a Viper engine with a stroked 427. Would a 331 or 363 equate to an SRT4 turbo engine.

I like how the turbos build power. The SRT4 when staged has more tq than hp. What spec'd N/A V8 would build/deliver power similar to a turbo?

 

[Seymour Snerd]

....This power delivery - of the SRT4 kind - is what I desire from an engine to go into a GT40.

I would like to hear your comments on this "acceleration affect" or difference between the two engines. I am concerned that a stroked 427 will leave me with a "flat" or "boring" acceleration curve similar to the Viper.

Would a 363 (500 hp/450tq) fulfill my needs in a similar fashion to the SRT4 style of acceleration?

I am equating a Viper engine with a stroked 427. Would a 331 or 363 equate to an SRT4 turbo engine.

I like how the turbos build power. The SRT4 when staged has more tq than hp. What spec'd N/A V8 would build/deliver power similar to a turbo?

You are describing the effect of the shape of the torque curve, and your preference for that is for it to increase with RPM. Torque curve shape is not inherent to any size engine; in principle you can have a stroked 427 with a torque curve like the SRT4; you just have to ask your builder to build it that way.

However, when you are talking about a 2400 lb GT40 with the amount of torque a 427 will produce with that torque-curve-shape, you are describing a vehicle that could be extremely dangerous in the hands of a less-skilled driver because the amount of torque applied to the drive wheels will change very rapidly without the driver's input or ability to correct. In the middle of a corner this can be suicide, literally.

So the wise thing to do would be to go after that torque curve shape with a much smaller engine that a 427. Maybe a 289 or 302. A 363 is really not that much smaller than a 427 (15%), so when you are talking about mountain-shaped torque curves you are talking about 600 ft-lb of torque more, possibly much more. This is not to be taken lightly (pardon the pun).

Your typical viper is at least 40% heavier than the GT40, so your 500 ft-lb in the Viper is like "only" 350 in the GT40. A ruler flat torque curve in a modern 427 will still be in the low 500s. I have one like that and I asked for it that way because I didn't want any surprises coming from the stuff downsteam from my right foot.

By the way, the technical term for the effect you're describing, increase in rate of acceleration over time, is "surge". So your SRT has high surge, and your Viper has little or no surge.

 

[William Quantrel]

Alan,

Thank you for that and very much so.

These are exactly the answers I have been search for and for a very long time.

Yes, it is surge I am after. I had it with my sportbikes and turbo car.

I didn't know the builder can build it into the engine regardless of size. I assumed a 427 with 500/500 hp/tq always delivered large amounts of tq at low rpm. I assumed to have a 427 with surge meant a solid roller to bring tq higher up in the rpm and thus, make it unstreetable. Goes to show you what little knowledge I have of these things...

Excellent point - 15% difference between 363 and 427. That was a complete oversight.

Another excellent point about building surge into a large cube engine. Using your explanation, is it fair to relate this to the old 911 Turbos that suddenly came on boost and with it a pile of tq to ruin your day especially in a corner?

I also see your point about wanting a "predictable" power curve i.e. flat torque curve. I can now see how knowing that you have 500 lb-ft, once at a given rpm, makes for "carefree" driving.

I need to think this through carefully because I have been spoiled with modern day driver aids like traction control/ESP a.k.a. "let the ECU sort out where my right foot is and what position the steering wheel is". Excellent point you made by stating that an increase in tq with no increase in pedal travel can be hair raising by having a surge-y power delivery.

 

[Andrew Robertson]

Hi, you may also have felt the effect of increasingly effective charge cooling on the turbo car - most OEM turbo cars get pretty heat soaked pottering around and staging, and will gain power through the increasing effectiveness of the intercooler as speed rises which lowers intake air temp and therefore the mass air flow into the engine, along with an ability to tolerate greater ignition advance which also aids power. This phenomenon can be really pronounced on a vehicle that makes aggressive use of a knock sensor to manage detonation on a highly strung pump gas turbo car, and does not really exist in a mid engined naturally aspirated environment.

And as an aside, the guy who has an extra 50 or 100 ft/lbs off the turn, all else being equal, is going to waste the guy with the nice soulful high-winder. Cubes rule!!

Cheers, Andrew

 

[roaldin]

Nah, turbo's win.
Turbo's make crazy torque, they just need to be driven properly.

Please excuse the rather massive generalisations.

Tim.

 

[Seymour Snerd]

...is it fair to relate this to the old 911 Turbos that suddenly came on boost and with it a pile of tq to ruin your day especially in a corner?

In a sense, although my understanding of that particular car is that it's handling behavior was a strong contributor. AFAIK a GT40 wouldn't have so strong a "power-off oversteer" effect. But it's the same fear, that is, the interaction between driver and car have the same problem, and the accident scenes would be similar. :stunned:

 

[Andrew Robertson]

Nah, turbo's win.
Turbo's make crazy torque, they just need to be driven properly.

Please excuse the rather massive generalisations.

Tim.

Tim, I agree with you, but cubes still rule! I'm a proponent of both (my last road race car ran a twin turbo 461BBC, and my road car is an old '72 XJ Jag with a twin turbo 383SBC in it). It's nice to have something that's still quick and responsive offboost, and then on-boost.... as you say the real fun begins. Cheers, Andrew

 

[William Quantrel]

Thank you to all who have replied.

Putting the flat tq curve question into a GT40 context, am I to safely assume that due to the mid-engine layout of the 40, one can use/apply more tq due to the weight sitting over the rear tires? More grip?

Gordon Levy has stated he prefers a 306 or 331 in 40 and he was making reference to a lesser amount of tq and pushed higher up in the rpm stating that the car will be easier to control.

I see this statement as one meaning a 306 or 331 with "surge". I interpreted his views as "500 lb-ft @ 2000 rpm" in such a light car = trouble.

Of course we cannot speak for Gordon.

This line of thinking poises an interesting question; throttle pedal travel.

350 lb-ft over 1 inch of throttle travel is harder to modulate/control than 500 lb-ft over 6 inches of throttle travel.

Can we safely have 500 lb-ft @ 2000 - 5000 rpm but over 6 inches of throttle travel???

I recall Jaques Villeneuve preferring a VERY short throttle travel in his F1 car.

 

[Seymour Snerd]

Thank you to all who have replied.

Putting the flat tq curve question into a GT40 context, am I to safely assume that due to the mid-engine layout of the 40, one can use/apply more tq due to the weight sitting over the rear tires? More grip?

Gordon Levy has stated he prefers a 306 or 331 in 40 and he was making reference to a lesser amount of tq and pushed higher up in the rpm stating that the car will be easier to control.

I see this statement as one meaning a 306 or 331 with "surge". I interpreted his views as "500 lb-ft @ 2000 rpm" in such a light car = trouble.

Of course we cannot speak for Gordon.

This line of thinking poises an interesting question; throttle pedal travel.

350 lb-ft over 1 inch of throttle travel is harder to modulate/control than 500 lb-ft over 6 inches of throttle travel.

Can we safely have 500 lb-ft @ 2000 - 5000 rpm but over 6 inches of throttle travel???

I recall Jaques Villeneuve preferring a VERY short throttle travel in his F1 car.

I don't have answers to the above interesting questions, but just some thoughts:

1. I was reading David Hodges book last night and he observed that early on, and even with a small block the GT40s weight distribution was more rearward than ideal, and that test drivers said it was OK, but don't let the rear end too far out or you are in trouble. So perhpas there's a hint of "Porsche 911" to watch out for.
2. I hadn't heard the Gordon Levy comment; can you point me to the context?
3. I don't think you can safely have 500 lb-ft of torque at all. Well, at least I can't.
4. One thing to remember about long-travel controls for humans is that feet and hands have travel limitations of their own where as you move to the extremes your own degree of control diminishes. Think of your foot in full ballet-point mode: you can't really quickly and accurately modulate the throttle like that. But aside from that it sounds like a good idea to err on the side of long travel for something over-powered, not only for fineness of control but also because it might tend to inhibit too-sudden control movements.
5. Yeah, well, he's Jacques Velleneuve....

 

[Howard Jones]

You need to settle on tires. First! Then build the power to suit the grip of the tires. I say tires because you will be well served to install a LSD in the gearbox and use both rear tires.

Then there is rev range. 6000 rpm engines will last a very long time and cost a huge amount less than 8500rpm engines. Both can make 500hp. (more than enough)

 

[EGLITOM]

Good performance tires are a must for a 500hp engine. So on 15´s there is a small choice of Avons, Pirelli, Michelin and Hossier to scope with those power levels. On 17´s there are more.


I would say engine choice and torque curve shape should also be seen in context with your gearbox choice. If you have a bigger displacement engine with a flat tourque curve and broad powerband a 4 speed or 5 speed will do it. If you go for a small displacement, small powerband engine you need more gears so at least a 5 speed is needed ( and if so this should have closed ratio race gearings with a long 1st and the other 4 closely spaced) or a 6 speed. with the correct ratios. Correct ratios are much more important on small powerband engine than on a broader one.

It realy is the complete package you have to take in account.
TIRES
GEARBOX
DIFFERENTIAL ( open Diff, Platestyle LSD or ATB)
ENGINE
this perfectly harmonised will give you a great package.

If it wouldn´t be for the sound of a high revving V8, i would decide for a 408 with app 550 to 580 HP with hydraulic roller tappets, max RPM6300 , torque over 530. flat torque curve. This will give you great drivability and reliability if you chosse the right components and engine builder.

One comment to the pedal travel. It will more depend on the feeling and smoothness. You will find out that you will go for the max travel you can get because of the long cable and the resistance from the throttlle plates. The more plates you have to move, the more your ratio have to be adapted to that, to give you a nice feel. A short way of course can be realised if you run a Fly by wire solution.

TOM