What's the point of a high revving LSx?

[Bryan]

So I've been researching the slc for years. I have to build it myself and I just don't have the time and it's killing me.

So I just randomly popped on the board today and saw the new transaxle that is available, which is amazing btw.

So I was just randomly zoning out while watching TV and thinking about high rpm engines and their benefits. I see a lot of talk of building a high RPM small block, I actually thought of building one myself one day for the slc.

Now I think I had a bit of a revelation in my theoretical build. I have to admit, I did work on a car in my garage today, and the exhaust vent hose slipped off the exhaust pipe, and I didn't notice it for a very long time, so I could possibly be out of my mind.

Is there no point to building a high rpm lsx for these cars? I mean, do people even know why they want an engine to rev high? I think people just associate high revving engines with racing. Or think the powerband might be more manageable when the engine is not making "low end torque" so to speak.

After some thought, something which I've known for awhile, the only real reason any race cars, sports bikes, any car for that matter opt for a high rpm engine, is simply because torque requires more metal. Lets take a 600cc sport bike, 130hp, 17,000 rpm. Without tracking down a dyno graph, just using ol 5250 to estimate, that's probably around 40ft lbs of torque, which allows you to use extremely small and lightweight drivetrain components.

Now why many of you probably already knew this, did we ever think about the purpose of a high revving v8? I mean I really don't think people that want these are simply trying to do so to make use of a lighter transaxle. If you're using say a graziano rated for 600ft lbs (probably conservative), why would you want your ls2, or whatever base you are using to actually rev higher if you aren't coming close to it's torque limitations.

Now say if you want to argue the old traction debate. A high rpm motor will have more traction since the the lack of low end torque doesn't hit you like a brick wall every time you touch the throttle, and you can actually control the car on corner exits. Without driving one, I would imagine it's probably the biggest downside to owning an slc with a SBC. Torque madness.

That is all very true and a good argument. The problem is, I see people spending huge sums of money for high rpm lsx's to solve this problem. It's certainly not cheap to decrease the rotational mass and the entire valve train. I think there is a very cheap way to solve this.

There are plenty of ways to make serious power on a budget with these ls motors. 500hp is pretty cheap. 500hp and 8000 rpm is not. The difference between the two just being the powerband.

Then I had a thought, in order to make something lightweight like an slc more controllable with a torque monster like an lsx, why not simply limit the torque down low with some sort of variable air restrictor. I would think that would be pretty darn easy to design. It will be mostly unheard of since it is actually taking power away from the car. (not many cars have the power:weight ratio of an slc)

For example, if you had say a 500hp lsx, only rev's to 6000rpm. Which would mean torque is around 500 as well. Why not just cut the air flow at lower rpms. Essentially you could easily have any torque curve you wanted. In essence, you might even be able to take the dyno graph from your favorite car, and tune the restrictor to mirror the line on the graph. If you took that lsx and it's theoretical 400ft lbs of torque at 2000rpm, and turned that into 325ft lbs of torque using your restrictor, and simply kept gradually increasing the torque throughout the rev range, you engine would essentially just have the behavior of your favorite high rpm screamer.

This idea really only works for cars like the slc since the people building the engines and pairing the transaxles are not race teams or car companies. There are limited drivetrain selections. In some of the higher end builds, someone might be tailoring the engine to maximize the lightest drivetrain possible, but not in most home built cars. The point of this car is to keep in on a budget. Also the slc is in a very rare class as far as power:weight ratios go. In most cars more power at any rpm would likely lead to more fun. In the SLC, and other cars with ridiculous power to weight ratios, the power band can make or break your experience and/or lap time.

cliff notes:variable air restrictor, or probably just an electronic throttle body, tuned for less power at low rpm, could make the car far more enjoyable to drive, and faster on a track, and significantly reduce the cost and need of a high rpm build everyone so desires.

Anyway, where I come up short is the actual implementation of such a thing. While I have rebuilt a few engines, work on my own cars, done a little racing, and a little ecu tuning, I am for the most part a keyboard racer, and you guys would probably have a better idea if such a design would work.

 

[Roger]

Is there no point to building a high rpm lsx for these cars?

IMO, it's mostly for the sound. You can get more power by revving more, which is the reason that F1 engines do. Your torque reasoning has the flaw that you get higher losses (more heat in the gearbox) and still the same output torque to handle.

why not simply limit the torque down low with some sort of variable air restrictor.

That's call the accelerator

For me, one of the advantages of a car like the SLC is that it is raw - you feel and control everything. I'd go with traction control to limit the trouble that high power to weight could get you in.

 

[James]

Well, one more advantage you get,, and it does come down to transaxles again, or should I say the limited selection (without spending $$$$$$) is you get a little more speed with each gear selection. Let me explain

My G50/20 giving an RPM redline of say 6000,, will have the following

6000rpm vs 8000rpm

1 -- 36 vs 48
2 -- 68 vs 90
3 -- 98 vs 131
4 -- 124 vs 165
5 -- 149 vs 199
6 -- 178 vs 237


So,, with the extra 2000rpm,, you are getting much more speed out of 1st,,, otherwise you could be changing as soon as you got a little grip,, and not that it's very applicable,,, but,, you could reach the cars top speed. All with the same transmission, and not changing gear ratios.

 

[Bryan]

IMO, it's mostly for the sound. You can get more power by revving more, which is the reason that F1 engines do. Your torque reasoning has the flaw that you get higher losses (more heat in the gearbox) and still the same output torque to handle.

That's call the accelerator

For me, one of the advantages of a car like the SLC is that it is raw - you feel and control everything. I'd go with traction control to limit the trouble that high power to weight could get you in.

Yeah, but actually having a good power band is no replacement for a computerized traction control system. We also know that throttle control is far easier said than done, and increasing the chances of human error almost always decreases lap times. Hence why most people posts better lap times in a nissan gtr than a corvette z06, when the z06 crushes it on paper.

More power is not the reason to rev more in my opinion. If you need to increase hp, producing more torque is almost always easier on the rotating assembly then getting a power increase from spinning faster. I have seriously come to the conclusion the sole reason is to keep the weight down of the powertrain.

 

[Bryan]

Well, one more advantage you get,, and it does come down to transaxles again, or should I say the limited selection (without spending $$$$$$) is you get a little more speed with each gear selection. Let me explain

My G50/20 giving an RPM redline of say 6000,, will have the following

6000rpm vs 8000rpm

1 -- 36 vs 48
2 -- 68 vs 90
3 -- 98 vs 131
4 -- 124 vs 165
5 -- 149 vs 199
6 -- 178 vs 237


So,, with the extra 2000rpm,, you are getting much more speed out of 1st,,, otherwise you could be changing as soon as you got a little grip,, and not that it's very applicable,,, but,, you could reach the cars top speed. All with the same transmission, and not changing gear ratios.

You can just change the ratio the differential and achieve the same result

Edit: I realized it right after I hit send

 

[John Ross]

To the original poster: I think people want high revs for the sound. They like that howl.

If you believe (perhaps correctly) that a typical LS motor will have too much torque to get best lap times in an SLC with an average driver behind the wheel, instead of designing in an intentional air restriction to reduce torque (NASCAR guys would want to skin you alive), pay for a custom 3.000" stroke crank and be done with it. This is a trivial expense in relation to an $80,000 project, particularly if you're building up an engine in the first place.

I believe the circle track guys have already solved any valvetrain issues with running at 8000 RPM.

FWIW there's a new guy on the board who just came over to look at my car. This is exactly what he's planning for his build.

JR

 

[Jac Mac]

I believe most if not all guys building these cars still believe they have the ability to control the throttle etc, if not they still want to give it a go!

What your describing is something akin to the fly by wire setups on BMW etc where once the limit of tire adhesion has been reached the computer takes over and wont allow any more power to be delivered regardless of how hard you push on the loud pedal...... bit like driving with your missus....where is the fun in that

 

[James]

You can just change the ratio the differential and achieve the same result

Edit: I realized it right after I hit send

Normally, I would agree with you. However, because every choice of trans-axle we have is pretty big money, and even more to change anything, it seems like the engine is the next best thing to change. You can build a fully forged bottom end on an LSx for just a few grand, and a pretty stout top end for about the same or maybe a little less.

A G50, you are going to spend at least $3-4k on just changing to a taller gearing diff. When you do this, you also put more strain on your transmission. Each individual gear I believe runs around $600 to $900.

If you achieve a little more HP by RPM,, you can effectively keep the same transmission. Not saying it's the best way, just,, another way.

 

[Mike]

Now say if you want to argue the old traction debate. A high rpm motor will have more traction since the the lack of low end torque doesn't hit you like a brick wall every time you touch the throttle, and you can actually control the car on corner exits. Without driving one, I would imagine it's probably the biggest downside to owning an slc with a SBC. Torque madness.

Then I had a thought, in order to make something lightweight like an slc more controllable with a torque monster like an lsx, why not simply limit the torque down low with some sort of variable air restrictor. I would think that would be pretty darn easy to design. It will be mostly unheard of since it is actually taking power away from the car. (not many cars have the power:weight ratio of an slc)

My right foot cannot out think the RaceLogic Traction Control unit I have in the SLC. I am thoroughly pleased with this control box and it's "fuel cut" is highly adjustable, smooth and/or can be turned off or adjusted with the control knob from the drivers seat in seconds.

 

[Cam]

I've always heard its more of a cost issue than the sound of the engine. You look at most European OHC, short stroked engines. They generally turn more revs than the U.S. counterparts.

It's more expensive to build DOHC/destroked but there is less destructive torque. U.S. cars make a bit of a compromise, long "stroker" / pushrod engines make cheap horse power but cant rev as high. They take the tons of low end torque as a by product and live with it to get more HP cheaper. So, an with LS3/7 on track, you better be on your game during trackout or the Tq will be a TKO due to oversteer.

This is an interesting article on it..

Low-End Torque or High-RPM Power?

 

[Howard Jones]

This a interesting subject. I would only suggest that comments be designated as pertaining to real racing cars in real races OR other performance type cars on track day outings OR street cars only. There is SO much difference that they really don't have anything in common.

I have said my piece on open track cars a lot on this forum. Build a big motor, right, 400-450 CI, and run it a bit more than 1 hp per inch on pump gas. This will result in a lifetime of use with a reasonable level of maintenance.

Street cars can be built with a smaller version of the track car, 300-350 inches, maybe not run a oil cooler, vacuum advance, and a carb with a choke. FI guys good luck. I'm old and hooked on Holley's.

Never had a race car but I suspect you better build it by the rulebook and spend as much as the willing teams or don't bother. That's the way it works in the bike would where I have a little bit of experience.

The engine use and thus the build should set the rev limit not the other way around.

 

[marc]

So go with something else. The key of the LSX is it is best bang for buck. Aluminum, more and more aftermarket parts are coming out for it all the time. Much more reliable than a SBC or SBF for leaks and maintenance. FI with a good tuner will make it sing. The alternatives Toyota's/Lexus V8, new Coyote from Ford, and for real beat your head against the wall 3 rotor Mazda. The difference is right now you have more support with the LS3/7 available. The SBC/SBF are capable of some unique sound (180 crank to go with the headers) but carbs are great if your cruising, FI is the way to go. You can use some ludicrous cams and still make them drive on the street.

 

[Al Jones]

Don't see where the problem is.....I like TORQUE. It makes the car far more tractable on the street in most everyday situations and easier to live with than a peaky engine. Even for a race only machine, the lower revving torque motor has some desirable features, such as recovery from some turns on the course and letting the driver concentrate on the course without rowing the machine around with the gearbox lever.

 

[Jac Mac]

So go with something else. The key of the LSX is it is best bang for buck. Aluminum, more and more aftermarket parts are coming out for it all the time. Much more reliable than a SBC or SBF for leaks and maintenance. FI with a good tuner will make it sing. The alternatives Toyota's/Lexus V8, new Coyote from Ford, and for real beat your head against the wall 3 rotor Mazda. The difference is right now you have more support with the LS3/7 available. [[[[[[[[[[[[[

The SBC/SBF are capable of some unique sound (180 crank to go with the headers)]]]]]]]]]]]]

but carbs are great if your cruising, FI is the way to go. You can use some ludicrous cams and still make them drive on the street.

You might want to re-phrase that bit in red brackets above- if you had a 180° crank then there would be no need for the 180°/crossover headers- all production ford, chev, mopar etc cranks have throws displaced @ 90°.

 

[John Milmont]

Much more reliable than a SBC or SBF for leaks and maintenance.

I have built a leak free SBC and it kinda scares me because it's not normal..:worried: I have a timken teflon seal on the rear main. They say these cut into the crank over time, but I figure that by the time that happens, its probably time for a new engine anyway.

FI is so easy these days that I cringe when I hear people talking about putting carbs on a new motor. An LSX with carbs? :thumbsdown:

But a high reving pushrod motor is going to be trouble if its going to be a street motor. I guess it depends on what you consider high reving; but in my experience, if you are going to build power at high rpm, the cam is not going to want to idle well or drive at normal cruising rpms.

 

[Bryan]

Don't see where the problem is.....I like TORQUE. It makes the car far more tractable on the street in most everyday situations and easier to live with than a peaky engine. Even for a race only machine, the lower revving torque motor has some desirable features, such as recovery from some turns on the course and letting the driver concentrate on the course without rowing the machine around with the gearbox lever.

I think everybody likes a lot of instant torque until they drive something with under 5 lbs per hp.

 

[John Ross]

I think everybody likes a lot of instant torque until they drive something with under 5 lbs per hp.

Ask Brian Thomson about the time he supercharged an LS7 to 950HP.

In a Pontiac Solstice...

 

[Keith]

There is another vital component to consider in to this argument: the weight of the vehicle to be moved.

One of the reasons European engines are small is that historically, production engines have been small (most race engines are derived from street/production motors) because the weight of the vehicle to be moved has been relatively low.

A torque monster in a very light car is going to be 'interesting' unless you have the kind of electronics that Jac is talking about.

Colin Chapman just about summed up European Road racing philosophy in a nutshell: "Build it stiff and add lightness".

In removing unnecessary weight from a vehicle (which equals a horsepower hike), consider not just one or two heavy pieces but a thousand pieces weighting less than an ounce.

 

[Mike]

I think everybody likes a lot of instant torque until they drive something with under 5 lbs per hp.

I don't know, I had a 1,480lb. lotus super 7 replica with a GM LS6... this car was nothing but fun and very controllable once I got the throttle cable/pedal linkage dialed in & good tires installed, also over 25mpg - very eco friendly. I should have kept that car...:cry:

 

[John Milmont]

It's already been said in this thread how the transmission choice is a limiting factor. But if it wasn't, one of the upsides of a lot of torque is that you don't need many gears. Look at the old le mans cars and can - am cars that had large displacement. They had very few gears, mostly 4 speeds and they could go over 200mph.

Those 6.5 liter V12 Lambos went just over 60mph in first gear despite weighing 4000lbs. And it did it in what, 4 seconds? I don't think a low torque/ even higher rpm engine could do the same thing.