Grimace

Stuff in a V8.

You could turbo it, but only do so knowing that the last gen. RX-7 didn't respond well to turboing. Heat issues. Rebuilds after 60K miles. Etc.

One large turbo rather than two turbos might do the trick. Guarentee either way it will either be cost prohibitive or nearly technically impossible.

zoom44

i just read something in one of the mags thats doing a rebuild on a twin turbo. they basically said the same thing. out with the twins and in with the single larger one. the twins made too much heat and combined with the restrictive stock exhaust was just killing the engines. the guy doing the work for them said thats the first thing he does when rebuilding a third gen rx7, getting rid of the twins.

wakeech

ya, the twin turbos aren't as efficient as a single one, but give better boost at low RPM, and way higher boost at high RPM than a supercharger...
but, one thing first... i don't think that the exhaust system is as "restrictive" as teh aftermarket guys say it is... sure, it looks that way, but restriction isnt' always a bad thing, it's waht gives you torque, but robs you of HP... you need the back pressure (for the opposite and equal reaction of pushing the car) to make teh torque...
and i don't hink that factory enigneers are so easily being outdone by these after-market folks... they are, after all, the best and teh brightest, and optimize it to work a different way, not "worse" than aftermarket ones...
and another thing, FI will add the "replacement for displacement" you want down low... remember that torque of this kind is unparalleled in ANY OTHER TYPE OF GASOLINE ENGINE of coparable displacement...
so, i think you're looking for a super charger system... lower heat, no need for ******* around with a special exhaust system (i mean any aftermarket one would work) and less heat problems than a turbo system, and cheaper and easier to install... though, not as much HP

zoom44

good point about the factory tecs. has anyone had a supercharged version instead of a turboed one?

babylou

Dude, we ain't talking about jet engines here! Exhaust back pressure increases pumping losses which robs power and efficiency. The only viable reasons I know of to have any exhaust backpressure are:

1. Impart a measure of exhaust gas recirculation which reduces combustion temps to reduce NOX emmissions.
2. Back pressure can be a byproduct of having small enough exhaust passages to allow exhaust scavenging by way of momentum effects. This is usually only important in the cylinder head and outlet manifold/header.

KayakDaddy

You can't engine brake without exhaust back pressure. Really high flow exhausts minimize the effectiveness of slowing down this way.

I do a lot of engine braking or transmission breaking as some people call it. It's nice to have the car in the gear you'll want it in coming out of the corner as you are going into the corner. You'll break out of the corner faster and be able to keep a cleaner line through the turn. Plus it's fun.

cOz

Engine braking is not the fastest way to drive. Using the brakes, double clutching, and rev-matching is the best way to drive a car fast. It is hard to learn and even more difficult to master while dealing with all the othertasks of driving. Engine braking is fun, but keep in mind its not great driving.

karmavore

I'm no expert but to me it sounds like your logic is correct but your implications are wrong.

* If two cars both produce 150 pound/feet of torque, one at 3000RPM and one at 7000RPM the second might be better off because it can take advantage of gearing, but... won't the car that produces its 150 FT/LBS @ 3000RPM have an advantage off the line because it got all its torque early where as the other car had to wait until it hit 9000RPM?

* If two cars produce DIFFERENT amounts of torque, one 250 FT/LBS @ 3000RPM and one 150 @ 7000RPM it seems to me that the one that makes 250 will still have an advantage because not only did it produce more torque earlier, but by the time it too hits 7000 RPMs it's probably also producing at least as much torque as the other car, so it should, IMO, win in both regards, no?

wakeech

you're rigth about the engine with the low-down torque being quicker off the line if they both start at the same revs, but because of the gearing (and the fact that you can probably drop the clutch out higher in the rev range with the peakier engine) you can allow the higher revving engine to hit the higher revs at the same time the torquey lower revving engine hit the low revs, or at least reduce the time it would take to rev all the way up by using different ratios than the engine with teh torque down low.

now, in comparing two different engines of obviously different displacement, it's kind of spoiling the comparison. yes, having SO much more torque down low would imply having better horsepower at equal revs to the less torquey engine at equal revs (but it probably doesn't rev out at the same RPM as the less torquey one, most likely considerably lower) and yes, would probably have the advantage in making power at all RPM in this analysis. but again, kinda spoils the comparison, like a 302 with a 1.6L Honda (both in comparable states of tune, obviously).

zoom44

in your first scenario i think i see the diiference. the car making 150 at 3000 rpm has a shoter power curve. probably only 2500 to 3500 rpm. but the other one has a broad power curve something like 3500 to 8000 rpm. so it can pull at its peak torque for longer enabling it to pull ahead of the competition.

in the second scenario, i don't know. i'm not an expert either but i think the torque curve comes into play again. because of the gearing the first car's curve falls of sharply after its peak. so its probably (just guessing here) not going to be making more than the second car at 7000 maybe equal or less. so it would have to shift , which would drop it under its peak and it would have to climb into its tourque curve again allowing the second car to catch up, if there was enough track left.

i wonder where the break point is though. 280 ft/lbs? 300?

Rich

Man, I wish I had a white board.

The key in my comparison was that the two torque curves are identically shaped. If the shape of the curve is the same, but one is stretched horizontally (higher RPM) and compressed vertically (lower peak torque), then the comparison works.

The easiest way I can think of to explain what I mean is to put two engines in black boxes, with the only output being a drive shaft. Nothing can escape the box, except the drive shaft turning, so no sound, sight, etc.

Engine A has a max of 300 lb. feet of torque at 4k, and a redline of 5k. Engine B has a max of 150 lb. feet of torque at 8k, and a redline of 10k. Remember, identically shaped curves, just scaled differently. Inside the black box of Engine B, you attach a 2:1 gear, so that the output of the shaft is half the speed of the input to the gear. The torque is multiplied by 2 as a result of the gearing, and the RPMs are lowered by a factor of 2.

How can you tell the difference between the shafts at the output? Assuming no losses in the gearing, you can't! They would have exactly identical torque outputs at the shaft at the same RPM (of the output shaft).

Guess how the horsepower compares at those two points? It's identical!

Now, if you put both in a car, how would they drive? If the car is the same weight, and the gearing is identical except for the 2:1 gear on Engine B, they would accelerate identically. The sound would be different, and Engine B would rev more quickly. However, it would hit 8k RPM at the same time Engine A would hit 4k, and they would be making identical torque at the wheels.

I think the reason this is so hard to grasp is that for so long people have been told that "Torque rules", and that torque is what you get at low RPM, HP is what you get at high RPM. That's kinda-sorta true, but it completely misses the point.


Now, this is all an academic exercise. The main point that I think people should walk away with is that you can't judge a car by its stats! You can't say Car A has more torque/HP, so it's faster. You can't say Car C handles better than Car D because it's skidpad numbers are higher. You've got to DRIVE the dang things to see what they're about!

zoom44

SORRY RICH! i forgot about your earlier post saying the torque curves were identically shaped just different in scale.
thanks for the reminder. i understand better now.

karmavore

That's like saying a student who attended a class half of the time will do as well on the final as a student who attended all of the classes ...if the first student gets a hold of the exam and writes half of the answers on his hand. So what?

You're taking two engines that appear to offer different amounts of power, giving the 'weaker' one an advantage the other doesn't have (but almost assuredly would in real life), telling us they are performance equals, and playing it off like it's some sort of wonderful automotive conspiracy that's been taking place right under our noses!! Sure, at higher RPMs torque may be multiplied by gearing, but to insinuate that that means high strung VTECs and rotaries are 'better' off because of it is an invalid conclusion.

...but yes, Rich, if you insist on saying so, a turtle will always be faster than a rabbit ...if the turtle has roller skates!

Luke.

Rich

No conspiracy. I'm not sure where that came from. The gearing "advantage" is real, and the "stronger" engine can't be geared to get the same advantage. The key that you're missing now is that this difference in gearing is real. You wrote that the weaker engine would have this advantage in real life. It can't! The reason is that there's one gear ratio that will allow an engine at a particular RPM to power wheels that are turning at a different RPM. *This is important to understand*. Once you understand that, you can clearly see why Engine B can't have the gearing "advantage" that Engine A has, and why it's not being given "an advantage the other doesn't have". That's simply false. No conspiracy, no roller skates, no nothing. Just physics.

Another way to look at it is to ask yourself why, if gearing isn't real, can't the waterwheel in the link provided above and here propel a car to any reasonable speed, even though it's got 2600 lb. feet of torque? It simply rotates way too slowly. Gear it so that it can get to 60 mph in a car and it's got 43 lb. feet of torque! That's 6 HP!

One last example that will possibly help to explain. Here's what I wrote in the other forum, very slightly modified to match the hypothetical engines in this thread:

12 RPM at the wheels is approx. 1 mph in a car (I took this from the web site I cited. If wrong it just changes the final number, not the comparison).
Let's say both cars are geared so that max torque is available at 40 mph in 2nd gear.
That's 480 RPM at the drive wheels.
Engine 1: 150 lb ft. of torque at 8,000 RPM
Engine 2: 300 lb. ft. of torque at 4,000 RPM
Engine 1 is geared (this is just derived from the 3 above points) so that:
150 lb. ft. * (8,000/480) = 2500 lb. ft. at the drive wheels
Engine 2 is geared so that:
300 lb. ft. * (4000/480) = 2500 lb. ft. at the drive wheels

Keep in mind, this gear ratio is the ratio that MUST be to get an engine at that RPM to power wheels that are spinning at a rate that will move a car 40 MPH. There's only one gear ratio that does that, given the engine RPM and wheel radius! You can see that because it's not just made up, and nowhere did I arbitrarily give the "weaker" engine an advantage. It's just the physics of the situation.

The engine with lower torque at higher RPM gets the same amount of power to the drive wheels despite being "weaker", because it can take advantage of *gearing*.

The gearing advantage is real. If my explination has failed to convince you, maybe that link I provided above will.

Last, I'm not trying to say that high strung rotaties and VTECs are better in any way! Please don't put words in my mouth. I actually just test drove an S2000, and hated the lack to torque until it got to 6k. I understand the physics involved that explain how they get a very low torque engine (143 lb. feet, 2.0 liters) to propel a car to 60 in under 6 seconds. However, I didn't like how it handled at the limit, and didn't enjoy its performance off the line either. I think the thing I've been strongest in advocating is exactly the opposite of what you said I was implying. Drive the dang cars and leave this information at home. If you don't like high strung VTECs (I don't), don't but an RSX or S2k! If you don't like revving a rotaty to 9 grand, don't buy an RX-8!
The only point I ever tried to make other than that is that you can't simply say the RX-8 has no torque without understanding the issue of how RPM and gearing play into the picture. I know this is tough stuff for most people to understand (I was a physics tutor in college), so I've tried to be as clear as possible. If I've failed to make it clear, I take full blame. However, if you've still got questions, I'll try to help clear them up if anyone's read this far.

*Edit for spelling error and clarity*

boowana

I think i got it I hope it work.:D

rpm_pwr

I just wanted to say I love the level of education on here. I was sure the rx-8 board was going to be full of idiots with half the posts about lack of torque. Now I can see that there are some clued up people. Cool

zoom44

well, some of us (read that as me) only have half a clue, but we're learning

Styjan

Thanks for the explanation Rich. I hope people are starting to understand the hp/torque/rpm relationship. I think your best point is that people that have doubt about rotary powered cars should go drive one. It's not for everyone, especially if you are in love with muscle cars. But, if you are like most of us here who really love rotary powered cars you will become a rotor-head too.

rpm_pwr

Couldnt agree more - you have to drive one to understand. My brothers SR20DE powered pulsar has the same redline as mine and is generally regarded as one of the best 4 cylinders ever made. But I hate to rev the car. Once a piston engine gets to the top end the valvetrain noise/shudder becomes disconcerting - you feel like you should change gears. The rotary has no valvetrain and thus no mechanical feedback that says "this is getting harsh - better change gears". In fact that is why all rotaries since the first Cosmo 110s had redline buzzers, because you can't "feel" the RPM.

Ask a rotary driver how many times they've smacked into the limiter or heard the buzzer go off and thought "damn I wasnt ready to shift yet.."

NOTA V6

My answer: I hear the buzzer just about every time have to shift.

Although I have run into a couple of folks who didn't know they had one until I told them to try to run it up there once. A couple of original owners too even.

rpm_pwr

yeah there's always a few. I've heard of people complaining the 2nd turbo doesnt work but it's just that carbon buildup has left the actuator a bit sticky. Some people buy cars purely for looks I have my limiter set to 7250rpm so the buzzer means my car has turned into a flamethrower :D

FLPanther21

Bob Hall, one of the fathers of the Miata always answers this way when asked about the Miata needing more horsepower:

If you can't go fast with 90hp, 900 hp won't help you.

The RX-8 will be plenty fast enough for me!


Bill

Toadman

Exactly! No valve-train thrashing and a good description there, guy. To those interested in the RX-8, take heed. The power curve feels like no other car. It will rev 'til it blows or is fuel-limited by the ECU. That's why it is so addicting. Like a Timex, it keeps going and going..... Rotaries rock! :D

specmi80

well as for the s2000 which is a good comparsion man this thing is fast with 142 pound feet of torque it does good time i seen
some sc issue they have recored zero to sixty in 5.3,5.4
as for the rx8 which have a 10 hp advantage and a bit more torque is should as fast.I have a spec v which has 180 pound of torque boy let me tell you it a blast the thing peel out 1 2 and third
but is no good if you dont have top end horse power

zoom44

remember boys and girls- it's not how fast your car will go, it's how fast you are willing to go in your car!