Revs, torque and power : questions from a dummy

[radler]

Hopefully these don't sound to stupid to ask and a couple of points up front

So I finally got my xr6 t cat back on my car last year

Based on driving the car, it all seems good and I am happy

But the more I get into this stuff the more questions I get in my head

First up and from observation

When I look at my tachometer and speedometer it seems to me that the pattern of relationship has changed

Say at 2,000 rpm in 5th gear it tracks on 100 kmh when cruising. This from memory was the same before the catback

But everything up to that seems to have shifted

Say at 80kms per hour and in 4th , if my memory serves me properly, it would sit at 2,000 rpm but now it seems to rev higher at 2,200 when cruising

In trying to stick with a basic approach from things I have read ... i can grasp that torque and power have a mathematical relationship etc

So without going into complex maths and to put it in general terms I figure that with the new cat back that the power and torque curves have shifted

I am not too worried about that because I am judging the car by how it sounds and what it feels like to drive

In one way I regret not doing a dyno run before and after so that in part leaves me guessing

More revs as I understand is more power and whilst I don't have any speed over distance measures , I am guessing my car is making a bit more power , albeit on a different curve to when it was stock and I am guessing that on what I am observing in revs at cruising speeds in different gears

In asking all this stuff I am hoping to improve understanding and to see what if anything I can do to flatten the torque curve, if that is at all possible on an fgx xr6 n/a

I appreciate a lot if people focus on maximum power and with a view to improve 1/4 mile times etc

But for me, I am trying to learn about torque and for the purpose of considering decent modifications from a street car perspective if any are available

With exhaust as an example I get the gist of differing views and the science around velocity. I have read some of the 'stick's in this forum as well ...

Some would have it that the xr6t catback is overkill on an xr6 n/a and in time that may well be proved correct

But as a person going down this line I am not yet convinced that it is an adverse performance ... to the contrary it feels pretty good ...

So whilst there are arguments on the exhaust side, for someone like me it gets more complex when I move to look at cold air intake scenarios

There appears to be arguments for and against what if any benefits are gained on that

Say, improved throttle control ... that seems to be a comment

So to me, whilst a Cai may not increase power, or that it might shift the power curve a little towards higher revs.0, more throttle control, I think could be a good thing and something more related to torque as compared outright power ...

Anyway, hoping I have explained it ok, given my convoluted thinking because of poor kniwkedge and limited experience on my part and hoping I can get some responses

Just to add, I am not at the stage of going headers .... but possibly a tune if that could achieve a flatter torque curve , meaning I am seeking to discuss torque as compared outright power gains at maximum revs etc ...

If none of that makes sense, then please tell me to pull my head in .. apologies too if what I am asking is somewhere available in this forum ....

Also
I had my car serviced recently and I might have made a mistake in reading my consumption figures but I thought I was at 18

I figured that the cat back must be contributing to higher fuel consumption and figured, that's good because that means more power (obviously for people seeking efficiency that would be seen as a bad consumption number ..)

But whilst I am not a well developed rev head I am also not a greenie, so a bit of consumption for me is not something I am worried about

Today, a week after a service and having done a typical weeks driving I am at 13.5 ... so on these numbers I am confused .... and in all of tha it gets me researching and thinking more ...

I think the n/a xr6 is an interesting proposition , slow off the mark, but once off the mark, the car seems to have a lot of get up and go... and I a
In putting this in a thread I am open to comment or criticism, but as I see it, figure and feel, it is the torque aspect that now has me somewhat fixated ...

To me it seems that torque is n/a xr6 strength ...

[radler]

Ps, sorry for typos, I went back to edit and thought stuff it, I think it makes sense ...

[lucas2]

I'll be honest and say I didn't read everything you wrote. However, you need to look at gear and diff ratios to find your answer. A change in power or torque won't change where your car sits at a certain RPM and speed - that's determined by gear, diff and tyres. How quickly your car accelerates through the RPM is determined by torque and power.

[lucas2]

To add - torque is a rotational force. Power is essentially torque x RPM. I'd suggest watching some YouTube informational videos to help put it in perspective. Also - look up why turbocharged cars tend to make higher torque at low RPM compared to high revving N/A cars. For example - compare the Ferrari 458 to the now turbocharged 488. Or the GT3RS to the 911 Turbo.

Hope that helps!

[radler]

Thanks Lucas , much appreciated

Sorry for all the waffle up top ... I shudder when I read my stuff back ... I am taking things one step at a time and endeavouring to do my homework .

[sierra63]

The relationship between RPM and speed would be physically fixed, and based on the gear you are in, diff ration and tyre circumference. A change to exhaust, air intake, tune etc will not change this physical connection, unless you have slippage in your hydraulic transfer / clutch (auto / manual).
Changes to tune, air in / out of the engine could change the makimum power and acceleration rate that is possible, and your driving style will all play a part in how much fuel you will use in liters / 100 km.

[radler]

Thanks

Re the post from Lucas, I am happy with the gearing etc

The axle is 3.46 and with the 6 speed manual I figure it's a good baseline to work from

For purpose of a project and in context as my only car that I mostly use for daily driving, I am happy over all and the cat back upgrade since doing it has me more interested ...

Whilst I don't have a lot of technical knowledge I am picking up on things as I go and the torque curve has me curious

Ford spec it at 3,500 for maximum torque which I figure is smack in the middle

Looking at dyno print outs from other people, modifications appear to shift the curve up but the shape doesn't change too much

So in pushing this thread on a bit, I figure that if I went to the next step of a change to the air intake it will possibly put the curve up a little more, possibly with a small drop at the low end ...

Then I figure if that is likely it might equate to a small shift in power at higher revs but that is not, from what I understand, going to be a change in the torque curve in terms of it being flatter

From my perspective I am not chasing more power as an outright objective, albeit any small change is welcome, providing that whatever the outcome doesn't come at a cost of peaks or too much of a shift of where the max torque sits on the curve

I mentioned in my blurb above if it is not possible on an n/a to achieve this sort of thing , and I suspect that it is isn't , then I guess tinkering around too much is a waste of time ....

Having said that I have read that a change in the intake improves throttle control, and that would be of interest for me ...

But I figure to at least explore the options ... I am not seeking to rush in on the excitement of after market bolt ons and am taking my time ...

Thanks too for the info in the reply ...

[radler]

Just to add, and to clarify, I can see from basic explanation's that torque, rpm and hp are interelated, so I figure that to change one componet in the equation will naturally bring changes to the others etc ... whilst my knowledge of complex things is limited I appreciate that a basic understanding leaves one open to errors of judgement etc ...

But .to use another example in terms of exhaust... like I get the idea about fluid disbursement from the analogy of a hose and certain volume on a certain exit size increases or decreases the velocity etc

And how that translates to exhaust gas which I recall from high school science is classed as a fluid ....

And then I read from pundits that on an n/a engine too much loses power too little causes back pressure etc etc

I also figure the ecu or the sensors will deal with changes in velocity on the air intake and exhaust ... so then I think, well, some things can be modified and if within small increments it's probably not to risky on wear and tear on the drive line and engine etc

And then all this stuff goes round and round in my head until I go and get in the car, drive the bloody thing in different gears and rev ranges, observe what my dials are saying, what it sounds and feels like and get back home and go, ok ... yeah , that feels good now what's next ...

And it's the what's next which is the problem ... because I lose sight of what I have, is pretty good ...

But in terms of what's next , a flatter torque curve .... that is what I contemplate, but not from the perspective of making changes to gearing etc, but from the perspective of modifications to air flow at the in take and exhaust, without a custom tune, and if that's not possible, with a tune if that promotes as a solution ....

A lot of people seem to shoot for outright more power, but I am coming at this from a different perspective .... if any of that makes sense ...

[Ross 1]

Have a look at getting an OBDII and using that to gather data on your laptop or tablet. It will help you keep track of everything and will show you more than the gauges on the dash.
https://fordforums.com.au/showthread...ght=obd&page=9

[lucas2]

I think you're over thinking it. Especially in an N/A I don't think chasing power is worth the coin. An intake and a catback would be plenty for noise and go. You can spend a few grand and get 30rwkw more and still get dusted by a stock BA turbo. In my n/a BF I cut a hole in the intake (F6 size) and it made a dramatic difference to intake noise. That's all I did performance wise. Save your coin for a turbo or turbo kit. Once you drive a turbo you won't want to go back.

[radler]

Hi Lucas

Thanks and yep, I certainly overthink ... I use to have a bf turbo ... a lot of people have said the fg turbos were a big step up ... but after driving the n/a fg mk 2 I have no regrets on th the fgx n/a

I respect the turbos and the turbo community , but I enjoy the n/a world and just chipping away on my project ... I don't see myself buying another falcon turbo ...

[cs123]

This is a long, long subject.

At any given RPM the torque created by the engine is dictated by the engine Volumetric Efficiency (VE). If you are to alter the characteristics of the engine to improve the VE of the engine at higher RPM you make more power. Power is simply the rate of doing work.

For any engine you want to design the characteristics to produce torque where it is needed. ie the operating rev range of the engine. For street motors you want this to be pretty evenly spread over the entire RPM range.

What affects the VE of the engine? The three main things are intake runner length and diameter, Exhaust manifold diameter and length (Primary and secondary pipes) plus exhaust diameter. Finally valve timing.

For exhaust design the thing you are interested in is exhaust scavenging. When the exhaust valve opens a shock wave travels down the pipe, followed by the pulse of exhaust gas. The shock wave is travelling fast. When the wave hits a change in diameter like a merge or opening a negative wave is reflected back along the pipe. You will remember this from high school physics. The idea is to tune the length of the primary and secondary merges to reflect the negative pressure wave to assist in the evacuation of exhaust by having it arrive when the cylinder is nearly empty during overlap.This expels all the remaining gas and helps with the intake charge. The whole exhaust system plays a part in tuning. All the way....

Getting the lengths wrong results in loss of power. It can be significant. There are programs like pipemax which help design exhausts for NA engines.

The second part is intake design. Runner length affects cylinder filling and is ideally designed in conjunction with the exhaust so they compliment each other. On the Barra motors there is a dual length runner design to switch to a shorter runner length as RPM increases. You can look at changing when this happens to alter the characteristics of the engine.

Finally the Barra uses variable cam timing. Altering the timing of intake and exhaust valve events as the RPM Changes will allow you to alter the VE of the motor.

I haven't played with these engines at all but if I were I would look at fitting a slightly better exhaust, within reason. I would look at altering the the valve timing with a PCM edit.

I am not aware of anyone who tunes NA engines and the dyno time would kill your wallet. Id be looking at investing in purchasing editing software and learning how to tune it.

[radler]

Thanks and much appreciated

Within all this I also respect that I can't and should not rely outright on a forum, but there is certainly a lot of good information available within this community and it at least helps someone like me to gain some insight

In a practical sense I have started with the exhaust and am using a fg xr6 t catback

From a driving sense it feels good and I am happy with sound

And I regret not getting a dyno run before and after so I could see the power and torque curve before and after

But in context of the information you provided is it more or less likely to provide a issue given the diameter increases from the cat back that I am using. It's a 3 inch

One choice that I feel I have is that if the catback can be improved on again I could go down the line of a custom design ....

My sense of it is that whilst all components are important, the exhaust from the cat back is a decent place to start because some improvements might be achievable without getting into custom tuning ... etc etc

If I can find a way to figure my optimal point on the cat back I can then look at intake etc

[cs123]

A 3 inch is probably OK, bordering on too large IMO but should work OK.

One thing to remember is that if you make any difference to the engine VE a different RPM ranges you will throw out your fuel maps in the PCM. I found this out when dyno testing exhausts on a carbed V8. We were altering collector pipe lengths and watching the AFRs swing significantly as we approached the sweet spot

Say you stumble across an change that yields 10% improvement in cylinder filling at 4000 RPM WOT. Then your injector duty cycle will be wrong and you will need to alter it.

So what I am saying is that changes to exhaust should come with a PCM edit.

[radler]

Thanks

I am figuring to persist with the 3 " at this stage and I appreciate the sentiment about bordering on too large ... a lot to think about ... I will head off this thread for a while now and then come back with some updates as I progress

I think the way you explained ties it up nicely ... gives me a little confidence that I am not totally nuts !!! Lol .

I want to at least get the car onto a dyno at this point and see what the curves look like ...I will post it when I get it ... and work from there

Ross