K27 on a 951 - comments welcome

[Neil Haughey]

ORIGINAL: DivineE

I don't mean to offend but I think your missing the idea behind the phrase. Your argument is actually in line with the theory of creating a larger area under the curve.

What they mean is you cant just look at the peak bhp figures and think a car will be fast, actual speed comes from having a larger square area under your torque curve. i.e. power through the rev range not just one peak at the end.

Regards,

Ben

I don't want to confuse everyone but a flat i.e. square torque curve will give a linearly increasing power output. To get a flat power output you need a big torque hit low down that gradualy tapers off at higher rpm. This is because power is nothing more then torque multiplied by the rate at which that torque is applied i.e. the rpms of the engine. Its easier to understand if engine parameters where quoted using proper physics units like Newton metres and kilo watts.

This is also why so much nonsense is often spread around about how wonderful diesel engines are, completely fortgeting that 400 lb/ft at 2000 rpm is in many ways only 1/2 as impressive as 400 lb/ft at 4000 rpm, especially when one considers that a diesel application will have gearing designed to work at the low rpm ranges of diesel engines i.e. the torque multiplication factor generated by the gearbox is totally different so the real torque at the wheels is completely different.

 

[333pg333]

ORIGINAL: DivineE

ORIGINAL: 333pg333

as my hypothetical GT35 one before and saw that there was nothing, nothing, nothing, then bang for 1000rpm--change gear etc rather than something more suited that gave us a broader power delivery, that at any given rpm we can drop down immediatly underneath on a vertical axis to this supposed AUTC and we are looking at what preceeded it.

I don't mean to offend but I think your missing the idea behind the phrase. Your argument is actually in line with the theory of creating a larger area under the curve.

What they mean is you cant just look at the peak bhp figures and think a car will be fast, actual speed comes from having a larger square area under your torque curve. i.e. power through the rev range not just one peak at the end.

Regards,

Ben

Ben you're not offending me, and no offence implied either but you're just not getting me. Again, I am not suggesting that peak power is the be all and end all, not do I think it's relevant to this discussion. I am just saying that I don't even think there is such a thing as AUTC. To put it more simply I would term it 'Power/tq that has lead to you being where you are at any given point on the curve' lol. Everyone who uses the term AUTC seems to be saying this in relation to having a broad power curve v's a peaky one and the obvious general preference for the former. Some racers would differ. I just think it's a phrase that has gained momentum in certain parlance that is somewhat baseless. That's just my opinion of course.
Patrick

 

[Hilux]

Power is only a function of torque. Without torque the power is useless and in an unuseable band a la motorbikes

Loads of torque throughout the rev range means a MUCH quicker car which is why our standard turbo`d cars are so quick.

eg: mine (253+ish bhp) is as quick as a carrera 4s through the gears and starts to go past them at 130mph (honestly)

Dont ask how I know as the elbow bruises from the wife still hurts...........................

 

[Neil Haughey]

Patrik, I understand what you are saying. For a sports car or a racing car AFAIK you always want the biggest torque hit at the point in the rpm range that you drop down to after a gear change. This is to get the best possible acceleration. In many cars with a 6000 rpm max power point the drop on a gear change will be down to 4000 rpm in the next gear. Therefore you want the maximum torque at 4000 rpm. A huge torque hit at 2000 rpm is pretty pointless IMHO and in many ways most cars are more driveable if the power low down is kept to reasonable levels. However turbo lag is a different issue and in practise it would seem that maybe the approach of trying to have a pointlessly wide powerband is just a cover for trying to reduce turbo lag low down.

Back to what AUTC is to me just a roundabout way of saying power output integrated over a typical operating range. However it would be more applicable to actually calculate an integrated value say for example when accelerating through the gears rather than just pointing at pretty pictures which is meaningless.

 

[Hilux]

You want a linear increase in power and a flat/constant torque line as high as possible (unless you are a racing car) where you want peak power in narrow bands due to the fast/slow nature of circuit racing.

Note also that no matter what you do to an engine its natural chacteristics mean that the power and torque lines will always meet at a nominal 5252rpm (see here) everything else each side of this intersection is a compromise.

 

[sawood12]

Patrick - i'll have to hold you to your word about the KW's. Mine going into Promax this weekend for the KWv3 installation with Racers Edge camber adjustable top mounts and the Rennbay geometry correcting ball joint kit. I also reached for the phone today and rang up Chris (Adrians 'apprentice') at Exeter OPC and ordered a pair of M030 rear TB's. The 'sticker' price is about £135+vat each, he did them me for £113inc vat, so I can't grumble at that. Makes me feel better for not going the whole hog and fitting Weltmiester ARB's. To say i'm excited is an understatement.

Anyway just to try to steer the debate slightly back on topic. One think i've been pondering is that if you make a free'er spinning tubo by utilising ball bearings am I right in thinking that the turbo will be able to maintain boost higher up the rev range? The main reason why people go to bigger turbos is to deliver more boost at higher RPM, however if you utilise a free'er smaller turbo can you get away with a smaller turbo? I.e. instead of going for a conventional K27 turbo why not go for a ball bearinged K26 so you have the advantage of the better spool up speed and can maintain boost pressure at high RPM's??

 

[333pg333]

Scott, don't worry you'll love the KW's. With my brief foray in my newly rebuilt car before I destroyed #3 piston last Sunday (on my birthday but that's another very frustrating story) I had forgotten just how good my car handled and with the newly rebuilt stock 40%lsd in there while I wait for the 80% to arrive, it was even better. I couldn't believe how much better it went on the previously hated Kumhos so fear not you will love your 'new' car.

As for turbo lag, torque, power well again with my brief 4 days in my car while I was running it in I was only revving out to 3500 to 4500rpm and had the ebc turned off so it was only boosting on the spring itself which was reaching .4 bar. I had taken 2 days off work and after picking it up on the Wednesday afternoon, by Saturday afternoon I had over 800 kms on the engine and was letting it go once or twice up to just over 5000rpm. The best thing was even with the big turbo (rated over 700hp) the big head and cam (designed for mid to upper rpms) the car just had oodles of low end tq. I know this is really out of context with most 951's but even with only .4 to .5 bar and 4k to 5k rpms, with the extra 500cc's and changing to the S2 crown wheel and pinion the car felt better than my old engine on 1.4 bar. So in a round and about way of getting back to the original context of this thread and please beg my pardon George, but you can have your cake and eat it to a degree. It all comes down to how much you want that cake, but you can easily go up in turbo size on these cars for good gains without sacrificing tractability.

 

[Neil Haughey]

ORIGINAL: Hilux

You want a linear increase in power and a flat/constant torque line as high as possible (unless you are a racing car) where you want peak power in narrow bands due to the fast/slow nature of circuit racing.

My own personal preference on a road car would be for the boost to tail off at high rpm resulting in a flatter power band i.e. the torque is going down. Most car manufacturers in particular those that build turbo engines (Saab springs to mind)seem to go for a similar approach either by bleeding off the boost via a controller or designing the inlet to be undersized to get the same result. My thinking is that maybe the past 20 years of development has shown that high peak cylinder pressures are OK down in the mid range but up at 6000+ rpm will produce a less reliable package then one that gets its performance by having a big hit in the midrange.

 

[Neil Haughey]

Forgot to say that you can't get this effect by just using a small turbo run flat out. If the turbo is run flat out at high rpm you end up running too much back pressure and causing long term damage to the engine [ask me how I know, different car but I ignored expert advice and paid the price].

 

[sawood12]

Yes, but with a free'er running turbo then less back pressure would be generated due to the vastly reduced friction. I suspect that part of the reason the K26/6 and K26/8 turbos bleed off pressure at the top end is the power generated in the turbine starts to loose the battle with the bearing friction which will increase with the square of the speed and at 30odd thousand RPM there is a huge increase in friction over the operational RPM of a turbo. Obviously I know that with a 2.5 ltr engine running at 0.8 bar the physical capacity of the turbo means it cannot maintain boost at high rpm's however you might be able to stave off the boost drop off for 500rpm or so by utilising ball bearings. By utilising ball bearings you do get less lag by 500 rpm or so - so I wondering if you get a benefit at the other end of the rev range. I'd like to see a back to back of a stock K26/6 or 8 and a ball bearing'ed equivalent.

I'd like a turbo that can maintain boost pressure upto the red line for the track and you can always use an EBC to change the boost profile for the road. I think with 944's it is pretty well understood that you can run 1 - 1.2 bar boost pressures with a stock engine with virtually no significant detriment to engine reliability.

 

[Neil Haughey]

ORIGINAL: sawood12

I'd like a turbo that can maintain boost pressure upto the red line for the track and you can always use an EBC to change the boost profile for the road. I think with 944's it is pretty well understood that you can run 1 - 1.2 bar boost pressures with a stock engine with virtually no significant detriment to engine reliability.

Seems like a sound idea to me.