Power vs Performance

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Spanner Monkey
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Power vs Performance

Postby Spanner Monkey » Sun May 31, 2015 8:40 am

I had not intended opening the Pandora’s Box of power adders so early in my ‘top tips’ series, but as some of this is particularly relevant to some of the handling questions I’ve been asked, here it is. You’ll have to wait for the brake tips….

Power vs. performance.

There seems to be a general confusion between power and performance.
Put in simple terms, power is what the engine produces, performance is what the engine/car combination is capable of.
What we should be concentrating on is not an increase in outright power, but an improvement to the power-to-weight ratio.
A 2000kg car with 1000bhp will have the same power to weight ratio as a 1000kg car with 500bhp and therefore the same theoretical performance. The lighter car will also be easier on all of the components, particularly the brakes and tyres.
If you compare the early S197GT (300bhp advertised) with the S197 GT500 (500bhp advertised) you find that the GT at 190bhp/tonne is only 90bhp less than the GT500 at 279bhp/tonne. So that supposed 200bhp performance advantage is immediately halved by the extra weight involved to produce that power, and in the case of the GT500 all of the extra weight is added in the front of the car which really upsets the weight distribution. Remember a lighter vehicle will generally handle better than a heavier vehicle as its kinetic energy will be less.
So what I’ll try to consider here is not just more power, but better overall performance.

Step 1. Weight.
Free power.
The best, cheapest and easiest way to improve the power to weight ratio is not to increase the power, but to reduce the weight.
Look at all of those heavy things in the car: Seats, trim, spare wheel, carpets and the worst of all – sound deadening. Under your carpets, behind the seats, behind the dash there are kilos and kilos of power-sapping soundproofing. The average car has more than 50kg of soundproofing products stuck and clipped to the parts you can’t even see. Getting back to our S197GT, take out the sound deadening and the spare wheel and the PWR increases to 200bhp/tonne. Only 10bhp, but it’s a free 10bhp. Fit lighter seats, body panels etc., and the extra gains become obvious.
The added positive is the handling of the car will improve with less weight. It’s easier to change the direction of something that’s lighter.

Step 2. More in.
All of these ‘power’ mods are assuming your ignition system is in A1 condition and will cope with the additional air/fuel (and therefore cylinder pressures) involved.
With the later cars ‘more in’ starts with fitting a CAI (Cold Air Induction) system, earlier cars it just means fitting a better flowing air cleaner (K&N or similar). Outright power gains aren’t generally huge – in the 15-25bhp region for a good CAI – but well worthwhile doing as it will have an effect on any further mods you do.

Step 2. More out.
If you’re getting more air in, you’ll need to get more air out, so a good free-flowing exhaust is essential. Put an exhaust in that is larger in diameter and cuts out (some of) the silencers/cats and replaces the cast-iron manifolds with tubular headers and not only will you add power, you’ll also remove weight = better overall performance.
It’s perfectly acceptable (though not necessarily desirable) to merge the pipes into one directly after the headers. A single system will be lighter than a dual system. The exhaust note isn’t bad (makes a V8 sound more like a ‘screamer’ S4), but you must ensure the single pipe will adequately flow the output of twin banks of cylinders. The calculation of size is: R3 = √(R12 + R22) where R1 and R2 are the radii of the twin header pipes, and R3 the radius of the single exhaust pipe. This equation will also work for unequal sized pipes (not that any car I know of has them…..).
Obviously this is a sliding scale of exhaust mods depending on your usage. For a daily you want something that flows better than the standard system but without the brain-numbing resonance that a full free-flow exhaust will give, a compromise between performance and comfort. For an out and out track car full flow, full noise, no compromise.

Step 3. Even more in.
For earlier (pushrod) cars the next step IMHO would be a better camshaft, and following that better cylinder heads with larger valves and ported runners. If money allows, putting good aluminium heads on a Windsor, Cleveland, FE or 385 engine will not only add power but reduce weight AND improve the weight distribution %. If you are renewing your heads now is the time to review the inlet manifold. Some factory manifolds flow extremely well, some don’t. Cast iron manifolds weigh a ton, so even if you just change for an identical alloy manifold the weight loss (and effect on weight distribution) will be well worthwhile. Swapping iron heads and manifold for alloy parts on a small-block Windsor engine will typically save around 50KG!
Do some research on what works best on your engine/with your cam for the use you want as some really open single-plane manifolds will only give an advantage at high revs and rob low-down torque.
For the later SN95, S197 I’d bypass the above and pressure charge it.
Putting a supercharger on an S197GT will give you gains of up to 200bhp without the necessity for any other modifications. There will be a slight increase in weight but the overall performance increase will more than outweigh the weight gain. Whether you decide to go with the centrifugal (Paxton style) or rotor type (Eaton style) is largely irrelevant as the engine won’t care what’s making the boost. Again do some research as to what suits your engine/needs best as there are hundreds of kits out there and there’s bound to be one that works for you.
Turbochargers are the flavour of the month at the moment but all at the expense of extra heat under the bonnet, or if you mount the turbocharger(s) at the rear all of the weight and inconvenience of the extra plumbing. Heat robs power as it makes the intake charge less dense, hence the addition of intercoolers in all of the higher power turbo (and supercharger) kits to cool the intake charge down.
With the addition of extra air it’s necessary to add extra fuel. With an earlier carburetted engine this is as simple as changing the jets or fitting a larger carburettor. With later ECU controlled cars there is normally larger fuel injectors and fuel pump in the supercharger/turbocharger kits and a flash-tune to upgrade the ECU to ensure the efficiency of the new parts.
Given the stoichiometric ratio of petrol engines is approx. 15:1 air:fuel (by weight), it’s important (particularly with a forced induction engine) to keep the ratio above that. Simply put – don’t run it lean!

Step4. Still more in. Lots more out….
If you are going for out and out performance on any engine you will need to start from a blank page and completely rebuild the engine – better(stronger) crankshaft, connecting rods and pistons. Stronger fixing bolts for connecting rods, main bearings and cylinder heads. Improved oil pump etc., and possibly even an aftermarket block. The list goes on and is only limited by the size of your wallet. 3000bhp ‘streetable’ engines are commercially available in the US, but these do not share any components with production engines and the price reflects that. Fine if your bank account is large enough!
Nitrous Oxide.
First off, don’t confuse Nitrous with Nitro:
Nitrous Oxide is an inert gas that contains about 12% more oxygen than atmospheric air, it is not flammable or explosive if exposed to open flame.
Nitro is an abbreviation of Nitromethane which is probably one of the most volatile liquids on the planet. Nitromethane is the fuel that is used in Top Fuel dragsters.
NOS is the trademark of the company 'Nitrous Oxide Systems'.
Nitrous Oxide makes power in an internal combustion engine as it bears more oxygen than air.
As a power adder it’s extremely good. There are however a number of negatives when using nitrous:
1. You need to add more fuel to compensate for the extra oxygen. This can be done in two ways – a ‘wet’ system which injects extra fuel via additional jet or jets (usually used with carburettor induction), or a ‘dry’ system which relies on an ECU tune to add more fuel via the existing fuel injectors.
2. Nitrous does not last long. If you have a 10lb Nitrous bottle and hit the button until it runs out, you will be lucky to get a couple of minutes-worth of extra power. Yes, those 10 minute chase sequences in ‘The Fast and The Furious’ really are fantasy……
3. The systems rely on the pressure of nitrous to make the power. As the gas pressure (and therefore flow) drops the extra fuel flow does not, so the mixture gets increasingly richer to a point it actually harms performance rather than enhancing it.
4. It is quite expensive. At around 1200GBP for a decent kit and 80GBP to fill a 10lb bottle you will soon get into supercharger territory price-wise, and you will never stop spending money on a Nitrous Oxide system.
For these reasons it’s not really practical on a road car or a track-day car.


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Re: Power vs Performance

Postby badhand » Thu Jun 04, 2015 5:17 am

Not much left to discuss!


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Re: Power vs Performance

Postby cati » Sun Jun 07, 2015 11:49 am

Thanks monkey....

All makes a lot of sense... What would you do to a s197 to make it your ideal track day car and still be street able to the track plus get through a stringent noise test like those at Goodwood and castle Combe

Very interested to hear your thoughts...

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Re: Power vs Performance

Postby Spanner Monkey » Sun Jun 07, 2015 8:36 pm

S197 track-day car.
I think the way to go would be in stages:
1. Shell. Strip out anything you don’t need. Rip out all of the interior and take a heat gun to the sound deadening to remove it. Top Tip here: put it all in bin-bags and weigh it afterwards – you’ll be surprised at how much comes out. If you want to keep the carpets for comfort put them back in after the cr*p has been removed. Take out the radio, passengers airbag and literally anything else you are not going to need whilst going around a track. If you’re really serious replace the side and rear windows with Lexan or polycarbonate.
2. Discard the rear seat. Rivet/bond in a single-skin aluminium or composite full width bulkhead. Double whammy here – less weight and a bit more stiffness in the shell. Fit light-weight race seats in front. Replace the seatbelts with harnesses – again you’ll be surprised at how much the original belts/mechanisms weigh.
3. Poly-bush all of the suspension. Upper & lower rear arms, Panhard rod, front lower wishbones and even the top strut mounts. Lower & stiffen the suspension as required. Adjustable dampers will be good if you’re serious about lap times. Similarly, adjustable sway bars if you want to get down & dirty.
4. Stiffen the shell. Rollcage will add stiffness at the expense of weight. Good strut braces front and rear will be worthwhile.
5. Power-wise, do it in stages. Assuming you’re using the ‘base’ 4.6 manual throw a CAI at it. Quick-shift gear lever would give a better feel. Obviously with any upgrade in power you may also need to upgrade the clutch.
6. Somewhere in the above you’ll need sticky tires and possibly wider wheels. Lower profile tyres will assist in the stiffening, but don’t go too low as a bit of sidewall flex is desirable to allow the tyres to grip properly.
7. Better brakes. I’ll do a ‘thing’ on brakes as there are as many brake mods to do as there are handling/performance etc.
8. Better cooling. See above!
9. Exhaust and noise. Big-bore free-flow system with the longest re-packable glass-packs you can fit under the back. You may need to cut out the spare wheel well and fit them sideways. Glasspacks this size are great at cutting the noise, but they do deteriorate quickly so it is necessary to have them easily repackable. A lot of the aftermarket stuff that’s available have the end caps that unbolt, but after a few ins and outs they become difficult to use. I cut a rectangular hole in the silencers and put a bolt-on plate for access, that way you don’t have to remove any other exhaust components to repack them.

That’s it. I’d like to see this car built so keep us all in the loop!

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Re: Power vs Performance

Postby mattmoxon » Mon Jun 08, 2015 11:52 am

I think the Spanner Monkey has hit it on the head for a track car, though I’d add one more piece; a tubular K-member. I fitted the BMR one and it weighs half as much as the OEM steel box K-member. You do make big sacrifices in NVH though as the poly engine mounts don’t half transmit vibration.

I went down the chassis tuning, gears and Drag radials route for drag racing rather than just chucking power at it and I have so far shaved around 1 second off my ET.

So far I haven’t removed any excess weight sound deadening, interior et al (hell the spare wheel and jack are still bolted into the wheel well in the trunk) yet but as its daily driven I want to keep a level of comfort and usability, if I do add a bit more power I’ll be going down the NA tuning route rather than FI or N2O though as 12.00 is my ET max.

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