12psi - Is it worth it?

[VXR_Daz]

After looking through the VXR8 post I have been doing some "back of a fag packet" calculations and to be honest I am now rethinking my previous intention of going to 12psi!

My calculations were as follows:

Before Supercharger it was 336.7rwhp

Added on supercharger which in theory should give:

336.7/14.7*5 = 114.5rwhp

Obviously there will be losses though, as much as anything due to having to drive the SC in the first place

The actual result was 405.9rwhp which is an increase of 69.2rwhp. So losses are:

(114.5-69.2)/114.5 = 39.5%

If you then work out at 12psi the calculation is as follows (using the above losses):

((336.7/14.7*(14.7+12))-336.7)*0.605 + 336.7 = 503rwhp

Actual numbers coming out from Wortec are ~500rhhp which is about 468rwhp (using Wortec’s figures on losses through tyres)

So the losses are obviously a lot more than the previous 39.5%

With no losses we should see:

336.7/14.7*26.7 = 611.5rwhp (which is a gain of 274.8rwhp)

Actual gain is:

468-336.7 = 131.3rwhp

So losses are:

(274.8-131.3)/274.8 = 52.2%

12psi and an intercooler is a not insignificant investment (roughly £4,500 including clutch) and also means that both the engine and supercharger is then not covered by warranty.

My question is, is it really worth it? Am I really going to notice the extra 62rwhp enough to justify both the outlay and the additional risk?

Maybe I have missed something somewhere though!

[BO55 VXR]

[BO55 VXR]

What about the torque gains though

[AM04ARO]

No.

Personally I think a SC V8 with IC is more than enough.

I thought about going from 6 ti 9 psi but I now think my car is 'quick' enough. Pretty sure any further improvement is hardly likely to make any real world difference.

Am ready to be convinced that anything else will make any major difference.

[VXR_Daz]

BO55 VXR said:

What about the torque gains though

Mine was showing about 420lb/ft at 4k revs, wortecs show about 500lb/ft but that is rear hub so probably more like 470lb/ft at the wheels so gain of 50lb/ft going from 5 to 12 psi... nothing much!

[crisisjez]

Tub the back, put some proper rubber on, and THEN fit a 3.9 diff. (and cooler and cover)
500 horseys be plenty

[AM04ARO]

crisisjez said:

Tub the back, put some proper rubber on, and THEN fit a 3.9 diff. (and cooler and cover)
500 horseys be plenty

Nice idea but been there done that.

Really no idea how a more powerful car would put the power down without some serious change in grip at the back.

Talk all you like of BHP but 550 + without race logic = wheel spin galore with tyres that fill the arches already.

I had a 'moment' today changing from 3rd to 4th on full power so really not sure I would want anymore power.

[VXR_Daz]

Racelogic was on the shopping list!

[V6 JDT]

Tyre width would improve things no doubt about it. Just look at this weeks Autocar and see the size of rear tyres on the powerful cars against the Veyron to see how they do it.

[AM04ARO]

VXR_Daz said:

Racelogic was on the shopping list!

It will make you feel very very quick when you do.

Awesome bit of kit - flat out without wheelspin..........

[crisisjez]

Sorry i`ll rephrase my post, make more room at the rear for wider tyres 355`s? by cutting out the inner wings and refabricating(Tubbing )them.
More grip= better accel= lower ratio possible.
How`s that.

I know (before I get jumped on) its too much hassle and there will be engineering problems, what i`m really trying to say is the car`s a beast @500bhp with the size of rubber on the back,even with 3.4 diff first gear is a complete waste of time, prob only using about half power to prevent spinup.

Which is what racelogic does.

Edited by crisisjez on Thursday 31st May 23:06

[stevieturbo]

3.9.and FI is a silly idea.


in fact, straying off the std 3.46 with FI is a silly idea. 3.46 is a pretty good ratio.

[crisisjez]

stevieturbo said:

3.9.and FI is a silly idea.


in fact, straying off the std 3.46 with FI is a silly idea. 3.46 is a pretty good ratio.

Agreed, in fact 3.4 FI with the size of rear tyres is borderline.
The car needs more grip to best exploit what power it already has.

[greens vauxhall]

You need to compare the area under the curve of the dyno graph to tell whether it is worth it, peak gains are peak gains, drivability is the spread of torque etc etc. Also, adding the intercooler will give a more consistant level of power delivery in varying atmospheric conditions, and the clutch is optional but definately recommended!!

The std s/c install costs £5500 and that gives around the same gains as intercooler and 12 psi without a clutch thrown in. All down to personal choice and perceived value!!

[darren_r1]

All I can say is I took a VXR500 out for a spin and was not that impressed, took the VXR600 out and what a difference ! chalk and cheese in my view, I know what one I'm getting :-)

[gareth h]

Morning Alan, how observant where you yesterday morning? Warminster byupass...
A general question about traction, if you have adjustable ride height what effect would setting up the car with a rear weight byass have? Obviously this is the case with 911 s and their traction is renowned.

[LathamJohnP]

gareth h said:

if you have adjustable ride height what effect would setting up the car with a rear weight byass have

AFAIK you can only shift weight between opposite corners, not front to rear (unless you move static mass around).

John

[gareth h]

Surely if you raise the front or drop the rear the weight byass would be changed? Interestingly I measured the ride height on Alans car after his Avos were fitted and it seemed to have a nose down bias and was about 30mm lower than my VXR on standard suspension, the guy that fitted my Kws gave the car more of a tail down attitude.

[LathamJohnP]

gareth h said:

Surely if you raise the front or drop the rear the weight bias would be changed?

It depends what you mean by weight bias. Do you mean the proportion of static weight on each contact patch, the amount of weight transferred under certain dynamic conditions (accelerating, braking or cornering), or the ability of car to generate traction on bumpy/off camber surfaces.

If you mean static weight, then think of the car like a four-legged table. You can only transfer weight by "sticking beer mats under the legs", which is analogous to changing the height of the spring platforms. So you can transfer static weight diagonally, but the left-right or front-rear proportions depend only on the position of the centre of mass, which you can't change by fiddling with ride height.

Dynamic weight transfer and traction depend on things you can't easily change (height of CoM, wheelbase, track) and things you can (spring and damper settings, anti-roll bars, tyres etc).

ISTR the easiest way to get more traction is better tyres or a softer rear, but this will have other consequences - it's all about compromises.

I think I got this stuff from "Going Faster" (great book btw) but my memory is a bit hazy.

John

(edited a bit for clarity)

Edited by LathamJohnP on Friday 1st June 11:39

[AM04ARO]

gareth h said:

Morning Alan, how observant where you yesterday morning? Warminster byupass...
A general question about traction, if you have adjustable ride height what effect would setting up the car with a rear weight byass have? Obviously this is the case with 911 s and their traction is renowned.

Yep, I saw you on the last roundabout for me and the first for you - no chance for me to wave as you were too quick....

Wortec recommended the AVO set up I have and have kept it the same.

Interesting to see the view on VXR 500 to 600 being 'major'. IF I changd to the 9 PSI pulley would I need a remap too or just swapping over?? Would that give me another 50 BHP??