Power Output for Vx

[Steve Ashurst 63]

My car was bought in 1990 and at some stage had the BDR engine replaced with a 2L Vauxhall motorsport engine. It had a home grown engine management system which recently blew up and I have replaced with a Webcon unit.

 

Carbs are Weber 40s as opposed to the 45 you would expect although it has been re jetted by James Whiting.

 

I finally got it on the rolling road today to have it mapped and was surprised to find it giving 218BHP at the flywheel.

 

Questions are;

 

Does anyone recognise the car ? (reg is Q432 DPF)

What sort of output are people getting from similar engines ?

What sort of difference would moving to 45's make or should I move to injection ?

Power peaked at 6100rpm but the engineer felt their was a mechanical restriction ( dodgy valve gear ) , what should I expect it to rev to?

 

Apologies for the long winded message but I am still pleasantly surprised at the results and feel as if I may have got hold of something special - the car drives very well and is not in the least bit peaky.

 

Appeciate any info anyone can give.

 

 

 

[oilyhands]

Not wishing to be disparaging but.. 218BHP @ 6100 is rather a lot, this equates to 187ft/lb, given that peak power is usually made when torque is around 90% of maximum, that would make peak torque of around 208ft/lb which seems beyond credulity. Even massively developed engines find it hard to make 100ft/lb per litre, 93-95 is usually the best you can expect except in really exceptional cases.

 

I'd be suspicious over the power figures, applying some basic maths, even a *very* well developed VX16v would need to turn to 6800 or so to make 218BHP and if its peaking that low then it probably isnt using much cam.

 

I'd expect a VX 16v on mild cams and 40 webers to make 165-185BHP.

 

Oily

[EFA]

Oily is correct

 

On 40's and stock cams you would not be able to get sufficient flow to give over about 160bhp. I'd suggest a mild cam would have little effect as their is simply not the airflow theough the chokes on a set of 40's to generate the power. Any cam more race than mild would just give you a peaky undrivable engine. I would suggest the rolling road you used was rather less than calibrated.

 

On stock cams 45's will give around 175bhp and 48's 185bhp on a well mapped engine. All these power levels will require 7000 - 7200rpm.

 

Hydraulic cams and 45's will give around 210bhp max, and a much more aggressive cam on solid lifters and 48's can just about deliver 250bhp.

 

Even my 2.2 litre engine can not get over (or even that close to the magical 200lb/ft torque output.)

 

 

The VX needs to breathe well and the best results are obtained using throttle bodies which allow this such as the Jenvey Taper bodies, or if you are extremely wealthy (£400/set), the Swindon twin roller bodies.

 

 

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

 

Edited by - fat Arnie on 10 Apr 2002 23:46:29

[Steve Ashurst 63]

I agree with your view and now that I have got the car back the chart makes interesting reading.

 

The company doing the dyno work was Southern Carburetters, it would be useful to know the figures that other Caterhams have got from them - they always sem to have a couple there being worked on.This would give an indication of the callibration of their rdyno.

 

Southern Carbs view was that the engine had been very well blueprinted and that James Whiting had changed the size of the venturis when he re jetted the carbs so it wasn't a true comparison with stock 40's.

 

Power output at the flywheel was 215 at 5900 rpm and max torque of 205 at 4750 rpm. Yes I know, this is better than an M3 ! so I am taking it with a pinch of salt.

 

The chart shows a dip around the 5500 mark which they reckoned was down to some bounce on the cam followers suggesting new stiifer springs required. Any views on what should be done here and recommendations for people who could do the work ?

 

Appreciate any help people can give here.

 

 

[Roger King]

Ditto the above comments from everybody. That sort of torque figure would have delighted the touring car boys (old type touring cars that is).

 

I'm a BIT dubious about the dip being caused by needing stiffer valve springs. If this is the case, how come it comes good again at higher revs? This wouldn't happen if the springs were allowing valve bounce unless it was a resonance problem which is not the same as stiffness. More likely to be induction/exhaust length related.

 

I know that when we do a rolling road tune I can send the customer to a certain other rolling road for an instant improvement of around 30bhp.

[Steve Ashurst 63]

Roger,

 

Sounds like you may be seeing my car over the winter? I spoke to one of your guys a couple of weeks ago about a new exhaust and the decision was to leave it until after I had had it on the dyno. Techcraft seems the way to go still ?

 

Any quick tests I can do to establish what is going on and why the power/torque dip ?

 

Steve

 

 

[sjwb]

wink.gif

 

Steve B

[stevefoster]

The only James W choke mod I know of is to down size the chokes from 40mm to 38 or more usually to 36. Purely for road use and tractability. This give less power not more.

He may of course have done a special on larger chokes....

but the figures look way to high.

[EFA]

That JW mod is on Weber 45's as well!

 

The 5500rpm dip is far more likely the inappropriateness of any after market cam profile relative to the capability of the induction system.

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[Steve Ashurst 63]

looks like I need a second opinion on a rolling road. Any recommendations in the Surrey area ?

 

 

[oilyhands]

The biggest regular choke you can fit to a 40 is 36mm, and they dont work too well in the 40mm body, these are generally held to be good for no more than 150-160BHP. I've seen these turned out to 38mm, but they are likely to collapse at this size and dont give a good signal for the main circuit.

 

Oily