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Some of this week's Letters to AutoSpeed!

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Re Cam’d XR8 Dyno Graph #1

Re Cam-ing the Boss 260...

There seems to be something dodgy with the dyno chart in this article... It shows that at about 2800 rpm, the ‘before’ and ‘after’ engine specs produce the same power - but the torque figures at this engine speed differ by about 100Nm... We seem to be losing power in the cam’d engine - according to the speed and torque it should be showing about 130kW at 2800 rpm and about 320kW at 6500 rpm.

Peter Bodon

Re Cam’d XR8 Dyno Graph #2

The dyno graph in your article Cam-ing the Boss 260 shows torque at 2800 rpm increasing from 350 to 450Nm. Given that power is proportional to torque times rpm, how is it possible that power is the same at this engine speed?

Gavin MacLean

Re Cam’d XR8 Dyno Graph #3

I was looking at the article you wrote on the upgraded cams in the XR8 Cam-ing the Boss 260 and couldn't help but look at the dyno graph and compare it with what you said about the on-road performance.The power graphs for the standard cams and the modified cams as used in run number 96 shows power being very nearly similar up to 3500 rpm - however the torque curves are vastly different to this point. Mathematically this doesn't make sense.You mentioned that the low down performance wasn't very good - perhaps this would be obvious if the dyno sheet parameters were identical for runs 1 and 96.

G. Pring

Very well spotted! Here is the answer provided by Lachlan Riddel of ChipTorque...

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While this doesn't give the whole explanation, here are the graphs against road speed. The plots show the hardware changes we describe including a diff ratio change. With the increase in rev limit from 6200 to 6800 rpm, you would have to slide the later graph to the right to allow for the diff change if you wanted to line up the rev points. We have attempted to use the Dyno Dynamics software to do this using an rpm/kph calculated from the dyno rollers. Either we (as operators) have been insufficiently diligent with our rpm measurement, or the software has generated a higher perceived torque graph with the higher diff ratio, or (most likely) both. Sorry to any readers who have been misled by this oversight and all credit to those who did the maths on it. The power figures at the wheels remain as they are - a testament to how much improvement the changes have made.

Spoilt Munro?

Re Must-Have Monaro. This might be a very clean car but why, if he has had it since new, has "Daniel enlisted a panel beater to smooth the beautiful panels"? Hmmm...

Oh, and removing the originality of the Monaro has probably reduced its long term collectability as well. The most original GTHOs, Phase 3 XAs, 327 Monaros and A9Xs are the cars that are dragging the big bucks - not the ones that have been modified extensively.

Simon Brown

We must point out that the ‘slant’ for that article (ie being one of the last of the breed) is entirely ours. You’re right re the demand for all-original muscle cars. In the case of this particular Monaro, most of the mods could be easily removed and genuine replacement parts can be easily sourced. Not that the owner wants to take this route...

Sophisticated BOV Control

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I’ve recently purchased a cheap pair of blow-off valves for my Nissan 300ZX TT. Unfortunately, the valves won’t close at idle - no matter how tight I wind in the adjusting nut. So I was thinking about going down the "$70 Electronic BOV" route (as outlined in The $70 Electronic Blow-Off Valve). My question is, if I use the Delta Throttle Timer to controlthe BOV, won’t it also open when I change gears off boost (causing air to enter the intake system and cause a lean condition)?

Andrew Del Carlo

If the Delta Throttle Timer is adjusted correctly it won’t open the blow-off valve(s) during normal light throttle, offboost driving - the rate of throttle position change during a normal gear-change isn’t quick enough to trigger the unit.

Evo AFM Bypass?

Re AirflowMeter Bypass, Part 1... I would like to know if the bypass can work with a Mitsubishi Evo 6 running a Power FC using the stock MAF?

Brian Stone

There’s no reason why you can’t run an AFM bypass as described in our article. However, you should first check that your Power FC allows you to alter the airflow meter output.

Re Rotary Capacity

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Regarding rotary engine capacities in the article The Late-Model Mazda Rotary Engine Guide - yes, you probably knew this one would draw some fire!

The 13B has a single chamber swept volume of 650cc over 360 degrees of rotor rotation. However, there are 1080 degrees of crank rotation needed to complete a combustion cycle – so two 13B rotors displace 3.9 litres over 1080 degrees of crank rotation. The rotary engine is unique in that it requires 1080 degrees of crank rotation to complete the induction-compression-expansion-exhaust cycle - as opposed to a piston engine doing it in 720 degrees.

But - to keep the comparison of capacity between piston and rotary engines valid - one must compare apples to apples. The swept capacity of the 13B over 720 degrees crank rotation is 2.6 liters and the 20B is 3.9 liters.This does not please the rotorphiles who have a one-handed shuffle about driving a 1.3-litre engine - but facts are facts. CAMS back this up as well.

Peter de Vries

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