Internet experts

Regular readers will know that I am not a big fan of the internet but I do look at a few threads on various forums now and again.  One I liked was about intake lengths.  People were discussing the feature published here on inlet lengths and the adjustable system I came up with - If you missed it you can find it on

I did start to wonder if people think that I just sit here and make this stuff up!   I gave you all the results and included the test method yet there are still people out there who refuse to believe the results.  The problem is that they hold these beliefs based on what they have read, not what they have done or seen with their own eyes.  So here we go again this month, about to upset a few people who already know the answer to the question I have been asking myself for some time now.

Tapered vs Parallel Trumpets

Jenvey Taper ITB's Jenvey Parallel bore ITB's EMR ALI's

Are tapered throttle bodies really better than parallel ones?

Take one Renault Clio Cup car and convert it from the stock intake to individual throttle bodies from Jenvey Dynamics.  These bodies are available with a parallel bore (45mm in this case) or tapered (48mm down to 43mm).   We ran the tapered bodies first and recorded the power and torque curves.  The intake trumpets were Jenvey 90mm tapered ones, their standard offering.  Next stage was to convert over to the parallel bodies on the same intake manifold and check that the overall length was the same as with the tapered bodies.  I should point out that with the tapered bodies there was a slight step it the manifold where the body didn’t exactly match the manifold port but then the match with the parallel bodies wasn’t perfect either; we live in the real world here and I was not about to spend hours porting and matching to gain half a horse power.  After re-mapping, the power curves were compared and found to be just about the same, plus or minus one BHP, basically the thickness of the line on the power curve and within the limits that the rolling road can measure.

Tapered vs Parallel bore Comparison with EMR ALI's

The conclusion I came to was that the tapered bodies but do not work any differently to the parallel ones and you have to pay another £76 on a set of four to have the tapers.   The undeniable conclusion of all these throttle body tests is that it is the length that is the really important bit.   To prove the point we finally installed a set of our adjustable length inlets (ALI trumpets) and sticking them on full length gained torque right across the rev range from 3500rpm to 6500rpm.  In places this gain was nearly 10ft lbs, not an amount you can really argue with in terms of repeatability.  To be honest looking at the curve I would think that going even longer would be better still for mid-range torque but probably at the expense of a little top-end power; but that is just conjecture based on previous experience.

Bike bodies

In principle a nice setup... ...the reality was plagued by an annoying airleak

Okay, I am not a great fan of bike body conversions for reasons I have gone into before in these pages.  But the V8 I had in gave me a new headache in that one body constantly spat back up the intake, even though the lambda was showing the right (average) mixture.  The balance was correct as far as I could tell but the spitting back could not be cured.  The injectors all flowed the same so the problem had to be an air leak; even though there was no obvious leak that I could see.

The answer turned out to be an interference one.   The very nicely made manifold was catching on the edge of the rocker cover and holding it off ever so slightly.   The result was an air leak - but only on that front cylinder, the rest were making enough contact to seal okay.  Sadly we couldn’t map the engine like that so the car was towed home to be sorted and mapped at a later date.  The moral of the story is if you have a body spitting back, it is almost always the result of a weak mixture.  If you get it just as you open the throttle then suspect the mapping first, and if it's not that, look at the throttle pot after that.  If the ECU doesn’t know that the throttle is opening (because of a faulty TPS) you will get a weak mixture hesitation or spit-back on initial opening.


Rotrex supercharged RV8 Basically a mechanically driven turbo compressor Makes a fast car faster still!

I am seeing more and more Rotrex supercharged cars here and I actually quite like the system.  The theory might not be ideal in that the boost climbs with revs, rather than giving the traditional big mid-range torque boost you get from a conventional supercharger, yet the result in terms of drive and power output is very similar to a highly tuned aspirated engine.   This TVR probably didn’t need a supercharger but these days 500bhp has become the norm without which you can’t hold your head up in the pub.  Nice home-brewed conversion this one.

Another thread

One of the more interesting threads on Piston Heads has been pointed out to me by a couple of friends.  A regular contributor has been diagnosed with Non-Hodgkins Lymphoma.   The guys on the thread said that it was good that he didn’t try to hide it and they were generally supportive.  Basically he had developed a Cancer of the lymph system and I was particularly interested because I am in exactly the same boat myself.

As I sit here writing this I have no hair left and am going through the wringer a bit with the side effects of chemotherapy.  The reason I mention it is that if you turn up here and find me wearing a silly hat and not being in a very good mood, I'm blaming the disease, nothing to do with me turning into a grumpy old sod with little sense of humour left.   I find that having my life term somewhat under the microscope has made me a lot less tolerant.   For example, I was told by an engine builder the other day that my mapping had “bore washed” an engine on one cylinder while causing detonation on two others.  Nothing to do with shit machining or oval bores.   The experts involved are now putting in a rolling road so that these cars can be “mapped properly”.  The engine in question was never actually mapped at all because the injectors were too small and the mixture was going too weak at high rpm.  Bore wash – eye wash more like it.