Cold air intake tips for S30Zs - the CRX solution...

richiep

Club Member
"Short story - shorter trumpets/inlet tract length equals less top end but more response across the range/lower down; long trumpets - maximises top end but less low end performance."

Wrong way round!!!!
Correct answer is.......
"The very short answer to the final question is that short, wide inlet runners move the torque curve up the rev range whereas long, narrow runners move the curve down the rev range. BUT - there is a whole lot more to it than that. The effect is caused by 'pulse waves', waves of relatively high and low pressure in the runner."

For more info see Dave Walkers article on the development of his adjustable length trumpets......
DOH! thanks for the correction Pete! This is going to sound like an “oh yeah, pull the other one” moment, but I did know that - not sure what I was thinking putting it the other way around above... being a dumbass I suppose!
 

johnymd

Club Member
My v8 lexus engine in the red car has active variable trumpet lengths, as do a few bike engines. It is pretty crude but basically the long trumpet (12" long) are used up to 4k revs to give high torque and then over that an air valve lifts these trumpets out of the way and it then just leaves short (1" long) to give maximum power and unrestricted air flow. On the 300hp lexus engine it still only make a small difference with the 12:1 ratio in length. On a zed engine I'm not sure that you would actually "feel" much difference so probably not something to get hung up about. The trumpet entry point shape is probably more important along with removing any ram air effect by diffusing the air as it enters the airbox. If you look at well designed air boxes then you will notice that air often enters under the trumpets so there is less preference to the from cylinders. The old ford cosworths, Datsun s2000 and reveire zolder are good example of a well thought out design. I'm not saying I don't like the design your using Richie as it has lots of good design features and also has to fit the engine bay plus keep away from the hot exhaust. It is probably the perfect option for a zed based on value for money. I would probably chose the zolder but at £600 it's a big step up in price for probably very little extra performance.
 
They’ve more or less stopped using them on bike engines as a rule, there was a report somewhere that when over the 220bhp per/ltr (n/a) then they can help with how the torque curve is shaped. There’s also been variable height injection etc, which is amazingly cool to watch.

Looks like a tidy install, bet it sounds nice also.
 

jonbills

Membership Secretary
Staff member
Site Administrator
Here's a dyno pic from 2012.
One of the big differences between the red run and the black run is that black has the air box on.
DSC_0204.jpeg
 

SeanDezart

Well-Known Forum User
There are other factors involved than 'merely a cold-air intake. In an airbox, air-flow is stabilised but there are also advents of retaining mousse 'sock' over the trumpets to further stabilise/equalise airflows into the inlet pipe.....
 

jonbills

Membership Secretary
Staff member
Site Administrator
What filters did you have on before Jon or none at all ?
For the red dyno run, the lid of the air box was off, so no filters (but ram tubes were there)
Before I had the airbox I had trad K&N filters.
 
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Peato40

Club Member
Here's a dyno pic from 2012.
One of the big differences between the red run and the black run is that black has the air box on.
View attachment 32471
Hi John, out of curiosity, did you need re-jet when installing a cold air intake?
I tried one recently and it behaved horribly, as if the second stage pump jets weren't working. I removed the carbs to check how well the jets were squirting and all seemed to be okay so I re-installed, this time without air intake and velocity stacks and it ran fine. Believing the problem was cured, I then re-installed the cold air intake and it reverted to behaving like a pig. I have even removed the filter and hose and taken the lid off the air box and it still won't run very well.
Now admittedly I made a huge mistake as a friend of mine fabricated an air box from aluminium, which loves to absorb the heat, so I have abandoned the idea of using it but I find it strange that my car simply won't tolerate an air box even when it's just warming up and not really hot. Air box.jpg
 

SeanDezart

Well-Known Forum User
My advice - fit a full-size heatshield underneath the airbox - it won't cost much to try. (nice box btw).
 

jonbills

Membership Secretary
Staff member
Site Administrator
Hi Phil, no, didn't have to rejet.
Your airbox looks quite tight against the trumpets, and it's square. My guess would be its hindering the airflow. I'd suggest trying without the trumpets and see what difference that makes. If it makes a difference, maybe get some short trumpets.
But I think the shaped airbox that I have and Richie highlighted is better [emoji4] you can get one from Mr F
Also worth checking that your airbox isn't stressing or twisting the carb/manifold misabs or o rings as you tighten it and opening up air leaks. That would mess it up for sure.
 

SeanDezart

Well-Known Forum User
if you look at the period rally/hampster cage airboxes, they're quite similar to what you have there, maybe a little deeper?

A difference being that they can intake air from all around whereas the intake to that airbox for 6x trumpets is of a limited diameter - no ?
 

Peato40

Club Member
Thanks for the advice guys, I will most likely get one of those that you have @jonbills

Yes I did try it without trumpets and even without the lid on and it would not get into the second stage of acceleration. The curious thing is that it idles nicely and then when I go out for a run and encounter the stalling issue the AFR is displaying 19.0, which would indicate lack of fuel. This is what led me up the path of checking the pump jet circuit. The car runs fine with or without trumpets but just not if that air box is installed. The box fits nicely to the carbs and doesn't appear to be stressing the assembly but I will double check as that might be a good shout.
 

richiep

Club Member
The design and size of the box is clearly the problem. The shape and size of the airbox is critical. The one Jon and I use, as described early in the thread, has a shape that eliminates issues associated with air pressure pulses that can inhibit power. It would seem that your airbox is exacerbating that problem to an extreme. The size of the inlet pipe looks small too. The MJP airbox intake is something like 85-90mm if I remember rightly. My cone filter and tubing is all 3"/76mm too.
 
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