bonnet-cuts, ITBs, and heat management

[Austin David]

I've been replacing my perfectly fine stock Caterham 360 intake with ITBs, and in the process have been figuring out a good balance between mangling my beautiful car, max airflow, and minimum heat intrusion.  I happen to have a lot of data available from my ECU, as I'm also tuning it, so I'll share my findings here.  It's not what I was expecting.

 

My engine is a "stock" 360 with the milder cams.  The entire intake has been removed and replaced with an ITB setup from AT Power, with ECU from ME.  In this investigation the most interesting part is that the air filter is a vanilla pipercross-style aluminium backplate + foam filter, and is not isolated from the engine bay.  IAT sensor is at the top of the backplate between trumpets 2-3, not far above the throttle cable.  For this data (and until otherwise specified) the bonnet is cut from before cyl1 up to about cyl3, where the filter remains under the bonnet-line.

 

Measurement: WOT run in 3rd at 7k RPM, tracking IAT, Coolant, AFR, and lambda trim.  AFR is measured at the collector, an average of all 4 cylinders.  

 

GENERAL findings:

- lots of heat intrusion from the engine bay, like, a LOT.  With the filter open, IAT rises about 20C above ambient (25 ambient, 45 IAT) and mostly stays put.  No significant drop at WOT, and it stabilizes about 20C over ambient.  This was repeated before I started recording, which led me to try blocking engine heat.

- taping half the filter (pics below) will totally cure heat intrusion, but restricts airflow a little.  More analysis to follow

- just taping the sides of the air filter, particularly around the sides behind the horns, MOSTLY cures heat intrusion with measurable, but relatively little air restriction.

white: RPM

red: coolant temp

green: IAT

 

Bonnet in place, no taping on the air filter:

- no IAT deflection with airflow

- rises continuously, I suspect from the heat dump off the radiator at speed

 

Bonnet in place, air filter taped around the sides + short of #4:

IAT drops with airflow

 

Bonnet in place, air filter taped up to cyl 3:

note that IAT bottoms out to the minimum value, ambient.  This happened at every WOT event

[KnifeySpoony]

Any thoughts on just doing the full bonnet cutout? Or find someone nearby that has one you can swap on to do testing?

[Austin David]

It's hard to un-cut, so I've been making progressively larger holes and trying to prove I need more.  There aren't any close enough to test, I live in the sticks

 

But I will be testing with zero bonnet, to at least get a baseline set of measurements with zero restrictions 

[Austin David]

For reference: this got me started, and represents the minimum cutout to fit the bonnet.  Cyls 3, 4 are still under hood, only trumpets for 1/2 are exposed.

 

Here's a shot of the fully-taped filter, which mostly matches the above bonnet cut:

ignore the wiring and ignition coil, that will eventually come back out.  Some details in my build thread.

 

This configuration has the best heat management, but appears to restrict some airflow at WOT.  1-2% average enrichment + 1% fuel trim (out) from ECU.  Under throttle the IAT drops back to ambient, reliably.  Throttle > 98%, 7k RPM, average AFR is 13.19 + 0.99 fuel trim.  Target is 13.2

 

Finally, a shot of the bonnet as cut right now:

You can see the edges of the tape just inside the penetration, and removed up to the trailing side of trumpet #4 under the bonnet.  This setup seems to balance heat with much less restriction.  13.16 AFR @ 7k / 99% throttle, 1.0 fuel trim.

 

with tape 100% removed: 13.33 AFR, 1.0 trim -- concluding this runs a little more lean, less restricted.  IAT is also 45C vs 25C (with partial tape), not clear how much difference that should make.

 

 

The AFR differences are definitely subtle.  IAT and temperature swings with throttle (not speed) are much more dramatic. 

Edited October 20 by Austin David
linky to the build thread

[Austin David]

bonnet OFF, air filter "open" (no tape): IAT is now a function of speed, range was 27-34c, ambient about 20-25c.  Very little deflection with RPM.  For various reasons I didn't get a lot of data, but the data I do have shows 13.20 and 13.38 at two points, with 1.0 trim -- in line with "a little bit of tape", but without the better heat management.

 

The steps are degrees C, and the rise is about 2-3 C after the radiator starts dumping heat, then tapers back off with the airflow.

[KnifeySpoony]

I dunno, to me this just looks better too...

 

[Austin David]

you can compare a [Caterham] to every car every made, but not to another [Caterham]. 

 

My primary concern is that huge gap at the back of the filter where you might be able to see guts... if you're standing within 3'.

 

Yes, this is NOT A REAL PROBLEM.  I'm just trying to figure out the boundary between "you definitely need to cut more" and "now you're just showing off".

[Austin David]

minor update: I cut a few more inches out of the bonnet, with the intent to open about 1/2" in front of the air filter.  The air filter is taped "minimally", which basically means up to the hood penetration around trumpets 1-3, and slightly more generous under the hood by #4 but still behind the curve of the air filter.

 

I tried to get some data last night but it didn't collect correctly, BUT watching IAT on the dash I saw that it was holding 15-20C above ambient, and did not materially deflect under high load (WOT @ 7k RPM).  This morning I did some more typical commute-style driving and it was consistent, 15-20C above ambient without a lot of deflection.  Even with a few miles at 60 mph, very consistent +20C IAT. Ambient about 20C, IAT around 40C (39-41).

 

I grabbed the IR thermometer, and checked temps at idle with the fan running: 50C +/- 1 at the leading edge of the filter, and 30C +/- 1 at the trailing edge.  Ambient is 20C still.  IAT sensor is in the center of the air filter.

 

the surprises here: more airflow seems to have either made it worse, or definitely has not improved IAT.  But this does match observations with bonnet removed or most free-flowing filter, where IAT matches "heat dump from radiator" (coolant down -> IAT up).

 

 

Also, for anyone finding this thread: I'm not sure what impact IAT has on performance, but internet-sleuthing seems to lean toward 1HP per 5C OR 1% per 10C, "lower is better", and anything over 150F (65C) is bad.  So if the +20C difference is stable, I'm well into the range of "this is probably fine", especially in partial throttle.

Edited Wednesday at 01:16 PM by Austin David

[Austin David]

Two similar runs, only difference is "a little tape around the sides" and "no tape at all".  Ambient temp about 28C both days (yesterday, today)

 

White: RPM, peaks are just over 7k

Red: coolant, peak is about 100C, 1 degree steps

Green: IAT, peaks are 48-50C, 1 degree steps

 

No tape:

 

The scale makes it less obvious, but IAT swings wider, AIT actually rises under WOT, and rests about 2-3C higher overall. 

 

Tape:

 

Less variable, which includes less drop after WOT, which makes me think the drop is more due to highspeed airflow, and less through the actual intake.

 

AFR and trims are indistinguishable in these two configs. 

 

My net conclusion:

- hot air is coming up from under the bonnet, seeking the largest opening (the intake cutout), and getting pulled back into the air filter

- a little tape around the filter is not bad, and helps a little, like 3C, among an overall creep of about 15-25C heat rise over ambient

- blocking that hot air from the engine is possible, and would lead to near-perfect heat exclusion.

- the potential gain here is like 2-4 HP.  There does not appear to be any other negative impact from this intrusion: the engine runs well, no knock etc.

 

So now my math is whether I want to try and block that airflow from the radiator, and how to do it.  Taping the filter is "free", but a more effective solution would probably be a heat barrier ($50), formed around the air filter up to the edge of the chassis and around the bonnet, to help block the leading edge and bottom.

Edited Thursday at 09:34 PM by Austin David

[KnifeySpoony]

Caterham already solved this issue with the rad duct on the CSR. You could fashion up something similar if you have a vent in your nose cone. 

 

https://caterhamparts.co.uk/other/1724-radiator-ducting-csr-models.html

 

[Austin David]

Nah, this one's a sleeper 

[IamScotticus]

On 10/21/2024 at 9:36 PM, Austin David said:

My primary concern is that huge gap at the back of the filter where you might be able to see guts... if you're standing within 3'.

" and "now you're just showing off".

Therein lies the space between "what I think" and "what they think" and "what I want them to think".  Anyone who has served in the Army knows "there is a right way, a wrong way and the Army way".

Of course,  we want our car candy to pass the scrutinizing, ours ans theirs.  But fact is, the ogling admirer rarely knows the particulars of 7 details.  They just want to know if its fast and if it's a kit car?  Then you pull out the vomit bag you acquired from the last airline. 

But, even the uninitiated can discern between something that is half done and something finished.  For this reason, my vote goes to the full cut-out . If the gap needs filled,  get a wider filter.  Or just let the curious peek.  If they're looking, theyre interested,  you have a friend.