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Cedric
Joined: 27 Aug 2004
Posts: 2776
Location: Sweden
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 Posted: Wed Jan 07, 2026 7:47 am Post subject: Air box mod 931, reduced intake temps |
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After a looong time i finally took time to do some intake temp work, i even bought a spare air box many years ago to have something i could cut into or modify.
The original plan was to make a new larger snorkel out the fender to match my additional power, but first I had to check how the original intakes actually works. I know since my flow bench adventures that the snorkel is a bit small and creates some unnecessary restriction, but the intake temps were also a question. I didnt have the chance to try on track, but I tried to drive hard on country roads with full throttle, full brake, full throttle repeated many times until temps was stable to see how much it would rise, and then did some measurements around the airbox, and realised that there is not need for any complicated solutions, i just made some intakes to the front nostrils and it worked great, so good that im not sure i will seal of to the radiator as I planned.
The measurments were done with a thermocouple, with one sensor in the airbox or wherever i want to measure, and one reference sensor jammed in the door for a reference value to measure the difference.
Results(temp in the lower part of the airbox vs outside temp), difference in Celsius:
Orginal aribox:10-13 degrees
New holes and orignal snorkel open: 3.3-3.5
New holes only:2.2-2.5
So the OEM box isnt bad, but could easily be improved upon, i didnt have the time to measure the restriction differences, but I will do next season. Less intake temp means less compressor work needed, the same with restriction. More compressor work means more back pressure and less power, but mostly its fun to fiddle with stuff and make it work better.
Printed in PCCF, bellmouth in the inside and outside with a lip to cover the cut edge, m3 nut hidden on the inside parts. Made templates to facilitate cutting. Each hole is roughly the same area as the original snorkle.
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1980 924 Turbo
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924RACR
Joined: 29 Jul 2001
Posts: 9013
Location: Royal Oak, MI, USA
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| Posted: Wed Jan 07, 2026 10:41 pm Post subject: |
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Nice work.
I need to get off my butt one day and improve my own intake; currently running with just a big cone filter stuck on the turbo inlet side, but it's not ideal as I now have no washer bottle plus it lacks isolation from engine bay heat. Though the intercooler helps greatly! LOL
But it's one of a number of points on my IC install/EFI conversion that are good enough to work decently, but really want a proper overhaul with a better, more refined layout...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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Cedric
Joined: 27 Aug 2004
Posts: 2776
Location: Sweden
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| Posted: Thu Jan 08, 2026 5:52 am Post subject: |
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| 924RACR wrote: | Nice work.
I need to get off my butt one day and improve my own intake; currently running with just a big cone filter stuck on the turbo inlet side, but it's not ideal as I now have no washer bottle plus it lacks isolation from engine bay heat. Though the intercooler helps greatly! LOL
But it's one of a number of points on my IC install/EFI conversion that are good enough to work decently, but really want a proper overhaul with a better, more refined layout... |
A good way to start is just putting a sensor there and see how bad it is / if it is bad, it also depends how you use the car. If you just cruise around with the occasional blast its a different thing to running full blast, which puts a lot of heat in the radiator and thus in the engine bay.
The IC help, but not completely, since the increased compressor work will be there regardless of cooling efficiency. Its not massive at these levels, but if you pull a lot of heat it can be.
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1980 924 Turbo
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924RACR
Joined: 29 Jul 2001
Posts: 9013
Location: Royal Oak, MI, USA
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| Posted: Thu Jan 08, 2026 6:45 am Post subject: |
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Yeah, I do log IATs because EFI... but also you're entirely right in that the impact for me is pretty low since my usage is also pretty moderate. Rarely even get over 10psi boost!
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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morghen
Joined: 21 Jan 2005
Posts: 9087
Location: Romania
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| Posted: Thu Jan 08, 2026 10:58 pm Post subject: |
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Proper engineering stuff!
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chuck21401
Joined: 20 Feb 2005
Posts: 595
Location: Annapolis, MD
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| Posted: Sat Jan 10, 2026 12:12 am Post subject: |
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| 924RACR wrote: | | Rarely even get over 10psi boost! |
You know, turning up the boost makes more power.
I turn down the boost in my car in the summer...but turn it back up when the cold weather arrives. 14psi really wakes up the car. 
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1981 Porsche 931 w/S1 engine & euro g31 transaxle. Water-cooled intercooler |
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safe
Joined: 18 Mar 2017
Posts: 694
Location: Sweden
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| Posted: Sat Jan 10, 2026 12:25 am Post subject: |
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| chuck21401 wrote: | | 924RACR wrote: | | Rarely even get over 10psi boost! |
You know, turning up the boost makes more power.
I turn down the boost in my car in the summer...but turn it back up when the cold weather arrives. 14psi really wakes up the car. |
14 psi, that's the winter/slippry road setting! 
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/Magnus, Stockholm Sweden
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Porsche 924 -79 NA, EFI and Turbo.
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Porsche 911 -77, 3.2 Targa
Porsche 911 -69, 3.6, Coupe |
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924RACR
Joined: 29 Jul 2001
Posts: 9013
Location: Royal Oak, MI, USA
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| Posted: Sat Jan 10, 2026 3:17 am Post subject: |
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It takes too long just getting there!! LOL
Probably could use a re-inspection of my exhaust manifold, could be my weld repairs haven't held up...
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Vaughan Scott
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'79 924 #77 SCCA H Prod racecar
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Raize
Joined: 18 Sep 2013
Posts: 426
Location: Scotland
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| Posted: Fri Jan 16, 2026 7:31 am Post subject: |
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| That's a very good temperature reduction. I took the intake tube off my NA airbox for same reason but never actually measured it, just figured having it draw air off the radiator was obviously not good. |
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Cedric
Joined: 27 Aug 2004
Posts: 2776
Location: Sweden
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| Posted: Fri Jan 16, 2026 5:23 pm Post subject: |
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| Raize wrote: | | That's a very good temperature reduction. I took the intake tube off my NA airbox for same reason but never actually measured it, just figured having it draw air off the radiator was obviously not good. |
Did you have the n/a variant that goes over to the right side of the car over the rad i guess? Never understood that design, oddly enough the early ones go behind the pop up light out in the left wing, a much better sport, one could wonder why, maybe it was to fix intake noise legislation or something like that.
As an engineer having worked with engine development i rarely just do stuff, without at least some analysis, measurement, testing, calculations or similar 
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1980 924 Turbo
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Raize
Joined: 18 Sep 2013
Posts: 426
Location: Scotland
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| Posted: Fri Jan 16, 2026 7:53 pm Post subject: |
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| Cedric wrote: |
Did you have the n/a variant that goes over to the right side of the car over the rad i guess? Never understood that design, oddly enough the early ones go behind the pop up light out in the left wing, a much better sport, one could wonder why, maybe it was to fix intake noise legislation or something like that.
As an engineer having worked with engine development i rarely just do stuff, without at least some analysis, measurement, testing, calculations or similar |
Yes, it was that variant, from what I gather having a warm air intake improves emissions and economy due to a slightly better burn and lower volumetric efficiency (less dense hot air = more throttle opening = less pumping loss) but I can't imagine it being significant except at a corporate scale.
I did the calcs for my supercharger and chargecooler and 4-wheel disc brakes but this one just seemed to be common sense. It's good to see it quantified in your case. |
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