This was promised for next week but I’ll be traveling. And I’m bored right now. I called spacers a waste of money in another topic and I’m going to use physics to make the point.
The claim surrounding spacers is that they effectively lower intake temperatures by having a cooler intake manifold. Which is a reasonable enough sounding claim in and of itself. Sounds like common sense even so why wouldn’t it be true?
The second point is that they add a small amount of length to the runners and thus torque. The effects of this are probably measurable but constitute a slight shift of the torque curve downwards. Probably less than 100rpm going by the advertised thickness. Either way this is a secondary effect which shall be ignored for the sake of simplicity. I wouldn’t presume improving Audi’s design on the manifold is that straight forward anyway.
Issue #1: The intake temperature sensor on our cars is located before the manifold. It is practically impossible to measure the air temperature at the end of the intake manifold. Any proof of temperature differences is therefore impossible to come by.
Issue #2: Measuring the temperature changes of the intake manifold itself ignores how much heat transfer actually occurs from manifold to intake air during acceleration.
Solution: physics and maths! Boooo…
Premise 1: air isn’t a terribly good conductor of heat. So transferring heat from metal to air takes some doing. This is why charge coolers consist of many small channels and fins. An intake manifold has a different purpose thus wide wide channels for maximum flow - most of the air rushing through never touches it directly.
Premise 2: the intake air spends very little time in the intake manifold. So there’s really not much time for this poorly conductive gas to heat up at all.
How much time is spent in the intake? I’ve had to make some ballpark assumptions about runner diameter and the total length the air travels but if we take 40mm round runners (they’re not exactly round in reality, so 40mm is probably generous for the calculation) and 45 cm (about a foot and a half) average distance we can calculate the necessary speeds at various points:
- Throttle valve is 90mm in diameter. We’ll ignore the obstruction of the valve itself.
- Guesstimated diameter of the runners 40mm. If anyone can measure one please tell me what it actually is.
- The BNS has pretty awesome VE, I’m assuming 100 percent for simplicity, it’s close enough.
The runner needs to feed a single cylinder. Let’s take 6000 RPM. The required velocity through the runner would be 69ft/s at 6000 and 92 ft/s at 8000.
That’s the slow end of the manifold. 8 times as much air goes through the 90mm TB. Same calculation nets us 108 ft/s at 6000 and 144 ft/s at 8000. That’s 100mph!
But even at an average of only 90 feet per second, the air goes from throttle valve to end of the runner in 0.015 seconds. I guesstimated a foot and a half of distance inside the manifold. Either way the time spent is very short.
How much would you imagine the air heats up in the wide open channels of an aluminum manifold after 0.015 seconds? It’s not going to be 10 degrees. Even one degree is starting to sound like a lot considering a “hot” standard manifold is 145F and you have to subtract ambient from that to get the delta T.
Whichever gain/drop you get in air temperature is a small fraction of the total drop in manifold temp. On my car I logged IAT before the manifold as being roughly 10C over ambient. Making delta T with the intake manifold even smaller.
And there’s another angle:
Mass air flow during hard acceleration for a stock RS4 will be between 200 and 300 grams of air per second. The MAF tells us that. Air has a specific heat capacity of (near as makes no difference) 1 kJ per kg per degree K or C.
At an average flow rate of 250 g/s under hard acceleration the manifold would need to transfer 250 J per second to raise air temperature by ONE degree. Now, you’re not going to notice one degree difference. 5 maybe. I’ve seen people claim 10 degrees (not here, different brands) from spacers.
To achieve a 10 degree lower intake temp the stock setup would have to be transferring 2500J every second to the air. Considering the specific heat capacity of Al is slightly lower than air at around 930J/kg/K and the manifold weighs what, 5 kg? (someone know?) it would be transferring heat so quickly that every 2 seconds the manifold itself would cool down more than a degree C. A bit of fun on an on-ramp and the manifold would have lost 10 degrees supposedly. Obviously it doesn’t work like that. If there was that much heat transfer between the engine and the manifold the manifold would be running a lot hotter - basically as hot as the rest of the engine block is. In reality heat transfer from block to manifold isn’t that brilliant to begin with. It would be running much hotter than 145F if it was.
The intake air does cool the manifold and so the manifold does heat up the air slightly, it’s just that such huge amounts of air pass through it that the actual increase in air temperature is minimal. The math simply does not work if you assume much bigger numbers. So the effect from intake spacers is NOT equal to zero but it will be a small enough amount to render the exercise rather pointless. At least the money vs gain ratio is off the chart.
It is true that there is some performance loss as everything heats up under the bonnet. The intake temp before the manifold can go up several degrees, but this cannot be attributed to the intake manifold. It’s also worth taking in mind that the whole engine runs hotter when you’re thrashing about OR going very slowly. Hotter valves, hotter pistons, hotter cylinder walls,… oil temp is a pretty good if slightly delayed indicator. The detonation threshold becomes lower at higher temps and ignition timing will become slightly less advanced. The intake manifold is the wrong place to look for solutions to the supposed problem.
I’ve seen phenolic spacers sold for all kinds of cars over time and never was there an actual agreement or properly organized testing of them. My turbo Volvo with a decent intercooler had an intake manifold that ran only a few degrees over ambient when I was thrashing it about. The faster you went the colder it got. No spacer. The only difference is that it was mounted at the front of the engine rather than on top, meaning little convection. And spacers do nothing against convection anyway.
Bottom line if you tuned out halfway:
- Air, which has poor heat transfer properties but a large specific heat capacity, spends only a few hundredths of a second in the warm intake manifold at most.
- Delta T between the air going into the manifold and the manifold temp is relatively small. It’s not a hundred degrees, far less. And you can only expect to heat the air a fraction of the delta T in 0.01-0.03 seconds.
- The very small drop in air temperature should not have a noticeable effect on engine performance thus the mod is not worth the money from this perspective. If it makes you feel better, go and buy them, it can’t hurt. But you’re only really doing the company selling it a favor.