Another goofy test reveals the obvious

I bought my GT4 with aftermarket K&N air filters. They looked good and sounded good but recently I became curious on how hot the intake air is with this setup over the stock airbox. As you all know there is not much air movement in the engine bay when the car is not moving. My car runs just fine, but it does run the roughest when I am stuck in traffic on a hot day.

So in typical engineering overkill, I decided to combine another project I have going in the garage with this air intake question. I have a homebuilt Arduino data collector with a voltage mode temperature sensor collecting data that is saved to micro SD flash memory.

I ran the test in three configurations. 1) K&N, 2) K&N with cheesy fan, and 3) Stock Airbox. The fan was never thought to be a permanent solution; it was just another data point.

I wanted to get comparable data so I ran the test three times pretty much the same way. I warmed up the car on a typical 90 degree F hot day here in San Diego. After about 30 minutes of warm up I would start the test. First I shut down the hot motor for 1 minute to get the air over the engine warm. Then I start it up and run it hard up a hill, then run it easy back down the hill then I let it sit still and idle for 5 minutes. That is it. I just wanted a repeatable and comparable test to see how the three configurations compare. The figures below show the setup and the results.

As you can see the airbox is really needed to get that cold intake air. I am not sure how much performance can be gained with 20-40 degrees F, but I just know that the car seems to idle better when hot now with the stock airbox on. On the negative side, it is quieter. I seem to have lost that angry sucking sound when you are really into it. I did record sound files for the three configurations but the quality is not so hot with just using my cell phone.

So I am pretty sure I am going to stay with the stock configuration now. The K&N filters really look good and show off the 4 Webers, but I don’t think that out weighs the need for the cold air when idling on a warm day. This is probably where most of you go NSS (no s Sherlock), but hey, my hobby collecting data is not as weird as someone ice fishing for fun, is it ?

But now unfortunately I find the rubber accordion piece that connects the air box to the air scoop is ripped and in pretty rough shape. Any advice on where to get a new or better one? Any advice is appreciated. Thanks
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Factory engineering is not so bad is it?

 

Can you do a comparison in motion at say 30-40-50-60 MPH increments? That would be interesting to see.

 

Is your temp probe up inside one of the K&N filters or outside the filters just hanging in the area?

 

+1

 

Great experiment, great set of data. Thanks for doing the experiment and posting the results. Superperformance (UK) has the Air Intake Flexible Rubber Hose for £27.40 ($44.54) but shipping and customs will add a bit to that. They have some pretty good prices on gasket sets, belt change kits, etc., so check them out the next time you need that kind of stuff. I'm doing a valve job and swapping out the exhaust valves on my GT4 and even with the shipping and customs I ordered all the gaskets, etc. from them at a pretty good savings over US suppliers. I should have thrown in one of those hoses with my order, it probably wouldn't have added much to the shipping.

 

Yep. Now I am wondering what the previous owner was thinking when he took off the cold air intake to put on the aftermarket filters.

The only configuration that showed the speed effect was the K&N without fan. As you can see from the chart it starts hot then cools down driving up the hill, then heats back up as I turn around and idle for 30 seconds or so, cools back down on the way down the hill then really heats up during the 5 minute idle. I am probably not going to go back to the K&N configuration but I could do a test like you describe with the temp probe just out in the engine bay.

The temp probe was outside the filters in the center attached to that bracket shown in the first figure. I was trying to keep it in the same location for all three tests. Do you think the temperature would change much from that position to inside the filter?

 

Thanks! And since I am going back to stock filters, is the Baldwin the best?

 

I would just think that the probe is affected by the radiant heat from the top of the engine with the K&N set up vs the stock set up where the probe is somewhat protected from that radiant heat via the OEM air box. But then again that is the whole point too... whats the temp of the air "available" to the filters. It's kind of splitting hairs

 

Yea, you have a good point. I guess in hindsight I probably should have wrapped the temp probe with foil to minimize the radiant part. Oh well, next time...

 

I'm not any kind of engine tuner or engineer with enough knowledge to say in any way your test was wrong. I doubt you would get drastically different results if your probe was inside the K&N filter.
In reality what you were looking to find out what was the temperature of the air around and available to the carbs, and you did that very successfully. Without the protection of the OEM air box the engine raises the air temp of the air which immediately goes in for combustion with the K&N set up and hotter air is obviously less dense and has less oxygen. Thus the hotter could effect the fuel to air ratio for the carbs at certain temps/idle. Once you have the car underway and are feeding the carbs cooler/denser air you probably get into the "sweeter zone" the carbs are tuned for.
Thats just a purely laymans observation. I've got a couple single carb mid 60's corvettes and they for sure idle better when cold and or in cooler weather.

 

The effect of the air's temperature supplying the carbs is well established, it roughly equates to a percentage of horsepower gain for each degree of drop, but if i understand it correctly it really equates to a gain in octane, as lower air temperature slows combustion.
I do wonder if NOT having the stock filter on allows the carbs to cool better when the engine is turned off, as you do not seem to be experiencing the "won't start when hot" phenomenon (caused by "fuel boil"?)
I have to wait about 30 minutes to be able to start mine when hot, if it is off for more than a few minutes at most. I think the large, flat bottom of the air cleaner assembly may help to hold heat in the carb area.
I am considering adding some small muffin fans under the air filter to force cooling when the engine is turned off...

 

Cold humid air contains more oxygen than the dry hot air in the engine compartment. That is a main problem with air filters getting the air only from inside the hot engine compartment (leading to a richer mixture and moving the air-fuel ratio outside the ideal stoichiometric mixture range).

 

Yes! It is all making more sense now. When I rebuilt the engine there was evidence that it had been running rich for a while (large ring end gaps). And the car always seemed to be rich with its smell and the condition of the plugs. Now that I think about it when tuning the carbs the lid is open and the engine is getting cold air. Then the K&N setup gets nothing but hot air when the engine bay lid is closed, thus making it run richer...

The stock air box is the way to go. But the K&N setup had a few good things going for it:

-Looked cool
-Easy to get at carb adjustment screws
-Easy to inspect for fuel leaks
-Sounds much much better.

So last night when going out to dinner my wife commented on how much quieter the car is now and we can have normal discussions like in a regular car. Sigh...I do miss the K&N setup...

 

I did the same test about 8 or 9 years ago. I've been running the individual filters for quite a while now. The results that I got were traffic, the air temp around the filters was substantially higher than ambient (something like 20-30 degrees F higher). But, once at speed (probably around 30+ mph), the scoop provided enough flow that it was only 2-3 degrees higher. This was on a GTS so the scoops are different. I posted results here somewhere.

I figured the 2-3 degrees was negligible and wasn't too concerned with the higher temps during traffic conditions. Also, the added benefits of the "cold air intake" can somewhat be negated by the restrictive design. If you do use the OEM air box, I highly suggest removing the baffles.

Henry

 

I do not have a GT4 but I do appreciate your time end efforts. always good to learn something new.