I think I tested that void area in front of the firewall and it indicated a negative pressure at 65mph as well. I plugged up the VS firewall hole on my Speedster when I made sure that the engine compartment was totally sealed.
I then had 4 oil temp sending units plugged into my engine so I could monitor the oil temps simultaneously. I used a rotary switch to quickly cycle thru all the temps. (so I didn't have to buy 4 gauges) One on the inlet and outlet of the stock oil cooler, one on the outlet of the aux. oil cooler out in the fender well and the last one directly in the oil sump near the pick-up tube..
The reason I did all this was because I could tell that my engine was running hot and I wanted to know why and where. I used a very accurate thermocouple probe down the dipstick tube to verify this.
After reading about all the methods tried by many of you members here, I decided that the best method was to seal off the engine compartment like the VW/Porsche engineers intended. They obviously went thru great pains to do this so that is what I did. My logic dictated that the modified engines we now use in these cars will and do generate more heat, Hence, an external oil cooler in the left-rear fender well in addition the stock cooler. By doing what I did, I solved my engine cooling problem. About this time I was curious if the grill was a restriction because All the air for the engine now had to come thru the grill. Alas, I measured a negative pressure ! For me, I had already eliminated my cooling issue but perhaps I could get my temps down even further by eliminating this air flow restriction. I mentioned different methods in my previous post but what I did was remove the most material I could on the rain guard under my hood. I found these photos of what I was looking at. I borrowed this hood and rain guard from Greg. Please note that most of the actual rain guard that covers the distributor had been removed. This was OK because I was more interested in the restrictions on either side and that is what I measured.
The end result is that I removed a lot of the rain guard on the right and left sides , leaving the center part over the distributor. This eliminated the negative pressure and slightly lowered the oil temp. Only around 3 f in the sump.
An interesting note and surprising to me was that the stock oil cooler only had a difference of 4 degrees F. between the inlet and its outlet. It's a very high volume flow but I didn't expect that kind of number and still don';t really know why or how it cools the engine at all. Even on a stock engine. ............BRUCE
