Been chase one of my project car 2gen ( NA 13b bp ) low oil pressure for while, it will reach oil pressure 90psi around 3000rpm but drop to 60-70psi at redline
almost check everything, oil pump, F/R oil pressure regulator,
Oil pick up , different grade oil
Finally come to cooler itself
Previous owner instsll one 2gen and one third gen oil cooler in series, i know this will loose oil pressure but how much? Is it really the case, so i ask shop to machine a adapter so i can T fit a pressure gauge at front cover
Result is suprise
U can see with this setup u loose roughly 70psi , or in other world the sec gen oil cooler inlet is too restricted for this psi


At Dyno



So we find another oil cooler from my other project but twice the size of sec gen
with proper big size endtank and fitting (an12)





The end result look more normal
Even with such big oil cooler

 

Second gen coolers have been used in race applications with good results for decades. When using multiple coolers, they’re to be plumbed in parallel, not series.

 

I used 2nd gen oil cooler at 120psi with the MFR race pressure regulator and full wet sump oil mods for 15 years.

The issue you are having is high oil pressure forces the front cover away from the front iron and oil and the front cover o-ring shoot out if the gap.

You have to dowel the oil pump outlet to front cover (even when using side oil galley as oil pump outlet.

I will find pics of my mod for you.

 

What I ended up doing is have the front side housing machined and a hollow brass dowel pressed into the front side housing that just fits into the front cover. That way the o-ring cannot be pushed out of place and into your front cover and it helps prevent the low pressure drop at high rpm when the oil pressure pushes the front cover away from the front side housing and creates a big leak.


Make sure that the dowel cannot be pushed into the front side housing and obstruct the oil passage- these types of engine mods can easily cause catastrophic failure if you do them wrong.

 

I was expect some loose from series , never expect this much , low some thing new ~

Yeah I been using Sec Gen oil cooler for long time , on my other Rx7
so I try to swap another Sec oil cooler at shop only find out, it develops internal crack ( one of cooling fin brake off ) , this let me to use another oil cooler in shop , we have to remember Sec gen rx7 is build all way back in 80s ~ so might be wise check oil cooler condion time to time

 

But my front cover read over 150 psi with cooler connect in series ~

 

Besides plumbed in series, I’d wonder what kind of trash may have been in one or both of those coolers. Like you said, they’re old.