Hi guys. So I rebuilt my rb25 last year after burning out exhaust valves and scoring a couple cylinders and ever since have had a large increase is turbo lag.
I am running a Hypergear ATR43G3 with a .82 exhaust housing.
Engine was rebuilt with:
CP 8.5:1 forged pistons 86.5mm
Spool forged rods
Acl bearings
Crank collar
All new valves with stiffer springs
Balanced and block decked + head skimmed
Cometic MLS Gasket
Added an adjustable exhaust cam gear but left it set at 0
Now I know that I have lowered compression but I find it hard to believe that such a small decrease in compression (after accounting for the decking and skimming of head) would result in my lag. I currently reach full boost of 21psi at 5000rpm, whereas before the build I made full boost of 18psi at 4000rpm.
I inspected the turbo and found no damage, checked wastegate was sealing correctly, checked for vacuum/boost leaks and pre turbo exhaust leaks. The car is tuned with a powerfc, z32 afm and 740cc nismo injectors on 98 making 301rwkw. The only other change I have noticed since the build is that manifold vacuum has dropped from 24inhg to 16-18inhg, leading me to thinking that possibly the exhaust cam is retarded thus causing increased valve overlap, unless the drop in compression would cause such a drop in vaccum (I expect some loss of vaccum). I lined up the cam timing marks and crank timing mark correctly and triple checked they were spot on, however I didn't degree the camshafts.
Does this sound like a possible cause of this increased lag? Or does anyone have any other theories as to what could be causing such a large increase in response. The turbo comes on hard but as a streeter it is far too slow to spool. I'll also add that I have checked that VCT is operating correctly.
Thanks in advance