Increased Lag After Rebuild

[Daza33]

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

Edited February 11, 2015 by Daza33

[GTSBoy]

1. Skimming head and decking block will INCREASE compression, not decrease it. So no point looking for the problem there.

2. My bet is that regardless of you setting the adjustable cam at zero, the cam timing is in fact somewhat different to what it was before you took the engine apart.

3. You say you have checked that VCT is working correctly. If you trigger it at idle, what changes?

[Daza33]

I understand that skimming the head and decking the block will increase compression, I was more getting at the fact that with the 8.5:1 pistons it would bring compression back closer to 9:1

After the rebuild the VCT solenoid crapped out and fried the VCT driver in the powerfc. I fitted a new solenoid and repaired the ecu and can verify it is working because I have disconnected the solenoid and the car drives like a pig.

[GTSBoy]

Sorry, I missed the pistons in skimming your list.

So we fall back to my point #2.

[Badgaz]

so with all those changes you expected it to be the same with the current tune?

[Daza33]

Oh no I had the car retuned after the rebuild.

So GTSBoy you think degreeing the cams may solve the issue?

[GTSBoy]

I'd be immediately suspicious that it's not right, regardless of all the marks reportedly lining up.

From my point of view, I would never re-assemble an engine without slapping the degree wheel on it and checking the cam timing - especially if I've changed to adjustable gears or somesuch. The peace of mind that results from knowing absolutely what the LCAs etc are is worth the price of admission alone.

[AngryRB]

I'd like to know if using stiffer springs causes the lifter to collapse a little and decrease the amount of lift of the cam? I recently pulled some shagged lifters and I know the engine wasn't breathing too well, but maybe not relevant here.

[Daza33]

I pulled my lifters apart and cleaned them out, however I believe the springs shouldn't cause any issues with lifters.

I was always under the impression that over revving the engine is what causes hydraulic lifters to shit themselves, because they can't keep up as opposed to solid lifters.

[paulr33]

whatever is wrong is likely to be pretty obvious/basic given the lag descrepancy

id suggest looking at your base timing, check the cams and cam gear adjustments and also check and see if VCT is working

as youve gone through an engine build and turbocharger change a lot has been fiddled around with to keep it basic as to what to check

from memory stuff like this is caused by the cam gears being off by a whole tooth or an entire degree offset incorrectly

something like that might explain the mega lag

[jiffo]

Couple of things,

Are you dead set sure you have the pistons you ordered? Wouldn't be the first time wrong pistons have been supplied.

Decking the block and skimming the head will have the effect of retarding cam timing, BOTH cams so time for the degree wheel.

Adjustable exhaust cam is easy but you'll have to consider the inlet cam too.

From memory the stock VVT unit has adjustment on the rear, bit of a mission to alter but is do-able.

Really depends on how the numbers come out when you check.

[Ben C34]

If you have a Neo engine I can help out with a mod to be able to adjust the inlet cam without having to remove it.

[Daza33]

thanks for the offer Ben but it's a s2 r33 motor. After doing some searching on the net after starting this thread I'm pretty confident the cam timing could be my issue.

Everywhere I've read says retard both cams for top end, but have nothing down low which is pretty well what I have. Gunna suck adjusting it in the car but oh well live and learn

[GTRPSI]

The standard rule of play is to advance cams for more bottom end and retard their timing for more top end.

[Daza33]

Ok I'm going to dial in the cams this weekend, one quick question. Does anyone know how much adjustment can I get out of the standard intake cam gear?

[iruvyouskyrine]

Quite a lot, I'd say 6-7 degrees either way