Chris White

A leak down test will tell you the condition of the rings and valve seats. If the leakdown results are good then it rules out the pistons and leaves you with the valve guides as the culprit.

To be sure you should do a cold and hot leakdown test.

I am 99% sure that its your guides.

BTW - not that I am trying to convince you on it, but adding a 16v head to your other wide stock bottom end would also up your compression ratio and that helps to keep the off boost torque higher than stock as well as responding to throttle better. A really big compressor would not be a great match!

Dougs951S

Well, I've been throwing the idea around for awhile truthfully. Being that I am now at a crossroads, the cost of a 16v swap vs rebuilding my head with all the "while you're in theres" starts to look less steep. I need to think on this some more, but its definitely an option. My question is, if I went 16v wouldn't I be sacrificing part of my power band since it wouldn't be prudent to exceed the factory 8v redline? Or is that 6400 rpm on the 8v partly dictated by valve train mass? IIRC isn't the bottom end basically the same between the 8 valve and 16 valve cars with the exception of pistons?

Just to make sure I'm not missing anything, the only things I'd need would be the head, the headers modded to fit the X-over, and the intake to hack and weld to my turbo plenum, correct? Truthfully I'd probably just make all new manifolds. I bet if I found a good 16 valve head at a decent price, it would actually end up costing me the same or even less than rebuilding my head properly. Chris, are the S exhaust valves up to the task of turbo duty? I understand that the switch would bump me up to ~9:1 CR which is actually ideal for me. Thing is, I'm not sure how to map it. I did all the tuning for my megasquirt myself so I'm comfortable tuning the car so long as I can get a rough guideline, obviously I'll add in lots of fuel first and tune down from the rich end of thing to be safe, but what about the timing maps for a higher compression 16 valve car? I imagine that contrary to intuition they'd be similar to an 8:1 8 valve car since the 16 valve pent roof head with the center plug location is less prone to detonation, but I'm no expert on this. Do 951 pistons need valve pockets machined? I suppose even if it were not required, fly cutting the pistons would lower the compression a bit and make it non interference. Oh yeah, the belt drive. Chris, could you please elaborate on what I'd need to do in that department? I'd like twin sprockets, but if I did the conversion I likely wouldn't do that at first. My car is a total mongrel and I would feel no shame modding the hood when the time came in exchange for

Lex_GTX

I have a similar issue where the car goes thought about 1qt of oil in 1000km (600 miles). Leakdown is below 2% in every cylinder and warm idle vacuum at sea level is 21 inches of mercury.

There is no smoke at idle.

The car has 135k miles ... so the guides is what I will be doing and topping up the oil until I do so.

Crazy Eddie

Huumm...
Well it looks like I need to do a few tests to see where I stand...
The car has 49.900 miles on it ... I did track the car in the late 80's early and early nineties. Not heavy,( but did run the revs up to 6000 on a regular basis). I always lost a quart everything 1000-1200 miles or so ... but lately I seem to smell oil on deceleration as well as getting a high loss of oil? I had done so many changes to the car in the last 5 yrs ( bigger turbo/ replaced a ton of stuff ( including new AOS o-ring and cleaned up everything.
Don't get me wrong the car runs great and is strong as hell, but there is still a low vac (16 ish when hot) and rather high oil consumption
I think the biggest error I made is that I drained the oil out of the car, when I first stared this rebuild, and then left it empty for 5 yrs ... Maybe the seals dried out ??
So it looks like I will check for vacuum leaks, and then move into a leak-down test etc, and see where that will take me....
Thanks guys
Best regards
Ed

Dougs951S

Just wanted to come back to this, pulled all 4 plugs today chasing a cold stumble issue and found they were all toast. the new plugs totally fixed the stumbling issue but the burning oil continues to get worse. all 4 plugs had normal amounts of tan ashy deposits on them, but no oil. They were very worn, gapping at .035-.040. The oil burning is not enough that I notice it and have to add oil, its consumption is well within reason at under 1 qt/1000 miles. No smoke on start up, idle, boost, and really no smoke ever that I can tell, but the smell of burning oil is getting stronger when I decel in gear. I have a spare head but I'm unsure how to check it to know if the guides are any good in it. valve stem seals are cheap so I'd probably replace those and then slap the head on while I build my 951 head into something a bit more performance oriented. Just want to know if I'm still heading down the right path.

Eric_Oz_S2

The easiest way to check guides is get a valve spring compressor and remove valves springs from one exhaust valve and check how much the valve rocks. Limits are in the factory manual. On the 951 the guides often ovalise as they are such big heavy valves.

Dougs951S

Are the exhaust valves that much heavier than the intakes? How much lighter are na exhaust valves vs the sodium filled turbo valves?

mahoney944

Just my 2 cents, this could be a classic case of exhaust manifold cracks, most times they can go unnoticed because of the heat shields. i know my #2 pipe slightly leaks and is on my list. and most times you dont notice anything but get a fume smell under high decel and somtimes high boost shifts. when the windows are down, nothing noticable with windows up.

Eric_Oz_S2

They are just big compared to say the 16v engines. Doesn't matter what type, there is a lot of mass hanging out on that valve stem which makes them wear out in 100k miles or less.