antman89iroc

Just did a compression test on my L98 IROC.

The results are...

1) 120
2) 119
3) 118
4) 125
5) 114
6) 116
7) 114
8) 122

Does this seem on the low side?

The sort-good news is the cylinders are within 10% high/low but I expected a little higher readings. This is a ~9.8 compression motor with a short duration cam. The starter was cranking good with a charger keeping it up. I did not warm up the engine but did keep the throttle cracked open.

Has anyone noticed nitrous use causing accelerated ring wear?

This is a fairly low mile engine with <20k miles. Mostly regular driving but has seen a little drag and spirited street use. Probably 40-50# of nitrous through it so far. N2O levels were usually 125 or 150 WHP level but a couple of nights at the strip were at the 200 WHP level. This engine does use a little oil and the plugs show some evidence. It doesn't smoke out the tail pipe at idle or at WOT. I am considering a re-ring as it seems to have lost a little power. It still runs good so I don't know whether to bite the bullet and pull the motor or just run her til she really needs it.

Please share your experiance, commments and suggestions.

1gary

The ring gap is set differently for power adder engines to keep the ring ends from butting against each other. We actually add .005 to the chart shown:

1gary

And you know what.Guys think just by a second fuel source of pump gas while using nitrous, the amount of the shot doesn't matter. Nitrous is hard on a engine and kind of in effect it is raising the compression ratio. When we are using the bottle we also use 114 racing gas which helps some.

antman89iroc

So since my build up didn't plan for N2O I may have ended up jamming up the rings when dosing hi HP levels and that may account for the even-but-low compression numbers. My 20k miles = 150k of wear?

1gary

I could be a number of variables. Certainly it isn't where you have cylinders at 0 where alarm bells should be going off. Could depend on the finish hone of the cylinders from one cylinder to another to the type of rings you have and them being chrome rings that are going to take a long time to break-in/seat. What happens with gap on rings and power adder's is heat is power and the rings(top rings mostly) get hot needing more gap. They butt against each other and sometimes will fold over scoring the bore. The other thing is the kind of piston your using should be a consideration when you are on the bottle. Big shots really should be forged pistons that in it's self has clearance standards that are somewhat looser to allow for expansion. There are a big difference in how forged pistons handle the heat. Moderate nitrous use like 100/125 is one thing,but 200/250 is a big change. Also the bore size does come into play where your forcing the same adder into a smaller bore and it has a bigger effect on that bore/piston/rings.

But I am getting way off topic here. I would hate to see you take it apart only to find the rings haven broken in yet. I would lay off the nitrous for awhile putting on more miles and see if the oil use gets better and do another compression check. Just keep a eye on it for now.

Damon

You got the cam specs? Agreed that is low by about 30 across the board at least. And it's not from your nitrous use. You can have terrible leak down and it'll still give you good cranking readings. It's just low.

You ever double checked your gague against something? Even a lawn mower engine should push about 80-90 PSI. The 8:1 383 blower motor in my Malibu huffs up about 130-135 PSI against a little 214/224* cam (installed on the 4* retarded position of the timing chain). So 120 is just flat out too low a reading across the board.

Either your parts aren't what you think they are, the cam is installed WAY retarded (like off by a whole tooth) or your gague is not reading right. Simplest answer usually being correct, I'd double check the gague first.

1gary

I respectful disagree. Chrome rings do take forever to seal. It's for that reason I am suggesting he lay off the nitrous for abit. Also the posted chart clearly states a ring gap for nitrous and blown engines.

I do agree with your post on the cam spec's and the possibility of it not being degree'ed in correctly. Certainly a leakdown test would show that. But if the leakdown tests showed blow-by the rings,I am not sure I would tear it down just yet. I think we need to know which rings he has first. The idea of low tension rings on the street is a terrible idea.

sofakingdom

iTrader: (1)

Looks like you didn't have the throtle blocked all the way open.

#s should be in the 175ish range.

1gary

Where the hell ya been??. Been kind of lonely around here lately.

If you have a friend who is a pro mechanic with Snap-on tools. You might be able to scope some of the low cylinders at BDC to see what they look like.

sofakingdom

iTrader: (1)

NYC, DC, LA, Cleveland, Atlanta, Erie, Chicago, Baltimore, Cinci, KC, Philly, Tampa, CT, WY, Pittsburgh, WV, .... that I can recall off the top of my head, maybe a few others, in the last coupla weeks. They all turn into a blur after a while.

antman89iroc

Here's a little follow up info.

The compression tester I used wouldn't reach a couple of the plugs due to the header clearance. So I made an adaptor and hose extention out of SST fittings and about 1 foot of 1/4OD x 1/8ID teflon tubing. Possibly the extra hose and fittings posed a bit of restriction.

The gauge was suspect. I checked it against a couple of other pressure gauges I had on hand and it showed 5PSI lower than the other two. So the numbers posted were actually 5PSI HIGHER than what I read. IOW the average was around 113 on the gauge and I added 5PSI in my post. I should probably get a different compression tested and re-do a few of the cylinders I can get to without the adaptor just to have a comparison.

I don't have the cam data sheet in front of me but it's a CC roller with 212/218 @ .050, with a 113 or 114 LSA. I can't remember the centerline but it's an off the shelf cam for computer cars. I did install it with a 3' advance but didn't degree in the cam so who knows, I could have lined it up incorrectly especially since it's a 3 location (+3/-3) set. If I do have it retarded a bunch would it run ok? I have always felt it should have a little more bottom end but with 3.70s it's kinda hard to tell. I may be able to get a degree wheel on the balancer and check it at the pushrod if that would be close enough to tell me whether it's off a tooth.

Now the rings were originally molys but I had freshened it up a couple of years ago (maybe 5k miles) and I don't remember if the set in it now are moly or not. The first set didn't ever seat and it used oil from day 1. When I re-ringed it I probably didn't use moly in hopes to get them to seat. This car doesn't smoke, puff out the breather when the PCV valve is pulled or anything like that. It does use some oil though. It runs good but seems to be down on power a bit.

The nitrous was an afterthought not considered in the build. I haven't just beat it to death a lot and haven't used any for quite a while. I usually keep it at the 125WHP level for "everyday" use but did use the 200WHP jets for 1 night at the drags just to see how low it could go. BTW this car got BIG on that setting pulling a best of 7.69 1/8mi. Not bad for a daily driver- 3600lb car. It did worry me that I was playing with fire on my stock bottom end so I backed it off after that day. The "WHP" figures are supplied by the manufacturer of my kit, Cold Fusion. They point out these are WHP levels and not engine levels but I have not confirmed whether they are accurate or not. Basically, it should be fairly agressive setting.

Damon

OK, so we've got a little detective work to do here.

The fact it feels "a little low on power" is entirely understandable given the very low cranking compression readins. I'll take it at this point that the gague might be a little off, but it's unlikely you're off by 30+ PSI. (Agreed with above, I would think you'd be seeing more like 170-180 PSI with that combo of parts). Maybe test your existing gague on your neighbor's lawn mower or another known-good engine in the vicinity just to make sure it's not reading silly-low. A simple thing like replacing the special low-tension check valve in the end of the hose with a "standard" one typically used in a common tire valve stem can drop the readings by an easy 25-30 PSI across the board (trust me on this one- I know this to be true). So let's try to rule that out absolutely.

I doubt your home-built fittings to get to difficult plug holes caused a problem in th readings. You would see only those cylinder's readings noticably lower than the rest where you screwed on the hose directly, if that was the case.

So now the big question..... did the motor always feel sluggish even before you started using nitrous or did it only feel weak AFTER you used the spray a few times? While nitrous typically just flat-out breaks parts in a very obvious way if there is a problem, it can also cause more wear on the parts, including rings, that's true.

If it always felt sluggish then it's highly unlikely the nitrous use CAUSED this problem. If it used to kick butt and take names but now feels doggy then it's possible the nitrous DID cause a cylinder seal problem, resulting in the low compression readings.

Still it's unlikely that rings in all 8 cylinders would "wear" and lose seal by the exact same amount causing across the board radically low readings. Usually some of them hang on a little better while others scream "check please!" and totally bug out.

So...... bring on the info! You got my A-game waiting on this one.

1gary

Note the quote:" It could be a number of variables".

Interesting the first set of rings(now that we know that) didn't seal either,but you said you didn't break them in right. Did the bores have a nice cross pattern from honing on the first set??.

antman89iroc

[QUOTE=Damon;5360959]OK, so we've got a little detective work to do here. A simple thing like replacing the special low-tension check valve in the end of the hose with a "standard" one typically used in a common tire valve stem can drop the readings by an easy 25-30 PSI across the board (trust me on this one- I know this to be true). So let's try to rule that out absolutely. [QUOTE]

Ok I sorta remembered replacing the schreader valve some years ago so I checked it and it did seem stiff so I swapped the one on the hose with the one on the gauge. I also repaced an o-ring and screwed the gauge hose directly into a cylinder I could access. Gauge readings were 175/180 so I reinstalled the "adaptor" I had used the first time and it made 170/170. e Is this too high for a ~10 CR motor with a short duration cam? Since the previous test was pretty even should I go back a re-check them all? The only other differances were I left the other 7 plugs in the engine and it was warmed up.

Isn't there saying about the simplest answer usually being the right one. Hazman? Thanks for all your suggestions and comments.

1gary

Yep-sometimes the simplest is the right one. Of course the oil use might be more to look at. Got-a wonder why that is happening.

Damon

Ya know, I didn't actually expect that to be the answer. I just mentioned it on the one-in-a-million chance somebody else had made that same oddball mistake I did 15 years ago.

I'm glad something that simple was the issue because you were probably not going to like where the diagnosis would have taken us otherwise.

170 is about exactly where you should be with that combo.

antman89iroc

Damon I would have never guessed the valve would have so much affect on the gauge. I was going to get a different tester to compare but your suggestion got me thinking. There was a notable differance in the two valves plus the o-ring I replaced may have been a leak too. I actualy condemed an engine with that gauge Would the fact that I had all the plugs removed for the first test and the other 7 plugs installed on the second test make a differance? The cranking speed did seem better with the plugs installed.

And Gary you are right about the oil use. It's not just a fantom leak cause the plugs are showing it. I have put 4 sets of plugs in this motor since the head swap. Part of the reason was just chosing a type and heat range and since I am tuning my computer I have had it tooo rich at times. I now have a set of ACs for an L98 vette with AL heads and they seem to be performing best so far. The strange thing to me is how the plugs load up. One side will be "normal" and the opposite side will be black and shiney. Almost like you held the plug and spray painted it. If left in for a while the black side will build up and looks like pine bark under magnification, kinda krinkley. Best I can tell the black side is facing the head when installed, the piston side looks better. Any ideas on that?

Damon

I usually do a cranking compression test one plug at a time, replacing as I go. If you do it with only one plug in there's no mistaking how many times you hit the compression stroke on that cylinder, though!

1gary

For god's sake,I wouldn't think you need to index the plugs...........

1gary

Wait a second.What was done to those heads before you put them on??. Where they used??.

Damon

If your valve guides and/or seals are shot (or your intake isn't sealing up on that bank, creating a vacuum leak into the lifter valley) you will get oil coming in through the intake valve with the A/F mix. The center electrode will get get black on the side that faces the intake valve first, slowly getting worse and building up all around the plug over time. The threads on the plug will also be oily.

How do you know it's the side pointing to towards the intake valve? You make a mark on the outside (porcelain) of the plug with a sharpie that points at the intake valve for that cylinder before you remove it. Don't mix-n-match that plug with a different cylinder- the plug threads are all different from the factory and will point the plug in a different orientation in each cylinder.

If it's rings the whole thing will get black more evenly and the threads typically wil have little to no oil on them.

Just some shade tree mecanic diagnosis I've learned over the years.

jdracer13

This can be caused by an improperly honed cylenders, incorrectly filled rings, rings not broken in properly, amoung many other things. I would get a bore scope and take a look inside and see if there are any tracks on the walls or if they are discolored and see how wet the piston is. The culprit is probably some blow by and the cam not being degreed in properly makes a HUGE difference on power, and tuning.

antman89iroc

The heads were new Pro Comp 190/58 aluminum. When I started noticing this strange loading I marked a couple plugs before I removed them. There certianly could be an issue with the intake gasket or rings, even the valve stem clearance at this point as I am losing a confidence in the quality of these heads. Not to condem them yet but I have had issues with the studs, rockers etc, There was a mis match at the intake requiring a .125 thick gasket etc. Don't get me wrong here, the engine does run well, decent vacuum smooth operation. I'm just trying not to overlook the basics here.

1gary

I agree with this.

I said for him to buddy up with someone who has a scope.Might not be able to get to all the plug holes because of his headers.

A leakdown test would help lead him in the right direction.

I have posted many times how I felt about some of the brand heads being used these days and how I am suspect I am of the castings. It is based off another brand where a friend of mine is a huge mid western distributor that had to eat about 2/3's of his inventory because of cracked heads right out of the box.He doesn't deal in that brand anymore and he was never reimbursed for his losses.

It is kind of interesting you talking about a mis-match of the intake,and you saying I guess,had some valve train geometry issues.I am thinking more towards the valve train problem creating wear over the intake gasket because your also saying it has good vac.Could it be both??. Sure. You could suck oil and not have a vac leak.

As you know.Timing on these things sometime is a key. You can have a strong runner that hasn't really show serious issues yet and your catching it before it becomes one.

So your down on power alittle,and using alittle oil being shown on the plugs.

Yeah I would leak it down.

1gary

Also-like I said before,if it where me,I would lay off the nitrous until I figure out what was the problem. No sense in stressing it more right now.