for history, read the earlier post....

http://forums.corvetteforum.com/zerothread?id=309694


Heads have been milled, possibly .020-.025 plus head gasket compressed height of .015.... I'm guessing this has had an effect on compression ratio? Some mentioned maybe 1 full point. My 1990 never had problems running on 87 octane gas and I never saw any spark retard in Diacom. After doing recent head gasket job, now the car pings like mad even when running 93 octane. I've tried taking about 6 degrees of timing out in my chip at the low rpm, low load areas that the pinging occurs the most and it did help quite a bit. But there is still quite a bit of pinging. Fuel pressure is stock, the motor is pretty much bone stock.

Stock CR on a 1990 is supposed to be around 9.5:1.

I stuck a compression guage on it tonight and it looks like they are around 200-210 lbs. My 1985 vette was around 170 lbs. Does the 210 sound high? Would that compression need 93+ octance fuel?

 

Remember all the problems we had tuning my car and having to run at least 50/50 racing gas and premium?... only to find out that my heads had been milled too much. The fact that I have iron heads makes my motor that much more sensitive. I put a thread out here in this section awhile back asking about possibly thicker head gaskets, or perhaps doubling them up, to compensate for loss of milled off head material. I got some very informative responses from some of the guys that frequent this section. I seem to remember that increasing gasket thickness will help in "recovering" the loss of head material but at the expense of quench area. One person also stated that increased gasket thickness can actually help in promoting detnonation. So, I was kinda stuck between a rock and a hard place. I know you probably don't want to hear this but the only way to truely determine your current compression ratio is to remove the heads and have chamber volume measured. Cylinder compression is more of a function of valve and ring seal than it is of compression ratio... obviously since you have fresh heads put back on your rings are still in excellant condition. The fact that you were able to tune some of the problem out leads me to believe that the machinest probably took more off the heads than you were lead to believe. I only say that because that seems to be my case... also knowing my heads were milled at least twice. Did you check piston to valve clearance when putting the heads back on? I'll try and stay tuned to this thread because I'm eager to hear what some the pro's have to say. :seeya

:cheers:

 

Sounds like you should pull one of the heads and see where you really are. Measure the head chambers (you can get a Comp Cams ecomony cc kit from Summit for $40), then measure how far down in the bore the pistons are at TDC. You can then figure out your real compression and quench.

-Joe

 

if you are near a NASCAR short track check to see if they have a KATECH WHISTLER for checking the compression ratio. this machine can check the ratio by plugging into a spark plug hole and they use them at the track to make sure that the race engines are not over the allowed spec. :chevy

 

are you sure the Stock CR is 9.5:1 ???

I though both the 90 and 91 were 10.25:1

:confused:

 

:yesnod: Yes, it's 10.25:1 but from the factory the cr is usually lower than advertised. If they say it's 10:1 then in reality its closer to 9.5:1

 

well, that's been something I've never been able to pin-point. I think my owners manual says 10.25:1 and the Factory Helms manual says 9.5:1 and of course you get 100 opinions on the forum when you ask!!!! :D

I guess if it was 10.x:1 and now I've bumped that up by about 1 point, then I'm a bit high!

 

Hmmmm.... I live 10 miles from Texas Motor Speedway. How would I go about getting access to one of these things? I don't think I can just drive up and ask, can I????

 

Bill,

So do you still have those iron heads on? Or have you pulled them...

Unfortunately, I can't pull these off anytime soon. It was hard enough going without the car for the 5 days while I did this the first time since it's my daily driver. I'm going to put some high octane stuff in and see if the problem goes away. If it does, then I'm just gonna have to tune for it the best I can with timing and fuel.

 

you have to find a local short track that runs the winston racing series rules because they would have their own unit. NASCAR has their own unit when the go to the big tracks because NASCAR is in charge of the inspections. :chevy

 

Bill Reid is about 95% right on the money. In fact it MAY have been me that warned of the detrimental effects of lowering the CR by increasing the quench distance (thicker gasket). Tearing it a part to cc everything and determine the numbers would be very helpful, but you KNOW the problem. You've had the heads shaved and probably used a thinner head gasket. It sounds as though you have raised the compression ratio, considerably.

To debate one of Bill's points, unless the rings or valves are leaking excessively, cylinder pressure is more a function of compression ratio and cam timing. A longer duration high performance cam bleeds off cylinder pressure, especially at low rpms.

You could try thicker head gaskets and keep your fingers crossed, but a tight quench clearance promotes turbulance that induces fast burning that helps reduce the tendency to knock. The fast burn also requires less spark advance for best power. In this case you're not hurting power to retard the spark. Try trimming the whole curve. Less total, should help promote more power throughout the whole range.

If that doesn't solve the problem, you have two viable choices in my opinion. One is to try a hotter cam. The other is to remove the heads and enlarge the combustion chambers to lower the CR. Don't reduce the size of the quench area, unshroud the valves and maybe make the sides of the chambers match the bore, more closely.

Carefully managed, you may be able to turn chicken crap into chicken salad. Good luck.

 

could it also be that with the higher compression and its associate extra heat the spark plugs are glowing hotter and causing some preignition?

i'd try a couple heat ranges cooler before pulling the heads

 

Good point!


I did take a bunch of timing out in my chip, and it has helped a lot and I don't notice any difference in performance or driveability. I did have the car dyno'd last February, so someday if I get back to the dyno I'll know what's up. There are just a few areas that I hear a ping now, and only under certain conditions. I figure if I could spend a day tuning it, I can get it all nailed down. I only see this as being a temporary band-aid though, but I can't afford to pull the motor apart to do any further investigation.


anyway, it seems to be getting a lot better.

 

do you have cold air intake? that will lower octane requirements!

 

nope. Just an open air filter lid with K&N.


I did some more tweaking in the timing tables with the eprom last night. This morning on the way to work with Diacom I hardly saw any knocking at all. The car seems to run the same by SOTP feel, but I'll know for sure when I get to the strip or dyno if it's really the same or not. I had to take out quite a bit of timing though, which doesn't make me feel good.

The machine shop says they only took .010 off the heads, and with smaller compressed height head gaskets I don't see those two changes needing 10 degrees of timing taken out! :eek:


I'm wondering if there is something else going on. It does seem to have developed a slight miss at high rpms at WOT. Around 4000 and above. I don't know if it's ignition or fuel yet. I've got new ignition parts on there, but need to change the fuel filter. I might even need to hook up the fuel pressure guage and tape it to the windshield to make sure it's getting enough fuel at those rpms.

Anyway, the band-aid is applied for now. :(

 

if there is a weak injector, it will ping, miss, and lose power

 

Same if the EGR valve is stuck open.