Detonation?!?

[Bolisk]

No. Detonation sounds like marbles in a coffee can, not a rhythmic clatter like in the video.

Hmm. . .i would never say the noise sounds like "pining" or even marbles in a can. It sounds VERY similar to that video. I was trying to describe it as a drummer doing a series of quick rimshots on the snare drum. . .directly in sync with RPM speed. A fast "tack tack tack" sound.

I wish "pining" and or "detonation" existed as a good audio recording on the internet. . .for reference.

 

[Hib Halverson]

I don't know where the "best place" is for SCE. When I buy their stuff, I buy it direct from them.

 

[Bolisk]

I don't know where the "best place" is for SCE. When I buy their stuff, I buy it direct from them.

LOL!!!! Then that would be the best place!! :L

Clearly you trust their products. I've never heard of them. . .but that's not surprising.

 

[Peer81]

So hopefully for you detonation isn't your problem. I would say big thanks to Gerry for posting the vid!

Greetings Peter

 

[Bolisk]

So hopefully for you detonation isn't your problem. I would say big thanks to Gerry for posting the vid!

Greetings Peter

Yes!! Big Thanks!!! Just need to confirm and fix. Hopefully that's it.

 

[Hib Halverson]

LOL!!!! Then that would be the best place!! :L

Clearly you trust their products. I've never heard of them. . .but that's not surprising.

They make great high-performance and racing gasket stuff.
Sce Gaskets | Premium Gaskets for Racing | Street Performance | Engine Service

Another vendor of very high end head gaskets is Cometic.
Cometic Gasket, Inc.

In fact, looking at Cometic's latest offerings for the 18/23° SBV8, they have head gaskets .080 thick and even thicker. In fact, they have a gasket for SBV8 which is a 4.060 bore and .140" thick.

So, I was wrong about gaskets thicker than .060. I need to do better research.

Only problem is, while Cometic's stuff is very good, it's also pricey. Figure almost 500 bucks for a set of those really thick gaskets.

 

[Bolisk]

Only problem is, while Cometic's stuff is very good, it's also pricey. Figure almost 500 bucks for a set of those really thick gaskets.

YIKES!!!!!!

But if it lets me keep my original equip stuff, it might be worth.

Is there any known cranking psi to compression ratio charts? I'm guessing I will only get one shot to get the right. My current cranking psi is close to 220. If 220 was say 11:1, then maybe I can guesstimate how thick a head gasket I would need to get down to 10.5:1 if in fact detonation is my problem.

 

[Bolisk]

Check my logic please.

So I'm going to try nearly all of the suggestions that were given. In the event however that there does not appear to be an issue with my distributor, balancer, or timing chain. . .I think I'm back to too high of compression. Which means I have to work on reducing pressure.

I've done some research, and according to several publications, anything over 200 cranking psi is asking for trouble on premium pump gass. Anyone agree / disagree with that?

I previoisly reported that my cranking psi was 220. What I failed to remember at that time. . .was I was at 220 before I had my hardened valve seats done. . .which is when the heads were milled.

So I'm gussing that I'm actually over 220 at this point. So if I wanted to lower compression we talked about using a thicker head gasket, and that one company makes very thick gaskets at a high cost. So I may have a way out of this without having to buy new heads.

Now, obviously. . .I want to get closer to 200 psi or even lower if possible. I don't want to buy a new head gasket at high cost, only to find that I was way off on my guesstimate. . .and therefore blowing too much money on an expensive head gasket.

So I was thinking of the following test and wanted to run it by every one.

1) Test cranking pressure with engine cold
2) test cranking pressure with engine hot
3) calculate the % difference between the two numbers in order to detemrine the ~ cold cranking pressure equivelent from the hot number.
4) Pull one head off, and start stacking cheep head gaskets until I get a cold cranking pressure that equates to something close to 200psi hot using the percentage difference found in step three

It's important to note that I would never RUN the car with stacked stock gaskets. . .just used to determine cold cranking pressure. Would the gaskets seal well enough for this basic test?

I think this would allow me to get a ball park guess of what compressed thickness head gasket I would need to lower the psi to a number more in line with use of 93 pump gas.

Is this a reasonable approach to getting close to where I want to be in what I hope would be one expensive head gasket purchase?

 

[Vettehead Mikey]

Have you checked the timing yet?

 

[Bolisk]

Have you checked the timing yet?

Not yet. I plan to, but with two kids, (ages 3 and 1) it's hard to get garage time. Stay tuned. . .I will update when I get a chance.

I was just trying to think about plans for what if scenarios.

 

[Hib Halverson]

YIKES!!!!!!

But if it lets me keep my original equip stuff, it might be worth.

Is there any known cranking psi to compression ratio charts? I'm guessing I will only get one shot to get the right. My current cranking psi is close to 220. If 220 was say 11:1, then maybe I can guesstimate how thick a head gasket I would need to get down to 10.5:1 if in fact detonation is my problem.

When dealing with detaonation, don't get too caught-up with cranking pressure. It's dynamic cylinder pressure–a combination of static compression ratio, camshaft profile, spark timing and rpm–which affects detonation.

For example you could have an engine with low cranking pressure but which detonates like crazy due to a high level of dynamic cylinder pressure. Conversely, you can have an engine with killer cranking pressure but no detonation due to a really wild camshaft and lazy spark timing.

Yes, it is possible you have some carbon deposits raising your cranking compression, but, when you change head gaskets, you'll address that.

There are places on the Internet with formulas by which you can determine your actual CR if you know combustion chamber volume, cylinder volume and head gasket volume.

Get down to 9.75-10.0:1 and you'll be rockin' on pump gas.

My last Gen 1 SBV8 was a 400 with stock crank and rods, forged 9.75:1 BME pistons, a very mild crane roller, L82 Corvette heads fitted with 2.02-in intake valves, 1.6 Crane Race Rockers, headers and Rochester fuel injection, it made 425-hp@4900 rpm. On that engine I used Cometic head gaskets.

The engine was not in a Corvette but in an old 65 Malibu. In 09, confronted with a big bodywork bill to derust the car and paint it, I parted it out. The engine was sold to a guy in Arizona who put it in an '82 Vette. Far as I know, he's still got the motor.

 

[Peer81]

If detonation is the problem why not changing the cam for a bigger one so the DCR goes down?

Greetings Peter

 

[Bolisk]

Understood.

Looking at the gasket maker, the biggest size I can get would be .093 with a bore of 4.060 and not requiring o-rings. I think that might reduce me by close .75 of a ratio point. So maybe near 10.25:1

Sound right?

Any chance I would have issues with my intake fitting right?

 

[Bolisk]

If detonation is the problem why not changing the cam for a bigger one so the DCR goes down?

Greetings Peter

I was under the impression that the lt-1 cam that I already have is plenty big for street use. Perhaps not. Secondly. Easier and hopefully cheaper to put thicker head gasket in.

Having said that still need to try the other things mentioned in earlier posts.

 

[GerryLP]

I agree with Mikey. Just either prove or eliminate timing as the culprit, and then target another posibility.

Let's pretend you discount the timing as the cause, and you decide to pull the driver's side head, well then you are in prime territory to measure the CR on your engine. Don't go by,"advertised" values. Measure the volume displaced by the piston, and have the head chamber volume measured. Then calculate the CR.

And if all is GTG, then perhaps you can improve the dinamic CR and change the cam, like Peter suggested.

Having said this, if the engine was running fine, but it now detonates, then the likelihood is that it would be a relatively simple issue.

Good luck!

 

[Hib Halverson]

If detonation is the problem why not changing the cam for a bigger one so the DCR goes down?

Greetings Peter

Sure you can put a monster cam in the engine trying to reduce cylinder pressure and you may have some success but you also will move the power and torque peaks higher and closer together and that will make the engine hard if not near-impossible to drive on the street.

Given the engine is properly timed and the distributor is not faulty, the right way to get this engine to run well is to reduce cylinder pressure and the best way to do that is to reduce the compression ratio to about 9.75:1

Any other crackerjack ideas are "bandaid" fixes.

To figure compression ratio you need to know the bore and stroke of the engine, the head gasket volume and the combustion chamber volume. The first two can derived with geometry if you know the bore and stroke of the engine, the second number can be obtained from the gasket manufacturer and the third needs to be measured, ie: one chamber needs to be "cc'ed".

 

[Bolisk]

To figure compression ratio you need to know the bore and stroke of the engine, the head gasket volume and the combustion chamber volume. The first two can derived with geometry if you know the bore and stroke of the engine, the second number can be obtained from the gasket manufacturer and the third needs to be measured, ie: one chamber needs to be "cc'ed".

The bore is .030 over stock at I think 4.030
The stroke is stock LT-1 ???
Current head gasket is .041 thick with a 4.060 bore
The head CC will probably be different for each cylinder. Remember, some machine shop guy hand clearanced the intake and exhaust valves for better "flow".

This raises a good point. Suppose I check all CC of all chambers. How close should they be to one another. If they are not. . .should I get the die grinder out and try to make them all the same? I'd be surprised if whoever did this CC'ed the head chamber to see if they all matched.

 

[JohnZ]

The head CC will probably be different for each cylinder. Remember, some machine shop guy hand clearanced the intake and exhaust valves for better "flow".

You need to find out what's inside that engine. 220 psi cranking pressure is "off the charts", assuming it was checked properly, with the plugs removed, engine warm and the throttle held wide open; normal cranking pressure is 160-170 psi, and even the 302 Z/28 with 11:1 compression and domed pistons is 180-190 psi (and that iron-head 11:1 Z/28 engine will run all day long on pump premium without detonation due to the late intake valve closing with the "30-30" cam).

You also need to map the advance curve so you know:

1. Initial timing (vacuum advance disconnected and plugged).

2. Centrifugal advance (when it starts, how much is max, and at what rpm that occurs), also with the vacuum advance disconnected and plugged.

3. Is the vacuum advance working properly (how much advance is it adding, and at what level of manifold vacuum), and what is your manifold vacuum at normal idle.

This is all basic tune-up stuff, and you need a dial-back timing light and a vacuum gauge to check it out, which you need anyway to maintain a 40-year-old car. I wouldn't spend a dime on head gaskets or pulling cylinder heads or any other internal parts until you understand what you're dealing with. The Comp rocker arms don't have anything to do with it, and you didn't need them anyway.

 

[Hib Halverson]

"JohnZ" brought up an interesting point when he said...

You need to find out what's inside that engine. 220 psi cranking pressure is "off the charts", assuming it was checked properly, with the plugs removed, engine warm and the throttle held wide open(snip)

Describe how you conducted the compression test discussed earlier in the thread.

 

[Bolisk]

Exactly as described.

Car hot, all plugs out, screw driver in carb to WOT. Started at 1 and cranked engine with starter until needle stoped moving. I proceeded through all cylinders until I found one that was very low when compared to the others. At the time I was trying to diagnos a rough idle and was looking for a bad cylendar. All read between 205 and 220 except for one that was 176. Did a leak down test after and found a bad exhaust valve on that bad cylinder.

Pulled the heads, had hardened valve seats done, shop would not ware tee the work without milling the heads. So they did that. I reassembled, and she ran fine except for this noise at high rpm under load. I never rechecked compression.

One year later, I had the rocker fail. Put new ones on and noticed that the noise happened earlier in the rpm range somewhere between 3500 and 4000. Lived with it for the summer. Now ready to tackle it. Just waiting for some free time to open.

Thanks for everyone's input to this point.