Detonation?!?

[Bolisk]

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.

Inital timing, with the vacuumed advance off and plugged, with dial back timing light was set to 12 degrees when I first bought the car. After valve job, and noticing the problem I retarded it to 10 then 08.

I have to recheck the vaccume, but I think it was slightly lower than a normal l46 engine. Like 15. When I was looking at he rough idle issue, way back when, the vaccume needle was bouncing arround, which lead me to the compression and then the leak down test. After the valve job, I did recheck the vaccume and it was rock steady.

Yeah I know, I got suckered into the extra 15 hp hype with those comp cams.

 

[Bolisk]

Just checked my notes.

Pistons are TRW L2304 .030 over.

The numbers were stamped into the dome of the piston.

 

[Hib Halverson]

Just checked my notes.

Pistons are TRW L2304 .030 over.

The numbers were stamped into the dome of the piston.

That's a +.030 replacement piston for a '70 LT1. With a 64 cc chamber and a .041 head gasket, you should be around 11:1.

Next question is...why is the compression pressure so high?

I believe you said earlier that the heads had been milled. Do you know how much? If not, you need to CC all the chambers and find out what the "average" chamber displacement is.

 

[Bolisk]

I believe you said earlier that the heads had been milled. Do you know how much? If not, you need to CC all the chambers and find out what the "average" chamber displacement is.

No I don't. The machine shop forgot to write it down. ad

May be worse that I know, because the engine had been rebuilt twice by the previous owner. Once when he spun a main bearing. Second one when he converted it to lt-1. Heads could be way down.

I will test compression tommrow and report back.

 

[GerryLP]

Bolisk,

If you measure the chamber volume and piston displacement, you'll be able to approximate the amount of milled head surface. Of course, on the assumption that it was a true 11:1 CR.

I was looking in the web for a calculator I came across sometime ago that would return to you how many thousands of an inch a head had to be milled to achieve a particular chamber volume, but could not find it. Instead, I found the following article explaining surfacing of cylinder heads and engine blocks. While reading it, it reminded me that a 1986 Toyota pickup I restored for my daughter 7 years ago, needed a head saver metal shim to restore the proper head height and chamber volume. So, another option for you is to CC the heads and get one of those metal shims for cylinder heads.

Common Mistakes to Avoid When Resurfacing Cylinder Heads & Blocks: Engine Builder

 

[Bolisk]

Is there a cheap, or DIY head cc measuring tool out there?

 

[Hib Halverson]

Ok....it's time for me to eat some crow in re: this thread.

Earlier I said....

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. (snip)

Considering the high cranking pressure number, that was bad information. I should have added that the above is a valid point only if the engine's cranking compression pressure is within specs.

When I made that statement, I assumed the 220 psi you saw on your compression gauge was acceptable and I made that assumption because my service information only goes back to 1971, the first year of low compression engines. That is, I did not know what the factory spec for cranking pressure was for a LT1 from MY70.

Fortunately "JohnZ" knows more about some of this stuff than I and has service data going back much farther than I.

If 220 psi is "off the scale", then what I said earlier about static compression ratio vs. dynamic cylinder pressure doesn't apply and the first problem which must be investigated is why the cranking pressure is so high.

Prior to hearing what's inside the engine, I'll offer the following speculation...

1) Cylinder head milled
2) Block decked
3) Compression test done improperly.
4) Compression tester faulty.
5) Pistons incorrectly identified (ie: higher compression ratio than originally thought)

I do disagree, somewhat, with "JohnZ's" statement about 70 LT1s being "immune" from detonation because of the late exhaust closing of the camshaft. While it is true that a late exhaust closing will kill some cylinder pressure, in a practical sense, if that were true to the extent implied by "JohnZ's" statement, I'd have thought that, back in the day, one could have run those engines on lower octane fuel, but GM recommended premium gas for LT1s. Back in 1970, "premium" ranged from 100-103 research octane, which, today, might be around 96-98-oct (R+M/2 method).

With the best fuel available in the area which I believe the OP resides being 93-oct (R+M/2) and his ECT being posted as around 200, my belief is even if the engine had "normal" cranking pressure, with 11:1 compression, given optimum fuel and spark delivery, the engine may tend to detonate between peak torque and peak power when run on fuel with less than 95-oct. Even if the engine might not have a problem with knock, I'd think you'd be better safe than sorry and either spike each fuel load with a little racing gas or lower the CR just a bit.

If I had to run the engine on pump gas, I'd rather give away a little compression in exchange for being able to run an aggressive spark and fuel curve rather than having to run the AFR a bit rich and the spark curve somewhat conservative in order to live with the 11:1 compression.

 

[Vettehead Mikey]

GM recommended premium gas for LT1s. Back in 1970, "premium" ranged from 100-103 research octane, which, today, might be around 96-98-oct (R+M/2 method).

The only station around here that sold gas higher than 100 in octane back then was Sunoco. At other stations, 'premium' was 98 octane. These cars ran just fine on 98 RON- which is today's 93 AKI. The only Corvette requiring 103 RON was the L88.

 

[Hib Halverson]

The only station around here that sold gas higher than 100 in octane back then was Sunoco. At other stations, 'premium' was 98 octane. These cars ran just fine on 98 RON- which is today's 93 AKI. The only Corvette requiring 103 RON was the L88.

Ok.
I stand corrected.

Put 93 in the car, run it hard and post here if it detonates. I'd be interested to know.

I would add, however, that back then, Aamoco, Chevron and perhaps other brands both sold a 100-103 RON fuel.

Lastly a question...for a 98 RON fuel to be a 93 using today's rating system, it would have to be also be an 88 MON fuel. A 10-oct number spread is pretty large. Are you sure of your facts on a 98 RON leaded premium being the equivalent of 93-oct (R+M/2) unleaded? If you're sure, please direct me to the documentation so I can read-up and be better informed.

 

[Vettehead Mikey]

Lastly a question...for a 98 RON fuel to be a 93 using today's rating system, it would have to be also be an 88 MON fuel. A 10-oct number spread is pretty large. Are you sure of your facts on a 98 RON leaded premium being the equivalent of 93-oct (R+M/2) unleaded? If you're sure, please direct me to the documentation so I can read-up and be better informed.

Sure- about half way down the page is a listing of many fuel in the 92-94 AKI range with a 10 point spread between RON and MON

Octane rating - Wikipedia, the free encyclopedia

 

[GerryLP]

Question: In the spirit of experimentation, would a bottle of octane booster (Octane 104+ I think it is called) sufice for testing whether the engine detonates or not?

 

[Hib Halverson]

Sure- about half way down the page is a listing of many fuel in the 92-94 AKI range with a 10 point spread between RON and MON

Octane rating - Wikipedia, the free encyclopedia

Virtually all of those are current gasoline blends. I think you misunderstood my question. I was asking about your statement...

(snip) At other stations, 'premium' was 98 octane. These cars ran just fine on 98 RON- which is today's 93 AKI. (snip).

The only gasoline which comes close would seem to be...

[TABLE="class: wikitable sortable jquery-tablesorter, width: 1"]
[TR]
[TD="align: left"]"BP Ultimate 102 - now discontinued"[SUP][14][/SUP][/TD]
[TD]102[/TD]
[TD]93–94[/TD]
[TD]97–98[/TD]
[/TR]
[/TABLE]

Looks like a four pt. spread between RON and MON for that, but, admittedly, that's a "super premium" not a "premium".

Do you have any better information on the leaded premium fuels of that period? That is 98 RON leaded fuels which were 88 MON?

I, also, am trying to find an answer. I have a friend who works for a racing gasoline blender and who worked for Union Oil back in the day but I'm having trouble reaching him.

 

[Bolisk]

I added 4 gallons of sonoco 100 and the issue got better. I used to run on 93 with no issues. Now it makes the noise under load past 3500 rpm on 93.

 

[Hib Halverson]

I think, between all who've contributed to this thread, you know most of the things which need to be done to find out what problem your engine has.

It might be time now to work on getting some answers.

I'll be interested to come back here in a few days and see what you found in checking out your distributor.

 

[Bolisk]

Just got some time in the garage.

I got the car good and hot, made sure it was idling at 750 using an analog inductive tach.

Pulled the cap, looks good, rotor looked good. Pulled the rotor and checked the mechanical advance.

Fist issue, the weights would not move smoothly or snap back like I would want it. Springs looked ok. Lots of red dust, maybe rust?. I used electrical parts cleaner and compressed air, and got it all clean. After cleaning, it worked much better.

Is there a grease I should use on the weights to keep them moving freely?

After that I ran the engine again to set the initial timing. Checked idle again, and then set initial timing at 12 degrees. Note, vaccume advance was off and plugged. After i got that set and I locked down the distributor, I rechecked to make sure it was still 12. Then I reconnected the vaccume line. Checked timing and it was 26.5 with vaccume. Note the vaccume advance is stamped with 360, and not 355 that I have seen in parts catalogs.

I did not get a chance to drive the car after. I will do that tomrow.

I turned the engine off, and pulled the plugs. They all look ok.


I wedged several screw drivers into the cab to hold WOT. Started at 1 and took compression readings on all cylinders. Here is the results:

1) 190
2) 200
3) 190
4) 205
5) 200
6) 200
7) 200
8) 195

Clearly the compression tester I used ( borrowed ) way back when was faulty.

Head gasket is felpro .041

Pistons are the trw L2304 .030

Cam is not 30 30 but the correct lt-1 grind for 1970 according to my notes.

Ok, anything concern anyone so far. I will tell you if I reproduce the noise tommrow based on re checking settings.

Note I have not yet been able to pull the distributor to see if it's indexed wrong or jumped a cog. I will try to do that some time in the near future.

Thanks again for everyone's assistance.

 

[Vettehead Mikey]

Virtually all of those are current gasoline blends. I think you misunderstood my question. I was asking about your statement...


Quote Originally Posted by Vettehead Mikey View Post
(snip) At other stations, 'premium' was 98 octane. These cars ran just fine on 98 RON- which is today's 93 AKI. (snip).

The only gasoline which comes close would seem to be...


Quote Originally Posted by Wikipedia
"BP Ultimate 102 - now discontinued"[14] 102 93–94 97–98


Looks like a four pt. spread between RON and MON for that, but, admittedly, that's a "super premium" not a "premium".

Do you have any better information on the leaded premium fuels of that period? That is 98 RON leaded fuels which were 88 MON?

Hib-

I don't have a time machine. Gas available here was 98 RON and the cars ran just fine on it. No need for 103.

As to the wikipedia link, here's a better example

"Premium" or "Super unleaded" gasoline in US (10% ethanol blend) 97 (RON) 87-88 (MON) 92-93 (AKI)

That's today's gas with the ten point spread you asked about and the equivalency in AKI.

 

[Vettehead Mikey]

Fist issue, the weights would not move smoothly or snap back like I would want it. Springs looked ok. Lots of red dust, maybe rust?. I used electrical parts cleaner and compressed air, and got it all clean. After cleaning, it worked much better.

Is there a grease I should use on the weights to keep them moving freely?

Sounds like a big contributor to the problem. A very light smear of white grease on the pivot points should keep them moving freely.

 

[JohnZ]

I do disagree, somewhat, with "JohnZ's" statement about 70 LT1s being "immune" from detonation because of the late exhaust closing of the camshaft.

To clarify, my post referred to the 302 Z/28 with the "30-30" camshaft. The LT-1 used a very different cam, whose intake lobe closed the intake valve 10* earlier than the "30-30", which built higher cylinder pressure at low-to-medium rpm and made the LT-1 slightly more sensitive to detonation (and built better low-end torque than the "30-30"). However, LT-1's didn't have detonation issues in normal operation back in the day on pump premium either.

After several rebuilds, I suspect the OP's block (and heads) have been milled at least once (maybe twice), and he's probably running higher compression than he might think; the only way to know what the picture really is is to cc the heads and measure the piston-to-deck clearance so a calculator can make sense of the data to determine the static compression ratio. If he knows for sure what the installed cam is, factoring that data into the calculator will also yield the dynamic compression ratio, which is critical to the detonation issue.

 

[GerryLP]

Just got some time in the garage.
....After that I ran the engine again to set the initial timing. Checked idle again, and then set initial timing at 12 degrees. Note, vaccume advance was off and plugged. After i got that set and I locked down the distributor, I rechecked to make sure it was still 12. Then I reconnected the vaccume line. Checked timing and it was 26.5 with vaccume. Note the vaccume advance is stamped with 360, and not 355 that I have seen in parts catalogs...

At the risk of setting off a hijack bomb and diversion of topic at hand, please bear with me. No one else thinks that having 26.5 advance at idle is weird? So between idle and 3,000 rpm only a maximum of 9.5 degrees would be applied?

Bolisk, what was the timing at 3,000 rpm?

p.s. I know that the sweet spot in timing is perhaps slightly higher, such as 32 - 36 degrees, but that could only be confirmed on a dyno or at the track, so my comments do not deny that perhaps 26.5 would be the sweet spot for Bolisk's setup.

 

[Bolisk]

Sounds like a big contributor to the problem. A very light smear of white grease on the pivot points should keep them moving freely.

Actually the part that seemed to be hanging the weights up, was the top of what I would call to rivets underneath the center of both weights. The had wear markes in them.