67 L71, so cal nightmare

[Dave C.]

Re: 67 L71, so cal nightmare

Dave, Sorry about all of the trouble. I've been reading all of your posts. I can relate. Everyone here helped me when I had trouble on the last L71 I worked on.

I just read that you are trying to get a good reading of timing and engine RPM. What type of timing light are you using? I use this one, with a built in tachometer.....
Actron CP7529

http://shop.advanceautoparts.com/p/actron-digital-timing-light-cp7529/9090011-P#

You may be able to get one locally to help your cause and not have to rely on someone reading the dash tach during adjustments.

I wish you all the best to help to solve your problems. Hang in there.

Rich

I just emailed Duke with this info but I will share it here as well.
bought five gallons of 111 race fuel added half of it to a quarter tank of fuel.
pulled a plug , looked a bit white to me so I backed out the mixture screws half a turn.
car wouldn't start despite having lots of fuel and spark
ended up just pouring the fuel to it and it eventually started.
took my wife out for test drive, a couple miles in ( running great cooling at 190*) stopped to set the timing.
mechanical advance starts almost imediately above idle, and is all in around 3500, set idle to 900
drove for a couple more miles, car runs awesome , almost no detonation whatsoever
turned around headed home slowly temp starts to rise, starts knocking, back at the Rv park at 220* idle faltering but it won't die.
so that's where I am at.

[Richard M.]

Re: 67 L71, so cal nightmare

........mechanical advance starts almost imediately above idle..........

I was reading it all and it all seemed good, then.........Rats!

Seems like you're missing a counterweight spring in the distributor.....

or you're seeing a little bit of vacuum advance that's leftover if it's not all in at idle.

I'd pull the dist cap and rotor off and check the counterweights first.

Rich

[Duke W.]

Re: 67 L71, so cal nightmare

The OE centrifugal curve is 0@900, 30 @ 3800, so it sounds pretty close, and Dave has the idle set at about 900.

I think lack of fuel octane for the installed compression ratio is a primary problem. If you go through my 2012 San Diego presentation you will learn that even light, sustained detonation can cause rapid overheating, and heavy sustained detonation can disintegrate a valve and/or turn a piston into a pile of solidified molten aluminum as you can see in the photo. And the hotter the combustion chamber boundaries get, the more severe the detonation. It's a "death spiral" that needs to be corrected before serious engine damage occurs.

Dave is going to call me this evening during the basketball games. We'll go over the history, see where we are now, and figure out the next step. I think we're pretty close. I've helped a lot of guys sort out these kind of issues.

If the centrifugal advance curve on L-71 is OE and a B26 VAC is installed with full time manifold vacuum signal then the spark advance map is near optimum as long as it doesn't detonate. If it does detonate then you either have to run expensive high octane fuel or slow the centrifugal curve with stiffer springs, which is cheaper and more convenient in the long run even though it may reduce low and mid-range torque/power.

Duke

[Dave C.]

Re: 67 L71, so cal nightmare

The OE centrifugal curve is 0@900, 30 @ 3800, so it sounds pretty close, and Dave has the idle set at about 900.

I think lack of fuel octane for the installed compression ratio is a primary problem. If you go through my 2012 San Diego presentation you will learn that even light, sustained detonation can cause rapid overheating, and heavy sustained detonation can disintegrate a valve and/or turn a piston into a pile of solidified molten aluminum as you can see in the photo. And the hotter the combustion chamber boundaries get, the more severe the detonation. It's a "death spiral" that needs to be corrected before serious engine damage occurs.

Dave is going to call me this evening during the basketball games. We'll go over the history, see where we are now, and figure out the next step. I think we're pretty close. I've helped a lot of guys sort out these kind of issues.

If the centrifugal advance curve on L-71 is OE and a B26 VAC is installed with full time manifold vacuum signal then the spark advance map is near optimum as long as it doesn't detonate. If it does detonate then you either have to run expensive high octane fuel or slow the centrifugal curve with stiffer springs, which is cheaper and more convenient in the long run even though it may reduce low and mid-range torque/power.

Duke

Duke , I know my guy in Victoria tuned this car to the original factory specs, I looked them up today be fore you posted them, and if the factory only set the centrifugal curve to 30* in 1967 when they had high octane leaded fuel available at. The pumps, why would I be trying to achieve more advance than that considering the miserable fuel I am trying to deal with. Should I not be setting the mechanical advance at 30* to start with?

[Joe L.]

Re: 67 L71, so cal nightmare

I think lack of fuel octane for the installed compression ratio is a primary problem. If you go through my 2012 San Diego presentation you will learn that even light, sustained detonation can cause rapid overheating, and heavy sustained detonation can disintegrate a valve and/or turn a piston into a pile of solidified molten aluminum as you can see in the photo. And the hotter the combustion chamber boundaries get, the more severe the detonation. It's a "death spiral" that needs to be corrected before serious engine damage occurs.




Duke

Duke------


I think it's a strong possibility, too. As I've said many times before, I do not recommend a compression ratio of greater than 9.0:1 for all cast iron big blocks. Sometimes folks get away with higher compression ratios, sometimes they don't. This may be one of those times they don't. In my opinion, it's just not worth the chance when building an engine.

As I've also said before, GM all cast iron crate big blocks have, for many years, been built with compression ratios of 8.75:1 and require 91 minimum octane fuel. There's a message in that.

Big bore engines are more prone to detonation than smaller bore engines. At 4.25" bore, 427/454 big block engines are about as large a bore as you can get.

[Duke W.]

Re: 67 L71, so cal nightmare

Duke , I know my guy in Victoria tuned this car to the original factory specs, I looked them up today be fore you posted them, and if the factory only set the centrifugal curve to 30* in 1967 when they had high octane leaded fuel available at. The pumps, why would I be trying to achieve more advance than that considering the miserable fuel I am trying to deal with. Should I not be setting the mechanical advance at 30* to start with?

30 degrees centrifugal is about right for a SHP big block. With 6-10 initial you get the proper range of 36-40 total WOT advance at the point where centrifugal maxes out.

Most OE centrifugal curves are lazy i.e. slow, and performance can usually be improved without detonation by installing lighter springs that brings full centrifugal in earlier.

However the L-71 centrifugal is actually pretty aggressive for the more detonation prone big block. The L-72, which is basically the same engine with a different induction system that has no effect on spark advance requirements has max centrifugal of 28 @ 4600 with an initial recommendation of 8. So one approach is to slow down the curve with stiffer springs.

I can probably dig up a selection of springs from some of the guys and get them to the chapter meet where we can try some stiffer springs to see if that helps.

Call me in about an hour. I need more information on how long this engine has been in service and what your experience was driving it around Vancouver Island with their 94 PON fuel. Does it have ethanol? How much by volume. Maybe this is just a "Calilfornia problem" due to our lower octane 10 percent ethanol fuel and higher temperatures than you see in the Pacific Northwest.

I could help if you remove the weights and springs, place them on a white napkin, snap a photo, and post in here. I can sometimes tell if the parts are OE or aftermarket. Also, anyone with a L-71 that has known OE weights and springs can offer an opinion on the appearance or yours.

Joe is correct on larger bore engines being more prone to detonate. In fact due to this issue spark ignition engines are limited to about 6" bore, but I think there might have been a couple of highly boosted radials in the WWII era that were about 6.25". So in order to get more power you have to keep adding cylinders making the engine much more complicated. In contrast, diesel and turbine engines are scaleable. There's no practical limit on cylinder bore or core diameter. That's why modern marine diesels have as high as 36" bores and big jet engines are big enough to walk in the diffuser to the fan without hitting your head.

The mother of all spark ignition engines was the XR-7700. A few were built and tested, but it never got into production. It consisted of nine OHC inline fours on a common crankshaft - 36 cylinders - and was liquid cooled. Design objective power output was 5000 sea level takeoff horsepower. It was designed for the B-36 whose initial conceptual development actually went back to 1940/41 before Pearl Harbor. After several stops and starts the B-36 finally got into production in the late forties and the most powerful engine available was the P&W R-4350 - four row, seven cylinders per row, air cooled. But the "measly" 3500 HP was so little that six of them could barely lift an empty B-36 weight so four J-47 jet engines were added and the fuel systems were modified to burn avgas rather than JP-4.

Duke

[Dave C.]

Re: 67 L71, so cal nightmare

Member Since: May 2014 Posts: 207 Location: Victoria BC Thanks: 43 Thanked 2 Times in 2 Posts

Well I have had some success this morning, after I got the car started ( another issue to be dealt with) I warmed it up and pulled over at my usual spot, reset the timing to 30* mechanical, which gave me 4* initial, just like the factory specs say. No detonation whatsoever, and the car runs and cools great, still seems to have bags of power. But after a bit more driving, every time I come to a light the idle thing starts again, and I have to keep it from stalling. It was already up to temp when I stopped to set the timing, and it would sit there and idle fine. And the other problem is the cold starting, it will not start without pumping the crap out of the accelerator , then once it has started, you an shut it off and restart it with no issue.

[Duke W.]

Re: 67 L71, so cal nightmare

First check that when the engine is cold and you pump the throttle once, there is a discharge from the accelerator pump. Then check the choke valve and choke vacuum break adjustments. It's all in the CSM.

A stiffer set of centrifugal springs will likely allow you to set the total WOT advance in the optimum range. If you set the Total WOT advance to 30 and the initial is 4, then the centrifugal is only 26. I think we are having some nomenclature confusion.

I'm not sure what you mean by "mechanical advance", but when I refer to total WOT advance, which is simulated by disconnecting and plugging the VAC signal line, I mean the sum of centrifugal advance plus initial advance.

The relationship between the onset of detonation and spark advance definitely has a "hockey stick" appearance, so backing down the initial as little as two and certainly four degrees will often fully quell it, but then you may be below the optimum total WOT advance for maximum high rev power. In this case, slowing the centrifugal curve, which reduces total WOT advance at low to mid revs is a better choice.

Detonation is most prevalent at low to medium revs, so stiffer centrifugal springs will reduce total WOT advance in the low to mid rev range while allowing initial timing to be set in the range for best power total WOT advance at high revs.

Duke

[Richard M.]

Re: 67 L71, so cal nightmare

Dave/Duke, What was the final remedy, if there was one?

Last I read said there were still some issues with temperature.

Rich

[Duke W.]

Re: 67 L71, so cal nightmare

I don't know if Dave monitors the TDB, but we swapped an email in late June and he reported that the car is running fine, and he is enjoying driving around Vancouver Island with the top down. Daytime highs up there are usually in the seventies this time of year. The engine starts quickly cold or hot and engine temperatures are within bounds.

Detonation is still an issue even with his 93 PON pump gas. The CR is just too high, and he can only run 31 degrees total WOT advance, which is about one initial, but the full time vacuum advance helps keep engine temperature under control in traffic.

Pulling the heads, measuring CR, and reinstalling with the thicker gaskets to drop the CR to a suitable value is the long range solution. I know John McRae successfully tamed detonation on his L-71 with that method.

Duke

[Richard M.]

Re: 67 L71, so cal nightmare

Thanks for the update. If Dave has TDB notifications set in his profile he should get a email with your reply post. I recall the high CR issue being the ultimate problem and wondered if after using a higher octane fuel it helped reduce the detonation issue.

I also recall that the major helpful change to correct the low speed, off freeway ramp overheating issue was the switch to full manifold vacuum and the appropriate VAC and changing the advance map accordingly.

Thanks,
Rich
ps We fixed that other L71 issue. We replaced that spot welded football TI distributor shaft causing what you suspected was spark scatter.
You were spot on. Thanks very much for that diagnosis and correcting my note that I overlooked. I'll update that thread for future reference later. Runs perfectly now, however we did find a fuel pump problem too. Several cumulative intermittent faults which were tricky to diagnose.