Timing or Compression?

[BIGREDRASA]

IIRC, it was early 1974 when Uncle Sam mandated a universal switch to Unleaded gas. That meant an immediate drop in RON (Research Octane Number). I was driving a 1969 Torino Squire with 351 4V. It immediately started pinging, unless I retarded the timing. Then it ran like a three-legged dog.

After some research, I figured that replacing the flat-top pistons with the 2V engine's dishes would drop the compression. Much to my surprise, it ran better than ever. Other than lapping the valves (lightly), there was no other change made.

One thing that came to mind is that the dished pistons unshrouded the valves and gave improved breathing, and possibly better flame propagation.

Ever since, I've wondered about the fine line where increasing compression while using regular gas results in less power, because of the need to retard timing. I have the questions, does someone have non-technical answers?

 

[addo]

From a "simpleton's" point of view, I don't think there is a ready calculation or rule of thumb that covers most occasions. It's going to rely on interpretive skill of the engine builder and the motor's ultimate application.

 

[rbohm]

8) in my opinion flame propogation with dished pistons is slower than with flattop pistons thus preventing a pressure spike that causes detonation. with open chambers being detonation prone due to no quench in the chamber, reducing the pressure spike is a big help. how much a drop in compression is needed is hard to pin down, but a 1/2 point reduction is close enough that a litte extra ignition timing can make up the difference of the loss of compression pressure. remember though that valve timing plays a part in this as well.

 

[BIGREDRASA]

I had a problem with a coil that would heat-soak, and kill the engine unexpectedly. I took the Torino to a shop with an engine dyno, to see if they could find the problem. They didn't, but did report 190 HP at the rear wheels, on 89 RON unleaded. That was one sweeeeet motor. 8)

(I solved the coil problem by pouring cold water on the coil when it died on a hot day.)

 

[80broncoman]

IIRC the 4bbl 351W in 69 was like 10.5 to 1 . The 2 bbl pistons didn't drop all the way to 8.5 like one might think.
To reduce compression in the later years ford did many other things to reduce the compression including in 71 going to a .023 taller deck height. And of course in 75 they really opened up the chamber size in the heads.
The cam timing I am sure was changed also. which caused all problems is a real mixed bag I'd think.

 

[BIGREDRASA]

I'm pretty sure you have the compression #s corect. Whatever CR I wound up with by replacing the pistons, it was enough to allow the timing to wake the engine up once more.

 

[XPC66]

I think you'll find that by installing the 2V pistons you dropped the compression ratio from CR11 to CR9.5. In the process you would have moved the peak power and torque into a more driveable range i.e:

Stock figures were

351W4V 300 BHP @ 5,400 RPM 380 lbs./ft. @ 3,400 RPM
351W2V 250 BHP @ 4,600 RPM 355 lbs./ft. @ 2,600 RPM

but you presumably elected to keep the 4V carb, head, etc so effectively staged a 2V.

 

[BIGREDRASA]

I think you'll find that by installing the 2V pistons you dropped the compression ratio from CR11 to CR9.5. In the process you would have moved the peak power and torque into a more driveable range i.e:

Stock figures were

351W4V 300 BHP @ 5,400 RPM 380 lbs./ft. @ 3,400 RPM
351W2V 250 BHP @ 4,600 RPM 355 lbs./ft. @ 2,600 RPM

but you presumably elected to keep the 4V carb, head, etc so effectively staged a 2V.

The whole point is that the engine would not produce advertised Torque/HP on 89 RON gas with the 10+ CR, and "correct" advance; but produced very good power and economy when I reduced the CR and increased the timing. Hence the question. Will you really gain significant performance simply by increasing the CR without higher octane? Said another way, increasing CR only gains power if you use a higher octane #.

 

[XPC66]

And what I was trying to say is that by doing what you did, the peak power may have dropped, but the torque/power band where you drive has come back up to near stock levels.

By running the 89 on the CR11 setup, you probably had at least incipient knock and poor combustion characteristics.

 

[American Thunder]

As a rule of thumb, 9.5-9.75 with iron heads is about as much compression as you can get away with on pump gas. You can go 1 full point more with aluminum heads, due to aluminum's greater rate of thermal transfer. And you can go slightly higher yet if the cam is very aggressive.
A total timing advance of 34-40 degrees is ideal. If you have to set the timing back to less than 34 total advance to avoid detonation, you're running too much compression. (or fuel that's too wimpy, depending on how you look at it)

If your engine had 11:1 stock, it would take something like 100-102 octane to run it properly with iron heads. Moving the compression ratio down to 9.5:1 puts the engine back into the realm of regular unleaded gas, but it also moves the peak power and torque up in the rpm range. (and reduces both, along with efficiency/mileage)

 

[BIGREDRASA]

As a rule of thumb, 9.5-9.75 with iron heads is about as much compression as you can get away with on pump gas. You can go 1 full point more with aluminum heads, due to aluminum's greater rate of thermal transfer. And you can go slightly higher yet if the cam is very aggressive.
A total timing advance of 34-40 degrees is ideal. If you have to set the timing back to less than 34 total advance to avoid detonation, you're running too much compression. (or fuel that's too wimpy, depending on how you look at it)

If your engine had 11:1 stock, it would take something like 100-102 octane to run it properly with iron heads. Moving the compression ratio down to 9.5:1 puts the engine back into the realm of regular unleaded gas, but it also moves the peak power and torque up in the rpm range. (and reduces both, along with efficiency/mileage)

You're getting at what I was after. To rephrase my question: "If you're running 89 RON with an iron head, which will give better driveability and/or average horsepower improvement over the 2000-3000 RPM range: 1) An increase in compression that requires a timing retard, or 2) lowering compression and advancing the timing? I tend to believe that for a given load/speed, the settings that give better MPG are those that are more efficient. Again, in my case, in '73 when the octane dropped, and there was no longer 100 RON available (other than aircraft or racing fuel), a 9.5 CR rendered "better" performance than 10.5 CR with retarded timing.

 

[American Thunder]

Again, in my case, in '73 when the octane dropped, and there was no longer 100 RON available (other than aircraft or racing fuel), a 9.5 CR rendered "better" performance than 10.5 CR with retarded timing.

That's correct.