Engine - Compression - Timing Comparison

[hunt2elk]

I have a question for you engine guys. I am redoing my 69 Road Runner. Had a 451 put together 7 or 8 years ago using Source's kit with 4.380" -4 flat top pistons (measured compression 10.3:1). I had always struggled with a slight bit of detonation, but had it pretty much under control by backing the timing down to 31* from 35* (where it made the most power on the dyno) and slowing down the advance curve. Since the car is totally blown apart, I decided to have the motor taken apart for new bearings and gaskets, and try to lower the compression a bit.
Bought a set of Source's 4.382 -12 dished pistons and kept everything else in the motor the same. With these pistons, the CR measures 9.76:1.
My thinking is that I should be able to advance the timing to 35 and speed up the distributor advance. Also replaced the 750 Quick Fuel with a 850. In my mind, with these changes I wouldn't loose much for power and have the benefit of lower compression.
The new motor was on the dyno last week. The torque stayed the same at 543 ft. lbs. Horsepower was down from 544 to 529. So a loss of 15 there.
The big surprise and the reason for my question is that the motor made it's best numbers at 31* timing. We tried down to 28*, and up to 32, 33, 34, and 35.
How can this be when we are always told max power is at 35* on these motors?
Here are pics of the 2 pistons.

 

[Curiousyellow71]

What heads? I would guess its because of the quench pad on the new pistons. It has a more efficient burn if it makes power with less timing.

 

[dvw]

Depends on which head, piston dome, bore size. My -1 motor runs at 32. Even with a 4.5" bore
Doug

 

[hunt2elk]

What heads? I would guess its because of the quench pad on the new pistons. It has a more effecient burn if it makes power with less timing.

Stealth heads and a .039 gasket.

 

[diesel_lv]

Yes, the better the head design and the way the pistons match the heads, the more efficient the burn. The more efficient the burn, the less max timing needed.

 

[2quick]

My -1 (11.5:1) 440 likes 31° on 93.

 

[MoparLeo]

The factory engine may prefer 35° but timing will depend on the combination. Yours is no longer stock so your engine timing will depend on what c/r, fuel,cam, heads, etc...that you have put together. No book answer to your question . If yours like 32° then so be it...

 

[Curiousyellow71]

Generally speaking on the ones I have been around a open chamber heads like to make power at 36. Domed piston seemed to like 38. Closed chamber with quench will be close to 32.
Interesting dyno run for sure.

 

[4mulas]

@hunt2elk - I’m curious what camshaft you have? Those source heads- are they 84 or 88cc or something like that?

 

[hunt2elk]

Generally speaking on the ones I have been around a open chamber heads like to make power at 36. Domed piston seemed to like 38. Closed chamber with quench will be close to 32.
Interesting dyno run for sure.

Yes, I thought it was very interesting. Same dyno and gas was used on both motors, only several years apart. When the first motor was tested a few years ago, we threw a 850 carb on it and made a couple of pulls. It made 8 hp and 8 ft lbs more than the 750 I was using at the time. So if I wouldn't have went bigger this time, my numbers would have been that much lower.

 

[hunt2elk]

@hunt2elk - I’m curious what camshaft you have? Those source heads- are they 84 or 88cc or something like that?

Cam is a Racer Brown, here is the card on it. I had the builder measure both pistons when in the motor. Going by Source's charts on their stroker page, comparing the CR, they are 79cc.

 

[BSB67]

First, I think final best timing needs to be determined at the track.

What was the difference between the different timing settings? Repeatability and variability should be included in the overall analysis of the results.

Did you compare the horsepower curves (i.e. average hp) from 3500 to 6000 for the different tested timing?

I don't think 32 degrees is an uncommon dyno "best" timing with aluminum heads

 

[diesel_lv]

Here's a little comparison. Ford 4.6 all aluminum DOHC 4 valve head. Makes peak hp at 23° btdc. I run a Holley double pumper blow thru carb w Air to water intercooler in a jetboat. No need to pull timing as the air is super cold. Still peak hp at 23°. The heads w 4 valves are so efficient that they require very little timing advance.

 

[hunt2elk]

First, I think final best timing needs to be determined at the track.

What was the difference between the different timing settings? Repeatability and variability should be included in the overall analysis of the results.

Did you compare the horsepower curves (i.e. average hp) from 3500 to 6000 for the different tested timing?

I don't think 32 degrees is an uncommon dyno "best" timing with aluminum heads

I don't run my cars at the track, just hammer on them 0 - 70.
Their machine was setup to take the average data from 3800-5800.
36* average 520 ft lbs / 473.4 hp
33* average 521 ft lbs / 474.7 hp
31* average 525 ft lbs / 478.2 hp
29* average 519 ft lbs / 472.9 hp
We did at least 3 pulls at each timing setting and they were all very consistent. And as you can see, they did not vary much. But 31 was the magic #.

 

[hunt2elk]

Here's a little comparison. Ford 4.6 all aluminum DOHC 4 valve head. Makes peak hp at 23° btdc. I run a Holley double pumper blow thru carb w Air to water intercooler in a jetboat. No need to pull timing as the air is super cold. Still peak hp at 23°. The heads w 4 valves are so efficient that they require very little timing advance.

When the older guy running the dyno saw these numbers, he commented that some of the Ford motors they were doing were peaking at 26*. I didn't pay attention to which ones though.

 

[cdr]

Different Timing light than used the 1st time ?

 

[383man]

Here's a little comparison. Ford 4.6 all aluminum DOHC 4 valve head. Makes peak hp at 23° btdc. I run a Holley double pumper blow thru carb w Air to water intercooler in a jetboat. No need to pull timing as the air is super cold. Still peak hp at 23°. The heads w 4 valves are so efficient that they require very little timing advance.

But that sounds like a boosted eng ? If so thats a differnt timing ball game the a N/A eng. Ron

 

[hunt2elk]

Different Timing light than used the 1st time ?

No, same old Snap On light as before. Nothing ever changes in this old shop, lol.

 

[diesel_lv]

But that sounds like a boosted eng ? If so thats a differnt timing ball game the a N/A eng. Ron

No, for any N/A 4.6 4V engine, 23° is the max timing for hp. The 3V engines can use 24°. I was just saying that due to my cooling ability under boost, I'm also able to use 23° of timing. My 1250 CC v-twin stock used 34° of timing for max hp, w heavily modded heads, cams, etc... max timing is now 28° for max hp. Heads w matched pistons are very efficient. Require less timing and a much less aggressive curve.

 

[383man]

No, for any N/A 4.6 4V engine, 23° is the max timing for hp. The 3V engines can use 24°. I was just saying that due to my cooling ability under boost, I'm also able to use 23° of timing. My 1250 CC v-twin stock used 34° of timing for max hp, w heavily modded heads, cams, etc... max timing is now 28° for max hp. Heads w matched pistons are very efficient. Require less timing and a much less aggressive curve.

Got ya as I thought you were talking about a boosted eng only. Sorry about my missunderstanding. Ron