Test drove new heads, headers...finally

I don't know the answer to your question, but I wanted to post a comment.
72 was 8.5:1 while the 75 was 8:1, but I don't think there is a difference in pistons or chambers. Not until 76 for the chambers. However 75s, although identical to 72s, had more emissions plumbing to gum up the works.
Wasn't 73 when Ford used the taller deck height?
If so, that might be where the .5:1 compression disappeared to.
Anyway, back to your problem...
A low compression engine can detonate too under certain conditions.
Too high heat range on the spark plugs.
Too much carbon on the pistons or chambers.
Casting flash on the chambers or sharp edges on the pistons.

The carbon problem is common. You can have just a little and the carbon produces hot spots that preignite the mixture, or you can have a lot that actually takes up space. This will cause a higher compression ratio by virture of taking up CCs in the chamber or filling the piston eyebrows/dishes.

You have a good thought about the timing gear being retarded.
It also could be the added .020" deck height of the 74 up blocks. That is about 5cc added to the chamber.
I was told that high advance, like the 22* you mention, is okay, but you must disable your vacuum advance. Otherwise you slow down on the second half of your power range. No proof, but it sounds reasonable.

That cam you have there is what is known as a compression builder.
It does nothing to raise your physical compression ratio, but it has duration and timing that do not allow much pressure to bleed off, like a bigger cam.
It is PERFECT for a low comp engine in the 7.5-8.5 range. It will make your engine run like it has another point.
I didn't realize you were running such a small cam.
I ran a similar setup in a Sprint once.
9.1, 214 @ .050" cam, stock verter, 3.00 gears, and T289.
Folks were always commenting the car was way too strong to be so mild, or I would get comments about the high gear, small cam, mixed with the DP Holley and single plane. Folks would say it was way too mismatched, then they changed their mind when they saw/rode in it.
I love how light our cars are!

Dave

 

Earl, I'm thinkin 8.5:1, same as ratio411. He beat me to it.

ratio411, the 5cc added due to the higher deck is accounted for. Something in the math has to be wrong or something. Could be the gasket thickness isn't right.

Disabling the vacuum advance makes sense, I got better performance when the timing was set at 12-14* initial than at 22*. At 22* it really pulled off the line, kinda levelled out quickly. I didn't know as much about the timing then as I do now and I'm still working on that one.

 

Scooper, your cam is the exact same specs as the Ebrock Performer +. That's what I'm running, too. I added 1.7:1 roller rockers to the GT40Ps, so valve lift is now .502/.478, right where P heads like it.

 

I have been told previously that it is similar to Performer cam. I do need to invest in those 1.7 rockers, I think my stock ones are only 1.6?

Guys, I am glad that in my younger days of stupidity (last year!) I made a mistake that actually turned out OK! Sometimes, you can be lucky just flying by the seat of your pants.

 

The Edelbrock cams are made by TRW...

Summit house cams are TRW grinds...

Are we seeing a connection?


Now that I think about it:
I believe TRW owns Federal Mogul...
Federal Mogul owns Carter...
Carter makes Edelbrock carbs...
Maybe Edelbrock was bought by TRW?
I know the Edelbrock family is still involved, but they might just be figure-heads, or maybe the sale contract stipulated jobs for all of them.
I have seen it before.
Dave

 

Oh, I was also going to comment on putting 1.7 rockers on the car, but I forgot and was beat to it!

It is a good idea.
Some engine builders and cam grinders swear by low duration and high lift.
Then there is another camp that like long duration and low(ish) lift. But you need decent compression for high duration figures.

 

duration vs lift really depends on heads. It takes more duration on stock heads than on aftermarket. Lift over .5 seems to be wasted on stock heads.

Longer duration cams keep the valve open long enough for the air to enter the combustion chamber at low volume. With big heads, you don't need this, so you make more power at a lower RPM with lower duration and high lift.

 

Correct.

Correct.

Correct.

Incorrect.

Edelbrock simply buys cams and carbs from Federal Mogul. It's a contract deal. Edelbrock is still wholly owned by the Edelbrock family. And no, Edelbrock can't buy TRW. Intake manifolds would have to be $6000 each in order for that to happen.

 

I didn't know, just guessing.
TRW has their fingers in everyone's pies, so it just seemed possible.
Especially considering how cozy TRW is with Edelbrock.
Dave