[1965tripleblack]

There is a huge thread on this topic that has been moved to "The Dump". Why?

I didn't have time to go through the entire thread, but I immediately noticed that the duration used for the calc was the wrong duration.

The only way to figure DCR is by using the duration where the intake valve is effectively closed, which is the "seat-to-seat" duration, AKA, "advertised" duration. Comp Cams uses duration at .015" valve lift, and sometimes .012 amd .010 as well. This is known in the SAE as J604d duration, which is duration at .006" valve lift.

Was this ever addressed in the thread?

[F-BIRD'88]

Running seat to seat duration is not the same as Advertised duration. Althou there is a SAE standard.
Nobody uses it. In a strict sense. GM once used .001" .001 .009" "lash point duration" etc.
Just one of the flaws of this DCR theory stuff.

[hcompton]

Yep you got to measure it for most cam or call and ask what the actual measured specs are.

Is .006 or .001 best to use for dcr? When using a dail indicator right on the retainer.

[oldbogie]

Originally Posted by 1965tripleblack

There is a huge thread on this topic that has been moved to "The Dump". Why?

I didn't have time to go through the entire thread, but I immediately noticed that the duration used for the calc was the wrong duration.

The only way to figure DCR is by using the duration where the intake valve is effectively closed, which is the "seat-to-seat" duration, AKA, "advertised" duration. Comp Cams uses duration at .015" valve lift, and sometimes .012 amd .010 as well. This is known in the SAE as J604d duration, which is duration at .006" valve lift.

Was this ever addressed in the thread?

You are pretty much correct on this. The rising piston is applying a lot of pressure to the mixture in a situation not unlike the exhaust side during that cycle. So having the intake closure point at .015 inch and gussing an additional 15 degrees if that's even done is simply not adaquate as the pistong will be forcing much greater reverse flow than one would think if that information was related to intake flow at a depression of 28 inches of water.

Different manufacturer's give absolute degrees in some value other that truely shut valve degrees these are often measured from .001 to .006 inch from the seat. While not "perfect" this is a lot better situation than from .015 inch or guessing that the cam actually seats the valve with an additional 15 degrees as many cams, especially older grinds have extraordinarily long ramps and 15 degrees doesn't get close to true valve closure.

Bogie

[1965tripleblack]

Originally Posted by oldbogie

You are pretty much correct on this. The rising piston is applying a lot of pressure to the mixture in a situation not unlike the exhaust side during that cycle. So having the intake closure point at .015 inch and gussing an additional 15 degrees if that's even done is simply not adaquate as the pistong will be forcing much greater reverse flow than one would think if that information was related to intake flow at a depression of 28 inches of water.

Different manufacturer's give absolute degrees in some value other that truely shut valve degrees these are often measured from .001 to .006 inch from the seat. While not "perfect" this is a lot better situation than from .015 inch or guessing that the cam actually seats the valve with an additional 15 degrees as many cams, especially older grinds have extraordinarily long ramps and 15 degrees doesn't get close to true valve closure.

Bogie

Old flat tappet cams, like the "30-30" have ridiculously large spreads between .050 (254) and seat-to-seat (310) durations.

[bygddy]

Originally Posted by 1965tripleblack

There is a huge thread on this topic that has been moved to "The Dump". Why?

I didn't have time to go through the entire thread, but I immediately noticed that the duration used for the calc was the wrong duration.

The only way to figure DCR is by using the duration where the intake valve is effectively closed, which is the "seat-to-seat" duration, AKA, "advertised" duration. Comp Cams uses duration at .015" valve lift, and sometimes .012 amd .010 as well. This is known in the SAE as J604d duration, which is duration at .006" valve lift.

Was this ever addressed in the thread?

No idea why it landed in the dump, I started it, and wasn't offended, or offending anyone I don't think, course I do ask some stupid questions occasionally....but I was actually learning something sifting through it all.....ah well. Its not my sandbox.....

[thinwhiteduke]

I can't figure why it was dumped either, I learn a lot by reading through these posts too.

F'BIRD-88, this is for you in regard to you question in the 'dumped' thread. I sent to you PM also, before I found this one.

My aim was to achieve 11:1 SCR, or real close to it. If my heads were as advertised at 64cc , that's right about where I would be. The combustion chambers in the heads were measured accurately with the correct equipment by me and my machinist at his shop.

The block, a 2 bolter from a '73 Impala, was stock everything when I tore it down. Never had the heads of, factory.

Specs are now,
Bore: 4.030"
Stroke: 3.48"
Piston to Deck Clearance: .005"
Head Gasket bore: 4.060"
Head Gasket Compressed Thickness: .030"
Pistons: KB171-020 hypers with compression distance of 1.561", not rebuilders. Valve reliefs are marginal at 1cc, actually not even 1cc but I called it that. My machinist and I fly cut the pistons to give minimum piston to valve clearance as I was chasing compression.
Head Chambers: 61cc

Let's know what you get.

Duke

[techinspector1]

Originally Posted by 1965tripleblack

There is a huge thread on this topic that has been moved to "The Dump". Why?

Here's the explanation by Moderator Sam.....

"Rude personal attacks are not tolerated here and I am not sorting out any ones personal complaints. I am dumping this thread so if you guys wish to discuss this topic you can do so in a polite and courteous way.."

Sam
.................................................. .................................................. .................................................. .................................................. ...........
I personally would like to see this subject continued and I promise to be civil. I have no idea who wrote the straw that broke Sam's back and don't care.

Richard

[bygddy]

Originally Posted by techinspector1

Here's the explanation by Moderator Sam.....

"Rude personal attacks are not tolerated here and I am not sorting out any ones personal complaints. I am dumping this thread so if you guys wish to discuss this topic you can do so in a polite and courteous way.."

Sam
.................................................. .................................................. .................................................. .................................................. ...........
I personally would like to see this subject continued and I promise to be civil. I have no idea who wrote the straw that broke Sam's back and don't care.

Richard

Agreed....I'm completely ignorant when it comes to this stuff and am learning a ton.....and will happily keep any sarcasm in check. If you had a clue how much cash I spent 3 years ago on a build that never overly impressed me, and now feel very silly about. You would laugh your *** off. The difference between then, and now, is this place....and these sometimes ridiculously long winded threads that actually contain alot of good info. And I appreciate it. Nuf said.

[cdminter59]

Originally Posted by thinwhiteduke

I can't figure why it was dumped either, I learn a lot by reading through these posts too.

F'BIRD-88, this is for you in regard to you question in the 'dumped' thread. I sent to you PM also, before I found this one.

My aim was to achieve 11:1 SCR, or real close to it. If my heads were as advertised at 64cc , that's right about where I would be. The combustion chambers in the heads were measured accurately with the correct equipment by me and my machinist at his shop.

The block, a 2 bolter from a '73 Impala, was stock everything when I tore it down. Never had the heads of, factory.

Specs are now,
Bore: 4.030"
Stroke: 3.48"
Piston to Deck Clearance: .005"
Head Gasket bore: 4.060"
Head Gasket Compressed Thickness: .030"
Pistons: KB171-020 hypers with compression distance of 1.561", not rebuilders. Valve reliefs are marginal at 1cc, actually not even 1cc but I called it that. My machinist and I fly cut the pistons to give minimum piston to valve clearance as I was chasing compression.
Head Chambers: 61cc

Let's know what you get.

Duke

I hope this is misprint Pistons: KB171-020 hypers. Bore 4.030 anyway the cr. is 11.484.

[bygddy]

Originally Posted by cdminter59

I hope this is misprint Pistons: KB171-020 hypers. Bore 4.030 anyway the cr. is 11.484.

Weird......see bore

[bygddy]

And yet up top......

[techinspector1]

Originally Posted by bygddy

Agreed....I'm completely ignorant when it comes to this stuff and am learning a ton.....and will happily keep any sarcasm in check. If you had a clue how much cash I spent 3 years ago on a build that never overly impressed me, and now feel very silly about. You would laugh your *** off. The difference between then, and now, is this place....and these sometimes ridiculously long winded threads that actually contain alot of good info. And I appreciate it. Nuf said.

On another forum, 1Gary went through a long list of things that have been cussed and discussed in the past couple of years that we hadn't talked about before. I think the collective group of us fellows is actually making a dent in the thought process of younguns who are tackling their first engine build.

[thinwhiteduke]

Originally Posted by cdminter59

I hope this is misprint Pistons: KB171-020 hypers. Bore 4.030 anyway the cr. is 11.484.

Yeah, that's the cr I got too.
Misprint, yeah probably, I just used the 'narrow your results'
column in the search page on summits site and typed in 4.030 and that's what came up. I thought the p/n looked wrong when i typed it in. Anyway the machinist had the pistons before he bored the block so I'm sure it's ok, but I'll check my paper invoice when I get home on the weekend too be sure! Ya got me scared now! Thanks.

Duke

[OneMoreTime]

Here is a good article on dynamic compression ratio:

Classic Inlines - Compression Ratio's

As you can see the dynamic compression is what the engine actually sees in operation. This can be manipulated by valve event timing in other words at what point in the cycle of the cylinder does the valve actually close.

Sam