'76 F250 Turbo Build

jgavac

Active member
The DCR number is strictly a reference number we use to determine the 300 sixes tendency towards detonation and it also has relevance to the cranking compression.
We developed a DCR range for the 300 six based on engine build history using the .006" intake lobe closing point.
Any other lobe lift point is meaningless as far as detonation is concerned for the 300 six.

The Cam card I posted shows a .006" (advertised) duration of 268 degrees and a 114 degree LSA.
The .050" intake opening at -5 degrees and closing at 35 indicates the cam is to be installed 4 degrees advanced with the intake lobe center at 110 degrees ATDC.
That puts the .006" closing point at 268/2 + 110 -180 or 64 degrees ABDC.
All timing events are on the card using a little math.

So you would use 64 in the calculator to figure DCR.

The DCR should calculate as 6.6

The Felpro 1024 head gasket has a bore of 4.18 and a .039 compressed thickness.

Got it, this is based on historical data. That makes a lot more sense.

Also I don't have much experience in this realm, so I didn't know that advertised duration was equivalent to "duration at 0.006 lift". Thanks for clarifying that.

As it turns out, the GFM calculator might have an error in a formula somewhere. Putting in all the above values with a valve close at 64° and piston volume of 30cc, it turned back a DCR of 5.85 and not 6.6. This is a good lesson in putting trust in online calculators when you can't see the math working. After going through all the hand calcs, I arrived at the same 6.6 DCR

Piston math.jpg

The two numbers I calculated was 18.9 CC to get a DCR of 7.25, and 30.8 to get a DCR of 6.6. Pmuller, you are saying 6.6 would end up being a good way to go on this build? I can calculate theoretical bests, but I'll trust your empirical recommendations.
 

pmuller9

5K+
VIP
Supporter 2018
Supporter 2021
The two numbers I calculated was 18.9 CC to get a DCR of 7.25, and 30.8 to get a DCR of 6.6. Pmuller, you are saying 6.6 would end up being a good way to go on this build? I can calculate theoretical bests, but I'll trust your empirical recommendations.
Great job on the math.
You could have also used the Law of Sines with the inside angle of 116 degrees (180-64) to get the 5.257" side but it's not any shorter in steps.

If you could cool the intake charge back to ambient after compressing it with a turbocharger then the standard range for DCR would hold true.
Street driven intercoolers are less than 100% efficient so the compressed intake charge is still above ambient and we take at least a 1/2 point of DCR away for a safety margin.

This is the calculator I like to use.
Just enter the actual .006" intake closing point at the last entry.

 
Last edited:

jgavac

Active member
On a completely unrelated topic, I picked up a cam sync sensor for a 2001 3.0 V6 after reading up on modifying one to work. After some comparison to an old distributor, the only thing that doesn't look like it will work is the slightly smaller diameter housing (calipers are still on the way, so I don't have exact numbers... Roughly 5/16").

The drive gear on the sync was smaller, 14 teeth instead 16, but the shaft diameter and pin locations are the same.
20210420_192516.jpg

20210420_193154.jpg

There's a significant difference in length from the mounting face, with the shaft collar on the sensor being much longer
20210420_193251.jpg

But, the oil pump drive shaft slides in to the same overall depth on the sensor as it does on the dizzy, so I'm thinking it shouldn't be a problem.
20210420_194121.jpg

I haven't been able to check fitment with the engine and won't be able to for a while, but I think it'll work.
 
Top