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Purpleshaft cam
https://www.slantsix.org/forum/viewtopic.php?t=64216		 Page 2 of 2
Author:  Doctor Dodge [ Mon Nov 18, 2019 9:41 am ]
Post subject:  Re: Purpleshaft cam
Vote # 2 for taking the time and effort needed to mock-up, measure, calculate and degree-in the camshaft position.
This work shows you the effective compression ratio or "running" compression and small changes have a big impact on this number.
I target 8 to 1 effective compression on most street SL6 engines I build and reduce that to 7.8 for heavy / low fuel quality engines.
I use the compression calculator at:
https://uempistons.com/p-27-compression ... lator.html

Here is an example from a "street" SL6 225 engine I just built for a 1966 Dart:
54cc head chamber (head milled .045)
.038 head gasket thickness (compressed Fel-Pro)
-.115 Deck Height (I milled the block .065)
52 degree ABDC intake valve closing point (IC)
= 7.817 effective compression ratio.
I advanced the cam 2 degrees to get a 50 degrees IC:
= 7.904 effective compression ratio. (8.962 static compression)

Lets run the calc. with a -.080 Deck Height. (mill .100 off the block)
= 8.311 effective compression ratio. (likely to ping on pump premium)

These days... I spend all my time on finding & setting the engine's effective compression ratio and the static compression just comes out where-ever it ends-up at.
DD
Author:  Mikedodge [ Mon Nov 18, 2019 10:34 am ]
Post subject:  Re: Purpleshaft cam
I am now convinced to check it all out before doing any machining.
I think I'm just gonna reuse my stock cam for this and was wondering if there is any difference in the stock piston brands of compression height in the block since i should have this block bored anyway.
Author:  Charrlie_S [ Mon Nov 18, 2019 4:45 pm ]
Post subject:  Re: Purpleshaft cam
Quote:
was wondering if there is any difference in the stock piston brands of compression height in the block since i should have this block bored anyway
There are some "low" compression replacement pistons being sold online. Be carefull what you order.
Author:  DadTruck [ Mon Nov 18, 2019 5:35 pm ]
Post subject:  Re: Purpleshaft cam
the .030 Silvolite pistons that I used in the rebuild of the 83 D150 had the compression height set back to, keep the engine compression the same as stock with the .030 over bore.

what I did was have the block / bottom end built up 100% prior to doing any cylinder head work. I did have a 'clean up cut' on the block head surface, after the block was done, measured piston recession and calculated what the head needed to be cut to hit the compression ratio that I was after.

this pulls the process out, but you can set a target and order machining to fit exactly what you think is best.

a question for you
exactly what is the 'stock' cam and motor?
Author:  Mikedodge [ Tue Nov 19, 2019 4:22 am ]
Post subject:  Re: Purpleshaft cam
I have a few engines here i could use. I have a 1977 1969 and 1970 . I took the head off the 1977 and it has a big ridge in the top so i might check one of the others.
Author:  MJF [ Wed Dec 18, 2019 4:55 am ]
Post subject:  Re: Purpleshaft cam
Quote:
52 degree ABDC intake valve closing point (IC)
= 7.817 effective compression ratio.
I advanced the cam 2 degrees to get a 50 degrees IC:
= 7.904 effective compression ratio. (8.962 static compression)

DD
Quote:
Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees
Do you measure intake valve closing point fully seated? Or 050 lift plus 15 degree as that calculator says?
Author:  Doctor Dodge [ Wed Dec 18, 2019 10:40 am ]
Post subject:  Re: Purpleshaft cam
I measure the actual contact to seat point, with the valve clearance (lash) set to where I plan to run it.
I also do a second check, with the lash set at the end of the "lash ramp". This check shows the best effective compression you can get, before the valves start to "tick".
DD
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