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#1
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Looking for maximum safe over bore for R and RB engines. Also, head porting information.
Any help would be appreciated. |
#2
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On your overbore question. It depends on what year block you're working with. I know that in the mid 70's and into the 80's the thin-wall casting proceedure became more of a common practice to save material, weight, etc. If you're lucky enough to be working with a 60's to early 70's block you can most likely go 60 thousanths over wo/ a problem. Later blocks with the thin wall casting will only tolerate 30 thousanths over, w/ 40 thousanths you're really pressing your luck. I'm not exactly sure of when the change over to the thin wall casting actually went into affect so you'd best check with some authority. I just know it happened. If you're trying to gain cubic inches by overboring the result will be minimal. You shouldn't overbore any more than is necessary to get a round cylinder. If you've got say a Hemi or a MaxWedge block and you're trying to save it, but it's seen it's overbore limit already, sleeve it. It's expensive, but cheaper than trying to replace that block.
As for head porting for the street, clean up any imperfections in the intake or exhaust port area and runners with a grinding/porting tool taking out ang flash or metal that will get in the way of flow. You don't have to go bigger, as the bigger you go you will loose velocity. If you're going all-out race, send the heads out to a reputable machine shop that specializes in engine porting for race cars. Again for the street, port match both the intake and exhaust ports. Use the gaskets as a guide. First put the gasket over the ports ( putting them on the alignment pins that hold them in place ) and be sure the gasket material doesn,t hang into the opening. If it does take a utility knife ( or razor blade - becareful ) and cut the excess away while holding the gasket over the port and using the metal port as your guide. Open the ports up (if necessay) to the gasket size w/ the grinding tool and blend your runners into the same size as the ports, tapering from about 1 inch back into the runner to the port opening. This will give a smooth transition from the runner to the port. Put a straight edge to the vertical and horizontal sides of all ports and square up so they are all the same size in height and width. Be sure to use a radius on all 4 corners of all port openings - just use the gasket as a template. Use dykim steel blue ( scribing ink, you spray or brush on ) then use a sharp tip ( a scribe ) to "sketch" out the dimensions you want to end up with, again using the gaskets as a template to scribe with. Porting is to optimize flow by removing restrictions in the ports and runners. Hope this helps, Richard ps I did this to by buddies stock 350 Chevy heads and he dropped a full 1/2 second off his e.t. |
#3
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As far as porting goes what Richard said is true,and I agree with every thing he has said,but as a alternative to sending heads out to be professionally ported, Mopar Performance has porting templates that work real well too.Also some of the late 400 and 440 blocks will only go +.020 oversize due to core shift.(late 77 thru 78 year)
[This message has been edited by Christopher (edited April 21, 1999).] |
#4
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Addition to your porting question, in my answer,I left out to be sure to use the gasket "trick" on your intake manifold and the exhaust headers you plan to use. You want the mating surfaces to come together like this --, not this -_!
Richard |
#5
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Christopher, thanks for the additional comments & info, Richard
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#6
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76 and later big blocks 20 thou max bore.
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