I need some opinions on my current combination of parts for my next build. I have a new in crate short block from mopar performance p/n p4876908 which has h116cp pistons at zero deck. I measured across both banks and got a deck reading from .003 to .006 in the hole on all cylinders. I also purchased some mopar.performance heads p/n p5249574 which the chambers cc at 65cc. They are open chambered iron heads (early swirl port designed - pre magnum) with 2.02 / 1.60 valves. I ran the compression #s and with a .038 felpro it is 10.32 static and 7.66 dynamic. With a .054 felpro head gasket it is 9.92 static and 7.32 dynamic. Do ya'll guys think i will be ok to run on pump 93? Any help would be much appreciated.
Thought I had a good combo but now Im not sure!!
- Thread starter mmjhull
- Start date
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mopowers
Well-Known Member
What cam?
moturbopar
Well-Known Member
Does that short block have the .509 cam?
mderoy340
Well-Known Member
If you are basing your dynamic on the Intake closing event both gaskets will work. Did you CC the heads or is that the printed specs for CCs?
I'm at 8.2 dynamic and 93 is fine with a good ignition curve and carb tune.
I'm at 8.2 dynamic and 93 is fine with a good ignition curve and carb tune.
Yes it is the 508 cam with intake closing spec of 73 degrees (abdc). The heads did cc to 65 cc. That is why i became concern because of the small chambers with no quench. But the dynamic still seemed ok but i really wasnt sure.
Was your heads also open chamber iron heads?
mopowers
Well-Known Member
FWIW- My 340 has 9.6:1 compression and 8.87 dynamic CR and runs fine on 91 pump gas. It does have iron close chamber heads and .035" quench though.
Yeah thats what i have been worried about. Im pretty sure i would be ok with closed chambers and taking advantage of quench , but wasnt sure with open chamber heads .
moturbopar
Well-Known Member
Put it together with the .038, gaskets, at least 3.91 gears and a fairly loose converter and call it a day.
mderoy340
Well-Known Member
Yes 2.02 J heads 66CC smoothed combustion chamber, Fel Pro 1007 head gasket, sharp edges removed from valve reliefs, 180* thermostat, Autolite 65 Copper core plugs, old analog MSD 6AL, 24 initial/35 mechanical total.
nm9stheham
Well-Known Member
Your DCR numbers are well below the conventional wisdom limits for pump gas.
Thanks for the reply - makes me feel better about putting it together after hearing from someone with a similar build. Was your block zero decked also and are you using a large cam too?
mderoy340
Well-Known Member
Old 340 TRW Forged Pistons .018 above deck. XE262 Cam ICE 60* with stock OEM high stall converter and 3.23 gear. It's a fun street car that is my drive anywhere car. Need a good torque motor for street driving and air conditioning.
mopowers
Well-Known Member
Who told you that? I always figured dynamic was more important than static because dynamic is what the engine actually sees. Static cr is just a number.
I don't know that's correct. Static CR does NOT play in to engine dynamics like the Dynamic CR does. DCR is way more important. Mine is 11:1 static, 8:1 dynamic and it will click off high 10's on 91 octane fuel. No pinging or detonation issues with this particular combo. Based on your CR numbers with either head gasket, you should be safe with 93 octane. Rule of thumb is 8.0:1 DCR with iron heads and 8.5:1 DCR with aluminum heads. Depending on the combo, that can vary. SOme can take higher DCR and some need lower DCR. But it's a general rule so take that FWIW.
Thanks for the information. I thought as long as dynamic was under 8.25 i was good. Oh and by the way i have seen "nothing over 9.0 to 9.5 compression with pump gas and iron heads " mentioned several times on this forum. That is what had me worried. Good to hear from some guys that are actually using motors built this way. Thanks again you have all been very helpful and put my mind at ease.
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