First time swapping edelbrock aluminum for 915 cast iron heads.
AJ/Forms
Much of what your saying is beyond what I understand. Engine building definitely isn’t my forte. I’m learning as I go along. Not to discount that I’ve learned a ton from this website. My original motor ran really strong with just basic 915’s, no porting or anything special, older mopar performance 284 duration 484 lift cam, KB pistons, 9.8 to 1 comp. Harland sharp roller rockers, Edelbrock Performer intake, speed demon 650 cfm, 4 speed trans. It dynoed at 340 rwhp about 380 rwt. I’m changing out the 915’s to edelbrock rpm performers with a bump in compression to about 10.2 to 1 with the mild porting and milling which reduced the size of the edelbrock chambers to 72cc from originally being 75cc. I don’t understand why the motor is going to fall flat with those chambers. I know that a lot depends on how quick the cam reaches maximal lift and how long the valves stay open and close but I don’t know the original specs on the original mopar performance b/rb cam. Keep in mind this motor ran great on 93 octane no pinging even on hot summer days. I think that was due to original comp being 9.8 to 1. Now I know aluminum heads can run higher compression before pre detonation so I though 10.2 to 1 should work great. Why would you want to run up to 11.2 to 1 compression with the edelbrock’s and risk pre detonation on pump gas? Is the 180 your referring to the optimal cylinder pressure? I’m changing out the carb to a 850 cfm race demon and please explain where I going wrong because I don’t see how the motor is going to fall flat? Can people please explain in layman’s terms?
Hang on, I didn't say anything about falling flat.
There's three basic reasons to go with alloy heads.
1) detonation resistance
2) workability
3)efficiency.
If you spend all that money, for just one of the three, you are kindof cheating yourself. Sure, you bump up the flows pretty good, and slam a cam in there and go have a good time.
But if you don't take advantage of the heat characteristics of those heads, you're A) sticking way more cam in there than you need, boosting the power-peak up into an area you only rarely drive, and/or
B) leaving some performance on the table at other rpms.
...By increasing cylinder pressure from 155s to say 175plus, you can bring the power-peak down and fatten up the midrange, and if you care; design a cam to drive a little further down the road on less dollars, burning a lower grade of fuel.
...As to optimum pressure I can't say.In my 367, I have run 175 to 190,and more, on 87E10 (R+M)/2 Canadian gas with full timing, since 1999. IMO the bottom end is outrageous(lol) for a 367 streeter, And I give the credit to the pressure. This with a 230cam now
Here are some example of what happens to the low-rpm performance, with changing pressure. This is kindof important to a stick-car. I'm gonna assume you have the 114LSA cam, on account of you don't mention the idle lope. All these are alloys except yours in red.All should run on pumpgas, with proper Q.
These are just for demonstration purposes.I will list the Wallace results as ;
-Scr---ICA---Dcr---psi---VP
9.80/74*/7.13/138/110VP this is your iron headed 383,and 284/284/114 cam
10.2/74*/7.41/145/116VP your proposed combo; alloys at 10.2
11.5/78*/7.97/159/121VP compression upped/ cam retarded
11.5/72*/8.47/172/141VP cam advanced
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11.8/74*/8.52/173/139VP compression upped,cam retarded
11.8/76*/8.36/169/133VP compression upped, cam retarded to kill some VP.
I didn't bother going any higher, cuz I see the Scr getting excessive.
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With a
276/284/110 cam
11.2/68/8.62/176/151 in straight up. one size down cam,60* o/lap versus your 70*
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Notice the VP numbers;
compare the blue to red. The VP is just 116/110=
plus 5% stronger than your current combo.
Next, compare the violet line, to the blue; At 141/116=
plus 21.6% stronger than your proposed combo.
Finally compare the green to the blue;151/116=
Plus 30%. Where do you want your power to be?
At 151 (same as I have), this is a dump-it-and-go deal with a 10.97 starter gear. Just a blip of the throttle to put some energy into the flywheel and more or less dump the clutch and go. With 3.55s and a 1.91 second gear, this will break the 295s loose at up to 50 mph.
Remember, VP is just a number we use to help understand low-rpm performance changes .
Read about VP here
V/P Index Calculation
Wallace Racing: Dynamic Compression Ratio Calculator
