Budget 318 build
To my understanding the 85-89 318 had a higher compression 9to1 due to the closed chamber heads (302) 9 to 1 is higher than any 360 and higher than the later 340 so how would a 340 cam (basicly what the sum6900 is) kill my power?
Well, the Mopar 'factory' numbers were always high. I just ran some numbers with:
- the 302 heads and 62 cc chambers
- Felpro 1008 head gasket thinner than the standard Felpro head gasket but you can go 1 step thinner).
- the best CR pre-Magnum factory pistons, which sit around .075 down in the hole and have 4 small valve eyebrows (And we don't know which ones you have)
That combination is right at 8.3 SCR; that is pretty typical, and I find the real SCR numbers usually are around .7 to .8 point lower than the factory SCR numbers. So you are starting with an SCR that is about 1.4-1.5 points lower than the real SCR of a factory (early) 340. That is how it will hurt the low RPM torque.
Now lets put your cam in there and run the DCR (dynamic compression ratio number); this is what sets the low RPM torque, which is what gets the car moving from a stop unless you rev it up a lot to get into a higher RPM range (hence the suggestion for a lot higher rear gears and a lot higher RPM torque converter). The DCR is at 6.5 now, even with the cam advanced 2 extra degrees to help the low end DCR. Your cylinder pressures are gonna be in the 115 +/- range IF the rings seal well. Again that is lower than stock 318's..... which is sensible with a larger cam.
For Perspective: 6.5 is lower than the stock 318 DCR and lower than the stock /6 DCR. So, for low RPM's, this engine is going to 'lazy' at best.
For reference, I once ran a 300+ duration cam on a 7.6 SCR.... DCR was in the high 5's LOL. That engine was truly, truly dead below 3700 RPM: had to rev it up there to get a good launch; yes, I could putt around below 3700 RPM but it would not take off if I pushed the throttle. The only things that saved that engine was that it could rev to 7700 RPM all day long so I had at least a 2:1 usable RPM range, but that is narrow for street use. Oh, and that was with a 4 speed....
So the suggestion of a 2800 RPM torque converter and high rear gears is spot on. At least you have the better breathing on the high end, but with the heads, your real usable torque range is going to be narrow. OK for drag racing, not so much for street cruising. This cam is an older, low stress-on-the-valvetrain design. Low cost (which is good for sure) but sloooow ramps and long tails for a late intake valve closing which is lowering the DCR as explained above. It is a good cam for plonking into a stock setup and valve springs, since it does not have a lot of lift or fast ramps.
I indeed 'get' your plan to just get this 318 running and then move onto the 360; it certainly does not justify much more than a simple rebuild. I would just not put that cam in there and expect it to be a perky street cruiser. So I'd suggest a different cam for this with 2-3 steps lower duration. Something like a Lunati 10200205 is the ticket IMHO:
- .441" intake lift vs the K6900's .421" lift so bit better mid and high RPM HP, so the top end is maybe extended a few hundred RPM (though that may be more limited by the valve springs)
- 18 degrees shorter duration so the DCR is now 7:1...getting back to stock 318 levels so the lower usable RPM range has been extended down around 500-700 RPM.
- Moderate ramps and lift so you can run it with the stock stuff
- Only a $19 more on price from Summit, but you have to wait 2-3 week for it
Hope that helps. Are you still looking at valve springs?
