Compression question 383

plemon61

Member
Hello. This is my first post. I'm doing a rebuild on my low mileage 383. My question is if 10.3 compression is too much for xe 268 comp cam which I have already? The motor has kb 197 dish pistons, .025 down, .015 shim head gasket, TFS 195 heads with 64 cc chambers. Edl RPM airgap, Demon 750 dp and 1 5/8 headers. It's going into a 83 chevy wagon with 200r4, 2200 stall and 373 rear gears. Do you think this combo will work with 93 octane, or do i need to lower the compression ? Thank you in advance for any help. Patrick
 
well welcome to the web site!
ok lets look at the math a bit,
youll generally be fine with 93 octane fuel and using aluminum cylinder heads if you keep the engine coolant temps under about 215F
and the DYNAMIC compression under about 8.2:1 ,
dynamic compression is based on where in the crank rotation the valves both close as the piston can,t compress anything until the valves seal.
the charts based on iron cylinder heads and yours are aluminum which will generally allow about a 1/2 point in dynamic compression higher without issues.
your pistons are listed as having a 12cc dish volume, and being .025 down the bore adds about 5.6 cc more
https://www.summitracing.com/parts/UEM-KB197-030/
thus you have about 10.25:1 static compression
383po.png

383poi.png

the math shows your dynamic compression will be near 9.5:1 which is a bit too high for reliability without an additional octane booster
heatvscpr.jpg



if we look at your cam we see it is listed as having 224 duration on a 110 lsa

http://www.compcams.com/v002/Pages/388/XE268H-10.aspx
ringgr.png

if we look at this chart we see that your intake valve closed at about 38 degrees after bdc.

CrowerCamTimingChart_108-110.jpg


wpid-valve_timing_chart.jpg

CAM%20TIMING%20DIAGRAM%20BY%20CAMSHAFT%20ANGLE.jpg


you might want to read thru these link's carefully

http://victorylibrary.com/mopar/cam-tech-c.htm

http://members.uia.net/pkelley2/DynamicCR.html

http://www.wallaceracing.com/dynamic-cr.php

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

OK, first fact! the piston can,t compress anything being trapped in the cylinder by the piston compressing it as it raises,until both valves seat & seal
definition.jpg

postiongraph.jpg


http://garage.grumpysperformance.com/index.php?threads/dynamic-vs-static-compression.727/

http://garage.grumpysperformance.co...-calculators-and-basic-math.10705/#post-46768

http://garage.grumpysperformance.co...octane-for-compression-ratio.2718/#post-35581

http://garage.grumpysperformance.com/index.php?threads/crowers-valve-timing-charts.4299/

http://garage.grumpysperformance.com/index.php?threads/how-to-read-a-cam-spec-card.1477/#post-3329

yes you'll ALWAYS NEED to verify clearances carefully when assembling any engine and no, you can,t assume everything will fit just because its part of a component list thats suppose to fit correctly, simply because manufacturing is an imprecise process and mistake do get made occasionally!


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OK, now what? well we can index the cam to 4 degrees retarded instead of its current 4 degrees advanced location
(assuming your installing it DOT-TO-DOT on the timing gear set) that will result in a reduction from about 9.5 to about 9.2:1 in dynamic compression
you can swap to a carburetor designed for E85 and that ETHANOL laced fuel will run just fine on that compression ratio
383po2.png


you can convert your carburetor to run ethanol/E85 or ,
better yet, get a carburetor, fuel filter, fuel pressure regulator, fuel lines and a fuel pump designed for E85

https://www.summitracing.com/parts/qft-3-208qft

or you can lower the compression with a set of larger combustion chamber heads or different dished pistons
retarding the cam to 4 degrees after TDC , swapping to 74cc heads and a piston with a bit larger dish would be ideal
(except of course for your checking balance)
 
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When I used the Wallace Racing Dynamic compression calculator, which I believe is the one that you used, I came out with 8.5 dynamic, but I used ivc 61* abdc as the calculator said to use seat to seat timing for intake valve closure. So my effective stroke changed as did my DC from the figure you have so I'm a bit confused.
 
Now for a piston question. If I switch from KB 197 to KB 135 18cc dish will I have to rebalance the rotating assembly ? The KB 135's weigh 10 grams less than the 197's. Thanks for your help with this.
 
GOOD CATCH!
(I'm glad someone reads and understands the posted info)

yes,your closer,using the seat too seat timing, yeah, I forgot to use the extra seat to seat duration and incorrectly used the .050 duration in the calcs
yes the larger dish volume of the 18cc 135 KB would be a better match, especially if you install the cam retarded to 4 degrees past TDC.
 
Balancing ??

If you are able to get the combination of piston, pin, and ring pack to weigh the same as previous, then you don't have to rebalance.
One way to do this is with a heavier (10 gm) wristpin. But that might not be easy to find (or not be expensive).
This assumes you are reusing your same connecting rods.
 
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