heads or short block first?

grumpyvette

Administrator
Staff member
I get asked this frequently.
the question usually goes something like

" look Ive got a VERY LIMITED budget, which should I go for,
great heads or a good large displacement short block,
IVE only got $1500, or $2000, or $4000 to work with??"

well having been this route a few dozen times Id put my two cents worth in,
OBVIOUSLY youll need BOTH to maximize any combos potential, and it depends a great deal on how large a displacement increase over stock your thinking about,
now if your only going from a 350 to a 383, the heads will obviously be a good value,as better heads and a matched cam can boost the hp noticably,but if your seriously building to maximize the torque and hp in the complete project and your eventually going to build the engine to maximize the power..
and if your thinking about a serious upgrade in displacement , say from a 327, to a 427 in a sbc or a 396 to a 496-632 in a bbc,...
I ALWAYS suggest going for the large displacement short block thats well put together FIRST.
youll never make killer hp without the good heads but you can use stock heads on a well built short block to make killer low rpm tq UNTILL you can budget and save for the heads you want, ESPECIALLY on a BBC combo as the heads, even the peanut port door stop design heads, flow more than the VORTEC sbc heads ,now of course theres the potential problem of differant clearances, and compression ratios with differant pistons and combustion chambers, but if it will fit and fuction ID go FIRST for the displacement an have a driveable car with the old heads while saving for the better ones. YOULL NEED TO THINK EACH STEP THRU CAREFULLY.

example
lets assume youve got a 350 hp 396 in your 66 chevelle, nova or camaro, and want a killer combo.
youve saved up $3000-$4000 and its burning a hole in your pocket, now you can buy a 496 short block or killer heads and a cam, and theres little doubt that the killer heads and cam sellected correctly will make more power than your old heads and cam on the new 496 short block...but think it thru THEN WHAT?
in a few months or years as your budget allows, youll want to upgrade, if the heads and cam were sellected for the 396 its not likely that they are near ideal on a 496 with its greater displacement and lower average rpm range..
if you sellected the short block first, the heads and cam can now be sellected to match.
NEXT
its simply faster and easier too swap heads on a good short block than it is to swap short blocks out at a latter date when the funds and parts become available

keep in mind the displacement and compression dictate to a large degree the torque, while the cam timing and heads flow rates effect the rpm range and peak power potential that displacement can reasonably achieve on a limited budget.

http://maliburacing.com/patrick_budd_article.htm
 
keep in mind swapping heads gives you the option to increase or decrease the compression ratio of the combo, and once the compression ratios increased you can use a longer durration cam timing without sacrificing a great deal of low rpm torque, that you would loose with the lower compression ratio if the same cam were installed in the lower cpr combo

"(assume on a stock 350 small block)

Speaking in generalities and assuming no other changes, what's the relation between cylinder head combustion chamber size and overall engine compression?

Does just changing heads from a stock 76cc head, to a head with a smaller chamber make that much overall difference on engine c/ratio? Such as a 64cc or 58cc chamber. How about changes on necessary octane requirement?

Just curious to see what can be expected. I've read plenty on what performance gains can be had and what would work best for 350 TPI, but curious to see the other factors may play out.

Thanks"

Ok lets look at it a bit, theres two types of compression ratio, static and dynamic, keep in mind its MOSTLY dynamic compression ratio, that effects your results.
youll gain about 3% in hp increasing the effective static compression ratio one full point, so swapping from a 9:1 to a 10:1 cpr boost torque about 3%
swapping from a 76cc head to a 58cc head is a 1.84:1 cpr change so you can reasonably expect a 1.84 x 3% or a 5.52% boost in torque from that change alone.
if your current engine made 330hp that would jump to about 5.52% higher if the tq curve remained consistant, so youll see about 350 hp.


one of the the main functions of compression is to pack the fuel/air mix into a tight area for both fast effective ignition and to provide a mechanical advantage for the piston & rod assembly to push against the crank throw,as that mass in the combustion chamber burns and rapidly expands.

lets look ast your question, given identical 350 displacement engines with flat top pistons and a comon .032 thick head gasket, a .023 deck and 5cc valve notches, heres what your going to see in STATIC COMPRESSION,as a result of combustion chamber changes

58cc=10.61/1
60cc=10.36/1
62cc=10.12/1
64cc=9.89/1
68cc=9.47/1
72cc=9.09/1
74cc=8.91/1
76cc=8.75/1

keep in mind you want to stay at about 8:1-8.5:1 in DYNAMIC compression

to run comon pump gas without getting into detonation
that depends on the fuel octane, cylinder head temp. and several other variables but generally 8.0-8.5:1 dynamic works out well if your going to run mid grade pump gas
I think the hamburger oil pan is a better value At $242 than most of the common mid priced oil pans
notice the better oil control features

https://www.summitracing.com/parts/ham-1088/overview/
ham-1088.jpg




http://www.milodon.com/oil-pans/street-oil-pans-sbchevy.asp

http://www.jegs.com/p/Moroso/Moroso-Street-Strip-Oil-Pans/763991/10002/-1

https://www.summitracing.com/search/brand/hamburgers-performance/part-type/oil-pans

https://www.cantonracingproducts.com/cgi-bin/commerce.cgi?search=action&category=1301

http://www.stefsperformance.net/ste...oil-pans/aluminum-oil-pans-pumps#!prettyPhoto
0311phr_compress_07_z.jpg


calculators


http://kb-silvolite.com/calc.php?action=comp

http://www.csgnetwork.com/compcalc.html

http://www.bgsoflex.com/cr.html



threads/info

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

http://victorylibrary.com/mopar/piston_position-c.htm

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

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=727

http://forum.grumpysperformance.com/viewtopic.php?f=50&t=499
 
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