Veterans please bear with me, this is my first post. I own a matching # '71 vert, small block, automatic, with about 140k miles. I'm not a mech and deferred to my mechanic for rebuilding details. I wanted a mild 350 rebuilt to take advantage of modern technology, here's what I got.
350 bored .30 over, decked block, KB 10.1:1 pistons, Comp Cams thumpr cam, original 1.94 heads, rebuilt Qjet. First time I put car in gear it stalled. The Comp people told me I had way to much cam and no vacuum, so I replaced it with an XE-262 with good results. Regarding the piston issue, are the pistons listed above to much for the car in this configuration? I know that motor questions open Pandora's box but I could use some help. So far, this rebuild has been a disaster. Thanks in advance.

 

did your builder do any head work?did u have good rods to start with?10 to 1 is pushing the comp.with pump gas,but by the time u add head gaskets and chamber it will be lower but i cant say with so little info

 

I run 10:1 in my daily driver, but it has a computer to cut timing, etc. if I start to ping. I run low grade pump ****, not the good stuff. You should be fine, I'd start with the best pump gas you can find and beat on it, try to get the engine loaded so it'll ping if it's going to, then you can move down the octane scale if you need to. With good machine work and quench in the .040 range, you should be fine, IMHO.

 

I agree. As long as it's not pinging it'll be fine. If it is pinging try backing the timing off a little and see what happens.

 

u guys in tx must get some damn good gas,up here in ind. my harley wont even run with out pre.

 

10:1 isnt to much compression, premium gas is a must though.

10:1 May not be an optimal match for the rest of your components. Meaning, a solid 10:1 needs some good Flow thru. That much Bang may not ever tune well if theres restriction somewhere before the tail pipes.

If you determine that is accurate, at least you have the option for a thicker gasket to get the CR to 9.5-9.7 to 1.
My 2centavos

 

First ... TO ALL ... ALL should include an approximate geographical location in their profile so we can have a sense of what region your car is at ... helps w/ parts availability, weather-related diagnostics, emissions/smog restrictions etc etc.

ewashark:
as you likely know ... 71 has large chamber heads about 76cc
if it now has 10:1 that would indicate it has DOMED aka POPUP pistons ...
OR flattops WITH heavily-cut decks & have shim gasket ...
But you say number-match ... more often than not a heavy cut to deck also cuts off numbers ... some shops can avoid it. What do you have?

are you certain it's 10:1 ?
exactly which p/n KB piston did they install ? Are they dome/popup?
are your original numbers clearly visible?

 

When you say you got 'good results' with the XE262, did that solve your stumble/stall issues? You say it has been a disaster...

Was yours the base small block engine or L46? KB 10:1 pistons alone should not be a problem for your set-up. With a properly setup carb and distributor, your engine should run fine with your setup (Thumper or XE cam).

If you do in fact still have stumble/stall/performance issues:

How was your carb setup after the rebuild? Was the carb just 'rebuilt' without changes to primary/secondary metering rods, jets and mixture settings? Is your carb's cold start circuit/choke working as expected?

I would verify the #'s from your primary/secondary metering rods and jets that are in the carb. They may have to be replaced/tuned.

Also, how is your vacuum system (hoses/fittings)? If you are losing vacuum, you may have difficulty getting a stable idle even with a cam change.

Also, what type of advance curve did your rebuilder put into the distributor? If I recall correctly, my '80 L48 'stock' was set at 6-8 degrees BTDC at idle and had a very LAZY advance curve; it may have never gone beyond 20-25degrees total BTDC at WOT. Now, at idle, I'm at close to 15degrees advanced (BTDC), about 25degrees around 1500rpm and at 35BTDC by 2500rpm. Very nice advance curve and close to what you want to achieve with your configuration. Also, ported vs. non-ported vacuum to the distributor will matter too. Lars set up my carb and I had a tuner set the advance curve on my replacement distributor.

Oh, that reminds me, what is the health of your distributor? Has it been refreshed? If not, it may need attention (bushings, springs, vacuum canister).

 

To all who replied, a thousand thank yous. Geographically, Utah at 6800 ft. No ping now, but I use premium fuel. I am running the original rods and crank. Still looking for pt# of pistons but they are flat top and not domed. Engine got reassembled before I could determine if decking "erased" my #'s. As far as my block goes, it was a base 2=-bolt block with a 3970010 casting #. Just fixed another rebuild problem. Car was leaking oil due to an incorrectly (backwards) installed lower rear main seal. I'm the poster child for due diligence in hiring a mechanic. When you know you have the right guy, look a little longer. Thanks again to all.

 

You can still see them after the engine is assembled.

 

yep ... go look at ID stamp pad at front of passenger deck ... right where head meets deck. Some motors have Alt /AC bracket bolted across there; maybe have to loosen to see pad.

again ... y'all put your approx locale in profile so it's seen in each post.

afaik ... all OE 71 C3 sbc have chambers approx 76cc ... w/ flattops I seriously doubt it's more than 9.5 : 1 scr ... it's probably closer to 9.2 or less ... unless LOTSA, LOTSA race-type milling of heads & decks

seems to me motor likely built to about 9:1 or so ... then textbook error ensued ... they put in too much cam for too little compression ... then tried to tune their way out of a poor choice.

??? They may've used some combo of KB's crappy cr calculator and/or KB's goofy convention of publishing both dish & FT dome volume as positive to arrive at 10:1 ??? Plenty others've unintentionally built low-comp due to junk KB info.

FYI ... in the real world, both dish & flattop (valve reliefs etc) have a Negative volume while most popup/dome have Pos volume.

here's a very good static CR calc:
http://www.power-21.com/federalmogul...lculator4.aspx