72poncho

iTrader: (1)

Evening guys,
been a long time lurker on here. do most of my forum stuff on protouring.
swapping in LS to 1972 Pontiac. Currie 9", t-56 from american power train, tubular control arms, yadda yadda..
the time has come to purchase an engine.. have been planning to go with a TSP408LQ9, either PRC ls3 CNC heads or the PRC 255/260 ls3 heads.

On to my issue, looking to stay pump gas friendly, with 67cc chambers, -3cc wiseco pistons, unsure how far out of the hole, and .054" head gaskets i was told i should be right around 11.36:1 SCR.

With suggested off the shelf cam specs of 235/248 .649 .615 on a 112 LSA that gives me a valve overlap 17.5. i can get the same cam on a 114 also, which would get me closer to 13.5.

I know lots of LSX guys run 12.x:1 pump gas cars, is my cam specs putting my DCR to far out into left field? should i be looking for different cam options?
I am concerned about pump gas friendliness, enough vacuum.. estimated operating range will be less than 6.75k rpm.
open to suggestions from the forum..

JimsNovaLS

With an iron block get your quench (distance between piston flat and cylinder head surface) down to .040 (.036 with aluminum block). See how far the piston is above the deck and adjust the quench with head gasket thickness. Tight quench promotes swirl and makes the engine less prone to detonation even though it raises the static compression. With tight quench, aluminum heads and fuel injection you will be fine even at 11.5 static.

JimsNovaLS

The above post assumes you have 93 octane available in your area. If you are worried about idle vacuum and driveability, go with the 114 LSA installed on a 111 intake centerline.

72poncho

iTrader: (1)

Jim,

Yeah i gave TSP a shout.. they said they normally shoot for -.002 to +.004 out of the block, so that would give me a quench of .056 to .050 with a standard ls3 Head Gasket of .054.
i should probably just request that they use a cometic .040 to achieve a quench of .038 to .044. depending on the variability of the piston location.

i was told my dynamic compression would be 8.24:1 with the above combination in the first post.
i have 93 in my area.. even e85! the 112 LSA has a 109.5 intake centerline..

would changing the head gasket have an enormous effect on the SCR or DCR from .054 to .040? they claim the above cam isnt that big for a 408.

KW Baraka

iTrader: (7)

Worry less about the SCR and more about quench. Being down in the .035" to .040" range is worth ANY increase in compression that results. In your case, you will see about a .3:1 increase in compression, ending up with about 11.6 to 11.7:1 SCR and about 8.5:1 DCR.

With a good tune, I see no reason whatsoever why you can't run engine specs like you're looking at getting.

I run a TSP 429ci with 12:1 compression in my SS; .036" quench with no issues at all (244*/256* cam) using 93 octane. I have 18* overlap but I'm researching new cam specs to get me closer to 10* overlap.....for better drivability.

So if drivability is important, I echo Jim's recommendation of going with the cam with less overlap.

Best of luck!

KW

72poncho

iTrader: (1)

Are you running an auto or manual? What kind of heads are you running?

JimsNovaLS

They're right, that's not too big for a 408. In fact you want the duration to help lower low rpm cylinder pressure to help with detonation resistance. It's the overlap that will cause you the low vacuum and driveability issues. I would go with the same cam ground on the 114 LSA.
Felpro makes a .041 MLS gasket as well, I'm using them on my latest build. You want that tight quench.
Looking at KW's sig., I would say he's using LS3 heads with an auto. How about it KW?

KW Baraka

iTrader: (7)

You absolutely do!


All 2014 Chevy SS's are auto.....6L80E.

I have the 260cc PRC LS3 heads from TSP.

KW