L31 with LT1 bottom end

[Crazydavez28]

I just brought home an new to me 99 4x4 2-door Tahoe with a nasty knock. I have a couple standard bore blocks around but the crankshafts for both are toast. I found a cheap LT1 Impala engine nearby and after bringing it home the bottom end looks fantastic. I test fit the setup in my Vortec block and the piston is perfectly .025" in the hole at TDC. Trying to stay on my on-the-shelf budget I'm fitting this engine with a set of big valve Pro-Comp aluminum heads. I ordered some 0.015" Fel-Pro 1094 SS head gaskets puts me at 0.040" quench and 10.26:1 static CR. I have a Comp 08-416-8 marine cam I plan to use in this and installed at 0 i should get 8.27 Dynamic CR if i go 4 advance i'm at 8.67.

I guess I can use a thicker gasket later but just wanted to see if there were any thoughts on the numbers. Not sure if this will play nice with E10 pump gas or 91 only.

 

[Schurkey]

How are you getting ten-and-a-quarter compression from flat-top pistons sunk .025 in the hole?

Am I wrong about the pistons? If they're genuinely flatties, you've maybe got 9.5:1 with typical 63--64 cc chambers.

 

[Crazydavez28]

How are you getting ten-and-a-quarter compression from flat-top pistons sunk .025 in the hole?

Am I wrong about the pistons? If they're genuinely flatties, you've maybe got 9.5:1 with typical 63--64 cc chambers.

^^That's what i get with a 64 cc chamber and .041 gaskets. The LT1 pistons are flatties, I'm also figuring 5cc dish from the valve reliefs.


Im planning on trying .015 gaskets to bump that up bc the heads are aluminum. I will also be running a HD radiator and 160degree thermostat.

 

[eshaw]

Just so you know, the LT1 crankshaft balance is way different than the 350 you're trying to put it in. It may have the same casting number that the Vortec has but the LT1 is closer to a 305's balance specs than the 350. The balance change was dictated because the LT1 used light weight pistons.

 

[Crazydavez28]

Just so you know, the LT1 crankshaft balance is way different than the 350 you're trying to put it in. It may have the same casting number that the Vortec has but the LT1 is closer to a 305's balance specs than the 350. The balance change was dictated because the LT1 used light weight pistons.

I am using the pistons, rods, and crank from the LT1

What will be different? I know the balancer on the front end is different than the L31 but i was considering using the LT1 hub and a 1" spacer block to drive the accessories (have access to a lathe). The flexplate has the same size counterweight in the same spot.

Is there anything special about an L31 front balancer?

 

[Supercharged111]

Yes, front balancer is 2 piece hub and pulley. I'm pretty sure it's neutral balanced though. LT1 aluminum heads were 54cc and the iron heads I believe were 64cc like the Vortecs. Not sure what the compression ratio was on the Impalas, but definitely south of 10.5.

 

[Schurkey]

How are you getting ten-and-a-quarter compression from flat-top pistons sunk .025 in the hole?

Am I wrong about the pistons? If they're genuinely flatties, you've maybe got 9.5:1 with typical 63--64 cc chambers.

^^That's what i get with a 64 cc chamber and .041 gaskets. The LT1 pistons are flatties, I'm also figuring 5cc dish from the valve reliefs.


Im planning on trying .015 gaskets to bump that up bc the heads are aluminum.

http://www.diamondracing.net/tools/
I enter:
Cylinders = 8
Finished Bore = 4.000
Stroke = 3.48
Rod length = 5.7
Gasket Bore = 4.100
Compressed thickness = .041
Deck Height = 9.025
Top Ring = .200
Cylinder head cc = 64
Dome/dish = 5
Piston to deck = .025

Compression ratio = 9.55.

Change head gasket thickness from .041 to .015

Compression ratio = 10.17

This is actually higher than I expected at first, the pistons seem to have full compression height of 1.56, many pistons are reduced by .010 or .020 (or more). Note that I took some guesses at your head gasket bore, and top ring distance from top of piston.

I will also be running a HD radiator and 160degree thermostat.

Not a fan of low-temp thermostats. You're going to lose a lot of energy through the aluminum heads that could be put to work moving the piston down. OTOH, it's unlikely to actually hurt anything.

I just built a TBI engine using a Vortec short-block and aftermarket TFS aluminum heads. The Vortec pistons drop the compression to about 9.4:1. so I have zero problems with "regular" 87 or 85 octane fuel even with the 195 thermostat. I would have liked ~9.5--9.75, which should still use "regular", but I wasn't going to swap pistons. Even with the low compression, I've got more power than the original engine, or the service-replacement TBI Caprice engine that went in when the original engine popped 18 years ago.

 

[L31MaxExpress]

The factory Impala head gaskets are 0.028" compressed. Typical compression on a flat top piston and 64cc chamber was 9.6:1. The LT1 iron heads are 64cc. The aluminum heads 54cc. Aluminum head LT1s ran a 0.051" thick gasket. The thinner head gasket is a common trick to up the compression in the aluminum head engines.

 

[L31MaxExpress]

http://www.diamondracing.net/tools/
I enter:
Cylinders = 8
Finished Bore = 4.000
Stroke = 3.48
Rod length = 5.7
Gasket Bore = 4.100
Compressed thickness = .041
Deck Height = 9.025
Top Ring = .200
Cylinder head cc = 64
Dome/dish = 5
Piston to deck = .025

Compression ratio = 9.55.

Change head gasket thickness from .041 to .015

Compression ratio = 10.17

This is actually higher than I expected at first, the pistons seem to have full compression height of 1.56, many pistons are reduced by .010 or .020 (or more). Note that I took some guesses at your head gasket bore, and top ring distance from top of piston.







Not a fan of low-temp thermostats. You're going to lose a lot of energy through the aluminum heads that could be put to work moving the piston down. OTOH, it's unlikely to actually hurt anything.

I just built a TBI engine using a Vortec short-block and aftermarket TFS aluminum heads. The Vortec pistons drop the compression to about 9:1. so I have zero problems with "regular" 87 or 85 octane fuel even with the 195 thermostat. I would have liked ~9.5--9.75, which should still use "regular", but I wasn't going to swap pistons. Even with the low compression, I've got more power than the original engine, or the service-replacement TBI Caprice engine that went in when the original engine popped 18 years ago.

I am a fan of running the engine cooler. The minute difference in heat escaping is more than equally made up for by a denser intake charge and the detonation resistance. I run a 170°F thermstat in my 9.6:1 aluminum head 350 Vortec at 34° total advance on 87 octane in my Express van even with the travel trailer behind it. My Vortec was 9.4:1 as it was assembled by GM. I am running 9.6:1 by using a 0.028" rather than 0.051" gasket with .020" milled off the heads.

 

[Crazydavez28]

I am a fan of running the engine cooler. The minute difference in heat escaping is more than equally made up for by a denser intake charge and the detonation resistance. I run a 170°F thermstat in my 9.6:1 aluminum head 350 Vortec at 34° total advance on 87 octane in my Express van even with the travel trailer behind it. My Vortec was 9.4:1 as it was assembled by GM. I am running 9.6:1 by using a 0.028" rather than 0.051" gasket with .020" milled off the heads.

Yeah, the idea is to run cooler to try and avoid detonation. Pretty easy to swap out later if the datalogs look good. Not sure my cam is wild enough to compensate for the increased CR. Also, im pretty sure the heads were milled .020 to clean some damage when they were rebuild. Lost the paperwork....

Shurkey the calculation I used didn't account for the top ring height but the rest of the numbers look spot on.