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Old 03-12-2011, 01:27 PM
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compression ?'s on a tbi 350

iv asked ?'s about this b4 but i want to make sure iv covered anything i mite have left out, ok i want to build a good 350 for my 90' chevy 4x4, now id like to get the compression up to 9.1, and doing that without milling down the heads that are already 64cc, and without decking the block, like useing the stock pistons with a .015 thickness head gasket? flattops with no milling the head or deckn the block and a .042 head gasket ? thanx guys for all your ideas and help!

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Old 03-12-2011, 01:36 PM
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Are you sure this motor actually has a flat top piston.? And not a shallow dish?

The only way to get it right is to CC your heads, verify the piston type and measure the deck clearance at TDC of your motor.

Then select the gasket and/or do the required milling to get the real cr you want.
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Old 03-12-2011, 03:50 PM
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no the motor has stock pistons (dished), what i was refering was how i could get 9.1cr? the heads are the swirl port heads, everyone that i talk to on here has told me they are 64cc, im going to try to port them or at least get the swirl vane out...
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Old 03-13-2011, 07:37 AM
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another words what can i do to get to the 9.1 or even 9.3CR, somewhere in the range of 87 octane with out a whole lot of cost? like what i was saying was useing the stock pistons with no deckn or milling the head, use a .015 thickness head gasket, what would the CR be then? or use a flattop with no milling or decking and use a .042 head gasket, what would that do too the CR?.....thanx guys!
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Old 03-13-2011, 04:50 PM
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Quote:
Originally Posted by gofastz
iv asked ?'s about this b4 but i want to make sure iv covered anything i mite have left out, ok i want to build a good 350 for my 90' chevy 4x4, now id like to get the compression up to 9.1, and doing that without milling down the heads that are already 64cc, and without decking the block, like using the stock pistons with a .015 thickness head gasket? flattops with no milling the head or deckn the block and a .042 head gasket ? thanx guys for all your ideas and help!
The LO5, 350 Swirl Port with 191 or 193 heads has 9.4 to 1 compression as it is specified from the factory. To get that with 64cc heads; the piston has a round dish of about 12 ccs in volume. Theoretically, the Swirl Port engine will run on regular 87 octane fuel. I suspect this is possible with TBI having a detonation sensor that will pull the timing back till it doesn't ping. I rather think they all ping on 87 since everyone I've been in with headers talks to ya, while you don't hear detonation per se' you do hear a ring in the headers which tells me it's right on the ugly edge. I've noticed in the same truck with 92 octane the ringing in the headers goes away. To run like this with a carb and no detonation sensor is to risk the sudden surprise of buying a set of pistons.

Yes, you can grind the vane out of the port and end up with a pretty good flowing head, but you will trade a lot of time for the money you can buy a better head with. Maybe with where the American economy is going, returning to past hot rodding techniques will become the new way of doing things instead of throwing cash at problems.

Bogie
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Old 03-15-2011, 04:33 PM
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iv been told the tbi 350 engines are around 8.2-8.5, from the local machine shop guys? so what is it? im planing on useing the summit 1102 camshaft, and a 454 trottle body unit, other than that just grinding out the vane in the heads, and hopeing to get some good relieable power out of it...
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Old 03-15-2011, 06:28 PM
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Quote:
Originally Posted by gofastz
iv been told the tbi 350 engines are around 8.2-8.5, from the local machine shop guys? so what is it? im planing on useing the summit 1102 camshaft, and a 454 trottle body unit, other than that just grinding out the vane in the heads, and hopeing to get some good relieable power out of it...
The LO5 comes in three versions, passenger/police, light duty full size truck, and heavy duty full size truck.

Frankly even I get wrapped around the axle with what Chevy did with these engines. Consider this:

- The 191 head at 76 ccs with a 12 cc dish in the piston for 88 ccs less gasket and clearance volume, this tends to be the heavy duty full size trucks and vans.

- The 191 head is sometimes listed as having 64 ccs and used with an 18 cc piston for 82 ccs in heavy duty trucks and vans. Frankly I've never seen this volume on this head, my previous statement of 64 for this head I suspected was wrong but by the time I saw what I wrote, which isn't always what I'm thinking, the edit window timed out.

- The 193 head at 64 ccs with an 18 cc dish in the piston for 82 ccs less gasket and clearance volume, these were also heavy duty full size trucks and vans.

- Removed duplicate paragraph to above here.

- The 193 head at 64 ccs with a 12 cc dish in the piston for 76 ccs less gasket and clearance volume, these were in light half ton full size trucks and vans with a mild flat tappet cam, and in passenger/police cars with a roller cam of bigger timing than the truck cams.

So getting wrapped around the LO5 as LO5-KHD and LO5-KLD both with flat tappet cams and LO5 passenger/police with a roller cam can get you totally messed up.

For sure the 193 head is the high compression version and it is also common chamber and ports without the swirl vane and with some variance to port volume to the 14011083 or 14096217 which don't use the original center bolt angles so they will accept earlier intake manifolds than the swirl port heads.

So lets wrap this up. GM claims the Heavy Duty LO5-K has a compression ratio of 8.75 to 1. They claim the Light Duty LO5-K and the passenger/police version have 9.5 to 1. I've always found Chevy's compression ratio claims to be optimistic by .2 to .5 to 1.


Ok my brain is tired after this exercise.
Bogie

Last edited by oldbogie; 03-15-2011 at 06:32 PM. Reason: crap I did it again
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Old 03-15-2011, 07:30 PM
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Police LO5's come with flat top pistons also.....and get the old L98 roller cam.

My 90 I installed a set of big valve 305 heads on.
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