another compression ratio question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

stknrdr · 12-04-2002, 06:12 PM

Making a long story short here, my 350 has about 10k miles on it from a (i thought) professional rebuild. Started running poorly so I compression tested and voila two dead cylinders. Pulled the heads off and sure enough 2 burnt valves (middle two cylinders, right hand side). Of course the engine is out of warrantly, so i guess its a good excuse for vortec heads.

When I pulled off the heads, I noticed that my supposed to be flattop pistons were, in fact dished. I called the rebuilder to tryand find out how many cc's of dish they have so I can calculate a compression ratio for the vortec heads....they didnt know, but said they are same as stock and the compression ratio for dished pistons and a 76cc head should be 8.5:1 - so by playing with the compression ratio calculator I come up with about 12 cc's of dish.

Does this sound correct?

If so, the builders mistake may be a blessing in disguise as with the dish ill be under 10:1 with the vortecs. I'm just trying to figure it out as exactly as I can with what info I can muster up.

Im trying to figure out what kind of hp and torque improvement to expect, this engine has always been a bit lackluster and now I know why.

Anyway here are the specs, any guesses on what the Hp/torque numbers should be?

350 - .030 over
edelbrock performer intake, performer carb (1406)
headers, 2.5'' exhaust with flowmaster 40's
cam 206/214 duration .432/.453 lift
vortec heads (yes i upgraded the springs)
apparently stock dish on the pistons

bigjimzlll · 12-04-2002, 06:37 PM

around 320 hp and 360 torque..if you give the make and number of the cam Ill run a dyno for you...12cc dish is normal your C/R should be around 9.5-1. The vortecs can handle more c/R than a regular cast head(because of chamber design)

vfrdude · 12-04-2002, 06:37 PM

I think I would be concerned with what caused the valves to burn up in the first place... you don't want to go thru the process of installing new heads and have burnt valves within 10k....

crazy longhorn · 12-04-2002, 06:40 PM

try this link... http://www.smokemup.com/auto_math/compression_ratio.php

stknrdr · 12-04-2002, 06:48 PM

The cam is comp cams' 4x4 extreme 12-231-2 is the part number in my summit racing catalog.

As to why the valves burnt in the first place, Im thinking it has to do with the fact that it always ran a little hot, my original radiator never seemed to be able to keep up with it. I never let it get THAT hot though, I think its a contributing factor (its corrected with a big 'ol 4 core now)- I'm thinking more along the lines of a crappy valve job (after i started poking around I find that others have had the same problem from this shop).

I can't think of any more reasons, the other cylinders were all fine, just the two center ones burnt (and from what I understand they are the hottest ones too)).

Hopefully with new heads and the new radiator it wont happen again

stknrdr · 12-04-2002, 07:15 PM

Heheh in re-reading my posts, in case anyone is wondering about me, I dont really blame the builder for the burnt valves, that could have been any number of things- (and then of course there was that pesky overheating I had for awhile, I didnt think it was getting that hot, but, how accurate is my 1972 gauge, and, the sender is on the other side of the engine from where the burnt valves were)

I am, however a little miffed at them for not giving me the ~9.5 compression ratio I specified when I ordered the engine built. Im not going to bother them with it at this point, as, like I mentioned before it will work nicely with the vortec heads that are bolted on now - (havent fired it yet, waiting for the intake from summit).

And other people have reported burnt valves from the same shop - using old valves a little too machined perhaps?

But at this point im just going to write it off and enjoy my new vortecs.

bigjimzlll · 12-04-2002, 07:17 PM

max HP @5000rpm is 315 and max torque @3000rpm is 405

stknrdr · 12-04-2002, 07:22 PM

wow 405 ft/lb torque? taaaaaasty

12-04-2002, 06:12 PM	#1
stknrdr Registered User Join Date: Apr 2001 Location: Utah USA Posts: 27	another compression ratio question Making a long story short here, my 350 has about 10k miles on it from a (i thought) professional rebuild. Started running poorly so I compression tested and voila two dead cylinders. Pulled the heads off and sure enough 2 burnt valves (middle two cylinders, right hand side). Of course the engine is out of warrantly, so i guess its a good excuse for vortec heads. When I pulled off the heads, I noticed that my supposed to be flattop pistons were, in fact dished. I called the rebuilder to tryand find out how many cc's of dish they have so I can calculate a compression ratio for the vortec heads....they didnt know, but said they are same as stock and the compression ratio for dished pistons and a 76cc head should be 8.5:1 - so by playing with the compression ratio calculator I come up with about 12 cc's of dish. Does this sound correct? If so, the builders mistake may be a blessing in disguise as with the dish ill be under 10:1 with the vortecs. I'm just trying to figure it out as exactly as I can with what info I can muster up. Im trying to figure out what kind of hp and torque improvement to expect, this engine has always been a bit lackluster and now I know why. Anyway here are the specs, any guesses on what the Hp/torque numbers should be? 350 - .030 over edelbrock performer intake, performer carb (1406) headers, 2.5'' exhaust with flowmaster 40's cam 206/214 duration .432/.453 lift vortec heads (yes i upgraded the springs) apparently stock dish on the pistons __________________ 72 C/10 Fully Restored Hugger Orange - 355, 4" lift 35's Auburn limited slip diffs front and rear, 4.56 gears. Vortec Heads, 700r4/NP208 combo. On the burner: Roller Cam, Blower 77 Ford Bronco Rockcrawler 93 BMW 325IS Have-fun-on-the-way-to-work-car.

12-04-2002, 06:37 PM	#2
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	around 320 hp and 360 torque..if you give the make and number of the cam Ill run a dyno for you...12cc dish is normal your C/R should be around 9.5-1. The vortecs can handle more c/R than a regular cast head(because of chamber design) __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

12-04-2002, 06:37 PM	#3
vfrdude Registered User Join Date: May 2002 Location: Oakley, Ca. USA Posts: 137	I think I would be concerned with what caused the valves to burn up in the first place... you don't want to go thru the process of installing new heads and have burnt valves within 10k.... __________________ Rudy Fierros '95 Honda VFR750F Used to have a '71 Chevy Stepside, Guards Red, Lowered 4" rear, 3" front, White Oak bed wood, 700R4 w/373 gears until some low life stole it!

12-04-2002, 06:40 PM	#4
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	try this link... http://www.smokemup.com/auto_math/compression_ratio.php __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

12-04-2002, 06:48 PM	#5
stknrdr Registered User Join Date: Apr 2001 Location: Utah USA Posts: 27	The cam is comp cams' 4x4 extreme 12-231-2 is the part number in my summit racing catalog. As to why the valves burnt in the first place, Im thinking it has to do with the fact that it always ran a little hot, my original radiator never seemed to be able to keep up with it. I never let it get THAT hot though, I think its a contributing factor (its corrected with a big 'ol 4 core now)- I'm thinking more along the lines of a crappy valve job (after i started poking around I find that others have had the same problem from this shop). I can't think of any more reasons, the other cylinders were all fine, just the two center ones burnt (and from what I understand they are the hottest ones too)). Hopefully with new heads and the new radiator it wont happen again __________________ 72 C/10 Fully Restored Hugger Orange - 355, 4" lift 35's Auburn limited slip diffs front and rear, 4.56 gears. Vortec Heads, 700r4/NP208 combo. On the burner: Roller Cam, Blower 77 Ford Bronco Rockcrawler 93 BMW 325IS Have-fun-on-the-way-to-work-car.

12-04-2002, 07:15 PM	#6
stknrdr Registered User Join Date: Apr 2001 Location: Utah USA Posts: 27	Heheh in re-reading my posts, in case anyone is wondering about me, I dont really blame the builder for the burnt valves, that could have been any number of things- (and then of course there was that pesky overheating I had for awhile, I didnt think it was getting that hot, but, how accurate is my 1972 gauge, and, the sender is on the other side of the engine from where the burnt valves were) I am, however a little miffed at them for not giving me the ~9.5 compression ratio I specified when I ordered the engine built. Im not going to bother them with it at this point, as, like I mentioned before it will work nicely with the vortec heads that are bolted on now - (havent fired it yet, waiting for the intake from summit). And other people have reported burnt valves from the same shop - using old valves a little too machined perhaps? But at this point im just going to write it off and enjoy my new vortecs. __________________ 72 C/10 Fully Restored Hugger Orange - 355, 4" lift 35's Auburn limited slip diffs front and rear, 4.56 gears. Vortec Heads, 700r4/NP208 combo. On the burner: Roller Cam, Blower 77 Ford Bronco Rockcrawler 93 BMW 325IS Have-fun-on-the-way-to-work-car.

12-04-2002, 07:17 PM	#7
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	max HP @5000rpm is 315 and max torque @3000rpm is 405 __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

12-04-2002, 07:22 PM	#8
stknrdr Registered User Join Date: Apr 2001 Location: Utah USA Posts: 27	wow 405 ft/lb torque? taaaaaasty __________________ 72 C/10 Fully Restored Hugger Orange - 355, 4" lift 35's Auburn limited slip diffs front and rear, 4.56 gears. Vortec Heads, 700r4/NP208 combo. On the burner: Roller Cam, Blower 77 Ford Bronco Rockcrawler 93 BMW 325IS Have-fun-on-the-way-to-work-car.



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