170 psi compression!?! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69GMC910 · 11-10-2002, 11:02 PM

Is this way too high for a pump gas 82 350 with a performer plus cam, performer intake, and 600cfm performer carb. The engine unfortunately has 305 heads. What would my compression ratio be approx.?

Thanx

*Fast68Chevy* · 11-10-2002, 11:35 PM

150psi is stock, no one can guess what ratio it is, the only real way to know exactly what it is is by measuring cylinder volume- with piston at TDC, and then at BDC, and then by dividing the TCD volume measurement by the BDC volume measurement

bigjimzlll · 11-10-2002, 11:41 PM

ok..with a standard bore..with .020 piston down....038 head gasket..12cc piston dish..and 58cc chambers..I come up with 9.62-1 C/R..92 octain area..watch your timing..you should be ok

69GMC910 · 11-10-2002, 11:42 PM

Thanks fast68, I thought i was WAY too high, i thought 130 was stock but i was wrong!

Still looking for horsepower #'s on a stock 82 350, and with my minor modifications an approx. hp.!

Thanks again

And thank you bigjim, 9.62 sounds pretty decent, it's just too bad I'll get very little rev out of those 305 heads.

Thanks for working that out!!!

cableguy0 · 11-10-2002, 11:44 PM

i dont mean to sound rude but u most likely killed all hopes of performance with those choked out 305 heads. if i were u i would start lookin for a set of 350 heads to swap in

69GMC910 · 11-10-2002, 11:48 PM

It's just a winter beater cableguy, so i really don't care at the moment what heads I have as long as it gets me to work and back! I will definitley have to look into some new heads though.

67shortie · 11-11-2002, 12:17 AM

You should be fine, I run 170 cranking psi on my 427 and use 89 octane on the street with no detonation. Give it a try, and see how the engine likes it.

alta-aj · 11-11-2002, 03:53 AM

I have 9.5:1 pistons in my 406 and it runs fine on 87 octane.

bigjimzlll · 11-11-2002, 07:10 AM

one thing that really makes a differance is the cam...the more cam you have,the more C/R you can run, without detonation. This is called dynamic compression ratio http://cochise.uia.net/pkelley2/DynamicCR.html
you can go to this link and it will explain it, also you can down load the program and figure out what dcr you have

69GMC910 · 11-11-2002, 02:57 PM

I removed all the pollution crap off the truck as well, so it seems to run alot better now, the 73-87 board says this engine would currently be sitting at anywhere from 230-250 hp, Sound right? I hope so!!, Next step is shorty headers and 3" exhaust!!!

*Fast68Chevy* · 11-11-2002, 03:09 PM

bigjim knows what he is taking about

i dont even know what dynamic compression is really, heh.

im guilty myself of runnign 305 heads on a 350, ran some 1406 heads on a 72 350 engine, but never drove the vehicle with it liek that, just ran it a couple times and well not long after, the great shape rebuilt .030" over 350 4 bolt spun the rod bearings out of it, suppose bearings didnt like the new high compression.. was a super nice clean strong engine, was not even the tiniest spec of carbon or sludge or metal wear material anywhere in it to be found, not in the lifter gallery or oil pan or anything.. stuck the engine in a high rear ratio th350 83 caprice 4 door with stock converter and .488/.488 cam and stock 80's spreadbore alum intake and stock truck ram horns.. wasnt a good combo, hard to get it to idle in gear like that,, but would burn off right rear tire very, veryyyyyy easily, better intake and bigger valves and lots of converter stall and some headers and 3.73 or 4.11 rear would have helped things.

gonna have to do basically same thing all over again for the most part in this 81 c01 out here, the LG( 305 in it has some kind of top end problem, different timing chain didnt help it any and im tired of messing with it and ive got an 85 truck 4 bolt 350 engine minus the heads here in good shape and it now has 82 305 heads on it and i may just have to install it in the truck cuz i realy have nothing else that will easily go into it with the ESC type exh manifolds it has(7 bolt), the 305 heads on the 350 are ESC heads, and so.....
id have to remove crank from the 350 and have rod bearing journals checked out and or turned for new bearings to feel safe running the 305 heads on it., if a new distributor doesnt fix this LG9, well then.,,,, here goes nothing.... i need a truck and they are very hard to find in complete and running shape- of any year- in the last few years now,
gotta do something..

good luck with your engine.

crazy longhorn · 11-11-2002, 04:26 PM

69GMC910, I dont think i would go bigger than 2 1/4" ex haust (duals) on your combo......too big & you loose all the low end tork that you really need on a heavy truck. Im runnin 2 1/2" on my longhorn behind an edelbrock rpm topped 383.......& thats plenty. You dont really need or want 3" pipe till you are in the 450 plus range on hp......

11-10-2002, 11:02 PM	#1
69GMC910 MOTORHEAD! Join Date: May 2000 Location: Devon AB, Canada Posts: 849	170 psi compression!?! Is this way too high for a pump gas 82 350 with a performer plus cam, performer intake, and 600cfm performer carb. The engine unfortunately has 305 heads. What would my compression ratio be approx.? Thanx __________________ 82 GMC High Sierra VortecPro 496 12 Bolt, 3.42's AFR 265 Holley 850 Performer RPM Air Gap Hooker 2455's TCI Turbo 400 Super Street Fighter TRRC 12 Bolt Eaton Posi Mark Williams Axles Cal-Tracs Hoosier 30x10.5 Radial Slick Full weight long box pickup Current best- 11.33@117mph

11-10-2002, 11:41 PM	#3
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	ok..with a standard bore..with .020 piston down....038 head gasket..12cc piston dish..and 58cc chambers..I come up with 9.62-1 C/R..92 octain area..watch your timing..you should be ok __________________ 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

11-10-2002, 11:42 PM	#4
69GMC910 MOTORHEAD! Join Date: May 2000 Location: Devon AB, Canada Posts: 849	Thanks fast68, I thought i was WAY too high, i thought 130 was stock but i was wrong! Still looking for horsepower #'s on a stock 82 350, and with my minor modifications an approx. hp.! Thanks again And thank you bigjim, 9.62 sounds pretty decent, it's just too bad I'll get very little rev out of those 305 heads. Thanks for working that out!!! __________________ 82 GMC High Sierra VortecPro 496 12 Bolt, 3.42's AFR 265 Holley 850 Performer RPM Air Gap Hooker 2455's TCI Turbo 400 Super Street Fighter TRRC 12 Bolt Eaton Posi Mark Williams Axles Cal-Tracs Hoosier 30x10.5 Radial Slick Full weight long box pickup Current best- 11.33@117mph Last edited by 69GMC910; 11-10-2002 at 11:44 PM.

11-10-2002, 11:44 PM	#5
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	i dont mean to sound rude but u most likely killed all hopes of performance with those choked out 305 heads. if i were u i would start lookin for a set of 350 heads to swap in __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

11-10-2002, 11:48 PM	#6
69GMC910 MOTORHEAD! Join Date: May 2000 Location: Devon AB, Canada Posts: 849	It's just a winter beater cableguy, so i really don't care at the moment what heads I have as long as it gets me to work and back! I will definitley have to look into some new heads though. __________________ 82 GMC High Sierra VortecPro 496 12 Bolt, 3.42's AFR 265 Holley 850 Performer RPM Air Gap Hooker 2455's TCI Turbo 400 Super Street Fighter TRRC 12 Bolt Eaton Posi Mark Williams Axles Cal-Tracs Hoosier 30x10.5 Radial Slick Full weight long box pickup Current best- 11.33@117mph

11-10-2002, 11:35 PM	#2
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	150psi is stock, no one can guess what ratio it is, the only real way to know exactly what it is is by measuring cylinder volume- with piston at TDC, and then at BDC, and then by dividing the TCD volume measurement by the BDC volume measurement

11-11-2002, 12:17 AM	#7
67shortie Green club Join Date: May 2002 Location: Central CA Posts: 1,059	You should be fine, I run 170 cranking psi on my 427 and use 89 octane on the street with no detonation. Give it a try, and see how the engine likes it. __________________ 71 short/fleet ECE 4/6 drop, LQ4/700r4/3.42 gears 70 SS396 Chevelle 427/200-4R/3.73posi 61 Lincoln Continental, black on black Son you are going to drive me to drinkin if you don't stop drivin that hot rod lincoln

11-11-2002, 03:53 AM	#8
alta-aj Registered User Join Date: Aug 2001 Location: Qualicum Beach, B.C>, Canada Posts: 117	Pump gas I have 9.5:1 pistons in my 406 and it runs fine on 87 octane.

11-11-2002, 07:10 AM	#9
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	one thing that really makes a differance is the cam...the more cam you have,the more C/R you can run, without detonation. This is called dynamic compression ratio http://cochise.uia.net/pkelley2/DynamicCR.html you can go to this link and it will explain it, also you can down load the program and figure out what dcr you have __________________ 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

11-11-2002, 02:57 PM	#10
69GMC910 MOTORHEAD! Join Date: May 2000 Location: Devon AB, Canada Posts: 849	I removed all the pollution crap off the truck as well, so it seems to run alot better now, the 73-87 board says this engine would currently be sitting at anywhere from 230-250 hp, Sound right? I hope so!!, Next step is shorty headers and 3" exhaust!!! __________________ 82 GMC High Sierra VortecPro 496 12 Bolt, 3.42's AFR 265 Holley 850 Performer RPM Air Gap Hooker 2455's TCI Turbo 400 Super Street Fighter TRRC 12 Bolt Eaton Posi Mark Williams Axles Cal-Tracs Hoosier 30x10.5 Radial Slick Full weight long box pickup Current best- 11.33@117mph

11-11-2002, 03:09 PM	#11
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	bigjim knows what he is taking about i dont even know what dynamic compression is really, heh. im guilty myself of runnign 305 heads on a 350, ran some 1406 heads on a 72 350 engine, but never drove the vehicle with it liek that, just ran it a couple times and well not long after, the great shape rebuilt .030" over 350 4 bolt spun the rod bearings out of it, suppose bearings didnt like the new high compression.. was a super nice clean strong engine, was not even the tiniest spec of carbon or sludge or metal wear material anywhere in it to be found, not in the lifter gallery or oil pan or anything.. stuck the engine in a high rear ratio th350 83 caprice 4 door with stock converter and .488/.488 cam and stock 80's spreadbore alum intake and stock truck ram horns.. wasnt a good combo, hard to get it to idle in gear like that,, but would burn off right rear tire very, veryyyyyy easily, better intake and bigger valves and lots of converter stall and some headers and 3.73 or 4.11 rear would have helped things. gonna have to do basically same thing all over again for the most part in this 81 c01 out here, the LG( 305 in it has some kind of top end problem, different timing chain didnt help it any and im tired of messing with it and ive got an 85 truck 4 bolt 350 engine minus the heads here in good shape and it now has 82 305 heads on it and i may just have to install it in the truck cuz i realy have nothing else that will easily go into it with the ESC type exh manifolds it has(7 bolt), the 305 heads on the 350 are ESC heads, and so..... id have to remove crank from the 350 and have rod bearing journals checked out and or turned for new bearings to feel safe running the 305 heads on it., if a new distributor doesnt fix this LG9, well then.,,,, here goes nothing.... i need a truck and they are very hard to find in complete and running shape- of any year- in the last few years now, gotta do something.. good luck with your engine. Last edited by Fast68Chevy; 11-11-2002 at 03:15 PM.

11-11-2002, 04:26 PM	#12
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	69GMC910, I dont think i would go bigger than 2 1/4" ex haust (duals) on your combo......too big & you loose all the low end tork that you really need on a heavy truck. Im runnin 2 1/2" on my longhorn behind an edelbrock rpm topped 383.......& thats plenty. You dont really need or want 3" pipe till you are in the 450 plus range on hp...... __________________ 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....



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