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

brennecke · 04-01-2002, 05:04 PM

Checked my compression, and all the cylinders make 130-145 psi. Somebody told me I should be getting 185-215. How bad off am I? The motor seems to run plenty strong.
71 350 4bbl

CoryM · 04-01-2002, 05:20 PM

Well it depends on what engine it is and what compression ratio it has. That is about normal for a stock smogger with some miles on it. I think 150 is what they run at best. As long as the cylinders are within 20psi of each other you should be fine.
CoryM

------------------
1970 heavy duty C-10 fleetside sport truck. Vancouver B.C. Canada
http://www.geocities.com/chevroletc1070

"Take up our quarrel with the foe:
To you from failing hands we throw
The torch; be yours to hold it high."
Lieutenant Colonel John McCrae, Canadian Army

69 longhorn · 04-01-2002, 05:51 PM

You are fine on a stock 71 350......

**MikeB** · 04-01-2002, 08:24 PM

It should be prox 140-150. Anything more than 160 will probably require high-octane gas. 185-215 and you'd have a diesel! Be sure to remove all the plugs and hold the throttle open when you test it.

------------------
MikeB
69 C-10 Fleetside, mild 383, Dart Iron Eagle 180cc heads, Crane Energizer 266 cam and 1.6 roller rockers, Edelbrock Performer manifold and 600cfm carb, HEI w/ MSD components, 1986 front spindles/disc brakes, Vintage Air in-dash, lowered 2"/3".

brennecke · 04-02-2002, 09:51 AM

Rock on! I thought it felt strong. The biggest difference is only 15. I didn't hammer on the throttle either. I just waited for it to stop increasing on the meter. Pretty good for a stock engine with 106K on then.

Custom 68 · 04-02-2002, 02:30 PM

Is there a way from this info to determine what compression ratio you are running? If you know what cc the heads are. My engine was rebuilt several years ago and I dont know what pistons were put in them.

Thanks
Dave

[This message has been edited by Custom 68 (edited April 02, 2002).]

*Fast68Chevy* · 04-02-2002, 04:08 PM

diesel engine compression is 220 or 225 if you look in specs 150 is average normal for most all sbc excp hi-pef 60's musclecar engines

to determine cr you owuld need ot find volume of entire chamber with head gasket and all, first with pisotn at bdc and then at tdc and divide bdc volume by the tdc volume, IE if you had 850cc at bdc and 100 at tdc then youd have 8.5cr

you could get pretty damn close if you know the volume of the head chamber(probably 58, 75, or 76cc most likely) and then added a few more for the head gasket cylinder opening volume and added those to the cubic volume of the entire cylinder with the piston at bdc and then divided that sum by the sum of the combined volume of the head chamber and cylinder gasket opening volume and cylinder volume with piston at tdc
if you measure the head chamber volume yourslef then make sure you put in spark plug, and use TSX plugs to take up more room and increase the cr a little bit

good luck

[This message has been edited by Fast68Chevy (edited April 02, 2002).]

Custom 68 · 04-02-2002, 09:24 PM

Thanks Fast Sounds like a lesson in Math...
Dave

69 longhorn · 04-02-2002, 10:01 PM

The 461 " Olds that I ran in the longhorn had 190-195 psi......didnt like pump fuel too much! It turned out to be a little wild for my bro inlaws cutlass vert also.....after he did a few doughnuts on the street,he sold it......69 longhorn.

Rob Dundrea · 04-02-2002, 10:20 PM

you can't really tell what the compression ratio is from cranking pressure, i used to drive a dirt track car that was 9 to 1 compression with a 2bbl and cranking pressure was about 205, it depends on what the cam timing is, that motor ran on 105 octane an made 550hp, not bad for a 2bbl!!!

------------------
69 CST 10 496/700R4 10.40 @ 135mph, and its street legal!!!!
89 F/S Blazer 388, vortec blower,(eats SST's and s@!ts Lightnings)
67 Chevelle 350H/327, L79
screamer!!
97 Grand Am GT (wifes car)its not all that fast but it handles awesome!!

Its not how much it costs, it how well you hide it from your wife!!!!

REPO1 · 04-02-2002, 10:27 PM

I agree with Rob, the overlap of the cam will make a world of difference. No overlap would give you alot more, where lots of overlap would reduce your cranking pressure.

------------------
Sylvan Lake, Alberta, Canada

1970 Chevy C20 Custom Camper
#s matching 402BB Turbo 400 trans.
Air conditioning, power steering, manual brakes.
73,600 Original Miles!!
(Add on 2000 more from raod trip!)

04-01-2002, 05:04 PM	#1
brennecke Registered User Join Date: Apr 2000 Location: Missouri Posts: 469	compression ? Checked my compression, and all the cylinders make 130-145 psi. Somebody told me I should be getting 185-215. How bad off am I? The motor seems to run plenty strong. 71 350 4bbl __________________ Never tire of doing what is right II Thess. 3:13 Northeast Missouri 71 LWB original 350 3/speed Soon to be 500

04-01-2002, 05:20 PM	#2
CoryM Registered User Join Date: Dec 1999 Location: Surrey, B.C. , Canada Posts: 874	Well it depends on what engine it is and what compression ratio it has. That is about normal for a stock smogger with some miles on it. I think 150 is what they run at best. As long as the cylinders are within 20psi of each other you should be fine. CoryM ------------------ 1970 heavy duty C-10 fleetside sport truck. Vancouver B.C. Canada http://www.geocities.com/chevroletc1070 "Take up our quarrel with the foe: To you from failing hands we throw The torch; be yours to hold it high." Lieutenant Colonel John McCrae, Canadian Army __________________ 1970 heavy duty C-10 fleetside sport truck. Vancouver B.C. Canada http://www.geocities.com/chevroletc1070 "Take up our quarrel with the foe: To you from failing hands we throw The torch; be yours to hold it high." Lieutenant Colonel John McCrae, Canadian Army

04-01-2002, 05:51 PM	#3
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	You are fine on a stock 71 350...... __________________ http://community.webshots.com/user/hotrodhorn

04-01-2002, 08:24 PM	#4
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,599	It should be prox 140-150. Anything more than 160 will probably require high-octane gas. 185-215 and you'd have a diesel! Be sure to remove all the plugs and hold the throttle open when you test it. ------------------ MikeB 69 C-10 Fleetside, mild 383, Dart Iron Eagle 180cc heads, Crane Energizer 266 cam and 1.6 roller rockers, Edelbrock Performer manifold and 600cfm carb, HEI w/ MSD components, 1986 front spindles/disc brakes, Vintage Air in-dash, lowered 2"/3". __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

04-02-2002, 09:51 AM	#5
brennecke Registered User Join Date: Apr 2000 Location: Missouri Posts: 469	Rock on! I thought it felt strong. The biggest difference is only 15. I didn't hammer on the throttle either. I just waited for it to stop increasing on the meter. Pretty good for a stock engine with 106K on then. __________________ Never tire of doing what is right II Thess. 3:13 Northeast Missouri 71 LWB original 350 3/speed Soon to be 500

04-02-2002, 02:30 PM	#6
Custom 68 Registered User Join Date: Nov 2001 Location: Springfield, MO Posts: 2,939	Is there a way from this info to determine what compression ratio you are running? If you know what cc the heads are. My engine was rebuilt several years ago and I dont know what pistons were put in them. Thanks Dave [This message has been edited by Custom 68 (edited April 02, 2002).] __________________ Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!"

04-02-2002, 04:08 PM	#7
Fast68Chevy Account Suspended Join Date: May 2000 Location: IL Posts: 0	diesel engine compression is 220 or 225 if you look in specs 150 is average normal for most all sbc excp hi-pef 60's musclecar engines to determine cr you owuld need ot find volume of entire chamber with head gasket and all, first with pisotn at bdc and then at tdc and divide bdc volume by the tdc volume, IE if you had 850cc at bdc and 100 at tdc then youd have 8.5cr you could get pretty damn close if you know the volume of the head chamber(probably 58, 75, or 76cc most likely) and then added a few more for the head gasket cylinder opening volume and added those to the cubic volume of the entire cylinder with the piston at bdc and then divided that sum by the sum of the combined volume of the head chamber and cylinder gasket opening volume and cylinder volume with piston at tdc if you measure the head chamber volume yourslef then make sure you put in spark plug, and use TSX plugs to take up more room and increase the cr a little bit good luck [This message has been edited by Fast68Chevy (edited April 02, 2002).]

04-02-2002, 09:24 PM	#8
Custom 68 Registered User Join Date: Nov 2001 Location: Springfield, MO Posts: 2,939	Thanks Fast Sounds like a lesson in Math... Dave __________________ Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!"

04-02-2002, 10:01 PM	#9
69 longhorn Registered User Join Date: Aug 2001 Location: rock island,il,usa Posts: 5,382	The 461 " Olds that I ran in the longhorn had 190-195 psi......didnt like pump fuel too much! It turned out to be a little wild for my bro inlaws cutlass vert also.....after he did a few doughnuts on the street,he sold it......69 longhorn. __________________ http://community.webshots.com/user/hotrodhorn

04-02-2002, 10:20 PM	#10
Rob Dundrea Big Block Power Join Date: Jan 2001 Location: Jersey Posts: 178	you can't really tell what the compression ratio is from cranking pressure, i used to drive a dirt track car that was 9 to 1 compression with a 2bbl and cranking pressure was about 205, it depends on what the cam timing is, that motor ran on 105 octane an made 550hp, not bad for a 2bbl!!! ------------------ 69 CST 10 496/700R4 10.40 @ 135mph, and its street legal!!!! 89 F/S Blazer 388, vortec blower,(eats SST's and s@!ts Lightnings) 67 Chevelle 350H/327, L79 screamer!! 97 Grand Am GT (wifes car)its not all that fast but it handles awesome!! Its not how much it costs, it how well you hide it from your wife!!!!

04-02-2002, 10:27 PM	#11
REPO1 Shortbox wanna-be. Join Date: Mar 2002 Location: Fort St.John, British Columbia, Canada Posts: 773	I agree with Rob, the overlap of the cam will make a world of difference. No overlap would give you alot more, where lots of overlap would reduce your cranking pressure. ------------------ Sylvan Lake, Alberta, Canada 1970 Chevy C20 Custom Camper #s matching 402BB Turbo 400 trans. Air conditioning, power steering, manual brakes. 73,600 Original Miles!! (Add on 2000 more from raod trip!) __________________ 1970 Chevy C20 Custom Camper 402BB Turbo 400 trans. (Slowly becoming a '70 shortbox 1/2 ton) '71 Camaro SS 402/T400 resto '97 Z28 (11.41@127MPH)



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