Compression testing
Thread Starter
Burning Brakes
Joined: Mar 2005
Posts: 1,050
Likes: 710
From: San Antonio Texas
Compression testing
My 70 L-46 is supposed to have 11:1 compression and 350hp. It just doesn't seem to have much "get up and go" to me. Granted my last vette was an LT-4 car and I am used to power, this one just seems anemic. I have pressure guages, but am ignorant on how to use them. Is there previous thread related to compression testing? Or can someone briefly explain to me how to test. This motor has been rebuilt, so I am wondering if it may have been rebuilt with the wrong parts. Also any recommendations on other likely culprits to check would really be appreciated. This car seems very sluggish..... As always, Thanks in advance, Lee
Race Director
Joined: Oct 2005
Posts: 12,464
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
My old L-79 327 with an L-46 cam was a screamer. Very responsive. Same advertised 11:1, 2.02 heads, basically stock motor except for ehheaders.
It had about 190-200 pounds of cranking compression.
Doug
It had about 190-200 pounds of cranking compression.
Doug
Thread Starter
Burning Brakes
Joined: Mar 2005
Posts: 1,050
Likes: 710
From: San Antonio Texas
I have no clue what cam/pistons etc is in it. It was "rebuilt" in the early 90's (15,000 miles ago).... It has a Holley on it instead of the QJ. I really don't know where to start. I figured checking the compression would be a good start.... yes/no???
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
No offense meant...but, if you don't know where to start, you shouldn't be "doctoring" your own engine. And I doubt that you are going to learn enough from asking questions on this Forum to be able to do a good job at 'fixing' it--if it even needs fixing. Your first order of business is to identify a competent, affordable mechanic who can assess the condition of your engine and let him/her clear things up.
Contact a local Corvette club or vintage car club to see who they might recommend. Stay with a private shop...not a chain store.
Contact a local Corvette club or vintage car club to see who they might recommend. Stay with a private shop...not a chain store.
Le Mans Master
Joined: Aug 2006
Posts: 7,411
Likes: 1,550
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Many others had no clue either but there are plenty of very helpful and knowledgeable people on the forum so ask away. If you don't mind grease under your nails and have some tools then you can do it! A 70 Service Manual and an AIM would help a lot.
Just because you have a compression tester doesn't mean that should be the first test.
You have points? Check or replace them,
Buy a dwell meter-dwell should be 29-31 and remain steady as you increase RPM.
Get a Dial Back timing light-disconnect and plug the vacuum advance hose and post back the numbers at idle and increase the RPM slowly to about 3,000 and see how much mechanical advance you have.
Let enging idle and reconnect the vacuum advance hose-post how much timing increases or not.
Check or change the spark plugs and now you can do the compression test.
Leave the carb alone for now.
Just because you have a compression tester doesn't mean that should be the first test.
You have points? Check or replace them,
Buy a dwell meter-dwell should be 29-31 and remain steady as you increase RPM.
Get a Dial Back timing light-disconnect and plug the vacuum advance hose and post back the numbers at idle and increase the RPM slowly to about 3,000 and see how much mechanical advance you have.
Let enging idle and reconnect the vacuum advance hose-post how much timing increases or not.
Check or change the spark plugs and now you can do the compression test.
Leave the carb alone for now.
Thread Starter
Burning Brakes
Joined: Mar 2005
Posts: 1,050
Likes: 710
From: San Antonio Texas
Many others had no clue either but there are plenty of very helpful and knowledgeable people on the forum so ask away. If you don't mind grease under your nails and have some tools then you can do it! A 70 Service Manual and an AIM would help a lot.
Just because you have a compression tester doesn't mean that should be the first test.
You have points? Check or replace them,
Buy a dwell meter-dwell should be 29-31 and remain steady as you increase RPM.
Get a Dial Back timing light-disconnect and plug the vacuum advance hose and post back the numbers at idle and increase the RPM slowly to about 3,000 and see how much mechanical advance you have.
Let enging idle and reconnect the vacuum advance hose-post how much timing increases or not.
Check or change the spark plugs and now you can do the compression test.
Leave the carb alone for now.
Just because you have a compression tester doesn't mean that should be the first test.
You have points? Check or replace them,
Buy a dwell meter-dwell should be 29-31 and remain steady as you increase RPM.
Get a Dial Back timing light-disconnect and plug the vacuum advance hose and post back the numbers at idle and increase the RPM slowly to about 3,000 and see how much mechanical advance you have.
Let enging idle and reconnect the vacuum advance hose-post how much timing increases or not.
Check or change the spark plugs and now you can do the compression test.
Leave the carb alone for now.
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Melting Slicks
Joined: May 2009
Posts: 3,079
Likes: 314
2024 C3 of the Year Finalist - Unmodified
Years ago I was told by the local vette guru the first thing he checks, when a customer complains about a lack of get-up-and-go on these cars, is the accelerator. Imagine that.
On the NCRS board a while back, somebody was trying to diagnose a lack of performance. I told him to first check and see if the throttle cable was opening the carb all the way. He did not, then followed the stream of advice from others and had the carb rebuilt...and just about everything else. Finally he noticed the accelerator pedal arm was bent, which is common after many years of mashing it to the floor. Simple fix just to straighten it out.
On the NCRS board a while back, somebody was trying to diagnose a lack of performance. I told him to first check and see if the throttle cable was opening the carb all the way. He did not, then followed the stream of advice from others and had the carb rebuilt...and just about everything else. Finally he noticed the accelerator pedal arm was bent, which is common after many years of mashing it to the floor. Simple fix just to straighten it out.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
Lots of folks think that using their hand to actuate the throttle linkage on the carb is the same as depressing the accelerator. Not so! There is a pedal assembly, a cable, and some other hardware in the system before it gets to the carb linkage. And, if you can't verify that depressing the accel pedal [with your foot] will fully open the carb secondaries, you shouldn't even be thinking of working on the engine...yet. 
Safety Car
Joined: Sep 2007
Posts: 4,443
Likes: 326
From: Indy Indiana
It depends on your cam profile as to what compression you are getting, the later the intake valve closes, the less compression (at cranking speed) you are going to get.
listen to all the above advice, you need good ignition, good carb setup , good mechanical and vacumm advance, etc..
listen to all the above advice, you need good ignition, good carb setup , good mechanical and vacumm advance, etc..
Le Mans Master
Joined: Aug 2006
Posts: 7,411
Likes: 1,550
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Lots of folks think that using their hand to actuate the throttle linkage on the carb is the same as depressing the accelerator. Not so! There is a pedal assembly, a cable, and some other hardware in the system before it gets to the carb linkage. And, if you can't verify that depressing the accel pedal [with your foot] will fully open the carb secondaries, you shouldn't even be thinking of working on the engine...yet.
Thread Starter
Burning Brakes
Joined: Mar 2005
Posts: 1,050
Likes: 710
From: San Antonio Texas
Years ago I was told by the local vette guru the first thing he checks, when a customer complains about a lack of get-up-and-go on these cars, is the accelerator. Imagine that.
On the NCRS board a while back, somebody was trying to diagnose a lack of performance. I told him to first check and see if the throttle cable was opening the carb all the way. He did not, then followed the stream of advice from others and had the carb rebuilt...and just about everything else. Finally he noticed the accelerator pedal arm was bent, which is common after many years of mashing it to the floor. Simple fix just to straighten it out.
On the NCRS board a while back, somebody was trying to diagnose a lack of performance. I told him to first check and see if the throttle cable was opening the carb all the way. He did not, then followed the stream of advice from others and had the carb rebuilt...and just about everything else. Finally he noticed the accelerator pedal arm was bent, which is common after many years of mashing it to the floor. Simple fix just to straighten it out.
Le Mans Master
Joined: Apr 2001
Posts: 5,941
Likes: 281
From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
What have you done to the car? Have you checked the timing? Looked at the spark plugs? Checked the wires, cap and rotor for wear? I would start with those first and get more complex from there. Is the car still on a points distributor, hei, msd or other type? The carb could need some work on jetting. The valves may need adjusting if it is a solid lifter cam. There are a lot of simple issues it could be before I would go checking compression. That is just the quick stuff I can think of off the top of my head. Start looking there and give us a full run down and we can suggest more ideas from there.
Burning Brakes
Joined: Mar 2005
Posts: 861
Likes: 0
From: WNY
I will say - My 383 Stroker doesnt feel 'all that powerful' in the low end on my car.. it REALLY wakes up in the mid-higher RPMS because of the cam profile and the gearing.
Feel is just too subjective. A torque monster can feel WAY more powerful than it is - instant torque just gives you that illusion. My car will pull like crazy, but it is from nearly 3000rpm - 6300rpm or so that it really FEELS like it has power. I know for a fact - I raced a buddy @ the strip last year who has an 85 4+3 lightly modded. His car makes ALL its power low - andif you ride with him to say 40mph, then ride with me to 40mph, you would SWEAR his car would stomp mine. Put them on the strip? I had a football field on him by the end of second.
Thats why the butt dyno is so unreliable for really gauging how fast a car is.
Feel is just too subjective. A torque monster can feel WAY more powerful than it is - instant torque just gives you that illusion. My car will pull like crazy, but it is from nearly 3000rpm - 6300rpm or so that it really FEELS like it has power. I know for a fact - I raced a buddy @ the strip last year who has an 85 4+3 lightly modded. His car makes ALL its power low - andif you ride with him to say 40mph, then ride with me to 40mph, you would SWEAR his car would stomp mine. Put them on the strip? I had a football field on him by the end of second.
Thats why the butt dyno is so unreliable for really gauging how fast a car is.
Safety Car
Joined: Sep 2007
Posts: 4,443
Likes: 326
From: Indy Indiana
use a compression test to not check for total compression per cylinder, but use it to check for a difference in any particular cylinder. low compression on any one particular cylinder that is say less than 90% of the rest may point you to valve/ring/head gasket/ flat cam types of issues.
this is similar to a troubleshooting trick of pulling one plug wire and measuring rpm drop at idle.. each cylinder should show equal or real close to equal result.
this is similar to a troubleshooting trick of pulling one plug wire and measuring rpm drop at idle.. each cylinder should show equal or real close to equal result.