High Engine Compression
Thread Starter
Instructor
Joined: Feb 2008
Posts: 201
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From: The Ham AL
High Engine Compression
Well, I have been working on my car for 6 months now on and off.
I have posted here that my car sounds like a dragster and I have been working to clean up the engine a bit.
Today, I ran a compression test with the help of a trusty forum member. We did it just like the stack of firewood (FSM) said to and this is what we found out. Some cylinders we checked multiple times for good measure. Keep in mind that the FSM said 100 psi is stock correct test.
Cylinder 1 - 190 & 210 psi
Cylinder 2 - 195
Cylinder 3 - 175 & 190
Cylinder 4 - 190
Cylinder 5 - 175 & 190
Cylinder 6 - 190
Cylinder 7 - 190
Cylinder 8 - 205
I thought my car might be a higher compression, but
Question 1: What is considered high? Is 190 typical or does it just vary?
Question 2: If a car has a high compression engine, is it typical to run at lower vaccum? My vacuum is around 12 in Hg. I understand that a stock engine should run around 20 in Hg.
Any thoughts?
I have posted here that my car sounds like a dragster and I have been working to clean up the engine a bit.
Today, I ran a compression test with the help of a trusty forum member. We did it just like the stack of firewood (FSM) said to and this is what we found out. Some cylinders we checked multiple times for good measure. Keep in mind that the FSM said 100 psi is stock correct test.
Cylinder 1 - 190 & 210 psi
Cylinder 2 - 195
Cylinder 3 - 175 & 190
Cylinder 4 - 190
Cylinder 5 - 175 & 190
Cylinder 6 - 190
Cylinder 7 - 190
Cylinder 8 - 205
I thought my car might be a higher compression, but
Question 1: What is considered high? Is 190 typical or does it just vary?
Question 2: If a car has a high compression engine, is it typical to run at lower vaccum? My vacuum is around 12 in Hg. I understand that a stock engine should run around 20 in Hg.
Any thoughts?
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 15
From: backwoods upstate ny
what the guage readings are is of very little importance...what is critical is that all be about the same, as yours are...reason for this is that there are dozens of variables that affect the total pressure reading, not least of which is the gauge itself...try a different gauge and see what you see, those do look "high" for a street driver.
Thread Starter
Instructor
Joined: Feb 2008
Posts: 201
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From: The Ham AL
You guys that mentioned that you measured readings as high as mine....what is your vacuum reading?
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Safety Car
Joined: Sep 2001
Posts: 4,150
Likes: 551
From: Glenbeulah Wi
Well, I have been working on my car for 6 months now on and off.
I have posted here that my car sounds like a dragster and I have been working to clean up the engine a bit.
Today, I ran a compression test with the help of a trusty forum member. We did it just like the stack of firewood (FSM) said to and this is what we found out. Some cylinders we checked multiple times for good measure. Keep in mind that the FSM said 100 psi is stock correct test.
Cylinder 1 - 190 & 210 psi
Cylinder 2 - 195
Cylinder 3 - 175 & 190
Cylinder 4 - 190
Cylinder 5 - 175 & 190
Cylinder 6 - 190
Cylinder 7 - 190
Cylinder 8 - 205
I thought my car might be a higher compression, but
Question 1: What is considered high? Is 190 typical or does it just vary?
Question 2: If a car has a high compression engine, is it typical to run at lower vaccum? My vacuum is around 12 in Hg. I understand that a stock engine should run around 20 in Hg.
Any thoughts?
I have posted here that my car sounds like a dragster and I have been working to clean up the engine a bit.
Today, I ran a compression test with the help of a trusty forum member. We did it just like the stack of firewood (FSM) said to and this is what we found out. Some cylinders we checked multiple times for good measure. Keep in mind that the FSM said 100 psi is stock correct test.
Cylinder 1 - 190 & 210 psi
Cylinder 2 - 195
Cylinder 3 - 175 & 190
Cylinder 4 - 190
Cylinder 5 - 175 & 190
Cylinder 6 - 190
Cylinder 7 - 190
Cylinder 8 - 205
I thought my car might be a higher compression, but
Question 1: What is considered high? Is 190 typical or does it just vary?
Question 2: If a car has a high compression engine, is it typical to run at lower vaccum? My vacuum is around 12 in Hg. I understand that a stock engine should run around 20 in Hg.
Any thoughts?
Thread Starter
Instructor
Joined: Feb 2008
Posts: 201
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From: The Ham AL
No, I do not know the history of the car.
BTW, the timing is set to 6 degrees TDC, but it was off when I got the car.
Might have a higher lift cam. I don't know....yet.
I am pretty sure I have a modified engine (understatement).
BTW, the timing is set to 6 degrees TDC, but it was off when I got the car.
Might have a higher lift cam. I don't know....yet.
I am pretty sure I have a modified engine (understatement).
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
compression
I have an 86 with flat top pistons w/ 4 valve reliefs. Stock bottom end. The old heads were 76 cc.
I installed 113 heads, with 58cc chambers.
It only likes 91 or better gas, but my mileage is high because now it has become a more efficient air pump. The static compression should be 10.75 with my setup.
but it doesn't ping, and I can't tell if it is pulling the timing back anymore since I changed the injectors. Runs pretty good.
Vacuum is pretty much determined by your cam. And all those vacuum leaks you have been ignoring
I installed 113 heads, with 58cc chambers.
It only likes 91 or better gas, but my mileage is high because now it has become a more efficient air pump. The static compression should be 10.75 with my setup.
but it doesn't ping, and I can't tell if it is pulling the timing back anymore since I changed the injectors. Runs pretty good.
Vacuum is pretty much determined by your cam. And all those vacuum leaks you have been ignoring
Last edited by coupeguy2001; Jul 15, 2008 at 04:41 PM.
Thread Starter
Instructor
Joined: Feb 2008
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From: The Ham AL
So, what I am basically hearing is that vacuum should be around 20-24 in Hg for every car on the forum or you have vacuum leaks that should be dealt with. I'm not trying to put words in your mouths, but I haven't recieved a response to indicate otherwise.
I plan on doing an injector replacement within the month and reseal as many areas as I can get to that might cause a vacuum leak.
Thanks for the replies.
I plan on doing an injector replacement within the month and reseal as many areas as I can get to that might cause a vacuum leak.
Thanks for the replies.
Safety Car
Joined: Sep 2001
Posts: 4,150
Likes: 551
From: Glenbeulah Wi
So, what I am basically hearing is that vacuum should be around 20-24 in Hg for every car on the forum or you have vacuum leaks that should be dealt with. I'm not trying to put words in your mouths, but I haven't recieved a response to indicate otherwise.
I plan on doing an injector replacement within the month and reseal as many areas as I can get to that might cause a vacuum leak.
Thanks for the replies.
I plan on doing an injector replacement within the month and reseal as many areas as I can get to that might cause a vacuum leak.
Thanks for the replies.