can cams lower the compression?

[boostinboy]

hey guys, I need some help. after i got my jdm engine installed my mechanic said the compression was 150 accross. But a coulpe of days after when he installed my 272 cams he said it's normal for the compression to drop down to 130. is he telling the truth? or should i go back and demand a motor with good compression?

 

[2.4 turbo Hyped]

hey guys, I need some help. after i got my jdm engine installed my mechanic said the compression was 150 accross. But a coulpe of days after when he installed my 272 cams he said it's normal for the compression to drop down to 130. is he telling the truth? or should i go back and demand a motor with good compression?

What kind of cams are they? I dont think the compression would change unless your cam timing is off a tooth or two.

 

[myblack98gst]

ditto

 

[92awddsm]

Well, my compression was 168-170 without cams but now(this past weekend, same location, same compression gauge, same everything) with the comp big cams, I got 173-175 on all cylinders. The only reason I could think of it being low is if the belt is wrong.

 

[Black94TSI]

If it was run under the EXACT same conditions, same temperature of the motor, same air temperature and barometric pressure, etc, the conditions SHOULD read lower on the compression tester. Just think about it for a minute, more duration means that the exhaust cam is going to stay open longer and it is going to open more than with the stock cams. The exhaust cam isn't going to close as soon as the piston hits BDC and heads towards TDC for the compression stroke, so you can assume that some cylinder pressure is going to be bled off with the larger cams. This is why cars with larger cams can run more boost on pump gas, and naturally aspirated motors can run more compression.

 

[2.4 turbo Hyped]

Well, my compression was 168-170 without cams but now(this past weekend, same location, same compression gauge, same everything) with the comp big cams, I got 173-175 on all cylinders. The only reason I could think of it being low is if the belt is wrong.

Exactly what I was thinking. Start there. Let us know if things work out.

 

[boostedinaz]

If it was run under the EXACT same conditions, same temperature of the motor, same air temperature and barometric pressure, etc, the conditions SHOULD read lower on the compression tester. Just think about it for a minute, more duration means that the exhaust cam is going to stay open longer and it is going to open more than with the stock cams. The exhaust cam isn't going to close as soon as the piston hits BDC and heads towards TDC for the compression stroke, so you can assume that some cylinder pressure is going to be bled off with the larger cams. This is why cars with larger cams can run more boost on pump gas, and naturally aspirated motors can run more compression.

This is correct. Larger cams will bleed off some cynlinder pressure at idle and low RPM. It doesnt actually lower compression it just lets some of that pressure bleed out.

 

[boostinboy]

thanks guys, so i guess my motor is healthy then

 

[kpt4321]

The exhaust cam isn't going to close as soon as the piston hits BDC and heads towards TDC for the compression stroke, so you can assume that some cylinder pressure is going to be bled off with the larger cams. This is why cars with larger cams can run more boost on pump gas, and naturally aspirated motors can run more compression.

No, it's not.

The exhaust cam is not opening the valve at BDC of the intake stroke, so your example is all wrong. The exhaust cam closes the valves sometime during the intake stroke, which is between TDC and BDC of the stroke BEFORE the compression stroke.

The intake cam is the one that will effect compression, because it keeps the valves open into the bottom of the compression stroke.

 

[boostinboy]

so do you guys think I should take my car back to him and ask him to put in another motor? He promised me at least 150psi across the board

 

[Black94TSI]

so do you guys think I should take my car back to him and ask him to put in another motor? He promised me at least 150psi across the board

Why are you getting so antsy because you think that the compression numbers are going to be low? If you havn't even tested the motor yet you're going to sound like a gigantic asshole barging in there and demanding a new motor, and chances are they're going to be assholes right back at you. Go do a compression test, if the numbers are close to each other you're fine. If you're still antsy in your pantsy go buy or rent a leakdown tester and test the motor for leakdown.

 

[my_gst95]

I wanted to reply to this older thread because I too noticed a drop in compression when I had installed my Comp 101200's yesterday. It went from a perfect 150 to 120 across, even with the timing marks lined up. My vacuum went from -20 to -10 at 700 rpm. I plan on raising my custom chip to 1000rpm idle and it should be perfect. The car doesn't sound like it has cams at all, has a nice deep tone without the lump. So to anyone else wondering about the compression loss, I too noticed this but have yet to see any performance loss.


PS: Has anyone been able to ground out their wire to set Base Timing? My car idles so low that I can't set base timing due to the car dieing. Any tips (I may start a new post).

my_gst95

 

[boostedinaz]

Why are you getting so antsy because you think that the compression numbers are going to be low? If you havn't even tested the motor yet you're going to sound like a gigantic asshole barging in there and demanding a new motor, and chances are they're going to be assholes right back at you. Go do a compression test, if the numbers are close to each other you're fine. If you're still antsy in your pantsy go buy or rent a leakdown tester and test the motor for leakdown.

There is no thinking, just knowledge of motors and how they work. It WILL have less cranking compression with larger cams. This is the way things work.

You are getting static and crank compression confused. Static is what you tell people you have and what is, well should be, calculated when building a motor. IE 9:1 10.5:1. Now depending on ALOT of factors the cranking compression will be different. Just because compression numbers are low doesnt mean the motor is junk or needs a rebuild. So, if has a 7.8:1 motor and had 150 across the board, then installed cams he will still have 7.8:1 static compression, but he will have maybe 130 cranking compression.

 

[TSIfreek]

There is no thinking, just knowledge of motors and how they work. It WILL have less cranking compression with larger cams. This is the way things work.

You are getting static and crank compression confused. Static is what you tell people you have and what is, well should be, calculated when building a motor. IE 9:1 10.5:1. Now depending on ALOT of factors the cranking compression will be different. Just because compression numbers are low doesnt mean the motor is junk or needs a rebuild. So, if has a 7.8:1 motor and had 150 across the board, then installed cams he will still have 7.8:1 static compression, but he will have maybe 130 cranking compression.

Very good reply!! I have a built motor with 2k on it with 8.5:1 pistons and comp 100 cams and I have 165 straight across. Compression tests are just really a base line it really tells you nothing. The leak down test is alot more accurate as far as how well the engine is sealing.
The guy with the comp 200s on stock block is fine.

 

[BlackHawkAWD91]

I have 264/272HKS on 9.1 ross and my compression is 188-189 across. This is on a fresh build engine. Could this mean without cams it could be in the 200's ?

Thanks

 

[Turbo Talon DL]

Blackhawk: I had Arias 9:1 pistons and same cam combo as you, my compression is mostly identical to yours, about 190psi.

My_GST, i'd try unplugging the idle switch just to see if you can get the car to idle a little higher, I had the same problem once, but I forgot what I did to fix it. I would try adjusting the idle screw all the way out, then start adjusting the idle switch position, (screw it up or down to open/close throttle). I got my idle to stay at 800rpm with hks 264/272 doing that. After you get a good idle, then ground out the test connector.

 

[boostedinaz]

PS: Has anyone been able to ground out their wire to set Base Timing? My car idles so low that I can't set base timing due to the car dieing. Any tips (I may start a new post).

my_gst95

Here is a nice write up with pictures on how to.

http://www.plymouthlaser.com/biss.htm

 

[1fast97gsx]

PS: Has anyone been able to ground out their wire to set Base Timing? My car idles so low that I can't set base timing due to the car dieing. Any tips (I may start a new post).

my_gst95

Actually what you can do to set base timing if the car is dying out now is just unplug all the injector wires to keep the car from starting. Then take a timing light and have someone else crank it while you check / adjust timing. A lot of 6 bolt 2g guys like myself do this as we don't have a plug to ground it out in the first place.

 

[1fast97gsx]

On another note ... my compression went from 180 across to now 155 across after my 272 install. Timing marks are perfect and car runs awesome. I have always heard it's normal for static compression numbers to drop 20 psi or so when putting in agressive cams. Did you check compression on a warm or cold motor?

 

[my_gst95]

take a timing light and have someone else crank it while you check / adjust timing. A lot of 6 bolt 2g guys like myself do this as we don't have a plug to ground it out in the first place.

Thanks! I myself have a 95 with a 6bolt and might do this method on it to make sure my timing is correct, as I've heard the ground clip on the 95's may not work in conjunction with setting the base timing. They really should add this method of setting timing on the Magnus writeup most ppl use for the 6bolt swap. Thanks again, i'm going to try this method on both my cars when I get home

my_gst95

 

[1fast97gsx]

Yea this method works great but it takes 2 people to do. Don't crank it for more than 10 seconds or so at a time.

 

[snox135]

Different sized cams can effect cranking compression. You can get this back with adjustable cam gears. But that usually isn't the best thing to do. It's best to adjust the cam timing so they make the best powerband for your type of driving.