FP2 cams and static compression

[Reese92tsi]

I did a compression test after installing FP2 cams and my compression dropped by thirty PSI in every cylinder. I know that aftermarket cams lower your static compression due to the fact that there is more reversion of the intake charge back into the manifold but this drop seems excessive. Is this type of drop normal?

 

[andymoraitis]

I did a compression test after installing FP2 cams and my compression dropped by thirty PSI in every cylinder. I know that aftermarket cams lower your static compression due to the fact that there is more reversion of the intake charge back into the manifold but this drop seems excessive. Is this type of drop normal?

If you have compression figures taken without the cams, I would call Brian at Forced Performance and ask him what's reasonable to expect in terms of compression drop. It sounds high to me, but I would check with him and also make sure that you compiled both sets of results on a warm motor, otherwise it will skew the outcome.

Let us know what you find,

Andy

 

[1fast97gsx]

I had the exact same drop with my 272s. I lost roughly 25 psi but I was told it was normal as long as the drop is equal in each cylinder and timing marks are correct.

 

[Reese92tsi]

Thanks 1Fast97gsx

 

[94awdcoupe]

surprised there is not more data here. my compression gauge is older and likely does not read as high as is should. I did a cam change last week and was miffed at the compression drop.

First numbers 125-155. stock 2g 8.5/1. head was clearly tired. burning lots of oil at idle. all rubber in head hard as a rock.

New numbers 135. changes Tomei 260 poncams. 9/1 compression pistons. fresh head.

Pistons should have given about 15psi. so cams dropped compression about 35psi.

 

[donniekak]

Changes in the cam gear settings can get all of that compression back.

During some recent testing I've gotten anywhere between 90-160 psi on the same bottom end with different cams, and cam gear settings. The settings with the larger cams that produced the best idle vacuum also produced the highest peak power.

 

[94awdcoupe]

wow cool post. what kind a cam gear adjustments gave best vacuum increase?

I had about 15psi straight up.
advanced intake 3 degree = 14psi vac.
advanced ex 4. = 16psi vac.

kinda got discouraged as I have played with cam timing on BC280 and was able to get 5-6psi swings at idle on those cams.

 

[19gsx91]

I'm gonna watch this thread closely, I have fp2 cams and with a HG swap my compression dropped about 15psi across the board and I'm only managing about -11-12hg of vacuum, I installed my cams "straight up" on stock cam gears, now I have adjustable ones but have been hesitant on making any changes.

Any advice you have Donnie, or Jerry, I am more then willing to take into consideration.

 

[94awdcoupe]

pretty sure i was getting 17-18in on FP2 cams on 2.3 engine. I think I still get 17in on the FP3

 

[1990TSIAWDTALON]

You can get past some overlap of the cams by advancing the intake 1* and retarding the exhaust 2*. It has been suggested to me by Waylen Speed to get more vacuum as I run about -8hg max vacuum.

 

[donniekak]

You want to take overlap out of the cams to get more vacuum at idle.

With the kelfords adjusted to exhaust +3 intake -3 I was able to pull 15-16 in/hg at idle and make peak power at 8750 rpms with a 3586htz.

With the fp2's at +2 exhaust -2 intake it pulled 19+ in/hg at idle. Although best power was with the fp2's at 0 0.

The fp2's spool the turbo 500 rpms faster than the kelfords, and make off boost driving feel like a normal car instead of a slug.

 

[1990TSIAWDTALON]

Some VERY good information there Donnie! Thanks for posting up your results, it helps everyone.

 

[henrysan]

GSC S2 cams with stock gears changed my static compression to 115 from 150 and 11 vacuum at idle, on a stock 6 bolt.

 

[donniekak]

A lot of what cam companies are selling now as a 272, or stage 2 cam are just a race profile from 10 years ago.

There is no free lunch. You want 40-50hp gains at 8,000+ rpms, well you'll have to pay for it in idle quality and off boost power, along with a little bit of spool.

People were running 9's on hks 272's decades ago.

Now for a race only car, especially an auto that holds the engine in a narrow rpm band they are great.

 

[94awdcoupe]

so just to clarify as I think I am

You want to take overlap out of the cams to get more vacuum at idle.

With the kelfords adjusted to exhaust +3 intake -3 I was able to pull 15-16 in/hg at idle and make peak power at 8750 rpms with a 3586htz.

With the fp2's at +2 exhaust -2 intake it pulled 19+ in/hg at idle. Although best power was with the fp2's at 0 0.

The fp2's spool the turbo 500 rpms faster than the kelfords, and make off boost driving feel like a normal car instead of a slug.

I think I am confused. I have some simple questions. looking at face of gears.
1. if you loosen adjustable gears, put 17mm wrench on cam and rotate wrench/cam clockwise, that is advancing cam or retarding cam??
2. if rotate ex cam and int cams toward each other, ie ex clockwise and int counterclockwise that increases or decreases overlap?
3. is no 2 reversed when referring to evo 8 ??
4. when you quote +3 ex and -1 int. does + mean adv?
5. when you quote +3/-1 are you referring to crank degrees? as each mark on a cam gear is 2 crank degrees
6. someone should make a thread on which gears are marked wrong. these skunk 2 pro series gears I have are marked wrong. they have +/- reversed of what they should be

 

[19gsx91]

Dumb question, but since you three are monitoring this I feel I can get a solid answer.. Having the adjustable cam gears, am I safe to make incremental (1-2degree) changes, and run the car and see how it performs without the cams being degreed? I'm not saying end up with like -8int and +6exh.. but from what I have now, being straight up, set it to +2int and +2exh and see if it's better or worse then what it currently is?

 

[donniekak]

To be sure of how much you can change the cam gears safely you need to check valve to piston clearance when you put the engine together. That being said a few degrees either way is usually well within the safety margin.

On a Dsm engine turning the cam clockwise with the gear stationary is advancing the cam.