If you've dyno tuned your cam gears..please post

[viperlp01]

This stuff is confusing. I did the +2/-2 and my car ran like crap. Terrible vacuum and noticable loss in power. So I went +2/0 and it got better. Should I go +4/0 and see what it does or does anyone have an idea where my "straight up is".

 

[Calan]

This stuff is confusing. I did the +2/-2 and my car ran like crap. Terrible vacuum and noticable loss in power. So I went +2/0 and it got better.

By "+2/-2"...are you saying you advanced the exhaust 2* and retarded the intake?

...or does anyone have an idea where my "straight up is".

"Straight up" usually refers to just dropping the cams in without any adjustment. If you mean where should they be adjusted to match their intended design specs, you have to degree them to find this out.

 

[viperlp01]

... it is like an epiphany. Like a light bulb turning on . By me advancing my exhaust I am allowing it to open ealier which will allow both valves to be open at the same time. So I make the exhaust cam open later by retarding it and by advancing my intake will do that same thing.. AHA!!!

So my 95 style cas somehow got loose so I think I have to pull that gear and lock tite the screws. I am going to mess with it today.

 

[Calan]

... it is like an epiphany. Like a light bulb turning on . By me advancing my exhaust I am allowing it to open ealier which will allow both valves to be open at the same time. So I make the exhaust cam open later by retarding it and by advancing my intake will do that same thing.. AHA!!!

TADA!

Don't forget, any time you change the intake cam, you'll need to reset your base electrical timing also since you have moved the CAS.

 

[delta448]

Unless I missed something, I think you mistyped Craig. By advancing the exhaust and retarding the intake, you'd be decreasing the overlap. The exhaust valves open at the end of the power stroke -for the upcoming exhaust stroke- and then the intake valves open as the exhaust valves are closing to bring the next charge into the bore for the following compression and then subsequent power stroke.

Matt (dsm-onster) shared his thoughts in this tech article he wrote on cam basics.

 

[Calan]

Unless I missed something, I think you mistyped Craig. By advancing the exhaust and retarding the intake, you'd be decreasing the overlap.

You are correct. I'm thinking in terms of centerline separation (which is just easier for me to visualize), and had a brainfart. Retarding the exhaust cam would decrease the lobe separation between intake and exhaust, and increase valve overlap since the opening events are on "opposite sides" of each other...if that makes any sense. Duh.

Nice catch.

It is very interesting that many people see an increase in performance when they retard the exhaust cam, when in theory it really shouldn't on a turbo motor according to several sources. I wonder if it has something to do with cams usually being dropped in straight up vs. being degreed to better match modified engine geometry?

 

[Boosted98gsx]

I'm on FP3's. I don't have any numbers save some seat of the pants dyno guesstimates. After advancing the intake 5* and retarding the exhaust 6* there was a monumental increase in high end power. The turbo spools up a little faster, and mid range torque is also very nice. It makes driving around on 15psi with the HTA3582 about the same as 20 psi felt prior to the alteration.

Edit, and it went from sounding like OEM cams, to this..

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[delta448]

Not to nitpick, but actually I believe Lobe Separation Angle is measured from exhaust centerline to intake centerline w.r.t. degrees of crank rotation, meaning the LSA would drop as overlap increased for a given set of cams. -?

I can't directly link the pop-up, but if you scroll to the bottom of this page and click the "FP cam vs Factory A-A" you'll see what I mean. For example: the FPCam1 LSA is less than the factory LSA.

 

[Boosted98gsx]

Not to nitpick, but actually I believe Lobe Separation Angle is measured from exhaust centerline to intake centerline w.r.t. degrees of crank rotation, meaning the LSA would drop as overlap increased for a given set of cams. -?

I can't directly link the pop-up, but if you scroll to the bottom of this page and click the "FP cam vs Factory A-A" you'll see what I mean. For example: the FPCam1 LSA is less than the factory LSA.

shame they discontinued the FPx cams and only have the FP4R's and FP5R's

 

[viperlp01]

Well if I am thinking right. If you advance the intake gear enough it will close before the exhaust cam opens the valves even if you retard the exhaust and advance the intake. Your opening everything earlier in the stroke. I might try that as well .. I should degree my car but I just don't have enough time before the shootout. I am going to advance the intake gear and retard the exhaust

 

[Calan]

Not to nitpick, but actually I believe Lobe Separation Angle is measured from exhaust centerline to intake centerline w.r.t. degrees of crank rotation, meaning the LSA would drop as overlap increased for a given set of cams. -?

Correct again. See...I knew what I was talking about...I just got into an endless babbling fit.

I fixed my post up there ^. I shouldn't be out in this friggin' heat frying my few remaining old brain cells.

 

[Boosted98gsx]

Well if I am thinking right. If you advance the intake gear enough it will close before the exhaust cam opens the valves even if you retard the exhaust and advance the intake. Your opening everything earlier in the stroke. I might try that as well .. I should degree my car but I just don't have enough time before the shootout. I am going to advance the intake gear and retard the exhaust

You're thinking in the wrong portion of the stroke. The intake cam will always close (2 strokes) before the exhaust cam opens.

What you are trying to obtain by advancing the intake is make it open before the exhaust closes. This increases overlap, and on turbo cars, helps push the exhaust out of the cylinder increasing the efficiency of the burn and in the end gaining more torque from that combustion.

 

[Calan]

Well if I am thinking right. If you advance the intake gear enough it will close before the exhaust cam opens the valves

The exhaust valves start opening WAY before the intake (about 180*). If you advanced the intake cam enough (which I don't know if you could even do), the intake valves would close before the exhaust valves finished closing.

 

[viperlp01]

Would allowing the intake valves to open ealier in the stroke be better so it has more room to fill? It seems like you can let more air in at a lower PSI if you open the intake sooner. I think advancing the intake is a good idea.

Edit: because I sound dumb. disregard what i said

 

[Boosted98gsx]

Would allowing the intake valves to open ealier in the stroke be better so it has more room to fill? It seems like you can let more air in at a lower PSI if you open the intake sooner. I think advancing the intake is a good idea.

Take the turbo out of the picture. Now you have atmospheric pressure working through both sides of the head. If you have overlap in this condition you will get exhaust leaving through both sides of the head (called reversion) until the exhaust valves close. It will cause reduced lower end torque due to the INCREASED amount of exhaust still left over in the combustion chamber and the intake manifold. This is what is seen in turbo cars at lower RPMs.

This is why it is vital in a N/A motor that is seeking performance to have a "tuned" 4-2-1 header. Each pulse as it enters the next stage causes scavenging on the other opposite cylinder, using the conservation of momentum (to create a vacuum) to draw exhaust out of the opposing cylinder and allow overlap to be beneficial.

Now add the turbo back in. The same overlap is now beneficial once there is + manifold pressure. If your turbo spools quickly enough, you can gain lower end torque, however, if your turbo spools too slowly, your torque curve will be drastically shifted to the right on a dyno graph.

 

[Calan]

I'm on FP3's. I don't have any numbers save some seat of the pants dyno guesstimates. After advancing the intake 5* and retarding the exhaust 6* there was a monumental increase in high end power.

Andrew... is that cam or crank degrees?

 

[Boosted98gsx]

Andrew... is that cam or crank degrees?

It's based on the markings on my unorthodox racing cam gears. I ASSume that it's cam degrees.

Also, during off-boost cruising, the car DOES NOT like to stay alive when unloaded (ie clutch engaged or pulled out of gear). Reversion sucks.

 

[viperlp01]

Ya I caught myself and called myself dumb .. Also wouldn't that help the turbo to spool itself with some overlap. The little bit of pressure that leaks through the exhaust will up the pressure that is in the exhaust manifold. My car was designed for street use.

I miss having my quick spool and nasty torque that I had with the 2.0l stock cams. My car feels slow until 4k. I need more down low power.

Advancing the intake gear that much makes it open sooner and close ealier as well but you get some overlap back by retarding the exhaust cam so much. I think I am starting to get it.

 

[Boosted98gsx]

Well the turbo would, in a sense, help itself once it begins to feed the engine. It would take a positive intake to exhaust pressure ratio to cause the reversion to stop, and the overlap to start functioning.

Just wanted to add, this is also why the big dawgs have those HUGE turbo headers with built in 4-2-1's. It helps spool the turbo faster, reduce reversion, and thereby reduce the torque curve shift, by making use of exhaust scavenging, since they normally use huge overlap cams. This is why it's harder to keep engines with huge cams idling, because there is so much reversion at idle and off-load, that the engine can barely breathe since it's recycling so much exhaust gas.

 

[delta448]

Advancing the intake gear that much makes it open sooner and close ealier as well but you get some overlap back by retarding the exhaust cam so much. I think I am starting to get it.

You're picturing the intake events happening before the exhaust events, but you should realize that there's much more rotation between the intake to exhaust than exhaust to intake. By advancing the intake you're increasing overlap, by retarding exhaust you're again increasing overlap.

I don't want to insult anyone by posting this next part, I'm sure you all understand well enough, but just to make sure that we're all on the same page...

Let's review the 4 cycle engine's process:

1) The intake stroke begins at TDC with the exhaust valves closing (or just closed) and intake valves opening, the intake stroke ends at BDC with exhaust valves closed and intake valves closing (or just closed).

2) The compression stroke begins at BDC with the exhaust valves closed and the intake valves closing (or just closed) and ends at TDC with all the valves closed, which produces a pressurized air-fuel charge for combustion. The spark plug fires before the end of the compression stroke during >95% of the possible engine operating conditions.

3) The power stroke begins at TDC with all valves closed and the charge already burning and creating pressure, it ends at BDC -usually with the exhaust valves starting to open prior to the piston reaching BDC.

4) The exhaust stroke begins at BDC with the exhaust valves open and the intake valves closed. It ends at TDC with the exhaust valves closing and the intake valves opening.

5) go to step 1, wash, rinse, repeat.

 

[Calan]

And if you want pictures: http://www.dsmtuners.com/forums/articles-miscellaneous/338831-4-cycle-combustion-process.html

It's based on the markings on my unorthodox racing cam gears. I ASSume that it's cam degrees.

If that's the case, wouldn't that be like 10* at the crank? How much before you run the risk of interference?

 

[Boosted98gsx]

If that's the case, wouldn't that be like 10* at the crank? How much before you run the risk of interference?

This cam degreeing was done by the Scott at forced performance, who actually designed the cams. I trust his judgment on these motors more than anyone else on any forum, period.

 

[viperlp01]

All I know is that i went -1 ex/ +2 in and dsmlink has never est that high. I was (straight up)pulling off 2.0 70-90s and tonight with the cams at that position I pulled a 1.84... Link was est over 400 the whole time. I need to turn the boost up.

 

[rx7ttlm]

shame they discontinued the FPx cams and only have the FP4R's and FP5R's

dude, look at the 4r's specs. you will understand why. Its a bad mother, I wish I would have bought my fp3's a year later, then I would have gone with 4r.

 

[99gst_racer]

^^^ Yes, the 4R is an awesome grind. It was my runner-up cam choice behind the Kelford 272's that I ended up going with.