Cam degreeing question

[czimpel]

I'm installing an older ATR RP309G cam into a budget 4.1 build. The cam is smaller than I would've liked, but with the word budget, it's what I have. I'm pretty sure the ICL is 104˚ according to the cam card, but I honestly don't know for sure being it says 104˚ @ .050.

I just measure the cam, and I have it at 104˚ ICL right now.

I'm wondering if I should retard the cam a couple degrees to soften up the lower end and move the power range up a little in the RPMs.

Here's the card.

 

[turbobitt]

Seems like a lot of advance on such a small cam and typically don't go over 4 degrees. I would say anywhere from straight up (0 adv. @ 109) to +2 (2 adv. @ 107) would be acceptable but there may have been reasons they wanted it at 104.
AG.

 

[bison]

They don't tell you if the open close events on the card are with the cam in at 104 or 109. If those events are at 109 I would not advance the cam anymore.


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[czimpel]

OK, I will check those tonight and see if they are at 104 or 109. Just a question, where are you guys getting the 109 from. I see it on the LSA, but what does that have to do with the ICL?

 

[fastblackracing]

The Info stated on the cam shows the open and closed points with the cam installed at 104* or 5* advanced. 2*btdc +180* + 30* abdc -Take 1/2 intake duration 106* = 104* atdc installed ICL. I agree with turbobitt + bison there is no need to advance the cam this much unless you are racing with a locked torque converter you would be better off putting it in at 107*-109*. The bigger problem that you have is if your heads have never been modified and you state "unopened 109" The exhaust guides may need machined down and this means the heads have to come off. The exhaust valve lift of .453 is getting real close to the limit of retainer to guide clearance and at a minimum you need to measure what that clearance is with a valve spring removed and verify what you have. If you dont have enough clearance you will have some big problems. HTH

 

[czimpel]

This is not the motor in my sig. It's a 4.1 build that I'm doing with Champion Irons. Clearances are all good. I'm thinking of installing it more towards the 107˚-109˚, but I'm going to check with what Bison asked about whether the events are happening at 104˚ or 109˚.

 

[turbobitt]

This is not the motor in my sig. It's a 4.1 build that I'm doing with Champion Irons. Clearances are all good. I'm thinking of installing it more towards the 107˚-109˚, but I'm going to check with what Bison asked about whether the events are happening at 104˚ or 109˚.

Those events are given at 104. At 109 they would look like the below @ .050".
IVO @ .050 -3
IVC @ .050 35
EVO @ .050 32
EVC @ .050 -6

 

[czimpel]

Thank you guys!

 

[fastblackracing]

The info I gave answers that question it is 104* If you follow the math I did you can see the numbers. With the lift starting at @ 2* btdc you take 1/2 the duration which is 106* and that puts it at 104* atdc ICL max lift. These are the numbers on the card, make sense?

 

[fastblackracing]

I like the numbers at 109 much better for a 4.1 build, good luck.

 

[czimpel]

Yes, it does now. I thought I understood cam timing, but this card was tricky for me to understand. I'm used to most of my small block Lunati cams that give you the words install at 104˚ intake centerline. Learn something new everyday. Now if I could only find a way to make it stick in my brain.

 

[fastblackracing]

Yes, it does now. I thought I understood cam timing, but this card was tricky for me to understand. I'm used to most of my small block Lunati cams that give you the words install at 104˚ intake centerline. Learn something new everyday. Now if I could only find a way to make it stick in my brain.

Me too weather its at work or out in the shop at home I always look too gain knowledge every day. Cool that you learned something.

 

[bison]

The info I gave answers that question it is 104* If you follow the math I did you can see the numbers. With the lift starting at @ 2* btdc you take 1/2 the duration which is 106* and that puts it at 104* atdc ICL max lift. These are the numbers on the card, make sense?

The numbers on the card are derived from a formula. I suspect Turbobitt ran a cam card to verify it based off the advertised duration and duration at .050". This assumes a symmetrical lobe also. If it were me I'd install that cam with the ex closing at .050" @ 6btdc. But then again that's just me. If you have a loose converter and low CR there may be advantage closing even sooner but the only way to know for sure is to try it. In the long run it won't make much difference at all. If you look at the open close events they are very similar to all the small turbo cams out there. Advancing to 104 will likely more closely match the stock cams events.


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[czimpel]

Bison, I'm so confused as to why everything is based off of the .050" measurement. Is it the same as saying that I need to have the exhaust fully closed at 6bdc? I'm just not following all of the .050" measurements and what that really means.

 

[czimpel]

Also, should I be measuring this at the valve or tappet? I would imagine the number will be different.

 

[turbobitt]

Bison, I'm so confused as to why everything is based off of the .050" measurement. Is it the same as saying that I need to have the exhaust fully closed at 6bdc? I'm just not following all of the .050" measurements and what that really means.

The .050 just seems like the industry standard. There can be a lot of variation in the actual cam opening and closing ramps due to grinding tolerance and also opening and closing rates of different cam families make it impossible to judge a cam based on only one set of numbers alone. Looking at .006. .015, .020, .200 numbers give a more clear picture of the cam.
AG.

 

[czimpel]

Perfect, thanks Turbobitt!

 

[bison]

Bison, I'm so confused as to why everything is based off of the .050" measurement. Is it the same as saying that I need to have the exhaust fully closed at 6bdc? I'm just not following all of the .050" measurements and what that really means.

I don't base everything off the .050" multiple points have to be used to determine the cam lobe type as Turbobitt mentioned. Actual ex closing @ .050" pretty much always falls between tdc as 12btdc. This is the actual measured event. Not what is listed on the card. If it's closing later than tdc it needs to be advanced or the lobes/separation are probably wrong.


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[czimpel]

Thanks for clearing that up Bison.

 

[87GN_70GS]

The Info stated on the cam shows the open and closed points with the cam installed at 104* or 5* advanced. 2*btdc +180* + 30* abdc -Take 1/2 intake duration 106* = 104* atdc installed ICL.

That's assuming the profile is symmetric. If it's non-symmetric, the lobe centerline could be any other number.