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As mentioned you can throw any cam in and make decent power. Higher you go on overlap the smaller the tuning window will be. I don’t understand the allure of the larger duration camshafts on SBE type builds and relatively low rpm. You are going to have 2:1 or higher back pressure ratios depending on boost. Even the 1.32 T6’s with the big 87/96 exh wheel is going to produce 2:1+ BPR’s at 25+lbs. Which is where the OP will need to be to hit the numbers he is wanting.
I don’t see how a cam like the TU2 with 50*+ of overlap can be the “best” choice with a 2:1+ BP ratio. Stock48 had the best running “small” single turbo SBE 6.0 I’m aware of (Weight VS trap) He ran a 218/218 @ 112 .56x lift cam with a T4 S476. Copying the guy that’s pushed the SBE 6.0 single “small turbo” combo the farthest seems like a great place to start IMO. Most of the guys running the larger duration cams have the 1:1 back pressure ratios and or methanol/alky based fuels to help with the reversion/detonation issues. OP will be running race gas and won’t have the additional CC cooling you get with alky based stuff. I’d go smaller if the TU2 wasn’t sitting there ready to try for nothing. Or run a larger single turbo capable of 1:1 back pressure ratios and cam it like an NA “race car”.
I came up with 11* for the TU2 cam, assumed this formula was correct: ((Intake Duration + Exhaust Duration)/2)-(2xLSA)
Originally Posted by rpturbo
This was exactly my point, he already has it, and since he has no data to work with so far, start there.
You are using the duration at .050, not the advertised duration... I believe that is what is usually used to calculate overlap.
Doh, details. haha!
Saw a post by LJMS saying a 225 exhaust lobe leaves a bunch on the table and the min exhaust duration should be a 235+.
Just seems the TU1 cam taps out a little early and a little bigger cam could be used to bring power out further. Bigger cam might even help traction not hitting so hard down low?
Since I don't know it myself, I looked it up and found this. Since I do not have a cam card with opening and closing numbers, this is what I used. Which is @.050
If the seat to seat opening and closing times are not known, the overlap can be calculated using the advertised duration and the Lobe Separation Angle (LSA). This formula works for both single and dual pattern cams.
Add the intake and exhaust adv durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Using the example cam above:
236+236=472
472/4=118
118-112.5=5.5
5.5x2=11
The overlap is 11º
Without a cam card, how do you find the numbers that aren't @ .050 ??
Since I don't know it myself, I looked it up and found this. Since I do not have a cam card with opening and closing numbers, this is what I used. Which is @.050
If the seat to seat opening and closing times are not known, the overlap can be calculated using the advertised duration and the Lobe Separation Angle (LSA). This formula works for both single and dual pattern cams.
Add the intake and exhaust adv durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2
Using the example cam above:
236+236=472
472/4=118
118-112.5=5.5
5.5x2=11
The overlap is 11º
Without a cam card, how do you find the numbers that aren't @ .050 ??
U A typical 236 lobe will be around the around the 280 range advertised. Which is what I used to make the guess above. Can shoot speed inc an email, I'm sure they would tell you exactly. It's gonna be over 50*
Saw a post by LJMS saying a 225 exhaust lobe leaves a bunch on the table and the min exhaust duration should be a 235+.
Just seems the TU1 cam taps out a little early and a little bigger cam could be used to bring power out further. Bigger cam might even help traction not hitting so hard down low?
You really can't make general statements like that. You can easily build an engine where the added duration is not beneficial... or the other way around.
So with this info, from what I have read, you add the intake opening to the exhaust closed. Which is 6+6= 12* overlap.
I emailed Speedinc again this morning to get them to give me the actual answer. I have a feeling that the open and close numbers are not actual "opening / Closed " instead the open is when the valve is at full open. So not starting to open.
I asked them to tell me when the intake starts to open and when the exhaust just closes.
Without a cam card, how do you find the numbers that aren't @ .050 ??
A typical LS hydraulic roller will generally be about 50-55 degrees larger at .006" than it is at .050".
I think if you start looking at the valve events, you might find some additional insight. This calculator is a handy little item for that. It is free for download:
Using this calculator it still ended up at 12* overlap when using the valve open and close numbers. When using the duration, lobe center and advance, it sets its own open and close numbers which ends up with 1* overlap. Problem is that Lobe separation, Advance and centerline changes when setting the other.
For some reason it wont let me copy the picture. But it lets me enter the duration, spread which I am assuming is LSA and the advance. Then you update and get the overlap of 11* and it presets its own centerline and valve events. Which aren't right.
Then if you enter the centerline, it changes the LSA to 111.5 and advance to 0. It also sets its own valve open and close numbers by .5 different than what the cam card says. and an overlap result of 13*overlap.
If I change the valve open and close, it then changes the centerline and LSA to 112.0 and gives a 12* overlap.
So it wont let me enter all the info and update it without it making changes to the things I already entered. Like I am not doing something right or the program is saying it can be like that.
I think the numbers being at .050 might be the issue.
I am using the info Speedinc sent me, which is noted above.
Duration of 236
LSA 112.5
Centerline 111.5
Advance 1
Intake open at 6
Intake close at 50
Exhaust open at 50
Exhaust close at 6
I believe the above screenshot illustrates what you are saying. Events not matching lobe centerlines by up to a couple degrees can happen when using actual measurements. The reason for this is there are multiple ways you can measure lobe centerlines. Specifically, you can measure the lobe at different lifts.
Since most modern lobes are non-symmetrical, the lobe centerline will measure differently at .050" off the base circle than it will if measured .050" off of peak lift. If this cam has the lobe centerlines measured at a different lift than .050" it will likely show slightly different valve events.
I am not sure if that is what your situation is, but it is a possible explanation.
Last edited by speedtigger; 07-10-2016 at 09:48 AM.
Yep, that's what its doing with me. So in the end, I still do not know what the exact overlap is. Speedinc is impossible to get to answer their phones. Drives me bat **** crazy. If your a retail company, answer your dam phones. I emailed them again in reply to what they sent me. Haven't heard back yet. Sad part is when I did get them to answer Friday, the guy I was speaking to had no clue what overlap was and had to ask someone else, then that guy told him to just send the emailed cam card template. Which still does not answer my original question.
So my question is , is it 11-12 degrees of overlap? Or is it actually much larger due to using .050 numbers?? If its 11-13 degrees, is this even a big issue for my setup? I really don't want to have backpressure issues or problems with detonation if I am mixing a bunch of exhaust gas mixing with the intake charge.
I did see where a lot of companies do not account for the first 6 degrees of the valves opening due to its so nominal, plus there are variances in the valve train that may not actually get the valve moving yet in the first few degrees.
It's 50* plus for sure, more likely 55*+ of overlap. You'd need the duration at .006 as mentioned above this is what you stick into the overlap formula. It's gonna be 280+ at .006.
While looking at duration at .050" is the current popular way to discuss camshafts, it is just one way to look at a camshaft's specifications. This method certainly does not tell you everything about a camshaft, but it does create a shorthand for communication.
Looking at duration using .006" has also been used for years on hydraulic cams. this number was also referred to by many as "advertised duration". I think this number was used because someone at some point figured that .006" was an amount of lobe lift that would likely be taken up by deflection and hydraulic lifter compression. Their goal was surely to pick a number that would allow them to predict when the actual valve itself would come off of the seat.
One problem with using duration at .050" lift when discussing overlap is that it gives birth to some nonsensical ideas like "negative overlap". Many people have literally thought that a given camshaft might have less than no overlap. The reality is there is no such thing as a true negative overlap in an a modern automotive camshaft. Even stock camshafts have some overlap when measured at .006". So, if you really wanted to know "total overlap" you might want to use duration at .006" or "advertised duration".
With all of this said, if you are going to look at .006" duration, make sure that the other camshafts and other peoples results that you are comparing to are also .006" duration numbers. Obviously, comparative data is only useful if the data is acquired with the same system of measurement.
Last edited by speedtigger; 07-10-2016 at 03:36 PM.
Most hydraulic LS cams will have between 50-55 more duration at .006" vs. .050". So, if we split the difference and use 52, your camshaft might be about 288/288 112.5+1. That would look like this at .006":
07-08-2016, 11:12 AM
