No, less overlap would be great, right now at the high rpm my turbo is blowing out of the exhaust. I have a 88mm turbo and I can't get boost over 26 psi. Motor just eats everything you throw at him. There are two possibilities: to go bigger with the turbo (98mm) or get a new cam. If I get lower gears then I lose my very strong first half of the track.
My first seven second run
- Thread starter vlzman
- Start date
No, less overlap would be great, right now at the high rpm my turbo is blowing out of the exhaust. I have a 88mm turbo and I can't get boost over 26 psi. Motor just eats everything you throw at him. There are two possibilities: to go bigger with the turbo (98mm) or get a new cam. If I get lower gears then I lose my very strong first half of the track.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
I don't think spool would be worse with less overlap. With your RPM, I wouldn't doubt your running the compressor near its limits.
Care to share any details on the cam ? Maybe we can shoot some ideas around.
Allan G.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
Cam is Comp cams #93-000-9.
Vizman,
I'm not an athority on 7 seconds cars but think I have some Ideas. Will send a PM.
Allan G.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
No, the cam doesn't look very big but does have a tight lobe seperation with and a lot of overlap. Those lobes are too lazy for that kind of lobe seperation IMHO.
If you really want to make some power I would consider the TK lobe profile of 4642/4641. This cam has a much more agressive lobe opening and will have much less overlap but with no sacrifice in duration. You can download the lobe details on the Comp website. This would be near the optimum cam I would prefer to run on my car but don't have the balls yet to make the leap.
For example , your current cam is rated at 286 duration, and with 250 duration @ .050" and 167 @ .200" lift.
The TK lobes will have a rated duration of 270, and 242 @ .050", and 168 @ .200" lift.
You can see from the above that the TK lobe profiles have a smaller rated duration but have a similar duration at .200 lift where it really matters. This would considerably reduce the amount of overlap, increase spool, and make way more torque. You could even stay with your existing LSA.
This type of cam would also require a very stout valve spring. I would guess at 300+ at the seat. For a race only application this would really not be a concern but for someone like me it is just to aggressive for the type of driving and durability I would like to have.
Allan G.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
Ive been analyzing cam lobes for high hp turbocharged v engines for a while now and I 100% agree with Allan about the lobes. They will leave a narrow window of cylinder pressure where the TK lobes will widen that cylinder pressure window quite a bit. You could run the same LS with the same duration @.050 and still have less overlap and pick up peak power as well as a considerable increase in power to the left and right of peak power. The engine will respond better in every way as far a an all out race application would expect. The spool time will be drastically reduced and the boost ramp vs engine output will be much more predictable. You will probably be able to drop 10% gearing and open up the time in the most usable part of the power band. It will help with the converter adjustment also. That car is flying with a relatively soft cam lobe. You will pick up ex energy with the TK lobe too. Chances are there isn't enough energy there to drive the compressor as hard as it needs.
Actually it's not so common around here to keep this kind of information as a big secret. So it didn't even come to my mind that sharing this would be something extraordinary
If I could get any good ideas out of it then it's a win-win....
What would more duration on both lobes , put on a 118lobe CL do for the entire combo?? Im asking because Im right smack in the middle of having some BBC sized cam cores rough finished to a 120 degree lobe CL. I have been talking to many people I respect , and Im getting quite different opinions on this. I may start another thread about my situation but that is my food for thought. Mike
- Joined
- Jun 18, 2001
Actually it's not so common around here to keep this kind of information as a big secret. So it didn't even come to my mind that sharing this would be something extraordinary
If I could get any good ideas out of it then it's a win-win....
There are no real secrets here, just a bunch of folks who think they have big secrets, lol.
Nice car and nice run. That thing must hammer hard in the 1/8!
Personally, I would look at other things before you start messing with the cam profile. It will change your combo around a bunch with converter and gearing plus swapping turbos and other parts may be easier and have bigger gains.
Looking at your cam specs, if I were to change anything I would spread the LSA. Back in 2007 I went from a 236* 110 LSA to a 248* 116 LSA cam in a stock style head motor with an 88mm turbo and picked up considerably on the top end and the turbo was actually easier to spool. Spreading the LSA gave the motor more area to work with while the narrower LSA may make more peak power the wider LSA has more power to use going down the track.
I haven't looked at cam specs or blanks in a awhile but I think with the current blanks from comp with 250* you may only be able to get to 112 maybe 114 LSA anyway. The cam I had/have was a crane blank which no longer exists.
CAUTION, spreading the LSA and decreasing overlap WILL increase cylinder pressure significantly. You are right at the maximum power level I would trust a carrillo rod maybe a little beyond. I had 6.5" billet crowers and managed to fold up 4 of them without ANY damage to the pistons other than scuffing from the bent rods. Fortunately the rods just bent and didn't break and the motor was salvageable. Carrillos tend to break and not bend and if they do they will take out everything! If you change anything I would recommend a NEW set of 6.300 billet crower rods.
I may also look at gearing and TQ conv along with RPM range you are operating in to see if it is optimal. You may find gains in less gear tighter conv and lower rpm operating range? These are also pretty easy to change and would give good data on what your combo likes.
Then again your car is hammering pretty good now and picking up MPH on the back end may not pick up big gains in ET.
JMO and if you have any more questions I would be happy to share all of my super secret Buick info for what its worth, lol.
Good luck and nice car!
Really good stuff, Chris. All very good suggestions.
If it were me, I think I'd throw that bigger turbo on the car first to see what sort of change you get from just that. One change at a time. You have got to be at the outer reaches of what that 88mm can do for you. I like the idea of the 98mm.
That does seem to be the trend. More duration, increased lobe separation to control overlap area. Start that thread. There seems to be a lot of input from people. I think it would be a very informative thread.
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
One important thing to note is that we can't blindly look at LSA without taking the lobe family into consideration. My next cam for example will have a tighter LSA and lobes with similar duration at .200 lift but with less overlap.
Allan G.
So as I have understood then actually this cam has played the major part in the fact that my motor has held together so well. Although I am spinning it so high. At the beginning of this year I had several occasions when I broke my rocker shafts, I still have 3 pcs in my toolbox just in case LOL. I got rid of the problems by fabricating a special retainers. As the lobes are so lazy then it is much more forgiving and the motor doesn't break so easily. Two seasons straight I have only changed oil and haven't experienced any more problems
By going on a more aggressive cam with less overlap I would increase my cyl pressure and put a lot of stress on the valvetrain?
Seems that switching turbo to a bigger one would really be most sensible move. Then again more power would probably take me to the other side of the 7.5 second barrier which is a problem as car is certed to 7.5
Dead end again.....
By going on a more aggressive cam with less overlap I would increase my cyl pressure and put a lot of stress on the valvetrain?
Seems that switching turbo to a bigger one would really be most sensible move. Then again more power would probably take me to the other side of the 7.5 second barrier which is a problem as car is certed to 7.5
Dead end again.....
But, like you stated earlier, valvetrain durability starts to become an issue with the more aggressive lobe profiles. There are generally accepted limits as to what makes a durable profile. Those limits should be known and fully understood before deciding to play with more aggressive profiles.
I see you understand the valvetrain durability concern.So as I have understood then actually this cam has played the major part in the fact that my motor has held together so well. Although I am spinning it so high. At the beginning of this year I had several occasions when I broke my rocker shafts, I still have 3 pcs in my toolbox just in case LOL. I got rid of the problems by fabricating a special retainers. As the lobes are so lazy then it is much more forgiving and the motor doesn't break so easily. Two seasons straight I have only changed oil and haven't experienced any more problems
By going on a more aggressive cam with less overlap I would increase my cyl pressure and put a lot of stress on the valvetrain?
Seems that switching turbo to a bigger one would really be most sensible move. Then again more power would probably take me to the other side of the 7.5 second barrier which is a problem as car is certed to 7.5
Dead end again.....
With the cam you are using now, I would be really interested to see what a larger turbo, and maybe, hopefully less exhaust backpressure might do for you.
What exhaust housing are you using now, and what exhaust housing would the 98mm be using? T4, T6?
turbobitt
Well-Known Member
- Joined
- Feb 14, 2002
Valve train durability would be comprimised and it depends on the user for there preference for exceptable limits. I know most of the really fast cars are running TK lobes and making them work.
Allan G.