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Old 01-25-2009, 04:36 AM   #1
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Default Cam selection for boost/high rpm

hey there guys... wrong wheel drive BPT guy here looking for some cam advice

this past fall my car put down 320whp/325wtq at 23psi. This is with a 323 GTR motor, completely stock from throttle body to turbine housing. My torque drops off fast at high rpm's, and i'm thinking it is most likely cam related. my boost level holds strong all the way to 7k, my intercooler is plenty big enough, and my exhaust is 3" all the way back...

here's my dyno plot:
Name:  320whp325wtq.gif
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Some details on the motor:

IHI ballbearing turbo, 65mm compressor wheel, supposedly fairly efficient to nearly 30psi
8.2:1 compression ratio

Intake cam duration 233 @ 0.003"
Intake cam opening 2 BTDC
Intake cam closing 51 ABDC
Intake cam lobe height 44.094mm (1.7360")
Intake cam base circle 35.941mm (1.415")
Intake valve lift 8.1534mm (0.321")
Exhaust cam duration 247 @ 0.003"
Exhaust cam opening 59 BBDC
Exhaust cam closing 8 ATDC
Exhaust cam lobe height 44.600mm (1.7560")
Exhaust cam base circle 35.941mm (1.415")
Exhaust valve lift 8.6614mm (0.341")
Valve overlap 10

any suggestions on a suitable off-the-shelf cam grind that might open up my top end a bit? would adjustable cam gears alone be able to yield some decent results? i cant go too extreme, this thing still needs to be able to pass a tailpipe test... and it is driven daily on occasion. i found a good deal on a pair of "web cams 505+2.5" but i'm worried they might be a bit too aggressive.
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Old 01-25-2009, 05:51 AM   #2
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1. search "intake manifold"
2. read all the threads you get
3. order an intake manifold from BEGi or make your own
4. ???
5. profit

Aftermarket IM is probably worth 40tq on your car at redline.
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Old 01-25-2009, 12:58 PM   #3
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install intake cam gear retarded by 12 crank degrees.
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Old 01-25-2009, 01:49 PM   #4
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I'd still put a set of rods in that motor. I know, I know, but at those power levels, I would.

Also +1 to new intake manifold. Also try retarding the intake cam ~6 degrees. That will take 6 degrees out of the overlap and put your intake valve closing at 57*ABDC. Youd probably retard it more and see larger gains after 5K+.
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Old 01-25-2009, 02:56 PM   #5
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You guys are suggesting retarding his intake cam, when his BPT intake cam is already way retarded compared to a BP motor.

therealdeal, note that your motor makes the same power at 4500 RPM as at 7000 RPM. If you don't raise your revlimit, that's actually pretty good.

Now I have to ask, what is your objective? If you increase your power at 7000 RPM but reduce it at 4500 RPM, the car may end up being slower instead of faster, in day to day driving. Do you drag race? If so, what does RPM drop to after say, a 2-3 shift?

If you want to raise your revlimit, then I would suggest increasing your intake cam duration by 10*. The guy who can design and regrind your cams given your specs and requirements is Dema Elgin at www.elgincams.com .
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Old 01-25-2009, 03:01 PM   #6
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Well hell, I see his intake valve doesn't open till 2* BTDC. A stock miata intake cam opens sooner than that. If the lifts are ramp rates are the same, I'd swap to a miata intake cam and then retard it, or do the exhintake swap and retard.
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Old 01-25-2009, 03:04 PM   #7
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DOes the miata intake cam have more duration?
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Old 01-25-2009, 03:11 PM   #8
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Yeah pretty sure the stock intake cam has more duration. Not sure on lift. I'll post specs in a sec.

EDIT: After looking at the specs below, this statement is false. Looks like the same duration, just retarded 3* and has a spec less lift.
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Old 01-25-2009, 03:13 PM   #9
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Old 01-25-2009, 03:15 PM   #10
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Goto integral cams. Your situation is paticularly difficult because of the high exhaust manifold and intake manifold pressures. My advice would be to get a cam that has 15 degrees more duration and .38 inches or so of lift. Then dial it in with a cam gear. The idea would not be to have more overlap, but to keep the valve open longer while the piston is coming up the cylinder because your intake manifold pressure will be higher then your cylinder pressure for much longer in your case.

Your developing a kind of miller cycle engine in a way. Expect power to drop off boost, but increase every where boost of a reasonable amount exists (more then 10 psi or so) Your intake cam centerline should be a much later opening then stock. Other then that professionals should steer you in.
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Old 01-25-2009, 03:16 PM   #11
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does anyone make NEW booosted cams for the bp? i know web can regrind which i was going to do unless i can find something better anyone?
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Old 01-25-2009, 03:18 PM   #12
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Quote:
Originally Posted by TravisR View Post
Goto integral cams. Your situation is paticularly difficult because of the high exhaust manifold and intake manifold pressures. My advice would be to get a cam that has 15 degrees more duration and .38 inches or so of lift. Then dial it in with a cam gear. The idea would not be to have more overlap, but to keep the valve open longer while the piston is coming up the cylinder because your intake manifold pressure will be higher then your cylinder pressure for much longer in your case.

Your developing a kind of miller cycle engine in a way. Expect power to drop off boost, but increase every where boost of a reasonable amount exists (more then 10 psi or so) Your intake cam centerline should be a much later opening then stock. Other then that professionals should steer you in.
Not sure that bold part is right. Cylinder filling and valve timing is largely a function of the velocity of the air going through the head.
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Old 01-25-2009, 03:27 PM   #13
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The pressure gradient and the velocity gradient are synomous. You can continue filling the cylinder until the spring in the cylinder is fully compressed, and begins a reversion flow into the intake tract. Force on the spring is dictated by pressure in the valve area. So when the pressure in the cylinder becomes higher then the pressure in the intake tract the spring begins to unwind and the velocity vector value becomes negative (assuming cylinder filling direction is positive.)

You are right and I'm right, we're just talking about it in two different ways.
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Old 01-25-2009, 03:36 PM   #14
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i shouldnt need rods... these motors are built up pretty well from the factory. its basically a motor straight out of a WRC car...

GTR---------------------------------n/a BP----------------------------------B6T (BF GTX)


heres some info on the motor: GT-R Engine

i'm not sure the intake manifold is an issue, but theres no way to know for sure without putting a custom one on it. the begi one won't work without severe modifications.... FWD bp's breathe from the opposite end :(

i never realized that the miata intake cams had more lift than the fwd intake cams, thats interesting! in theory if i put a miata intake cam in there and retarded it 3 degrees, i'd gain a little lift but leave the rest of the specs essentially unchanged. of course i could always then play with intake cam timing from there...

so the 505+2.5... would they be a good choice or no? i found a set brand new for about $300 but the deal isn't going to last too long...

yes i do drag race, but i also drive the car around town, to work, etc (in the summer). i cant remember my rpm drop off hand... and it looks like the gear ratio calculator i usually use on an f-body site is now blocked by firefox (wtf?)
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Old 01-25-2009, 03:38 PM   #15
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Those GTR rods look beefier, but I've seen people here post pictures of them broken at the top near the wrist pin, and were making less power than you. Up there, it's pretty similar to a miata rod. Just saying.
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Old 01-25-2009, 03:39 PM   #16
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I have an extra exhaust cam from a BP rwd, the famous "exhaust intake mod" I could sell you. I think the best bet would still be a to contact integral cams.

Last edited by TravisR; 01-25-2009 at 04:01 PM.
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Old 01-25-2009, 03:39 PM   #17
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Also an exhaust cam can be retrofitted in place of the intake cam, and they have more lift and duration than an intake. Though you may want custom cams, these are just cheap options.
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Old 01-25-2009, 03:41 PM   #18
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Quote:
Originally Posted by TravisR View Post
The pressure gradient and the velocity gradient are synomous. You can continue filling the cylinder until the spring in the cylinder is fully compressed, and begins a reversion flow into the intake tract. Force on the spring is dictated by pressure in the valve area. So when the pressure in the cylinder becomes higher then the pressure in the intake tract the spring begins to unwind and the velocity vector value becomes negative (assuming cylinder filling direction is positive.)

You are right and I'm right, we're just talking about it in two different ways.
Hmm. I see what you're saying I guess. It doesn't click in my mind though. I think what you're saying is the result of what I said.
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Old 01-25-2009, 03:58 PM   #19
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i know about the exhintake mod... but i really dont like how annoying it is to set up. X number of teeth off plus Y degrees... screw that lol.

i'll contact integrated to see what they think.

i have never heard of a GTR rod failing up near the wrist pin... ive seen a few that bent with around 400whp+detonation but i've never seen or heard of one that actually broke. do you have any pics, links to a thread?

gtx rods on the other hand have a much higher failure rate. mostly they seem to crack in the thin spots at the top corners of the big end.
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Old 01-25-2009, 04:00 PM   #20
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No link, can't remember who did it. But it's on this forum. A search may turn up the thread. Look for "GTR Rod" or something like that. I think they were at 250whp IIRC. But who knows if they knocked, or rev'd the snout out of it.
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