Yet another cam question
03-05-2003, 10:50 PM
#1
Drifting
Thread Starter
Yet another cam question
In the next few weeks, I'll reach the point where I must make a decision on camshaft. Engine is 427, 6" rods, 10.6:1, AFR 227cc heads, Holley Stealthram, LPE 58mm, Hooker 2151s; with the factory 6-speed and rearend. Street use, no slicks, want it to have a decent idle, but still get as much topend as possible.
I'm sure my engine builder will want to call CompCams and have a custom cam ground, but I'd like to give him some examples to compare with.
Throw some cam recommendations at me! :confused: :grouphug: :thumbs: :steering: :cheers:
I'm sure my engine builder will want to call CompCams and have a custom cam ground, but I'd like to give him some examples to compare with.
Throw some cam recommendations at me! :confused: :grouphug: :thumbs: :steering: :cheers:
03-05-2003, 11:26 PM
#2
Burning Brakes
Re: Yet another cam question (DiabolicalDave)
On that big cubic inch engine you want to go a little tighter on the LSA, but that
also increases overlap (i.e. idle quality) so there's one tradeoff. You don't
want the dynamic compression ratio to get too high either, and in order to
keep it down to a reasonable level given your 10.6 to 1 static compression ratio, you'll need a cam with some overlap, which again will affect the idle quality.
I did some math and I'd say you want a cam with about 280 degrees advertised duration on
a 108 LSA, installed at 106 intake lobe center will give you 64 degrees of
overlap, which will be a little rough, and a dynamic compression ratio
of 8.4 or so, which is getting a little high for pump gas but is doable if you
keep the engine cool (170 degrees) and run 93 octane gas.
If you reduce the overlap you're going to get too high on the dynamic
compression ratio.
How's that for a recommendation? I'm sure there are a couple dozen other
opinions out there.
Dan
also increases overlap (i.e. idle quality) so there's one tradeoff. You don't
want the dynamic compression ratio to get too high either, and in order to
keep it down to a reasonable level given your 10.6 to 1 static compression ratio, you'll need a cam with some overlap, which again will affect the idle quality.
I did some math and I'd say you want a cam with about 280 degrees advertised duration on
a 108 LSA, installed at 106 intake lobe center will give you 64 degrees of
overlap, which will be a little rough, and a dynamic compression ratio
of 8.4 or so, which is getting a little high for pump gas but is doable if you
keep the engine cool (170 degrees) and run 93 octane gas.
If you reduce the overlap you're going to get too high on the dynamic
compression ratio.
How's that for a recommendation? I'm sure there are a couple dozen other
opinions out there.
Dan
03-06-2003, 08:05 PM
#3
Drifting
Thread Starter
Re: Yet another cam question (danno85)
Dan,
That's exactly what I was looking for. I recognized the 10.6 was going to limit me because I do need to bleed that down some with the intake valve closing a little late, which of course will affect the idle and vacuum. You've put some numbers on it which is what I was looking for. My engine builder does 95% carbureted. I know he'll call CompCams to get their opinion, but it's nice to have an opinion from somebody close to what I'm doing too.
Thanks
That's exactly what I was looking for. I recognized the 10.6 was going to limit me because I do need to bleed that down some with the intake valve closing a little late, which of course will affect the idle and vacuum. You've put some numbers on it which is what I was looking for. My engine builder does 95% carbureted. I know he'll call CompCams to get their opinion, but it's nice to have an opinion from somebody close to what I'm doing too.
Thanks
03-06-2003, 08:26 PM
#4
Burning Brakes
Re: Yet another cam question (DiabolicalDave)
No problem. Let me know what your engine builder says after he gets
his info from CompCams or however. I'd be curious to know.
I thought about your setup some more and are you going carb or FI?
With that big an engine and that much compression it might be worthwhile
to retard the cam a couple degrees, install is at 108 intake lobe center,
which would further delay the intake closing and drop the DCR a little more.
I don't think that'd hurt your bottom end torque very much on your combination.
It'd be nice to have an engine dyno and play around with some different
configurations... :yesnod:
In fact, I'm thinking about getting back into the engine building business
(but don't tell my wife :D )
Dan
his info from CompCams or however. I'd be curious to know.
I thought about your setup some more and are you going carb or FI?
With that big an engine and that much compression it might be worthwhile
to retard the cam a couple degrees, install is at 108 intake lobe center,
which would further delay the intake closing and drop the DCR a little more.
I don't think that'd hurt your bottom end torque very much on your combination.
It'd be nice to have an engine dyno and play around with some different
configurations... :yesnod:
In fact, I'm thinking about getting back into the engine building business
(but don't tell my wife :D )
Dan
03-06-2003, 08:45 PM
#5
Drifting
Thread Starter
Re: Yet another cam question (danno85)
Dan,
I won't promise to let you know the cam I go with, because I'll probably forget by then. But, my website I'm building to document this little project will have the information.
The engine will use EFI; the speed density system with a custom chip, the Holley fuel rail kit for the Stealthram, and probably 34psi injectors, and a 58mm throttle body.
Actually, with the quench blueprinted and the excellent chamber design of the AFR heads, the compression might not be such a big issue. That's one of the first things I'll talk to Jeff about. I'd really like to be able to say I can run 91 octane. Requiring 93 octane means that I'm going to have to carry octane booster in the car in case there's no 93 octane available sometime.
The speed density system is more sensitive to the duration than MAF, as I'm sure you're aware. So, I'm balancing some things here.
Actually, after Jeff gets the recommendation from CompCams, I'll probably post the spec's they're recommending here to get feedback before I agree to it.
By the way, something that plays into this is the ignition. We have an MSD billet distributor sitting up there but I don't have a PN. This engine was originally going to be carbureted. I'm wondering if we have the right distributor. I think the timing needs to be controllable. Jeff was telling me that on most "hot" EFI engines the timing ends up being not controlled, and the computer just controls the fuel. Since I've never done one before, I'm not sure.
I won't promise to let you know the cam I go with, because I'll probably forget by then. But, my website I'm building to document this little project will have the information.
The engine will use EFI; the speed density system with a custom chip, the Holley fuel rail kit for the Stealthram, and probably 34psi injectors, and a 58mm throttle body.
Actually, with the quench blueprinted and the excellent chamber design of the AFR heads, the compression might not be such a big issue. That's one of the first things I'll talk to Jeff about. I'd really like to be able to say I can run 91 octane. Requiring 93 octane means that I'm going to have to carry octane booster in the car in case there's no 93 octane available sometime.
The speed density system is more sensitive to the duration than MAF, as I'm sure you're aware. So, I'm balancing some things here.
Actually, after Jeff gets the recommendation from CompCams, I'll probably post the spec's they're recommending here to get feedback before I agree to it.
By the way, something that plays into this is the ignition. We have an MSD billet distributor sitting up there but I don't have a PN. This engine was originally going to be carbureted. I'm wondering if we have the right distributor. I think the timing needs to be controllable. Jeff was telling me that on most "hot" EFI engines the timing ends up being not controlled, and the computer just controls the fuel. Since I've never done one before, I'm not sure.