I am running a "pure stock" circle track car on a 3/8 mile oval dirt track. I recently built my first engine, a 355, and I have about 15 nights on it.
Here were the specs: 10.2:1 compression ratio, flat top pistons and camel hump heads. 355 chevy. stock intake, heads, and exhaust manifolds, quadrajet carb. No porting whatsoever. 3.73:1 rear gear with a 1.82 powerglide. 3100 lb car with me in it.
I had a howards cam 110812 in it.
Duration at 050 inch Lift 248 int./252 exh.
Advertised Duration 286 int./294 exh.
Valve Lift with Factory Rocker Arm Ratio 0.534 int./0.542 exh.
Lobe Separation (degrees) 106
This cam was installed at 104 degree intake centerline.
The engine has about 165 cranking pressure and 9 inches of vacuum at 1000 RPM.
I've been watching some videos and what's happening is I'm getting absolutely killed on the starts, I start keeping up at the end of the straight (I'm at 6000 RPM at this point) then go into the corner where they're pulling away from me again coming out of the corner.
I talked to some other circle track guys on another board and they all tell me the same thing; my cam is too big. Apparently cam manufacturers have the habbit of giving me too big of a cam, probably because they're listening to the RPM I want to run (3500 or more coming out of the corner, the theory being to come out of the corner at a decent RPM where you make good power to accelerate quickly) out of a stock engine, I don't know.
But they're telling me to run something with about 235 degree duration. All I can say at this point is the cam company techs are giving me different suggestions than the guys who race.
So I ordered this cam:
Comp Cams 12-648-5
Duration at 050 inch Lift 240 int./246 exh.
Advertised Duration 270 int./280 exh.
Valve Lift with Factory Rocker Arm Ratio 0.510 int./0.519 exh.
Lobe Separation (degrees) 108
What throws me off is Comp's description is this is good for a 9:1 compression motor. Meanwhile I milled my heads down and will now get 10.7:1 compression. But I also notice there's a wider LSA so I'm thinking I may not be making any improvement??? These are all solid lifter cams by the way, flat tapet.
Someone else suggested this cam:
Comp Cams 12-646-5
Duration at 050 inch Lift 236 int./242 exh.
Advertised Duration 266 int./276 exh.
Valve Lift with Factory Rocker Arm Ratio 0.501 int./0.51 exh.
Lobe Separation (degrees) 108
I'm starting to think this may actually be better, given the wider LSA may offset the shorter duration some.
These are both Comp Cam's "XT" ramp cams, where theyre supposed to have a lot of "area under the curve" and have a good amount of lift for a short amount of duration. Anyone have any input between these two?
My other thought was to get an old Duntov 30-30 cam or the old LT-1 cam. Now I know everyone says these are old and outdated but they seem to be within the same horsepower range, 320-350 HP or so, as what I would expect my car to make. It sounds like-the old cams are longer duration and low lift. Everything I read about these cams on message boards from people who had these cams in their car back then (I wasn't around back then and I'm just learning about cams now) say these cams were poor performers under 4000 RPM but ZOOMED to the moon!!! All the way to 7000 RPM!!! BTW my engine seems to be making less power after 6000 RPM but that may be the stock camel hump heads, I have plenty of valve spring pressure. And besides, these guys back then may have just been joyriding, not running right beside someone else with another small block chevy doing everything he can to get in front of you. But I know if I get one of these old cams I may be in the ballpark, and that long duration/low lift may be a good tradeoff and work decent at high RPM's.
What if I installed the 12-608-5 at a 104 degree intake centerline?
I'm just trying to get the most power out of this engine and on other boards, other people going back and forth with similar specs as mine unfortunately seem to worry about power brakes, what it does at 2000 RPM, and how they can get that lumpy sound or whatever, me I just want to win races!