Cam intensity, overlap, ICA issues
Thread Starter
Burning Brakes
Joined: Jul 2002
Posts: 768
Likes: 3
From: Chicago Burbs IL
Cam intensity, overlap, ICA issues
Below is a table of possible cams for my in-process L75 (300hp) rebuild. The table is sorted by ascending overlap. Given an OE street motor (max TQ) with static CR of 9.8:1 and 461 stock iron heads, I am concerned with detonation issues. Originally, I was going with the an OE 929 grind but see that given the early intake closing my dynamic or effective CR would be very high ...8.5:1.
So...given a 9.8:1 static how best to select the cam?
Please vote and vote often....I do live in Chicago...
Notice, the large disparity between cam intensity (diff between advertised duration and .050 tappet duration). Seems the high intensity cams yield higher under the curve durations and lowered overlaps (ie improved low-end driveability) at the cost of valve train noise and durability. Is this trade-off worth it? My last L76 had a very noisy, very annoying valvetrain tickety-tack with a cam intensity of 50 (Comp 12-212-2). Interestingly, the OE929 cam has a aggressive 56 intensity while the current Speed-pros have a a gently 74.
So...given a 9.8:1 static how best to select the cam?
Please vote and vote often....I do live in Chicago...
Notice, the large disparity between cam intensity (diff between advertised duration and .050 tappet duration). Seems the high intensity cams yield higher under the curve durations and lowered overlaps (ie improved low-end driveability) at the cost of valve train noise and durability. Is this trade-off worth it? My last L76 had a very noisy, very annoying valvetrain tickety-tack with a cam intensity of 50 (Comp 12-212-2). Interestingly, the OE929 cam has a aggressive 56 intensity while the current Speed-pros have a a gently 74.
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
I'm curious why you are limiting yourself to these cams? None fit your build very well.
Since the engine is being built, what all is being done to it? I'm especially asking for quench. Has the block been decked so each cylinder is equal? Factory machining was often poor with different compression on the cylinders.
All your cams are short duration so I take it you value street manners and want a mild engine. If so, you really should consider installing pistons that give you a lower CR. With these small cams and iron heads I would stay under 9.5:1, probably closer to 9.0:1.
If you're set on 9.8:1 and won't consider aluminum heads I'd look at a longer duration cam. Something arouns 230 dgr intake, 235 dgr exhaust. You will be giving up manners though.
Also, why are you set on the 461 heads? If an original look is important, modern aluminum heads can be painted to look "stock".
Since the engine is being built, what all is being done to it? I'm especially asking for quench. Has the block been decked so each cylinder is equal? Factory machining was often poor with different compression on the cylinders.
All your cams are short duration so I take it you value street manners and want a mild engine. If so, you really should consider installing pistons that give you a lower CR. With these small cams and iron heads I would stay under 9.5:1, probably closer to 9.0:1.
If you're set on 9.8:1 and won't consider aluminum heads I'd look at a longer duration cam. Something arouns 230 dgr intake, 235 dgr exhaust. You will be giving up manners though.
Also, why are you set on the 461 heads? If an original look is important, modern aluminum heads can be painted to look "stock".
Thread Starter
Burning Brakes
Joined: Jul 2002
Posts: 768
Likes: 3
From: Chicago Burbs IL
I'm curious why you are limiting yourself to these cams? None fit your build very well.
Since the engine is being built, what all is being done to it? I'm especially asking for quench. Has the block been decked so each cylinder is equal? Factory machining was often poor with different compression on the cylinders.
All your cams are short duration so I take it you value street manners and want a mild engine. If so, you really should consider installing pistons that give you a lower CR. With these small cams and iron heads I would stay under 9.5:1, probably closer to 9.0:1.
If you're set on 9.8:1 and won't consider aluminum heads I'd look at a longer duration cam. Something arouns 230 dgr intake, 235 dgr exhaust. You will be giving up manners though.
Also, why are you set on the 461 heads? If an original look is important, modern aluminum heads can be painted to look "stock".
Since the engine is being built, what all is being done to it? I'm especially asking for quench. Has the block been decked so each cylinder is equal? Factory machining was often poor with different compression on the cylinders.
All your cams are short duration so I take it you value street manners and want a mild engine. If so, you really should consider installing pistons that give you a lower CR. With these small cams and iron heads I would stay under 9.5:1, probably closer to 9.0:1.
If you're set on 9.8:1 and won't consider aluminum heads I'd look at a longer duration cam. Something arouns 230 dgr intake, 235 dgr exhaust. You will be giving up manners though.
Also, why are you set on the 461 heads? If an original look is important, modern aluminum heads can be painted to look "stock".
Last edited by Talisman51; Mar 17, 2007 at 06:17 PM.
Race Director
Joined: Sep 1999
Posts: 11,360
Likes: 382
From: Plano TX
In that case your best bet will be a very low intensity cam. Try to bleed as much cylinder pressure you can before the intake closes completely.
You should be able to go to close to 220 dgr @050 without loosing drivebility, and then look for something with as much advertised duration as possible.
I built a 350 some time ago with a 214/224 cam on a 112 LSA that idles quite well at 600 rpm. Used 441 heads and flat top pistons. Ran real well for what it was.
You should be able to go to close to 220 dgr @050 without loosing drivebility, and then look for something with as much advertised duration as possible.
I built a 350 some time ago with a 214/224 cam on a 112 LSA that idles quite well at 600 rpm. Used 441 heads and flat top pistons. Ran real well for what it was.
Melting Slicks
Joined: Jul 2005
Posts: 2,060
Likes: 46
From: winter haven florida
St. Jude Donor '12
call comp cams, crane cams, and crower or lunati and tell them your specs and what you want out of your build and they can custom grind a cam for you or give advice on what cam to use. call several so you can see if they are all in the same ballpark.
Le Mans Master
Joined: Mar 2004
Posts: 7,477
Likes: 1,495
From: Little Rock AR
My first thought would be the "151" 327/350 cam advanced 4 degrees. You have the compression for it. The low end torque is a little low in the original configuration but advancing the cam 4 degrees will plump up the torque without a big top end penalty. Some cams will respond better than others when you change the cam angle. The "151" has a lot more RPM potential than it needs. This is above what the application calls for here so who cares if you take a little off the top to get more at the bottom. I ran one of these in my 69 Camaro with a 327 and a Muncie and found it rather nice.
Still stock, still quiet, still reliable.
The whole DCR/intensity empirical evidence works better on conventional aftermarket cams. The factory cams are designed differently so the DCR rules of thumb derived from experience with aftermarket cams do not apply. The rules of thumb are going to be different. Experience is more important here.
Just my 2 cents.
-Mark.
Still stock, still quiet, still reliable.
The whole DCR/intensity empirical evidence works better on conventional aftermarket cams. The factory cams are designed differently so the DCR rules of thumb derived from experience with aftermarket cams do not apply. The rules of thumb are going to be different. Experience is more important here.
Just my 2 cents.
-Mark.
Last edited by stingr69; Mar 18, 2007 at 09:08 AM.
Thread Starter
Burning Brakes
Joined: Jul 2002
Posts: 768
Likes: 3
From: Chicago Burbs IL
yep....
My first thought would be the "151" 327/350 cam advanced 4 degrees. You have the compression for it. The low end torque is a little low in the original configuration but advancing the cam 4 degrees will plump up the torque without a big top end penalty. Some cams will respond better than others when you change the cam angle. The "151" has a lot more RPM potential than it needs. This is above what the application calls for here so who cares if you take a little off the top to get more at the bottom. I ran one of these in my 69 Camaro with a 327 and a Muncie and found it rather nice.
Still stock, still quiet, still reliable.
The whole DCR/intensity empirical evidence works better on conventional aftermarket cams. The factory cams are designed differently so the DCR rules of thumb derived from experience with aftermarket cams do not apply. The rules of thumb are going to be different. Experience is more important here.
Just my 2 cents.
-Mark.
Still stock, still quiet, still reliable.
The whole DCR/intensity empirical evidence works better on conventional aftermarket cams. The factory cams are designed differently so the DCR rules of thumb derived from experience with aftermarket cams do not apply. The rules of thumb are going to be different. Experience is more important here.
Just my 2 cents.
-Mark.
I was hoping to get some feedback on intensity issues. Are the more harsh cams listed (as measured by low intensity numbers...diff of seat and .050 duration) prone to early lobe wear and seat recession etc...?
Any 300hp rebuilders out there? Remember I will be using OE everything...4sp...3.36 gear...cast intake...oe exhaust.
At this point I am tending to one of the first four from the table.
Thanks
JimV
Thanks
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Thread Starter
Burning Brakes
Joined: Jul 2002
Posts: 768
Likes: 3
From: Chicago Burbs IL
yep...other end of intake cycle.... Overlap effects low end TQ and idle while ICA effects DCR.... Both are issues....
