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Help with Cam selection (again)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 12:08pm
Originally posted by Greyhounds_AMX Greyhounds_AMX wrote:

Another thing that may help you is that for your application you can ignore the cam lift numbers. Any cam you use will be between 0.450 and 0.600 lift, and the flow rate of your heads peaks at about 0.450, so anything above that doesn't significantly increase flow. 

Certain relationships between duration and lift are required though in order to prevent mechanical problems, so lift will increase along with duration. There are exceptions (like dwell nose cams), but they aren't reliable or quiet enough for street use.

For example, here's a comparison of the flow you will get when using the lowly Edelbrock 2132 cam versus a much higher lift Engle, assuming your typical ported iron AMC dogleg heads. The peak flow rates are the same, only the increased duration adds to the flow area (the graph on the right). The increased peak lift doesn't really matter.

The car came with a functional cowl induction hood, but the breather is missing (naturally...) and I was considering doing some mild porting on the intakes.  If I were to do both/either of these things, would it be better to have some extra lift available?  Or, will it have much effect on overall flow?

Thanks.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Red Devil Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 12:49pm
If you read through the sticky on AMC cylinder head flows, I don't see flows stalling at 0.450" lift ... particularly with a bit of porting.http://theamcforum.com/forum/cylinder-head-flow-numbers-factory-aftermarket_topic159.html

If you are planning some mild porting work, as someone mentioned already, you could open up the chambers to unshroud the valves, increasing combustion chamber cc which will reduce your compression. Spec on the Wiseco PTS539A3 lists 10.5:1 w/ 58cc heads. Many AMC heads measure closer to 60cc.



Flat tappet cams have a pointy nose and move across the lifter from edge-to-edge. A larger diameter lifter allows this action to start sooner and end later without falling off the edge of the lifter. That's why a 0.904" lobe profile can have a greater rate of duration increase and greater lift for same seat duration, typically translating into more cylinder pressure, more torque and more power. If you're running pump gas and have too much compression, maybe that's a negative. If worried about valvetrain loading, maybe also a negative as spring forces will be slightly higher. Most off-the-shelf AMC cams are Chevy 0.842" or Ford 0.875" lobes on an AMC core.

Ultimately, tough to get a conclusive answer on a Forum unless someone is running your exact combination ... but a bigger cam is likely the right direction.

Greyhounds AMX: Do you have the Comp Thumpr cams in your spreadsheet? Curious how they would compare (not recommending one ... just curious)?

Thanks,RD.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 1:07pm
Yep, the Thumpr cams are in there, but they don't seem to fit much. Just too aggressive i think.

Jack, you can certainly use 842 or 875 cams with 904 lifters. Most AMC builds with hydraulic cams are using lobes designed around 842 Chevy lifter limitations.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 1:44pm
Originally posted by Red Devil Red Devil wrote:

If you read through the sticky on AMC cylinder head flows, I don't see flows stalling at 0.450" lift ... particularly with a bit of porting.http://theamcforum.com/forum/cylinder-head-flow-numbers-factory-aftermarket_topic159.html

If you are planning some mild porting work, as someone mentioned already, you could open up the chambers to unshroud the valves, increasing combustion chamber cc which will reduce your compression. Spec on the Wiseco PTS539A3 lists 10.5:1 w/ 58cc heads. Many AMC heads measure closer to 60cc.

Flat tappet cams have a pointy nose and move across the lifter from edge-to-edge. A larger diameter lifter allows this action to start sooner and end later without falling off the edge of the lifter. That's why a 0.904" lobe profile can have a greater rate of duration increase and greater lift for same seat duration, typically translating into more cylinder pressure, more torque and more power. If you're running pump gas and have too much compression, maybe that's a negative. If worried about valvetrain loading, maybe also a negative as spring forces will be slightly higher. Most off-the-shelf AMC cams are Chevy 0.842" or Ford 0.875" lobes on an AMC core.

Ultimately, tough to get a conclusive answer on a Forum unless someone is running your exact combination ... but a bigger cam is likely the right direction.

Greyhounds AMX: Do you have the Comp Thumpr cams in your spreadsheet? Curious how they would compare (not recommending one ... just curious)?

Thanks,RD.

Thanks RD--That is an interesting read.  If I'm reading the data right on the stock unported heads, flows don't stop after .450, but they do start to decrease at an increasing rate after that point and the gains get incrementally smaller.  

And, you're probably right on the cam selection--no one's going to be able to tell me what I need since everyone's set-up and preferences are different.  Going to have to just make a decision and go for it.  If I had to make the call right now, think I'd go with the bigger Crower.

Should have the pistons in tonight (unless the wife wants to go out to dinner...).  Hopefully can do a clearance check tomorrow, although it looks like the Wiseco's have pretty healthy reliefs on them so I doubt it will be a problem.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 2:16pm
More lift is almost always better as long as you do not exceed durability limits. More lift is still better even if the flow separates and falls off at high lift - a typical problem with too large a valve for a given port configuration and usually happens above .25 l/d. the point is peak lift is only for an instant, but with higher lift you get more overall flow area under the curve. And flow separation characteristics on a flow bench may not be the same thing in a running engine when the delta P is much greater.

Also I don't worry a lot with lifter diameter for street type applications. Yes a .904 lifter is better, but I had a conversation with a lobe designer one time and he told me there is not much in it on less than all out circumstances. to make a difference the contact patch has to move far enough from the centerline to be close to the edge. Much more of a concern on the race technology.

Edited by Ken_Parkman - Oct/12/2018 at 2:20pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 3:49pm
That's one of the things my spreadsheet is trying to illustrate via the graph on the right side. It a plot of the flow that heads will pass at the lift of the selected cam as is goes thru the travel.

The curve is then integrated to get the total area under the curve, and thats the "Intake Area" and "Exhaust Area" listed in the table. The difference in area under the curve of the two selected cams is shown on the graph.

Edited by Greyhounds_AMX - Oct/12/2018 at 3:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 7:08pm
I've noticed that in many cam designs the gross lift is either the same for Intake/Exhaust or the Intake is slightly higher than the Exhaust.

But in a few designs (like the custom spec'd by Bullet), the Intake is higher than the Exhaust lift.

What is the effect of that?  

Also, I have read that AMC engines like for the Exhaust lift to be slightly higher than the Intake--is this true or just another one of those "you shouldn't believe everything you read on the internet" things?

Thanks.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 7:46pm
Cam designers like to run a few more degrees of exhaust duration to help make up for a restrictive exhaust system.

A lot of the time they'll choose an exhaust lobe with less intensity so it can have that extra duration,
 while still having about the same lift as the intake.

In that case, the intake, or the exhaust lobe may have slightly more lift than the other,
 just the luck of the draw of using existing lobes.

I'd say only the Bullet designers would know if they're actively trying to maximize intake vs exhaust lift, or if the extra intake lift is just the by-product of the more intense intake cams they are using.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote jpnjim Quote  Post ReplyReply Direct Link To This Post Posted: Oct/12/2018 at 8:42pm
I copied your original two recommendations:
Quote
Howards:

.523/.520 valve lift

225/235 @ .050

279/289 advertised duration

112 lobe separation

108 intake centerline

Bullet:
Intake 
236 @.050
.350” lobe lift, .560” with 1.6 rocker
283 duration @.006”

Exhaust
244 @.050
.350” lobe lift, .560” with 1.6 rocker
294 duration @.006”

112 lobe separation 
108 intake centerline

 

The biggest problem is the 10.26:1 CR 360 would make me want a bigger cam to get the DCR down,
  and 2.23 first gear T-10 with just a 3.54 rear would want me to get a smaller cam to help me pull the gear.

My T-10 Javelin with a 360 had 3.91 gears and it was a chore coming out of the hole sometimes.

I like the 112-114 lobe center to close the intake valve a little later, and lower the DCR without going crazy on duration.

Howards cams are high quality, they make customs fast (mine took around a week) and the price is very good.

Their "Aggressive Flat Tappet Hydraulic" line of lobes are a pretty good compromise of intensity without the higher lifts you'd end up with the same durations in a .904 lobe.

File Name @0.006" @0.020" @.050" @.200" 1.6000 lift
3HF223323   270       247       223     137    .5173
3HF225327   272       249       225     139    .5227
3HF227330   274       251       227     141    .5280
3HF229333   276       253       229     143    .5333
3HF231333   278       255       231     145    .5333



The 904 lobes have a higher intensity & a bit more lift

File Name @0.006" @0.020" @.050" @.200" 1.6000 lift
HM2243450A  271     248       224      144   .5520
HM2263500A  273     250       226      147   .5600
HM2283525A  275     252       228      148   .5640
HM2303541A  277     254       230      150   .5666


I bet if you went with something like the
3HF229333   276       253       229     143    .5333 on a 114
you'd be at a healthy compromise between what the 10.26:1 compression ratio  wants to run on pump gas and what the (tallish) 7.89 first gear wants to pull a bubble fender Javelin in first gear.

If you want to drop the DNC a little, then maybe go a little bigger,
 or pick something from the "Standard Ramp Hydraulics"
 that have greater .006" duration, without bumping up the .050" duration too much:

File Name @0.006" @0.020" @.050" @.200" 1.6000 lift
1hf225310   279       252       225      137    .4960
1hf225327   279       252       225      138    .5227
1hf225330   279       252       225      138    .5280
1hf227313   281       254       227      139    .5013
1hf227320   281       254       227      140    .5120
1hf227327   281       254       227      140    .5227
1hf231300   285       258       231      139    .4800
1hf231313   285       258       231      142    .5013
1hf231320   285       258       231      143    .5120

^these would potentially bleed off some more cylinder pressure for the same .050" duration (since the gross, or .006" duration is higher),
 but they also give up duration at .200".

The second lobe from the top is the intake lobe Howards chose for you,
 this is the exhaust:


File Name @0.006" @0.020" @.050" @.200" 1.6000 lift
1hf235325   289       262        235      147   .5200

I'm not a cam designer, so I don't really have a recommendation
just putting some thoughts down Smile

Edit,
 it also makes some sense why Howards recommended the relatively mild 1hf225327 intake lobe,
 and the fairly aggressive 1hf235325 exhaust on a 112,
 trying to find a good balance between DNC and low end torque. Star


Edited by jpnjim - Oct/12/2018 at 8:54pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXrated Quote  Post ReplyReply Direct Link To This Post Posted: Oct/13/2018 at 1:02am
@jpnjim:  Thanks for the thoughts/analysis.  I am absolutely blown away at how much you guys know about this stuff.  The last time I worried about cam specs, we called them 3/4 Racing cams...probably dating myself.

I've heard really good things about Howard's, but never got a reply from the guy after I asked him to reconsider his spec's targeting a 1600-5500 RPM range.  I even sent him a follow-up email this morning.  


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