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Help with Cam selection (again) |
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AMXrated
AMC Nut Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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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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You can have my Hurst shifter when you pry it from my cold dead hand.
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Red Devil
AMC Addicted Joined: Jul/10/2007 Location: Canada Status: Offline Points: 1743 |
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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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Greyhounds_AMX
AMC Addicted Joined: Nov/14/2009 Location: Kansas City Status: Offline Points: 1268 |
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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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1968 AMX 390 w/T5
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AMXrated
AMC Nut Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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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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You can have my Hurst shifter when you pry it from my cold dead hand.
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Ken_Parkman
AMC Addicted Joined: Jun/04/2009 Location: Ontario Status: Offline Points: 1815 |
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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 1:20pm |
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Greyhounds_AMX
AMC Addicted Joined: Nov/14/2009 Location: Kansas City Status: Offline Points: 1268 |
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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 2:59pm |
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1968 AMX 390 w/T5
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AMXrated
AMC Nut Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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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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You can have my Hurst shifter when you pry it from my cold dead hand.
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jpnjim
AMC Addicted Joined: Nov/25/2007 Location: New England Status: Offline Points: 2752 |
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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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71 P-code 4spd Javelin/AMX
some Jeeps and some Fords |
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jpnjim
AMC Addicted Joined: Nov/25/2007 Location: New England Status: Offline Points: 2752 |
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I copied your original two recommendations:
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 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.
Edited by jpnjim - Oct/12/2018 at 7:54pm |
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71 P-code 4spd Javelin/AMX
some Jeeps and some Fords |
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AMXrated
AMC Nut Joined: Jul/26/2015 Location: Indiana Status: Offline Points: 290 |
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@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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You can have my Hurst shifter when you pry it from my cold dead hand.
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