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CAM SPECS |
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JERSEYJOE
AMC Addicted Joined: Oct/23/2008 Location: TUG HILL NY Status: Offline Points: 1718 |
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Posted: Nov/13/2018 at 2:02pm |
Am I doing the right thing here trying to tone down the motor for a more streetable ride?
These are the specs that the mechanical cam in my car had: INTAKE 258 DEG AT .050 .585 LIFT WITH 1.6 ROCKERS EXHAUST 262 DEG AT .050 .595 LIFT WITH 1.6 ROCKERS I did run this on the street years ago and it was a bit of a monster with 11.5 to 1. My goal is to tame this down to where it is a bit more drivable so I am going with the milder of the 2 mechanical rollers that CRANE sells but they are out of stock of blank billets |
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1985 J-20 PLOW TRUCK
1977 CJ 7 360 T-18 D 300 1970 AMX ex-SCCA car SPEED COSTS MONEY HOW FAST DO YOU WANT TO GO? |
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ghinmi
AMC Addicted Joined: Apr/04/2010 Location: Grand Rapids MI Status: Offline Points: 978 |
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Are you lowering the compression from the 11.5:1 and are you intending to run it on pump gas? What heads?
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1975 Cherokee S - Turbo Hemi stick shift autocross/drag race/street 9.97 @ 140.4
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Lime Green 70
AMC Apprentice Joined: Dec/01/2017 Location: pa Status: Online Points: 74 |
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I run a similar cam in my car . specs are intake 254 at .050 intake 260 exhaust at .050 on 110 lobe spread . it's a bit radical but runs excellent with my combo . I also have turn downs before the rear with 3 in exhaust . a lot depends on if your running full exhaust and the rest of your combo .
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JERSEYJOE
AMC Addicted Joined: Oct/23/2008 Location: TUG HILL NY Status: Offline Points: 1718 |
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Not lowering the compression. They are the 292 heads with 1.68 exhaust valves and they are ported a bit
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1985 J-20 PLOW TRUCK
1977 CJ 7 360 T-18 D 300 1970 AMX ex-SCCA car SPEED COSTS MONEY HOW FAST DO YOU WANT TO GO? |
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ChillyB
AMC Nut Joined: Nov/14/2018 Location: PA Status: Offline Points: 371 |
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I'm brand new here, so forgive me if I tell you what you already know. I have no idea what you know. With a high CR your engine will need a long duration cam with some overlap to run on pump gas. Otherwise your cranking pressure will be too high and you'll get pinging. What was it about the engine that was a little excessive? Rough low RMP performance? Jumpy on the throttle? Had a cam in my Harley that was a torque-monster. Low RPM torque like crazy, ran out of breath early, and some detonation problems at high temp. I could not lay into the throttle without pinging so what good was all that CR? Since the CR wasn't something I was willing to change (and wasn't actually that high, but the cam had moderate duration but very little overlap) so instead I put in a cam with more duration and overlap to bleed down the cranking pressure. Bike is way better mannered with the longer duration cam. IIRC it's the timing of the intake valve closing that was key to my problem, and my solution. It packed the air charge early, closed the intake early, built a ton of pressure, and without much overlap had gobs of low-mid torque but poor high end and too much cylinder pressure.
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ghinmi
AMC Addicted Joined: Apr/04/2010 Location: Grand Rapids MI Status: Offline Points: 978 |
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My opinion. The cam you had before isn't that radical. If you're planning on pump gas, you don't want to go smaller at all. If you do go smaller, you'll have to drop the compression. Or make the switch to e85 or race gas.
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1975 Cherokee S - Turbo Hemi stick shift autocross/drag race/street 9.97 @ 140.4
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Trader
AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6754 |
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Really, the easiest way to do this is put on 60 CC 502 heads and don't touch anything else. 10.5:1 compression ratio and should be pump gas friendly.
502's are cheap and easy to find. You will spend less building 502's and installing them then trying the custom cam and pushing your fuel/air out the exhaust not burnt - or it may be a cool flame thrower! Save your heads for another day or sell them for way more $$$ then what the 502's will cost.
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jpnjim
AMC Addicted Joined: Nov/25/2007 Location: New England Status: Offline Points: 2752 |
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Hi Chilly, welcome to the forum. More Jeep guys are always a good thing. I agree with everything you wrote, except for the overlap part. Overlap gets a lot of the credit/blame for bleeding off cylinder pressure but overlap & cylinder pressure are caused by opposite ends of the intake valve event. Overlap happens in the time period that the exhaust valve is almost closed, and the intake valve is begining to open, and intake & exhaust gasses mix. Cylinder pressure is determined by how early or late the intake valve closes on the opposite side of the intake ramp. So if you could advance the intake cam alone, without moving the exhaust cam (tightening the lobe centers) you'd increase cylinder pressure AND increase overlap. I think the confusion is because a BIGGER intake cam lobe would both open the intake valve earlier (increasing overlap) AND close the intake valve later (lowering cylinder pressure). But they really are opposite side effects happening on the opposite ends of the intake valve's timing. I know 90% of everyone puts in a bigger cam, gets more overlap and lower cylinder pressure and thinks "overlap bleeds off pressure", and you can find websites explaing camshafts that will outright tell you overlap is the bleeding off of cylinder pressure, but it's really the opposite. A wider lobe center cam will have lower overlap (all things being the same), and lower cylinder pressure A tighter lobe center cam (all things being equal) will have more overlap and greater cylinder pressure. More duration in the intake cam (all things being equal) will increase overlap and reduce cylinder pressure also Advancing the cam = greater cylinder pressure (no effect on overlap) Retarding the cam = lower cylinder pressure (no effect on overlap) So the original poster needs to not close his intake valve any earlier, or risk detonation from high cylinder pressure. A big intake lobe, retarding the cam, or having the new cam cut on a wider lobe center would all help with that, and all effect overlap in a different way. sorry for the length, I don't know how to explain all that with less words |
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71 P-code 4spd Javelin/AMX
some Jeeps and some Fords |
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ChillyB
AMC Nut Joined: Nov/14/2018 Location: PA Status: Offline Points: 371 |
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That's a great explanation and worth the reading of it. Appreciate the insight. On the topic, if I may intrude on the original post, I'll be rebuilding a 304 for my CJ5. I have no plans for spending a bunch of money on performance parts but I'm always willing to take advantage of some more or less free HP. Would the engine benefit from advancing the factory cam? Also, I have two 304 engines: the original 1973 and what I think was a 1980-something. Are the cam grinds identical?
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