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  #1  
Old 20-05-2018, 12:11 PM
69JavelinMan 69JavelinMan is offline
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Low Vacuum - Intake Reversion?

I've just changed the cam in my engine and now have an issue with rough idle and really low vacuum of 5 in-Hg at idle.

My understanding is that this can be either a vacuum leak or late intake valve closing. I can't find any vacuum leaks up top but possibly from underneath, it's a new valley pan and the same intake that was on there previously, so really should be ok.

Now I don't have any specs on the cam, so have installed it dot to dot and measured what I got:

Intake:
Open 32 BTDC, Close 86 ABDC, Duration 298
0.050", 7 BTDC, 45 ABDC, Duration 232
Intake center line 109
Valve Lift 0.510"

Exhaust
Open 73 BBDC Close 49 ATDC, Duration 302
0.050", 48 BBDC, 10 ATDC, Duration 238
Exhaust center line 110
Valve lift 0.510"

LSA 110

Seems like really long closing ramps on the lobes for some reason.

Now it seems like the intake closing angle is really late and I know this can cause reversion, but I can't find anything that says exactly how late is too late.

Possibly the cam gear is out a tooth, which would bring it back to 101 ICL (seems low) and maybe make some of the other numbers a little more reasonable.

Compression is down about 10-15 psi across the board with the new cam, which also seem to suggest late valve closing.
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  #2  
Old 20-05-2018, 09:59 PM
Geoff 4 Geoff 4 is offline
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What size engine? It makes a huge difference.

Most good cams have longer closing ramps to close the valves more softly & prevent valve bounce. Sign of a quality made cam.

232/238 @ 050 110LSA is 'getting up there' in cam size. 5" of vacuum does sound low. The rough idle is caused by exh gas dilution during overlap at idle. The spark is trying to ignite a mixture of fuel + air, but is instead being asked to ignite fuel + air + exh gas. Cannot do it & protests with the rough idle.

A single pattern cam [ 232/232 ] would have been much better & probably given better overall street performance. Reducing the exh component of overlap reduces exh gas dilution, smooths idle, increases vacuum. Very few engines need extra exh duration once the restrictive factory exh system is replaced with headers & large free flowing exh. I actually use cams with less exh duration! How does high 11s @ 236/230 @ 050 sound with a 3.23 axle, factory 4bbl carb & intake manifold, 3900 lb car without driver, 800 rpm idle?

Look at the Isky cams website Tech Tips: Is extra exh duration really necessary?

In the meantime, try this & report back: adjust initial timing to 32-38 degrees, not critical. Yes, 32-38 deg, not a misprint.
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  #3  
Old 20-05-2018, 11:57 PM
69JavelinMan 69JavelinMan is offline
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Engine is a AMC 360, nothing fancy, unknown history, most likely stock heads and compression.

Cam started out as a Comp XE262H with a bad lobe, caught very early, cam grinder didn't have that profile, but said they would put something similar on it, seems it got a whole lot bigger in the process.

Light car, 3100 lbs with driver installed, 4.10 gears, but low stall (haven't got to that yet).

Previous cam was a Crane unit from the markings on it:

H-272-2 272/284 adv, 216/228 @ 050, 484/512 lift 112 LSA 1800-5400 RPM

Wiped an exhaust lobe bad, again unknown history or miles on it.

Has always been a dog off the line, old cam ran 14 flat @ 98 mph with 2.15 sec 60'.

New cam has made it even worse off the line, so now 14.4 @ 98 mph with 2.4 sec 60'

I certainly wouldn't have chosen the cam that's in it now, but that's the story of how I got there.

I fiddled before I saw your post about ignition timing and I've advanced the cam 8 degrees (1 tooth on the cam gear), seems compression is now up around 160 psi, so feeling better about that. Intake valve to piston clearance is a bit scary, but hopefully it will work.

PS: not street driven, so that's not an issue, going faster down the 1/4 mile is.

Last edited by 69JavelinMan; 21-05-2018 at 12:02 AM..
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  #4  
Old 21-05-2018, 07:08 PM
69JavelinMan 69JavelinMan is offline
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Vacuum at idle is 7.5 in-Hg now, I guess that’s something.
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  #5  
Old 21-05-2018, 08:47 PM
Geoff 4 Geoff 4 is offline
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Darren,

Something doesn't sound right with the numbers. Light car, 4.10 gears & it is a dog off the line with a 360 engine? Sounds like you might have more wiped lobes. Have you checked for coil bind, valve train interference etc??

Wiped lobes. You CANNOT trust the stuff from the US, even brand names. I have had Isky, Comp & Crower lifters fail. The problem is not the cams, it is lifter quality. Soft metal, no radius, wrong radius or all three.

Could be made in China, Mexico or someplace else...

Don't the AMC engines use the 904 Chrysler lifters? If you want to avoid another wiped lobe, find a set of old factory lifters. Send them to Clive Cams in Vic & have them refaced. Contrary to popular belief, FT lifters can be refaced 2 or 3 times.
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Old 21-05-2018, 09:11 PM
69JavelinMan 69JavelinMan is offline
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I was very careful breaking the new lifters in, even used the old stock valve springs and changed the out afterwards.

The cam in there now is a bit ridiculous for the engine really, it's only about 8.5:1 compression according to the factory spec. Should have just bought a Comp 280H really or maybe even a 270H rather than going down the path I did.

I think the converter may be the issue, foot on the brake it stalls about 1400 RPM. I'll probably look at this over winter.

You might find this interesting, Holley Sniper datalogs with the old and new cams, new cam is the one with the bigger peaks, but you can see how it falls away off the line, going bigger on the cam was never going to help it off the line I guess. Both runs were off idle just to see what would happen:



PS: The old cam only lost the top off one exhaust lobe, so probably not going to hurt it that much in terms of performance.

Last edited by 69JavelinMan; 21-05-2018 at 09:14 PM..
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Old 22-05-2018, 08:30 PM
Geoff 4 Geoff 4 is offline
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The low CR is not helping vacuum. The single pattern cam would be better. The 268 Hi Energy cam would probably work well.

And a cam with short seat timing will help with vac & idle quality. Get a set of factory lifters refaced if you don't want another failure! You can be as careful as can be breaking in the cam, but if the lifters are crap..........

I have been using refaced lifters since the 70s. At one stage when I was drag racing regularly, I was changing cams & lifters every few weeks, using refaced lifters. Never had a failure.
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Old 22-05-2018, 09:32 PM
69JavelinMan 69JavelinMan is offline
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Certainly idled a lot better with the old cam and plenty of vacuum too. I'll give it a run at the Motorplex tomorrow night and then make some sort of plans for the winter break.
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Old 24-05-2018, 12:28 AM
69JavelinMan 69JavelinMan is offline
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Down a couple of tenths and 3 mph, so heading in the wrong direction by the looks of it.
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  #10  
Old 24-05-2018, 02:27 AM
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Angry Angry is offline
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tight convertor definately wont help........be like starting in 2nd all the time...

go 2500 stall
n more compression
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  #11  
Old 24-05-2018, 10:17 AM
69JavelinMan 69JavelinMan is offline
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That is pretty much going to be the winter project, change cam and convertor. More compression would be nice, but then it pretty much turns into a full rebuild and the dollars soon add up. I have a few other options to work through as well.
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Old 24-05-2018, 04:55 PM
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gke60 gke60 is offline
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headers will also help alot. cheers
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  #13  
Old 24-05-2018, 06:09 PM
69JavelinMan 69JavelinMan is offline
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Quote:
Originally Posted by gke60 View Post
headers will also help alot. cheers
They will also cost $2.5K for this application, ouch. Not sure if custom would be cheaper, might have to be home made.
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  #14  
Old 25-05-2018, 11:09 PM
69JavelinMan 69JavelinMan is offline
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At least it looks the part, assuming full beater spec is the desired look:



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  #15  
Old 28-05-2018, 07:40 PM
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Nev68 Nev68 is offline
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Here's your answer;
http://www.hotrod.com/articles/sneak...-getting-hemi/
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