Can't maintain 175 psi in cylinder

Discussion in 'Classic Mustang Specific Tech' started by my66coupe, Dec 2, 2003.


  1. my66coupe

    my66coupe Founding Member

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  2. my66coupe

    my66coupe Founding Member

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    Also another thing....what type of cam came with the mexican 302 stock? and what type of lifters? I have been adjusting my valves as if they were hydraulic.

    Mike
  3. mustangracer

    mustangracer Founding Member

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    Cranking psi isn't something you usually work your motor to attain, it's a function of cam specs and compression ratio for the most part.

    The stock cam is hydraulic, but if it's been changed :shrug:

    Ah...comp check is also supposed to be done after the engine is warmed up, preferably after the mainifolds have cooled tho. :nice: Cold mine ran about 125, hot about 160 before I did some upgrades.

    How fresh is the motor? If it hasn't been run a lot the rings could not be seated yet, which would make for lower compression also
  4. my66coupe

    my66coupe Founding Member

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    The car I know for sure has 74,000 original miles on it and it has doesnt have the original motor. The motor was done by the last owner I know and he didnt drive it much at all. Soo to answer your question Im pretty sure it is still a very low mileage engine.

    Mike
  5. my66coupe

    my66coupe Founding Member

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    Ok today at work I adjusted the valves, got it running, and did another compression check after it was hot. It still is at 125. Tommorow I will be swapping in my older valve train...stock rocker arms and push rods to see if that makes a difference.

    Mike
  6. SuperDave

    SuperDave Early-Model Mentor Founding Member

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    This questio remains confused because of the word "maintain" as used sveral times by the original poster. o "maintain" means to achieve and HOLD pressure, in this case, at 175 PSI. To maintain cylinder pressure. the rings, valves and pistons must be sealed to prevent compression leakage and cooresponding pressure reduction. Adding a heavy lubricant to the cylinders would help to "maintain" compression but that is NOT desireable.

    IMO, this thread is rotally confused by the use of the term "maintain".
  7. my66coupe

    my66coupe Founding Member

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    I already have achieved the 175 psi, and am pretty sure my engine is capable of holding it. I would like to know why I can't tighten down my rocker arms and still have 175 psi and only 125. Where to begin....Pushrods, Rocker arms???


    Mike
  8. RGS0907

    RGS0907 New Member

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    I once witnessed an engine trying to run BACKWARDS because the firing order was so far off! :bang:
  9. my66coupe

    my66coupe Founding Member

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    Ok today, I swapped out my comp cam dual valve springs, and put in my old ones, I first tried no Guide plates, Stock rocker arms and pushrods, I got excited but one of the valves was adjusted poorly and was loose. back to 125. Then i tried the longer push rods....Nothing. next was the roller rocker arms with guide plates. nothing. I didnt really have enough time to work on it. Tommorow I will try some other crazy combos. Any Ideas? One of the guys at work thought that I may have a collapsed lifter, but every cylinder is doing this, soo i dunno about that.

    Mike
  10. 3spd on floor

    3spd on floor Founding Member

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    IMHO i think your problem is just that you've got a big cam with a lot of overlap thats bleeding off your compression. by loosening up the rockers you're making it so the cam lobes dont open the valves when they're supposed to, therefore the overlap is reduced and the compression goes up.

    also, i might be off here, but wouldnt 175psi in a regular compression check be pretty high? unless i'm mistaken, thats a compression ratio of almost 12:1, whereas 125psi seems a little more reasonable at about 8.5:1. i got these numbers by taking cylinder PSI / std.atmospheric PSI, ie::175/14.7 = 11.9
  11. my66coupe

    my66coupe Founding Member

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    I told the guys at work about the Overlap and they thought it was complete BS. They said that a cam doesnt worsen compression, but makes it better, and shouldnt be bleeding out the exhaust. But I know there are alot of brilliant minds on this forum so I beleive you guys. Tommorow Im going to down grade all my cylinders to the lighter valve springs, reinstall my guide plates, reuse the longer push rods and roller rockers.

    Mike
  12. 3spd on floor

    3spd on floor Founding Member

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    ok, let me see if i can explain my reasoning for my beliefs, and please call me out if you think this is :bs: , i am no expert.

    the attachment a graph of the valve lift with respect to the crankshaft position for two cams, one a very mild cam and the other a very radical one, courtesy of Engine Analyzer 3.0. (TDC occures at 360* and 720* according to the graph, but not on this picture) note the intake valve position at the beginning of the compression stroke, the valve is still over halfway open when the piston begins traveling upward and remains open for almost half of the compression stroke. note how much more the valve is open for a higher lift,higher duration cam than it is for a small cam. it is my understanding that if the intake valve is open and the piston is traveling upward that part of the intake charge will travel back up through the intake valve into the manifold, thus reducing the amount of air in the cylinder, thus reducing overall compression in the chamber.

    on another note, perhaps this is why some cam manufacturers reccommend a minimum compression ratio for some of their bigger cams, because the bigger cam would bleed off some compression. if a huge cam was used with too low of a compression engine, it might bleed off so much that the engine wouldn't run properly. :shrug:

    technically i guess this isnt valve 'overlap', but it is definately a cam thing.

    DISCLAIMER:: I AM NO EXPERT ON ENGINE DESIGN ...but it makes sense to me... :shrug:

    anyone care to elaborate or school me?
  13. 2nd Mustang

    2nd Mustang Founding Member

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    I found the article that I had saved about cam questions. It's in the May 2000 issue of Car Craft Magazine. The LDA (lobe displacement angle) does contribute toward the cranking compression readings. The narrower angle can contribute toward LOWER CRANKING COMPRESSION NUMBERS depending on other factors on the engine build. It also goes on to state that a wider LDA in a street engine has higher vacuum and a smooth idle. To compare, my 302 has only 14 inches of vacuum at idle and a lumpy idle and 130 lbs. on all 8 cylinders. Granted, I really don't know how many miles are on my engine, but it does rev quickly, but I limit it to 5000 or so rpms as power seems to drop off at that point.
  14. SuperDave

    SuperDave Early-Model Mentor Founding Member

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    I'm following this but simply don't understand how having the valves properly adjusted LOWERS the compression. Help me, please.
  15. 3spd on floor

    3spd on floor Founding Member

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    Dave - what i'm thinking is that when the valves are improperly adjusted, they have a really big clearance between the pushrod and the rocker arm tip (valve lash i believe its called). for this reason, at the beginning and end of the intake lobe on the cam, the valve is not actually opening, instead the pushrod is taking up the 'slack' in the valvetrain. thus the valve closes sooner in the compression stroke, increasing compression.

    when the valves are properly adjusted, there is very little valve lash, and the valve very closely follows the cam lobe. this makes the intake valve open for longer during the compression stroke, decreasing compression.

    essesntially i'm saying that a loose valve train will slightly lessen the duration and lift of the camshaft.

    did that clear it up for you?
  16. my66coupe

    my66coupe Founding Member

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    Ok today, I decided to stick with the heavier valve springs. I only had the old ones on cylinder 1. Soo today I switched back, Put my roller rocker arms in, Hardened push rods, and guide plates. And im still at 125-130 all around. Also like 2nd mustang I have 15 pouds of vacumm and 125-130 with a really crappy idle. The car will idle but it sounds really rough. The guys at work said it sounded like a "miss" but i thought it was just the cam. Well I didnt get to drive it at all, but I will on monday.

    Mike
  17. mustangracer

    mustangracer Founding Member

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    I'm going to have to call :bs: on your fellow guys at work, 3spd has it right on about the overlap between the intake closing and exhaust opening on the compression stroke, and that as you loosen the valve lash it reduces the open time for the valves, hence reducing the overlap. Maybe your rough idle and crappy running just means the cam is too big for the rest of the combo? Just offering a possible option... Or it could be the timing is off, if it's too retarded (no comments from the peanut gallery) it can be hard to get the motor to idle, could be the distributor is off a tooth. Yes I know it seems like it shouldn't matter as long as the firing order is right, but there's something about distributor phasing that ends up being off slightly if the dizzy isn't in correctly.

    And calculated static CR of the motor is different than the dynamic CR that is measured on a compression check, due to the cam timing and other factors...

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