351c rebuilt - low compression help
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351c rebuilt - low compression help
hi all,
I have just finished building my 351c
- .040 over pistons,
- std 2v heads
- COMP Cams High Energy Camshafts 32-221-3
- Holley 600cfm 4160
everything else is standard.
im having trouble getting it to idle - I done a compression test on all cylinders and they are all around 75psi so very low. this could explain the bad idle. as everything is new on the engine im wondering what else could cause this low compression - ive wet tested them, dry tested them, left them compression for and hour each to test for leaks and they are all good.
my though with the idle is that the low compression is creating low vacuum for the idle circuit on the holley, I might be completely wrong.
the engine starts fine and runs great with throttle feathering.
any help will be very much appreciated.
thanks
Russ
I have just finished building my 351c
- .040 over pistons,
- std 2v heads
- COMP Cams High Energy Camshafts 32-221-3
- Holley 600cfm 4160
everything else is standard.
im having trouble getting it to idle - I done a compression test on all cylinders and they are all around 75psi so very low. this could explain the bad idle. as everything is new on the engine im wondering what else could cause this low compression - ive wet tested them, dry tested them, left them compression for and hour each to test for leaks and they are all good.
my though with the idle is that the low compression is creating low vacuum for the idle circuit on the holley, I might be completely wrong.
the engine starts fine and runs great with throttle feathering.
any help will be very much appreciated.
thanks
Russ
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Welcome to FTE.
First off, you have a C right, not the M?
If there is any doubt, measure your intake manifold width. 351c valley width measures 6-31/32", 400 measures 8-17/32".
What piston did you use? Do you have the manufacturers number off it?
Did you measure your rotating assembly, and did you look at where the pistons end up at TDC?
Cylinder pressure that low may also be caused by cam timing or profile, or both, that leaves the valves open too late in the process. How did you time your cam? Did you install screw in studs and new springs:
http://www.compcams.com/Company/CC/c...?csid=841&sb=1
First off, you have a C right, not the M?
If there is any doubt, measure your intake manifold width. 351c valley width measures 6-31/32", 400 measures 8-17/32".
What piston did you use? Do you have the manufacturers number off it?
Did you measure your rotating assembly, and did you look at where the pistons end up at TDC?
Cylinder pressure that low may also be caused by cam timing or profile, or both, that leaves the valves open too late in the process. How did you time your cam? Did you install screw in studs and new springs:
http://www.compcams.com/Company/CC/c...?csid=841&sb=1
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Hi, thanks for the welcome
Im sure it is a C i will double check just to be 110% but im sure it is.
As for the pistons ill double check the actual make but im sure they were speed pro flat tops .040 over.
At TDC the pistons Are practically deAd level with the deck. The crank is standard with .010 over.
As for timing i used the standard timing marks on the cogs. This is where im sure ive done bad.
Russ
Im sure it is a C i will double check just to be 110% but im sure it is.
As for the pistons ill double check the actual make but im sure they were speed pro flat tops .040 over.
At TDC the pistons Are practically deAd level with the deck. The crank is standard with .010 over.
As for timing i used the standard timing marks on the cogs. This is where im sure ive done bad.
Russ
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No thats the next thing,
I checked the timing at the weekend and might have been 1 tooth out but still it made no difference.
Although with the front off and checking for compression i can hear air hissing either into the intake or block.
So either valves, lifters or rings
with new rings i cant think it would be them but i suppose i need to check)
I checked the timing at the weekend and might have been 1 tooth out but still it made no difference.
Although with the front off and checking for compression i can hear air hissing either into the intake or block.
So either valves, lifters or rings
with new rings i cant think it would be them but i suppose i need to check)
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That cam needs screw in studs and different springs. If you are using stock springs, and it seems you are based on your post, and if you did not change to screw in studs, you might want to pull a valve cover and see if you've pulled a stud or 16, and if you've bent a pushrod or 16, etc.
From the Comp Cams site:
1 [1] Requires screw-in studs & guide plates.
3 [3] Standard weight and lightweight retainers
7 [7] Stock springs cannot be used.
From the Comp Cams site:
1 [1] Requires screw-in studs & guide plates.
3 [3] Standard weight and lightweight retainers
7 [7] Stock springs cannot be used.
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Hi thanks for your reply,
Im sure it would help tonuse compcams recomended parts but the engine does run and sound great above idle. No problems upto 6000 and over.
I figured those parts are needed if using compcams recomended parts on that page.
Im going to rerun the leakdown test to see
Im sure it would help tonuse compcams recomended parts but the engine does run and sound great above idle. No problems upto 6000 and over.
I figured those parts are needed if using compcams recomended parts on that page.
Im going to rerun the leakdown test to see
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Re-reading, I see pedestal rockers. My bad. Wade through this, it may or may not apply. I think it is more likely that you have a valve problem vs. new rings not sealing on all cylinders.
https://www.ford-trucks.com/forums/1...nt-thread.html
https://www.ford-trucks.com/forums/1...nt-thread.html
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Is it normal to have alot of hissing into the block when hand cranking?Also ive pulled the heads and filled the valve chamber with fuel to see if there is any leaking and all valves seal fine.
Could poor lifters or incorrect pushrod lengh cause this much low comp?
Could poor lifters or incorrect pushrod lengh cause this much low comp?
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Yes. Hot Rod Mag this month has an article about a guy with a 390 with low compression. Wrong valve train components were holding the valves open when both should have been closed. Did you check your pushrods when the valves were closed? If everything is tight when the valves are closed, there is still pressure on the valve, holding it open. (even the tiniest bit is enough to fubar things)
But you say the leak down is good? The cylinders hold air with the push rods in place?
The rings are 120 - 180 degrees apart, right?
But you say the leak down is good? The cylinders hold air with the push rods in place?
The rings are 120 - 180 degrees apart, right?
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Yes, i think it will be the valves to be honest. My only worry is the amount of air passing the pistons when hand cranking.
Because i used the stock pedastal rockers i just torqued and hoped for the best, this i assume is my mistake.
Now the heads are off im still tempted to change the rings? Not sure why but just to eliminate any factors.
Ive just ordered all recomended parts that go with my cam so they should be here in a week and half hopefully.
Its a real shame as the engine sounds superb over 2000 revs. But not 100% so no good.
Ill keep you informed and thanks for all your help
Russ
Because i used the stock pedastal rockers i just torqued and hoped for the best, this i assume is my mistake.
Now the heads are off im still tempted to change the rings? Not sure why but just to eliminate any factors.
Ive just ordered all recomended parts that go with my cam so they should be here in a week and half hopefully.
Its a real shame as the engine sounds superb over 2000 revs. But not 100% so no good.
Ill keep you informed and thanks for all your help
Russ
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