transam360

iTrader: (-1)

ok i have a 454 bored over .060 and has -25.55 dome pistons with a 119 cc chamber head also the stroke is 4.000" and 4.310 bore my question is what should the compression be when i am doing a compression test

Thanks Eric

fast82z

iTrader: (1)

Also need to know the intake valve close ABDC from cam card, rod length, and altitude. Search for dynamic compression calculator and there should be one in the first few to give you a psi number.

transam360

iTrader: (-1)

i got 143 psi that seems realy low

AlkyIROC

Depends on how it's compared to the rest of the cylinders. I consider 150 psi to be a good average. If they're all within 5-10 psi of each other then there's no big concern. If any are below 110, it's time for a rebuilt. If all are over 180, there's good ring seating and a good cam profile.

Cam profile and timing can change how much the cylinder compression can make. How long the valves are open and when they open and close will determine how much pressure it can make. Advancing and retarding the camshaft can change the cylinder pressures providing everything is sealing well enough.

Knowing bore, stroke, piston top, combustion chamber size etc can tell you how much compression ratio the engine will have but can't tell how much cylinder compression it will have.

A compression test is only part of cylinder testing. A leak down test can tell a lot more about what's going on.

transam360

iTrader: (-1)

ok i did the test today i was geting 130 in all cylinders the engine has about 100 miles on it will the compression go up anymore and if so how many more miles

fast82z

iTrader: (1)

Positive you're doing the test right? Have the throttle wide open, all spark plugs out, full charge battery and possibly a charger on it also, and crank it through 3-5 compression strokes. I've had a bad gauge before that read off by like 30psi, might check that. From what you've said about your setup, I'm approximating your compression at 10:1 and it would take a hell of a cam to get the psi down to 130's from that. My motor has read the same psi from new and now also, but mine being a blower motor might make some difference as it reads 95psi where it should be 120psi, but extra oil in the cylinders gets it to be 120psi. If nothing else shows issue, try putting some oil in a cylinder before you test it and see what psi it reads after that as this will get rid of any non-sealing on the cylinder walls from a new engine hone.

transam360

iTrader: (-1)

http://www.compcams.com/Cam_Specs/Ca...?csid=459&sb=2


here are the specs on the cam

fast82z

iTrader: (1)

That cam isn't as big as I thought it'd be, mine is actually a bit more than that. Mine closes the intake 64 degrees or so ABDC where as yours is 79 degrees, but big difference is mine is rated at .050" lift where yours is at .015" lift. From that info though, I'd say upper 130's lower 140's is fine. That low of a psi is why they recommend 11:1 compression with that cam instead of the 10:1 that you're running.

transam360

iTrader: (-1)

yea i want to get a diffrent cam because i was told with the pistons i have the compression was going to be around 11.5 to 1 the cam is like new so i will try to sell it

also what is a good Mechanical Flat Tappet Camshafts for me to buy with mysetup

fast82z

iTrader: (1)

I would just run the cam you have. Your compression isn't that far off to completely kill the performance. I wouldn't think a somewhat smaller cam would be all that much different. A couple years ago I was using a 246/254 dur @ .050 .558/.558 cam in my motor that had 9.7:1 compression and it still ran an 11.44@ 119. I'd just say to use what you have as best as possible is to get a bigger stall than recommended say like a 4000 or 4500 possibly higher, and realize that it will not have that great bottom end power. I guess if you don't have much anything special on the bottom end for revving to 7500rpm, then get a smaller cam, but my thought is you really don't know what it will run. On the plus side, you could probably run that on cheapo gas instead of premium. I took mine out to the track with this big cam and 8.5:1 compression, and that did kill it. It didn't really come into power until 4000rpm and took quite some time to get there in the first place, but once there it ran the rest of the track like a mid 12 second car. If I had the same cam with compression, I'd likely see a time of mid-upper 11's, but it can only muster up a 13.3 without the needed compression. The blower fixes the compression issue though to get it to 9.55.

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bored over .060 bad ? what are some pros and cons?