418W compression reading?

donraye · #1 (**permalink**) 03-27-2009, 05:55 PM

Does anyone out there have any idea how many lbs./inch to expect from a 418 stroker with about 10.3 to 1 static compression?
I just wanted to check mine to make sure I did not have any negative lash in the valve rocker adjustments. I do not have a leak down tester but I do have a compression guage.
Any ideas on what's in the ballpark to be expected? 185 to 200?
Who has or builds a similar set-up (408.418,427 stroker with about 10 to 1 and a moderately agressive street performance cam) and knows their readings?
D.

brettco · #2 (**permalink**) 03-27-2009, 06:44 PM

I have a sprint car with 15.2 to 1 that has 170 cranking comp. and a stock 02 mustang that is 9.5 to 1 thats 175psi- surprised me.Its all about the cam. My pump gas stroker 472 fe is 155psi

donraye · #3 (**permalink**) 03-27-2009, 07:32 PM

Quote:

Originally Posted by brettco

I have a sprint car with 15.2 to 1 that has 170 cranking comp. and a stock 02 mustang that is 9.5 to 1 thats 175psi- surprised me.Its all about the cam. My pump gas stroker 472 fe is 155psi

That's pretty interesting considering the very diffferent engines and yet the not so different compression readings. I would have expected higher and more differing readings, at least on the stroker and the sprint car. But I'm not at all "experienced" nor a pro. so that's why I ask this stuff.
This cam is a hydraulic roller and what I would call a "moderately" agressive street performance cam. The gross lift is .555 and .576 with 1.6 rockers and the duration @ .006 tappet of 287 and 293. So not a "monster" cam by any means.
Thanks for the comparisons and information it will help give me some idea of a range although I know it will not be in any way exact. Still it's a start to knowing what I might expect.
I would assume that if there is much leakdown, as due to improper valve lash, that the numbers would show a "considerable" loss of pressure not just a 20 or so pound one. However this is my very first build so I am only guessing about that. It might be a more subtle difference than I think, and therefore not a good indicator of anything at all.
I did adjust the rockers as the books say by having them at the point I could feel the slightest resistance while rotating the push rods then I gave them 1/3 turn more(a compromise between the 1/2 for street and 1/4 for racing that all the books called for) and locked them there. The lifters were full of oil and were not collapsed and I rechecked the rocker adjustments after starting and running the engine for a few minutes to make sure that it had not changed due to any possible "pump-up" of the lifters. It remained the same so I think all is fine. Just wanted to make any other confirmations I can with what tools I have before running the engine for a longer time and under load.
It will be going to the chassis dyno for a complete tune ASAP, so they can fine tune everything then.
Thanks again, D.

Got the Bug · #4 (**permalink**) 03-27-2009, 07:50 PM

Don - 185 to 200 should be in the right range. I think my 402SR Roush was about 190 when I check it when I though I had a problem. The problem ended up being a VERY loud exhaust manifold gasket leak that had failed.

jwd · #5 (**permalink**) 03-27-2009, 07:53 PM

Having the readings being all close to one another is more important than the reading itself. Cam timing will dictate the overall reading. I've seen motors with a 13:1 CR read 140 and ones with 9:1 read 180.

Jim

DAVID GAGNARD · #6 (**permalink**) 03-27-2009, 09:03 PM

Quote:

Originally Posted by jwd

Having the readings being all close to one another is more important than the reading itself. Cam timing will dictate the overall reading. I've seen motors with a 13:1 CR read 140 and ones with 9:1 read 180.

Jim

Exactly, cam timing/overlap probably has as much to do with cranking compression as anything.......

My 10.5 to 1, 351-W has about 130 lbs. cranking compression, but my camshaft, not being a monster cam does have a lot of overlap, thus bleeding off compression at low cranking rpms.
By contrast, I've seen a bone stock 351-W with 160 lbs. cranking compression, most stock cams have little or no overlap thus giving a higher cranking compression reading.......

What you need to be concerned about is you don't want more than 10% difference from the low to the highest reading on you cylinders.

After doing a cranking compression test, one at a time, squirt a couple of squirts of motor oil in each cylinder and re-test, if you reading goes up, your rings are leaking, if the reading stays the same, it's your valves leaking, assuming you have a low reading to start with...

David

bobcowan · #7 (**permalink**) 03-27-2009, 10:25 PM

what you're talking about is called effective cylinder pressure, cold cranking pressure, or dynamic compression. To calculate that proper number, you have to take into account things like intake valve closing point, static compression, altitude, rod length, etc. Lot's of differant facters.

Here's an excellent calculater for all that. It's for a Harley engine, but it works with any 4 stroke engine.

http://www.rbracing-rsr.com/comprAdvHD.htm

vector1 · #8 (**permalink**) 03-28-2009, 05:49 AM

i'm with the above guys on consistency, no matter where they're reading, if they're all similiar you'll be well off. on your new motor the readings will be lower and possibly inconsistent unless it has been ran.