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Once you’ve finished building an engine, the bombardment of questions soon follows: What camshaft did you use? How much power do you think it makes?…
Nice article, now we need one on how to degree a cam for dynamic compression with our new found data.
Please
I'll have to keep this article in mind if I ever have the Supra's engine out again. I really, really do not want to ever have to pull it. Not that it's overly difficult, it's just that an engine out usually means something went really wrong...
Actually you can just do a compression test and get the max psi and then from that calculate the actual compression of the motor. To me this is much more important than the theoretical max compression that a motor can make if the valves opened and closed at tdc and bdc
Measuring and adding up various volumes is fine, but I prefer to do it in a more foolproof fashion.
Every engine I build goes on an engine stand with the ability to rotate the engine.
Rotate the engine so the plug hole is uppermost. Bring the engine to TDC with both valves closed using a dial indicator on the top of the piston, through the plug hole.
Use a Burette to run light oil into the plug hole up to the bottom thread of the hole. That is your total combustion space, and needs to be adjusted only to account for spark plug tip (projected tip reduces total chamber volume, but the hollow around the tip increase it - usually close to a wash).
This means you don't need to worry about oddball piston crown shape with pop ups etc. You get an exact reading. Just remember to turn th engine upside down afterward to drain the oil out of the chamber. I had a not so fast thinking friend that I showed this method to, roll it upright after doing the compression test, bolted on a starter, forgot one hole was full of oil, and cranked it over on the stand. You might be amazed at how far the stream of oil travels as it comes out of the cylinder through the plug hole the first time that piston hits TDC. The one I saw went 10-20 feet over three adjacent cars.
dean1484 said:Actually you can just do a compression test and get the max psi and then from that calculate the actual compression of the motor. To me this is much more important than the theoretical max compression that a motor can make if the valves opened and closed at tdc and bdc
Do I remember something about cranking compression not necessarily being directly related to running compression?
Yes there are differences. I think it is due to the heat and expansion of gasses. And you also get better sealing of the rings from the pressures in the cylinder but all that being said I still use this as valve timing has a much greater effect than these things. I don’t know the percentages. That would be interesting to look at.
In reply to dean1484 :
Also, ram filling of the cylinders. And maybe a little bit of "compression takes time to bleed off, and the faster stuff happens, the higher the compression will be". I remember hearing stories of circle track drivers drilling small holes in exhaust valves so that cranking compression was lower than running compression (especially at 8-9k).
wspohn said:Measuring and adding up various volumes is fine, but I prefer to do it in a more foolproof fashion.
Every engine I build goes on an engine stand with the ability to rotate the engine.
Rotate the engine so the plug hole is uppermost. Bring the engine to TDC with both valves closed using a dial indicator on the top of the piston, through the plug hole.
Use a Burette to run light oil into the plug hole up to the bottom thread of the hole. That is your total combustion space, and needs to be adjusted only to account for spark plug tip (projected tip reduces total chamber volume, but the hollow around the tip increase it - usually close to a wash).
This means you don't need to worry about oddball piston crown shape with pop ups etc. You get an exact reading. Just remember to turn th engine upside down afterward to drain the oil out of the chamber. I had a not so fast thinking friend that I showed this method to, roll it upright after doing the compression test, bolted on a starter, forgot one hole was full of oil, and cranked it over on the stand. You might be amazed at how far the stream of oil travels as it comes out of the cylinder through the plug hole the first time that piston hits TDC. The one I saw went 10-20 feet over three adjacent cars.
This method sure sounds easier but how can you be sure that you don't have any air bubbles in the combustion chamber since you can't see into it to check? I can imagine that some combustion chamber shapes might be more problematic than others.
That method works perfectly on DOHC engines as the plug hole is right at the top - no air bubbles.
It also works on the OHV engines I've done it on, as you rotate the engine on the engine stand to ensure the plug hole is uppermost. It wouldn't work on an enigine already installed in a car, with the plug hole usually on the side of the head.
Not saying that there aren't some oddball chamber shapes out there that might not work well, but the majority certainly should pose no problem.
BTW, when I calculated the volume using the old addition of all volumes method, the result was always at least 0.5 points of compression off the actual.measured figure.
akylekoz said:Nice article, now we need one on how to degree a cam for dynamic compression with our new found data.
Please
I developed an Excel spreadsheet for calculating DCR with given cam parameters and engine stats. I can see if I can dig it up if you're interested.
Screenshot (yes, this was a horrible combo, just for illustration. Further work was done on 9 and 10:1 SCR.
You have to CC each chamber, should not be more than 0.2 cc difference - if more bring out the grinder. Might have to sink a valve. Of course you have the crank indexed, the rod length checked, etc.
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