Cranking compression

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mikesduster

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Whats the "GOOD,BAD AND UGLY" of cranking compression??? It was suggested that I do it,so I did.
I checked #1,#2,#7 and #8 and all were between 180-200.
 
thats good i'm between 205 / 210, would like to see yours just a bit closer but you could run 93 in your i do in mine.
 
Thanks!!!! I was talking to a friend last night and when I told him I had 12.2:1 compression he suggested I do a cranking compression. He thought I might be way over 200. My gauge may be off a little so it could be even a little lower like 175-195.
 
Do you know your static compression ratio? that PSI seems low for a true blueprinted 12:1. If it really is 12:1, the cam's too big.
 
Well, yeah, sorta. Are you really a blueprinted 12.1? Compression that high oughtta have a bunch of cylinder pressure. I built a blueprinted 8.5 360 years ago that had 175 PSI. About 2 years later, I had a 12.1 360 in a 65 Valiant that saw 205 PSI. Also, as Adam pointed out, the lobe separartion also has an effect. My guess is your camshaft is ground on 110, but probably 108. That combined with the big duration @.050" is where your cylinder pressure is going. Hard to tell without all of the cam specs, but I'll tell you someone who can help. Dave at Hughes Engines is really sharp. I wouldn't call him though, unless you are absolutely SURE about what your BLUEPRINTED compression ratio is, not what you THINK it is. If you don't really know for sure, my advice to get it DEAD right would be to get the heads off and measure it. It is a race car right? You do want it to go as fast/quick as possible on a given combination, right? then pulling that top end down to get it dead nuts on shouldn't be anything. If you have not measured the compression yourself, that's really the only way. any other way and you'll just be guessin. ....and just a little trivia for you......most people who build and or design their own engines end up overcamming them. That thing should have compression bumping or over 200 PSI. In other words, if it has iron heads, you ought NOT to be able to run pump gas. If you can, sumthin's gettin wasted. BAD. Alloy heads are a little different there, BUT the compression pressure does not change.....only the heat exchange rate. Lastly, WHERE is the camshaft installed? Is it straight up? Advanced? Retarded? Are you SURE? Did you degree it? THAT can make a night and day difference in cylinder pressure.
 
Ok,WHERE do I begin???? 1st,YES to all your question EXCEPT it is NOT a "race car" but a street/strip car.
When we put the motor together,we checked and re-checked everything and yes,it came out to 12:1 compression. The cam is ground on 108 but installed at 104 AND degreed in!!!!!!
My gauge is old and it may be off so Im gonna recheck today.
 
mikesduster said:
Ok,WHERE do I begin???? 1st,YES to all your question EXCEPT it is NOT a "race car" but a street/strip car.
When we put the motor together,we checked and re-checked everything and yes,it came out to 12:1 compression. The cam is ground on 108 but installed at 104 AND degreed in!!!!!!
My gauge is old and it may be off so Im gonna recheck today.
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Ok, good. At least you KNOW the engine. That's a lot better than guessin. Keep in mind, most cam grinders grind 4 degrees advance INTO the cam.....because most people choose too big and that helps make up for bottom end torque loss. So, you could be 8 degrees advanced. That's a ton. THAT should tell you how big that camshaft really is right there. Because as you advance camshaft timing, cylinder pressure goes up. So, if you're 8 degrees advanced, which it sounds like you are, that means you had to go 8 degrees JUST to get to the pressure you have now. Make sense? When I choose a camshaft, I always keep in the back of my mind what the factory used in the hipo version of the engine I'm building at that time. Those engines were torque monsters and they had much smaller cams than most people choose. Not saying you should go with something comparable to a factory cam, BUT, Chrysler did make an engine with 13.5 to 1. The Max Wedge. That engine had 300 degrees duration. Now, depending on how they measured .050" duration back then, it either had 225 at .050" or 255 at .050". It all depends on whether they used 750 as the checking standard, or 850 like Mopar Performance does. Even if they used 850, that puts that cam at 255 at .050" and they had 1.5 FULL points MORE than what you have. That's a lot of difference. I really think that your power would pick up with a smaller duration at .050". But like I said, I would talk to Dave and get his thoughts. I'm no cam expert, but I've built a frigton of engines and tuned and recammed an equal amount. I maybe off base here, but I don't think I am.
 
Again,I think my compression gauge is 10psi off. It zeros out at 10,not 0. I just rechecked #1,#2,#7 and #8. I checked them 2-3 times each and my gauge read between 190 and 200 which means it actually between 180 and 190. I dont have the money to buy a new gauge so im gonna have to use these numbers till I can get another tester.
 
Your fine, im running 93 octain with 205 psi, optimal street psi is 180 to 200
 
mikesduster said:
Again,I think my compression gauge is 10psi off. It zeros out at 10,not 0. I just rechecked #1,#2,#7 and #8. I checked them 2-3 times each and my gauge read between 190 and 200 which means it actually between 180 and 190. I dont have the money to buy a new gauge so im gonna have to use these numbers till I can get another tester.
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I would consider 190 pretty acceptacle for 12.1. In the end, what pump gas you can get by with really has nothing to do with static compression ratio or cranking pressure. It has to do with dynamic or "running" compression. That's what you need to calculate to see where you stand. There are several online calculators. United Engine and Machine has one.
 
How do I find this out???
Intake Closing Point (degrees)ABDC @ 0.050 lift plus 15 degrees

I went on that website and my static compression is 12.133
StrokerScamp said:
I would consider 190 pretty acceptacle for 12.1. In the end, what pump gas you can get by with really has nothing to do with static compression ratio or cranking pressure. It has to do with dynamic or "running" compression. That's what you need to calculate to see where you stand. There are several online calculators. United Engine and Machine has one.
Click to expand...
 
You need ALL the specs to your cam to find that value. Looks like you add 15 degrees to whatever your intake closin point is. That will be in your cam specs somewhere.
 
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