Blown Motor Compression?? - 460 Ford Forum
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post #1 of 7 (permalink) Old 03-15-2011, 08:08 PM Thread Starter
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Blown Motor Compression??

While looking at the threads in the engine tech section I came across the thread that had the dynamic compression calculator link in it from Sil-O-Lite. I ran my numbers and found that pretty interesting. In order to do that I had to go the the Lunati website to get the degress where the intake valve closes for my cam. I started poking around the cam section and wanted to look at the cam that was one bigger than mine. Below is the description of what I saw:

"Solid roller, rough Idle. Good cam for puller, lightweight mud runner or bracket racing applications with blown large cubic inch motors. Needs 5000-5500 RPM stall converter, headers, 13:1 compression ratio or better and 5.13 gearing or better. "

This is calling for a blown motor with 13:1 compression. I always thought that you needed lower compression for a foreced induction motor. With that being said this was a pretty big cam. I'm thinking that the 13:1 is referring to static compression and using this cam would probably bring it down to about 10 something to 1 dynamic compression. Am I getting all out race motors confused with more "streetable" motors? Now I'm thinking all I need is that cam and a blower. My motor is already at 13:1 static compression.

...Or is this cam for an alcohol motor that needs that high of compression? That high of compression really confused me for a blown application. Also below is a link to the cam I was looking at for a reference. I'm running the Lunati 51606 myself.

http://www.lunatipower.com/Product.aspx?id=2294&gid=249

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post #2 of 7 (permalink) Old 03-15-2011, 09:16 PM
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Originally Posted by MrScharfenberger View Post
While looking at the threads in the engine tech section I came across the thread that had the dynamic compression calculator link in it from Sil-O-Lite. I ran my numbers and found that pretty interesting. In order to do that I had to go the the Lunati website to get the degrees where the intake valve closes for my cam. I started poking around the cam section and wanted to look at the cam that was one bigger than mine. Below is the description of what I saw:

"Solid roller, rough Idle. Good cam for puller, lightweight mud runner or bracket racing applications with blown large cubic inch motors. Needs 5000-5500 RPM stall converter, headers, 13:1 compression ratio or better and 5.13 gearing or better. "

This is calling for a blown motor with 13:1 compression. I always thought that you needed lower compression for a forced induction motor. With that being said this was a pretty big cam. I'm thinking that the 13:1 is referring to static compression and using this cam would probably bring it down to about 10 something to 1 dynamic compression. Am I getting all out race motors confused with more "streetable" motors? Now I'm thinking all I need is that cam and a blower. My motor is already at 13:1 static compression.

...Or is this cam for an alcohol motor that needs that high of compression? That high of compression really confused me for a blown application. Also below is a link to the cam I was looking at for a reference. I'm running the Lunati 51606 myself.

http://www.lunatipower.com/Product.aspx?id=2294&gid=249
Yeah, that's getting up there in lift, and it has quite a bit of duration too....narrow LC means quite a bit of over lap, not necessarily a great blower cam (from what I understand about blower cams....and that's not a lot). I think they must be talking "either/or" because with a static of 13:1 you couldn't put enough bubble gum on 'em to not lift the heads if you added much boost...although the dynamic must be down because of the overlap. Still, from my understanding of blower cams, a wider LC (hence less overlap, 'cause you don't NEED it to help pull in the intake charge....it's being pushed in all ready, you'd only be bleeding off cylinder pressure)is used and of course much less overlap. Someone correct me if I'm wrong.....On the BDS site they suggest staying around 8ish for a street motor and not a lot of crazy boost. If you plan on running mondo boost numbers, I think they suggest around 6ish static. With the amount of converter, compression, lack of vehicle weight and deep gearing I'll bet that that cam has a pretty narrow power band and you'd need at least a four speed trans. Also, from my understanding of alcohol motors, you don't necessarily NEED high compression, it's works with lower compression engines too, as low as 8:1 if I remember correctly, but because of the cooling effect of the alcohol you CAN run the higher static compression numbers. Again, someone correct me if I'm wrong.
Rob

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Last edited by White Lightning; 03-15-2011 at 09:29 PM.
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post #3 of 7 (permalink) Old 03-15-2011, 10:16 PM
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post #4 of 7 (permalink) Old 03-15-2011, 10:59 PM
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Groan!
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Last edited by White Lightning; 03-16-2011 at 01:16 AM.
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post #5 of 7 (permalink) Old 03-15-2011, 11:07 PM Thread Starter
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I realize that alcohol motors will run on much less compression. I was just thinking that with that high of static compression and a blower that you would need to run something like alcohol to keep the detonation away. Because alcohol does burn so much cooler and with that high of compression the alcohol would be a high enough octane for that compression ratio.

...but none the less this still confuses me. So I will await that somebody to correct you!

But keep in mind. Your not alone. I didn't know either that is why I asked. I'm just trying to get my 460 education from these guys that are way more experienced than me.

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post #6 of 7 (permalink) Old 03-19-2011, 11:22 AM Thread Starter
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Come on now!! I know one of you smart guys knows the answer to this. I've been grinding on it for days now. Somebody put me out of my misery and tell me why!!

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post #7 of 7 (permalink) Old 05-17-2011, 01:38 PM
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the listing of 13:1 compression is if the engine is naturally aspirated. If you tried to fit a blower on a 13:1 engine you would end up with a blown engine - all over the place.
The lower the compression you start with in a blower engine the more power it will make using the same amount of boost - however with a lower compression you can use MORE boost and get even more power from it.
Ethanol is less prone to detonation because it has a high equilavent "octane rating" and it cools the intake and chamber through evaporation and a process call "latent heat" caused by water that may exist in the fuel. If you are using E-85 there is no appreciable latent heat because water in the mix would separate the alcohol and gas.
I hope that helps to clear it up.
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