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Well 18.5cc's would put you around 9:1, so that size or maybe a little larger.
 

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I'd have to know all of the specs of everything to be certain, but I'd expect you to be in that ballpark with that size.
 

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flat tops
 

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Sure, if he's got 116 octane to fill it with all the time. :rolleyes:
 

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If all you are going to run is 9lb's the I would go with a 9.0 CR. In case you ever have to put it back N/A for any reason it will not be a total dog. My engine is set up at 9.0 and was built by Renegade. Just my .02 :)
 

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Discussion Starter #8
yea, going with 18cc dish...6lbs on the street, and 9lbs for the track. of course I'll run 6lbs at the track too on street tires to see what she runs in "street trim"..then the MT DRs, and then the 9lb pulley :D
 

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Here's something to keep in the back of your mind while you're "engine engineering". :D

1. Boost from a roots blower is a lot more susceptible to detonation due to excessive CR than from a Centrifugal blower (assuming the intake charge temps can be kept reasonable).

2. Aluminum heads can usually handle about one more point of CR than an iron head (and again, keeping engine temps within reason).

3. Polishing of the combustion chamber (either on an iron head, or an aluminum head will also allow you to run more static CR.

Those are B&M’s words, not mine. That was back in the early 90’s when I was building my 351C.

For a roots blown engine, I would not go over 9.5:1, but on an aluminum head engine running a centrifugal blower, I think 10:1 (or maybe 10.2:1) would be very livable especially considering you’re going forged. As long as you get a good tune and don’t go silly with the boost.

I have read a lot of people that build a 4.6L and go with low CR only to find out that it runs like a dog. I know this is backwards from “old school”, and I’m just passing along what I’ve read and researched. :thumbsup:
 

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:rofl: :rofl: :rofl: I hear you!! Everything I've learned about boost does not seem to apply to the 4.6L.... :redmad:

03 Cobra = 8.5:1
Lightning = 8.5:1

But people are running gobs of centrifugal boost in 9.5:1 or 10:1 engines without problems.... :2huh:
 

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I plan on running ~9.5:1 when I go with my build up. With a roots/twin screw the lose in torque on the low end can be made up with the instant boost, while the centrifugal blower doesn't give much low end help. That would be the reason for a little bump in compression ratio, so that you could keep more low end and also keep the gas mileage up a little better. Keep in mind that I am going to be looking for 15 psi of boost or so.

Russ
 

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94 Daily Driven 4.6L said:
:rofl: :rofl: :rofl: I hear you!! Everything I've learned about boost does not seem to apply to the 4.6L.... :redmad:

03 Cobra = 8.5:1
Lightning = 8.5:1

But people are running gobs of centrifugal boost in 9.5:1 or 10:1 engines without problems.... :2huh:
Both of those are roots....so that would be just like you stated before about the compression ratios needing to be a bit lower. Also since it is a factory application they are looking to use one that will provide absolute durability...100k+
 

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It does appear to hold true for roots blowers, but with centrifugal it does not appear to hold true. For some reason 10 psi of “roots” boost is not the same as 10 psi of “centrifugal” boost… in my opinion. (I have my own theories on that, but no definite proof…) :leftright

And let’s not turn this post into roots vs. centrifugal “discussion”. :D

I’m just saying that IMHO some more research should be done before deciding on a CR for a 4.6L since it seems that you can get away with a lot more static CR than what would be considered “normal”. :thumbsup:
 

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Kodiak kid said:
or i'll just keep it at 9:1 :D
:thumbsup: Good choice... :D
 

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9.0:1 is a nice ratio for a mid-boost centrifugal supercharged engine. I know its not a "mod motor" but the 3.8L guys are running stock 9.2:1 compression engines with 11+psi all the time. Same basic engine principals, Iron block with aluminum heads.
 
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