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post #1 of 20 (permalink) Old 04-17-2005, 08:34 PM Thread Starter
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how much dish do I need?

ok...PI heads/cams/intake on a forged bottom end...what cc dish do I need to keep the motor happy with 9psi out of a centrifugal?

Shane

and this is actually gonna happen due to this: POS 4.6

Shane
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post #2 of 20 (permalink) Old 04-17-2005, 08:58 PM
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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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post #3 of 20 (permalink) Old 04-17-2005, 09:03 PM Thread Starter
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thank you so...22ccs would give me a ...8.5:1? I am horrible at this part of engine stuff.

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post #4 of 20 (permalink) Old 04-18-2005, 03:43 PM
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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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post #5 of 20 (permalink) Old 04-18-2005, 04:38 PM
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post #6 of 20 (permalink) Old 04-18-2005, 04:40 PM
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Sure, if he's got 116 octane to fill it with all the time.

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97 Thunderbird 4.6L LX /w Sport Package
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post #7 of 20 (permalink) Old 04-18-2005, 05:07 PM
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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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post #8 of 20 (permalink) Old 04-18-2005, 05:09 PM Thread Starter
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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
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post #9 of 20 (permalink) Old 04-19-2005, 12:00 PM Thread Starter
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oh yea, will it be bad if I get 16cc dish pistons instead? I can get 16cc dish wiseco pistons with manley rods for $900 shipped..good deal or should I keep looking?

Shane
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post #10 of 20 (permalink) Old 04-19-2005, 12:38 PM
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Here's something to keep in the back of your mind while you're "engine engineering".

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.

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post #11 of 20 (permalink) Old 04-19-2005, 12:42 PM Thread Starter
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damnit! you just confused me even more! thanx for the help though, so more research needs to be done on my end...
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post #12 of 20 (permalink) Old 04-19-2005, 12:49 PM
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I hear you!! Everything I've learned about boost does not seem to apply to the 4.6L....

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....

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post #13 of 20 (permalink) Old 04-19-2005, 01:16 PM
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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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post #14 of 20 (permalink) Old 04-19-2005, 01:19 PM
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Quote:
Originally Posted by 94 Daily Driven 4.6L
I hear you!! Everything I've learned about boost does not seem to apply to the 4.6L....

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....
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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post #15 of 20 (permalink) Old 04-19-2005, 01:33 PM
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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…)

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

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”.

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post #16 of 20 (permalink) Old 04-19-2005, 01:37 PM Thread Starter
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or i'll just keep it at 9:1
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post #17 of 20 (permalink) Old 04-19-2005, 01:45 PM
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Quote:
Originally Posted by Kodiak kid
or i'll just keep it at 9:1
Good choice...

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84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
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post #18 of 20 (permalink) Old 04-19-2005, 04:12 PM
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Quote:
Originally Posted by 94 Daily Driven 4.6L
Good choice...
That's what I have!

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post #19 of 20 (permalink) Old 04-19-2005, 07:45 PM
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That's what I have!
Me too. Got a good deal on my kit as well. http://www.modularperformance.com/mi...tegory_Code=EK

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post #20 of 20 (permalink) Old 04-19-2005, 08:59 PM
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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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