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Discussion Starter #1 (Edited)
Help! 97 Combustion chamber volume and compression ratio w/ forged pistons

Alright, from what I searched here, the combustion chamber volume on my NPI's is 49.77cc and 9.30:1. The NHRA stock car blueprints also attest to this fact. Here is the link I have yet to actually get some plexiglass and a buret and check myself but the heads are back at school so it'll have to wait. Someone on a board i read said they were 52cc, but i think that is too big, that those are the cobra/mark 8 heads. Now the probe industries webpage lists this info for compression ratio

Large Dish (-18.1cc)

Bore=3.552 in
Stroke=3.543 in
Rod=5.933
C/H=1.215
Weight=313g
Pin=.866 in
Part Number=10178-Std

Compression Ratio
42.4cc 47.7cc 54.2cc
9.00 8.23 7.80

Compression Ratio computed with .010 deck and .038 head gasket at bore +.125"
(weights and compression ratios are approximate)


Now I am looking for some gaskets that will be able to handle the boost I am going to throw at them, possibly upwards of 16lbs at the track with 103-octane+, and around 10-12 psi on the street, whatever amount i can get to run with pump gas. This means low compression pistons. I am looking at the std bore pistons.

My engine builder recommended Cometic head gaskets, http://www.cometic.com ,for MLS head gaskets but when i checked the webpage their listing for compressed height is only.030 and stock is .035. I also heard that the new ford GT and the 03 cobra get a four layer gasket instead of the 3layer that all the other 4.6's use and i believe that something like this would be best for my boost goals seeing as those cars are also s'charged.

Now when I go on the compression ratio calculator and type in their calculations, i get 8.6:1. I don't know How to calculate it apparently. ANd when i type on .030 for the cometic head gaskets, i get 8.74:1. This isn't looking good. I need to get this compression ratio down. I should be getting lower than 8.23:1 seeing as i have 2 more cc's than the 47.77cc values. :confused:

Someone tell me if i am looking at this wrong. I need to keep the c/r at or well below 8.5:1. I would like 8.0:1 so i can run more boost on pump gas. I know i can just put race gas but that isn't cost effective for me as a daily driver. I can do that at the track and turn up the volume. :) .

Any thoughts on how i may be calculating the c/r wrong?

Ideas on getting it down besides bowl work?

No, I am not getting rid of the intake swirl. I believe that JL said it was integral for efficient combustion. I may be wrong. I will need a dyno tune afterwards anyways.

Any help would be appreciated. Thanks.
 

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Well here's my opinion for what it's worth:

When you're building a true custom engine (i.e. different c.r.'s, bore, stroke, etc) that can't be pieced together, guess what: Custom pistons. :eek:

Of all the engines I built over the years, every time someone wanted a truly "x" specific engine, and I couldn't get it with head work etc. we ended up going with custom pistons. And when I say custom, I mean Vinolli, etc. You can get exactly what you want, not only in c.r., but also ring size, combustion surface design, compression height, etc… But you're talking big bucks. :leftright

Also, one thing to keep in mind that when you're building a blower engine and using aluminum heads and/or block, you can usually go with about 1 point higher in compression than a steel head/block combination. (Not my words, B&M's).

So if you're looking for an 8:1 ratio normally, you could safely get away with 9:1 with a good tune.

I have also heard that the Modular engines like compression even with boost. But I have no first hand experience with boosting the 4.6L (yet).

My recommendation would be go with the readily available forged pistons that will give you an 8.25 – 8.5 to 1 compression ratio. It’s a lot easier to “dial in” boost than to change the static compression ratio after the engine is assembled. :D

Again, just my opinions. :thumbsup:
 

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What your combination is after all the work I'm not sure, you're changing a lot of things at once. The stock numbers to use would be 9:1 compression, which gives a 71.89 combustion chamber volume. ~50cc's is about right, they do vary slightly. The stock pistons have a 10.5cc dish.
So using your numbers, 49.77+10.5=60.27 This leaves 11.62cc's in space that is used up in clearances (head gasket volume, volume between top of pistons and deck face).
The changes in the gasket will be very small, but without knowing exactly which gaskets youre using and what the measurements are, I can't do that yet.

The most major part is adding bigger dish pistons - 18cc's by your specs (technically 18.5 if they're stock PI pistons). So, we add 11.62cc's + 49.77 + 18= 79.39cc's.
79.39cc's gives a 8.24:1 CR.

(displacement of the cylinder/Total chamber volume)= (CR-1)
and vice versa
(cylinder displacement/(CR-1))=chamber volume
 

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Discussion Starter #4 (Edited)
I'm going to be using P&P'd NPI's with minimal bowl work, Stainless (larger) valves if available, cometic head gaskets p/n c-5502 and c-5503 with .030 compressed height, new block so no decking unless necessary, which i think is a .011 piston to deck clearance and arp head studs. i want to use the 18.1cc dish pistons i wrote about above, but i think some forged pistons are sold in the aftermarket with a -25 cc dish for the lightnings if the compression is not low enough with these 18.1's. I am going to get the new kennebell setup and see what it can do on this setup
 

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If you're using a stock unmilled block, then I'd expect the differences to be very small. The difference of the two gaskets is .005" or .127mm. That's maybe 1cc if youre lucky. The change largely depends on the pistons. If you go with 25cc pistons instead of 18cc pistons, the volume goes from 79.39-->86.39, which leads to a 7.66 CR, which is a bit lower than I think you wanted.
 

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Discussion Starter #6
I'm sorry, i just can't figure out how you get 11.62 cc's. yeah, you're right that 25cc would be too much. I won't be putting on the blower for at least a year as i will ahve to save the money up for it after this engine block is done. So i figure that enough time will pass that the rings will have had time to set properly by the time that i get the blower. I will have to gap them accordingly, so i wonder if that will affect drivability/ blowby during it's narurally aspirated phase.

As an aside, for street/strip life with a KB blower, what would be the best type of rings to go with? Everything I have heard(reliable or not) has led me toward the plasma-molys, but i have heard about stainless and the ones i gap or the ones already gapped or even the total seal ones.(the total seal gets controversy all its own regarding blowers. I have been recommended them and told not to get them by two people in the same conversation, one with a performance shop and 11 second car(who was in the process of building a 4.6, but quit due to cost) and one with a camaro that runs 11s (N/A)and uses the rings. So any recommendations that lead me toward the right path would be awesome.

Thanks so far. Your help is great.
 

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11.62 is just a leftover. For a given CR and displacement, you can find the total chamber volume. subtract piston/head chamber volumes and thats whats left. If you say swap the NPI head for a PI head and do the math, it comes out to the 10.2 or so that's been the accepted value. I havent had the time personally to rip open a 4.6 and go nuts with a micrometer, I'm just doing the math with the information I do have.
Rings in a 4.6 are an area I dont have a lot of experience in... I personally would try to get a set of 0 gap ones unless I knew of reliability/strength issues. Thats just my opinion though.
 

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Discussion Starter #8
The webpage says that it is forged of 2618-T6 aluminum. Will this cause a lot of piston slap in the cold mornings on the way to school or will it be ok?



Here is a picture, albeit blurry, of the pistons. They use a 1.5, 1.5 3.0 mm ring set. Will the ringlands be thick enough for boost? What are the advantages, if anyone knows, to using stainless steel rings over plasma moly for boost applications? would either cause more wear than the other or be better or worse on the street for any decent period of time? I want this thing to last..
 

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From what i've read, 2618 is what Manley pistons are made of, so I would think that will be strong enough... Can't tell you about noise, but I dont remember reading about anyone complaining about it either. Maybe someone who's been there already can chip in...
 
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