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post #1 of 6 Old April 5th, 2019, 10:55 AM Thread Starter
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4v Compression Ratio's

Hey guys,

Some of you might have saw my thread about my Bullitt's 2v ticking. After bore scoping and looking at the pistons, I have at least one with a couple nicks right on the edge, so I am going to pull it apart and take a better look. Well since it will be apart I thinking of maybe switching to a 4v.

I have a chance to purchase a full 4v engine with a 5.0 stroker kit. Engine has about 20,000 miles on it he says. Its a teksid block with B heads, 96-98 Cobra intake. I'm not sure of what CC pistons are in it but he claims it should be at 12.5.1 compression Ratio. I'm not even sure how you get the C/R that high. All the storker kits I can find only show the C/R being around 10.1, even with a low CC piston. Obviously 12.5.1 is way to high to be able to put my Novi 2000 on. What will I have to do to drop the compression? Will swapping to C heads lower me down enough to put boost on it? Or will I have to replace the pistons?

Any advice would be greatly appreciated. Thanks.
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post #2 of 6 Old April 5th, 2019, 11:23 AM
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I think the stroker kit changes your CR a little since your swept volume becomes bigger.

B head combustion chamber volume: 52cc
C head combustion chamber volume: 54cc
Swept volume (per cylinder) = 5000cc/8cylinders = 625cc

Compression ratio is equal to total volume divided by final volume, or-

(swept volume + dish + combustion chamber in head + head gasket volume)/(dish + combustion chamber in head + head gasket volume) if you assume zero deck and ignore crevice volume.

If you work that out for a 5.0 stroker and B head, it comes out to roughly a flat top, if not a slight dome.

If you throw C-heads and a 0.040" head gasket on it, that will bring you down to 11.3:1, which would be ok for E85. You can go really thick on the head gasket from somewhere like Cometic to get where you want to be.
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post #3 of 6 Old April 5th, 2019, 11:50 AM Thread Starter
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Thank you, That information helps a lot. I would have to be on 93 pump gas though. There isn't a lot of E85 around me, unfortunately. Will I be able to get it down low enough for pump gas using a really think Cometic gasket, and how would that effect the chances of the head gasket blowing under boost? What type of PSI would I be looking to get out of my Blower? I currently run roughly 20psi through my built 2v, but I'm guessing that would be to high on even the 11.3.1 compression. Thanks again.


Bruce
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post #4 of 6 Old April 5th, 2019, 01:40 PM
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Well it looks like Cometic goes as thick as 0.140", with a gasket volume of 23.638cc. That could potentially bring your compression down to 9.05:1, so anything is possible. I've never run anything that thick, but Cometic does have a reputation for sealing well. You might even try giving them a call to see what they say they can take. 20psi is pretty damn high on pump gas regardless.
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post #5 of 6 Old April 5th, 2019, 01:53 PM Thread Starter
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I'm guessing that 20psi on my 2v heads will probably be less then that on a better flowing 4v heads, correct? If I do end up with the engine, I will defiantly call comestic to get there recommendation. I'm still wondering if he decked those heads to get to 12.5.1. I cant find any stroker company advertising that high of a C/R. Also wouldn't running that thick of a gasket effect the heads height which would effect timing?
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post #6 of 6 Old April 5th, 2019, 02:48 PM
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Yeah you'll probably make more power with less boost with the 4V heads.

Decking the heads is a possibility. But doesn't sound unreasonable if he has flat top pistons. You can get domed pistons as well, but any of that is speculation at this point.

That much gasket may affect your "squish", which would affect performance to an extent, but without simulation software it's difficult to quantify exactly how much impact it would have.
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