commprestion ratio

[Pimpdady753]

Can some one explain to me or show me a good web site, I under stand some what what comprestion ratio is, but i dont understand why high comprestion is better for blowers and low is good for big inches, or witch ever way it goes. Im looking to build a 347 with AFR 185 or 205 heads but i dont know what comprestion ratio i should run, or the combustion chamber on the heads, it will be all motor for about a month or so for the brake in period then i will supercharge it after that. Any help would be great thanks.

[351wStang]

Ok, to start with you have a few things backwards.

High compression is not at all good for blowers/turbo(s). Superchargers whether they be centrifugal or positive displacement create boost (of course). Boost is merely a measurement of restriction. More boost is more air being forced into your motor. With all the extra air going into your cylinders you are obviously increasing the cylinder pressure. The air/fuel mixture that is channeled to each cylinder with forced induction is a greater amount, yet still contained in the same area. So therefore you want a lower compression motor, to allow for the "boost" created by the blower or turbo. This is just barely skimming the surface, if you want alot of info on it go do some research on turbomustangs.com. There is a huge wealth of information tobe had by reading tech threads over there.

Low compression & Big inches...where does that come from?

Now, you want to build a 347. AFR makes a real nice head, but I have a feeling that the 205 head will be too much for your motor and the 185's wont even reach full potential. That being said what do you want out of this motor? Stock block? What is your power goal? What style camshaft are you planning on? Will this be for a street car or a strip car? We are gonna need more details to provide the best answer here.

[Pimpdady753]

well what i want is street strip car to tinker with, im going to be bilding the 347 ive never bilt a engine but ive got all the books and im pritty sure i can do it. This car is pritty much a toy right now u know, I found a place were i can get a cheap stroker kit for 600 and the machine shop buy me said they can clean check for cracks hone it and do all that stuff for 200. So im going to be useing all the stock stuff, i do have a performance cam but not sure of the specs, at least thats what the guy i bought it from said. So i was thinking of changing the heads to afrs, i mean i mide aswell when i bild the engine and all. But im also going to be putting a 408 in like a year the 347 in temp, and going to let me learn how to build engines u know. That is why i was thinking 205 so i can just swap them over to the 408. Im also going to buy a supercharger for the 347 and run really low boost, i want to get like 500 hp or less with the 347. Then with the 408 run more boost with the same supercharger. I was just thinking ahead. thanks.

[351wStang]

I was pushing close to 600rwhp with a Vortech S-trim on my 357w. That was 14psi though. I would suggest that you buy some AFR 185 heads or TFS track heats. The Vic Jr, 205, highport, ect heads are gonna be way to much for a simple 347. I would suggest either Perf RPM, AFR 185, or TFS track heat/wedge heads, F303 cam, Holley systemax intake (for EFI). The middle of the road heads with the F303 cam will make good power. If you have any doubts about the F303 cam's potential go to turbo mustangs.com and check out Brent's car. 1200rwhp with a F303 camshaft. I say do your research, build your 347 and get used to it. Then add boost. You can up the cubes, change to a 9.5 deck block, ect. But, if I had it to do all over again I would go with a 8.2 deck Dart block and build a 363" motor. 3.4" stroke, 4.125" bore, 8.2" deck Dart block. That way you get good cubes, stout block, and you dont have to worry about all the stuff that comes with the motor swap.

[Pimpdady753]

ok thanks

[SkylineUSA]

Good advice, as always David

I think the Americans spend way too much time on cam profiles, F303 is a very good streetable cam, and it can be used in many applications.

[Pimpdady753]

so what ratio do u think i should run, knowing its going to be a 347 with afr 185 heads and for just a little while be all engine then a blower will be added after a little while.

[SkylineUSA]

I had 9.5:1 om my engine, with a two valve you can get a way with that, or go 10.5 and no blower makes for a better street car.

[351wStang]

9.5:1 will work just fine. But, you better be able to tune that thing. I suggest about 8.5:1. Sure there is a little more power to be had with the higher compression. But, the tune must be dead on. Also start out with cheap felpro head gaskets. Trust me when I say you would rather replace head gaskets then pistons.

I just pulled the 8.7:1 Probe forged pistons out of my motor. I replaced them with Mahle forged pistons that have me around 10.5:1cr. All motor and a 5spd in my old cougar will be fun. My machinist says I should have about 475hp to the crank, I'm thinking closer to 450, but who knows.

All I'm saying is if your sure you want to go the blower route then keep the cr low. Otherwise pump it up a good bit and have some fun.

[SkylineUSA]

Don't run 8.5, that is way too low for 12lbs of blower boost, at least go 9:1, so you will have better off boost power, trust me 8.5 with a two valve head is not good unless your planning on boosting over 15psi. To go 15psi, an 8 rib will slip like crazy,.

[351wStang]

I ran 14 with an 8-rib setup on 8.7:1. It was a hell of a lot more fun to drive then the turbo build.

[SkylineUSA]

14lbs with a belt tensioner is the only way to run it, but it puts a lot more stress on the front end of the crank, something I am not a fan of

I ran 12lbs with 9.5 without a tune, or any issues of det.

David,

The Turbo would have been a lot more fun, if you would have been able to drive it

[Pimpdady753]

ok so ill probbully go with the 9.5 to keep it in the midle, thanks.

[Pimpdady753]

i was looking what type of pistons should i get flat, dished.

[351wStang]

That depends on several variables. Cylinder head combustion chamber cc, piston compression height, head gasket compressed thickness, ect.

I'd say you will probably end up with a dished piston though.