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Discussion Starter #1
Hello All,

Almost a year ago I purchased a 2006 GT, 5 speed, and like many of you I've caught the mod bug. The car was basically stock, with the exception of JLA axle backs.

Well slowly I've been researching and modifying my wish list of things I want to do. Recently I added a JLT CAI to the car, along with a Brenspeed tune (love it)! And now it's time to finish it up. So with that, below is what I want to do to the car over this coming winter.

- Stroke to a 302 CID
- Detroit Rockers
- LT, X-Pipe w/cats, maybe new axle backs, although not sure
- New Clutch/Flywheel
- 4.10 Gears

I plan to keep the car NA, as i'm not really all that interested in FI. But I do plan to increase the compression from 9.8 (stock) to 10.8 - 11. With that said I have been doing some quick calculations, which is driving my below question...

All rotating assemblies, stock or aftermarket seem to have the top of the piston right at .012" below the block deck height. Is there a reason for this? I'm used to the old school motors, where you could run zero deck heights without worries, or close to it depending on configuration.

I have no experience with the 4.6, or any modular designs, so I don't know if it is a design requirement, or if it's just a safe number for all aftermarket manufacturers to follow. Has anyone played with the custom pistons to achieve a zero deck height relationship with the top of the piston? I haven't yet begun the research into block and/or head decking, so I don't know if that's an answer or a no-no.

I'm really curious why the .012" number is a constant, so if any of you have experience, or have read literature on this I'd love to hear about it. Thanks in advance for your time... Hopefully I sound somewhat intelligent. :bigthumbsup
 

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Shelby Gt 350 member
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I don't know why the kits out there are .012 in the hole. Until I read your post I didn't realize there was a consistent number. I'm not surprised they are in the hole though. The compressed gasket thicknesses available are all over the place so to be safe they are being set up with the pistons down in the holes. I think the thinnest gasket is the OEM ones at .036 (I think). Add to that the piston being down in the hole .012 and you get a piston to head clearance of .048, which is a good safe clearance. Even on a full blown race motor set up tight I wouldn't go under .040, especially a FI motor where the piston crowns could expand a bit more than a NA setup.

If I was looking to get every last bit out of a NA build I would probably push things a little more in an attempt to get the most compact combustion chamber possible. I've run 108mm (just over 4") bore pistons that saw 13K rpm as tight as .035. That was TIGHT though. At places like Daytona where you are WOT for long periods the pistons would rock enough in the bores enough to just kiss the heads. These were set up with titanium rods which stretch a bit at high RPM too.

And don't forget, when you change gasket thickness or deck the block you change the cam timing.

Now for my forced induction tangent.

Unless you swap to a Coyote motor I think you are insane going the NA route. I have never seen a 3v make more than 415rwhp (and not much torque) NA and that was a full tilt boogie, no expense spared, everything and the kitchen sink build. $12,000+. And a correspondingly large amount of labor that either needs to be paid for or done. And stroke doesn't do much good. And can be argued does as much to hurt as it does help in a NA situation. My tuner and I have been watching "torque trends" with stroked motors and we have come to the conclusion that stroke helps torque to the tune of 5-10ft lb up to about 4500rpm. Then after that there is no gain until about 6750rpm where it actually starts to drop.

I get wanting to take the road less traveled, etc. I do. But sometimes it is just too expensive. You can spend $10-14K and a huge time investment going NA and make 400ish hp. Or you can go with something like the DOB kit, using top shelf components and spend $3500-5500 to make 450hp/450tq. And do it in a weekend.

If I was going for something different and wanted to stay NA I would put a built up Coyote motor in (not terribly original) or a 427cu in Windsor (small block) between the strut towers. Both options will crush a 3v when it comes to making power and cost about the same.

Or put a 6.2L "truck" motor in the car. They are easy to get 500+hp out of at 6.2L. and if you bore/stroke them they will make 600+ without much trouble.
 

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Discussion Starter #3
Thanks for the reply sqidd! I absolutely understand the cost savings and benefits of FI over NA. I must also say I'm a little surprised that strokers typically yield around 5-10ft lbs. I would have thought that figure be a little higher, actually a lot higher. I was going by the premise of all things being equal, with a built NA motor yielding 1.25hp/ci, adding an additional 21 cubes would add an additional 26hp. But with no experience with this motor, what I think and actual are two very different things.


For a while I was kicking around the idea of purchasing a B326 from Brenspeed, but decided against it strictly due to my own personal need to figure this motor out. While I don't plan to max out the design, I do have a goal in mind, 350+ at the rear wheels, which I believe will require additional items to what I listed in the original post. Why not more? Well, I don't really know, except that I don't race at the local track, and I'm not really brazen enough to risk an all out speed run on the highway. I just want a nice sounding car that will keep up with the new stuff in the area. Then a few years down the road if I decide I need a change i would consider FI.

Thanks again for your advise. It was very methodical, well written, and did not go onto deaf ears. I'm just a geek when it comes to this stuff and i really want to figure it out.

I'll report back when I figure out the hard facts on the .012" in the hole. I would sure like to get it tighter to the deck, a lot tighter... but not at the risk of an engine.

Be safe, and enjoy the 4th!
 

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I always thought .008 was the factory deck height clearance. The biggest reason for deck height clearance is rod stretch. At high RPM's there could be enough rod stretch to slam the piston into the head if there is not adequate clearance. In order to obtain something other than the stock .008 deck clearance would require a custom piston ... which is just going to be really pricey (I know this first hand).

Also if you are going to a 3.75 stroke, you either need stroker pistons or stroker rods. Flat tops will put you in the mid 10's with a stock stroke. With a 3.75 stroker crank, a 6 to 11 cc dish will put you in that range depending on the combusiton chamber volume. Cometic makes gaskets from .036 to above .100 in. which can help in tuning the compression ratio.

I agree with Jason on the 3V NA hp. If you are want to stay NA, swap it out for a Coyote. I think going forced induction is the easier way and potentially the cheaper way to get a lot more power.
 

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Discussion Starter #5
I agree with you on the rod stretch... I was just surprised it was that much, and yes going FI would be the cheapest and efficient way to make power. But I do want to stay NA, and I think I'm going to have some fun doing it.

I will have to read a little into it, but I thought to swap out for a Coyote I would also need to change out the transmission. But like I said, I'll need to do some more research.

Thanks!
 
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