I've been slowly getting a 5.4L DOHC N/A engine into my '01 GT and after finally getting it bolted in and wired up, compression is terrible. That's what I get for trying to save a few bucks with a used block....

I'm thinking that so long as I'm having it rebuilt, I might as well up the compression a bit. The problem is, the stock engine already has flat topped pistons, yielding around 9.3:1, and there don't seem to be a lot of domed pistons on the market. This car is intended to be a fast daily driver, not a drag queen so I'm thinking something in the range of 11:1 to 11.5:1.

So far the only thing I've found are super expensive, custom grind jobs that take 3+ weeks. I don't suppose anyone knows where I might be able to find domed modular pistons that aren't custom orders?

Alternatively, I could just deck my block and use a thinner gasket but I'm worried my intake won't work if I start shortening the block....

Any advice is appreciated.


For those that are curious, to go with the old Navigator block I have new '01 Cobra C heads with '98 Cobra cams and an Aussie Boss 290 intake, courtesy of Birdman. Here's a pair of terrible pictures, hopefully I'll get some better ones before I pull the engine back out.

 

Probe would be about your only bet since they use the offset piston pin as compared to stock pistons. It'll be atleast 11.5 to one with 4v heads.

 

I guess your not worried about not running pump gas. Because with the much compression I would not run 93, well maybe if you pull a lot of timing out of the motor. Your setup looks pretty damn sweet and I would love to see some better photos of it.

Also You can't just use the higher compression pistons that the 4.6 uses? I know the 5.4 is a taller deck motor with a longer throw, but does it not share the pistons with the 4.6?

 

Thanks, there will definitely be nicer photos coming in the near future.

I could indeed use 4.6 pistons if they worked out to a high enough compression ratio.

I've given up on finding domed ones and I'm now looking at ones with offset pins. Ironically, the probe pistons would give me too high of a compression ratio. It would be around 11.7 and there's no way I could manage that without some radical cams which don't fit in my budget and wouldn't work well on a daily driver.

I've seen Cobra's that ran well at 11.5:1 on 93 pump gas but the more I look into it, the more I'm coming to realize that I'm not going to be able to hit that without spending a $1000+ on custom cams. If I can find a cheap set of FR500 cams I might be able to swing 11:1 but without them, I'm going to have to settle for something in the range of 10 to 10.5 which is still a decent improvement.

Can anyone recommend a lightweight but fairly cheap set of rods? It's an N/A engine that will always be below 500 HP so I don't need super beefy forged titanium rods, in fact I suspect I could get away with the stock rods if I'm only going 10:1 but lighter rods means less reciprocating mass which is always a good thing. So, any light and inexpensive rods?

 

I feel your pain there, I ended up getting a custom set from diamond. I got them to use the offset pins but with dished pistons so it came out to about 11to1. I think I paid $1200 but that was with coated face and skirts. That was with pins and rings.

As for rods.. theres the TMD rods that most of us are using, in the $330-$350 range. Basic forged H-beams

 

Have you thought about...
1. Milling the cylinder heads?
2. Decking the block?
3. Using custom rods that are slightly longer?
Decking the block should be effective since the pistons do sit in the hole rather deep. Just make sure you modify your manifold's width accordingly.

 

Sounds like a cool build! The 4v engines do like compression!!!

Thought about offset grinding the crank?