What is the most efficient way to reduce compression ratio?

[teters]

I have a serious compression ratio problem with a 2V Oz head on a 250 block. The head has been agressively milled and with a solid copper head gasket .060 thick,still, results in a 14 to 1 compression ratio. The desired ratio is more, like 8.5 to 1. I have several options available to achieve 8.5 to 1 but I am concerned about quench, flame travel etc. The combination of options under consideration are; enlarge the combustion chamber and unshroud the intake valve, which adds 30cc's, dish the piston 30cc's, build in a negative deck clearance of 30cc's, I need some combination of these or other solutions that add approximately 40 to 50 cc's to the combustion chamber volume, The current volume is 54cc's and the engine will be turboed to a maximum of 17 -19 pounds of boost.
Any help, ideas, creative insights , or strokes of genius will be greatly asppreciated. Another unmilled head with a 62cc chamber will only give me 11cc's more.
Thanks!

 

[68coupe200]

What about a cam to bleed off some of that compression? Or will this not work? I've heard talk of this used with big-blocks running forced induction, so i'm under the assumption that this trick will work here also. 'Fraid i can't recommend a cam though...but it is a thought.

 

[xctasy]

Real simple. Focus only on pistons or gaskets as the 2v head, 250 block are assets, while the rods and pistons are cannon fodder.

Low compression pistons or nasty SOB thick gaskets are fine for an instilation where way too much static compression is going to kill the block, head, pistons and rods. It's not a difficult thing to sort out, too much compression will kill you block and the one in 12 000 Aussie 250 2V heads.


1) Recheck your calculations on the Falcon 6 Performance website, the numbers don't add up buddy.

Even with a stock 6.5 dish 200 piston in 60 thou over with a 60 thou gasket, ablock shaved 80 thou down to give a 25 thou below the deck piston, you'd only get 10:1 compression, and maybee 11:1 if you decked the block 30 thou for an HSC 2.3 piston. (HSC pistons don't increase the CR).

2) Get someone here to send you a set of six ACL Ford cast pistons which have a 27.1 dome cc. That would give you about 8.3:1.

Although forged pistons run 100 degrees F cooler under load, most people with high boost engines limit the total advance conservatively, so a set of aftermarket FACTORY x-flow pistons should do a brillant job.

3) Cheep idea :idea:
You could off course keep the stock 250 i6 engines nasty 110 thou negative deck register and save on pistons and machine work. Adding a 180 thou copper gasket would get it back to the stock production disaster which cripples the 250 six in normally aspirated form.

Over in Aussie, cheapscape Holden 179 and 186 owners with turbo I6 OHV engines just shove in some shallower pistons from the 202, and have the piston stop short 125 massive thou, and they still run heaps of boost without doing nasty stuff. They've been doing that snce the late 70's with total sucess. In your case, no slightly shallower 250 pistons exist , so just one thick custom CNC machined copper gasket would do the trick.



Important Note 4.

Ross pistons in the US make custom forged slugs for sixes. Precsion in Australia are making forged pistons too. Contact them and ask for a deap trough design with between 22 to 27 cc's like the Aussie X-flows run.

Important Note 5.

Make sure your rods are up to scratch too,

Mating these pistons to some better conrods. Eg aftermarket 6.00 Chevy small block (0.927" wrist pin, 2.10" rod journal) or stock production 1998- 2006 Ford Falcon AU/BA/BA2/BF 6.06 conrods. (0.912" and 2.126" respectively). You can fit big block chev rod bolts to the Aussie rods.

 

[addo]

I'd pick up an uncut head. Then, relieve it a little to unshroud, and use a dished hypereutectic piston.

To ameliorate a 1/3 mismatch in ideal vs actual conditions, by machine work, will in my thinking never be a durable or happy compromise. I've seen people go on these chases and there is always another snag around the corner, which pushes the costs above "start over".

The other option is to change the build plans.

 

[gravelrash]

i used xf unleaded pistons in my turbo 250 2v as well as a decompression plate.

teters, PM me if you want to know more of my setup.

Cheers Greg

 

[Anonymous]

Assuming 0 deck height and the pistons are un-flycut (worst case scenario) currently you only have 11.57:1 compression.

54cc chamber
60thou gasket
250CI motor means that

BDC = 45.6CI
TDC = 3.94CI

Therefore CR = 11.57:1

 

[Lazy JW]

...1) Recheck your calculations on the Falcon 6 Performance website, the numbers don't add up buddy...

Yup. It would require a bodacious amount of domed piston to even get close to 14:1 compression. Either that or shave the head all the way down to the water jackets. I think you have way less than 14:1.
Joe

 

[rickwrench]

Silvolite used to stock Oz Ford 250 pistons with a 28cc dish.
CH = 1.525
Pin = .9122

Silvolite part# 3146.
Rick(wrench)