Chev 305 rods in 250 EFI. What mods?

[Unfalconbelievable]

OK I know a 56 thou overbore is required to make the pistons fit in the bore but what compression height should i get so I come out with close to 8:1 compression with the EFI head? And what has to be done to mate the bigends to the crank, mill the rods out, fit bigger bearings or grind the crank down?

THANKS ALL

 

[xctasy]

The reduction of compression by 10% from 8.7 to 7.9:1 results in a 6.5% power drop, and a 4% torque drop. That's like loosing at least 10 cubic inches. Any Falcon with an alloy head will run okay at 8.8 unless you boosting it with a turbo.

Most stock EFI 250'S had a about 22cc chamber pistons with a 49.6 cc combustion chamber. 8.8:1 compression.

Stock Chevy 229 V6 and 305 V8 pistons have a 12 cc dish which is less than 110 thou deap, so compression will take still take a huge hike upwards with them. If it is the forged piston, you can remove a little extra from the trough, but you won't get 22 cc's unless you are able to remove another 100 thou from the dish. You should get back to 8.8:1 that way.


There are other possible solutions. Open up the chambers to 57 cc by removing the swirl ramps if its an XF engine, or just removing a uniform amout with a die grinder if its an XD to XE head.

There isn't a lot of aluminum there to grind, the Honda casting dosen't have a lot of thickness in it, but turbo builders used to get down to 7.6:1 back in the 80's just by relieving the head, and it was safer than running a decompression plate with two gaskets. To reliev the chambers right, you have to duplicate the closed chamber Boss 302 or early 351 4V head. It will ruin the advance curve on the spark timing, but it's a last option.

Then the last, last option is to fit the thickest composite gasket you can get, or get a 62.5 thou spacer gasket (made of tin) and add two 41 thou gaskets. All Ford I6's, iron headed log, iron or alloy x-flow, all have minor issues with the rear water gallery rusting out after a few years, and adding another couple of gaskets doesn't help improve the long term gasket seal.

Rework the pistons!

 

[Unfalconbelievable]

As far as I know I can order various compression heights for the pistons, differing from stock measurements. Hopefully enough for the 8:1 or less comp i want with a little chamber work to unshroud the valves and I will also be adding a fraction to the piston dish with my lathe. Yes its a boosted application, planning on around 20psi maybe higher now if I can sort out a decent water injection system. Intercooler with water sprayers over 7 or so psi and water injection with 8:1 should run 25 pounds right? I have toyed with the idea of using a huge custom copper head plate and a couple of gaskets to increase deck height alowing a longer chev rod to improve rod/stroke ratio, but if you say it will only cause trouble I might throw that idea in the bin. Saw it done with a clevo V8 in the US engine masters comp, had to use stockish parts, block had to remain same height, but they forgot to limit head gaskets, pistons sat up 190 thou or sumthin crazy out of the deck, thats thinking outside the square!

 

[xctasy]

My first documented not of the concept of copper gaskets to allow the use of pop top pistons was a power boat engine used in an a 350 cube Chevy engined Capri piloted by a Melbourne guy, Stakov or Stadnic, I think. The Capri did low 10's, and was British Racing Green. Year was 1988, listed in a Street Machine magazine.

Anyway, the engine was ex power boat, where the regs didn't allow risen dome pistons, so the stock block was decked 125 thou, and the pistons were profiled to suit the angled plug fuelie heads. Result was a 12.5:1 compression with stock , formerly flat-top TRW forged pistons. Great idea.

I guess you have to decide on what you want, and don't let anyone pscho-bable you. If you build it right, a copper gasket, even on an alloy head, is the easiest way to drop the CR, and if you have to lift the head, it means there is no remachining required. Long term, over four or five years, the copper tends to be a sacrifical galvanic source, and the alloy tends to get marked and pitted. Sort of like pitching an old 2 cent piece into an alloy row boat.

Enjoy yourself, it'll be a rocket ship!