Getting lower compression...

[CobraSix]

Is there a good way to lower the compression ratio of an engine? I know using the thicker style head gaskets.

What about pistons. Are there any pistons anyone knows about that have a larger dish volume? What about different rods?

I'm looking to build either a 200 or 250 with a CR below 8:1 so I can run 10-14 psi. The other way I can think of getting there is having custom pistons made with lower lips and bigger dishes?

Also, I remember someone saying that the CR needed to run high boost is different for Turbos then for superchargers (or centrifugal versus positive displacement). Can anyone confirm or deny this? And if confirmed, why?

Slade

 

[Asa]

doing massive bowl work? if you increase the combustion chamber, the compression will go down, might only be good for a couple points, but it might be enough
are you going to stay with the head you currently have? if not, and it's an american head, i'd snag one of those that hasn't been shaved, after doing all the head work it should beat the price of having custom pistons done

 

[Anonymous]

I forget the details, but the '78 and later 250 head has the larger combustion chambers. It also has the larger log and bigger valves. I'd start there.

 

[CobraSix]

Except I want to keep my Aussie head. It's already been milled to 48cc on the combustion chamber (came that way from down under). That's why I'm looking for other ways.

Slade

 

[addo]

Build a 250. Use dished pistons, and maybe also lay back the chamber edges if possible... 48cc from Australia - hmm... It was probably gathering dust. Maybe trade it on a larger chamber one?

 

[Asa]

Maybe trade it on a larger chamber one?

i'd buy it offa ya... but i want to build a turbo motor for Susie runnin a good amount of boost

 

[CobraSix]

I may go 250 just for the extra cubes...especially since I'm looking at a positive displacement SC, the low end will be nice. May just have to go custom pistons...$$$, manley will like me.

Slade

 

[MustangSix]

Stock Aussie 250 crossflow pistons have a 12 or 15 cc dish. Same pin, same offset, same CD. Only problem is that they are cast, not forged.

 

[xctasy]

Two easy options

a) The David Vizard approach. Take a thicker 62 or 79 thou gasket, and o-ring the block with an Isky O-ring kit.

b) The Mike Vine way, a Queensland Ford Falcon turbo expert. In Australia, they used a 62.5 thou steel spacerplate with two 41 thou thick Monotorque gaskets on X-flow Turbos. They blew head gaskets only because of service issues. The idea is sound.

On a 200, use you use two composite gaskets, and a cnc cut 62.5 steel plate, it will work. Detonation is caused by hot surfaces of the fire ring heating up incoming gas and creating a flame front at the wrong spot, at the wrong time in the combustion cyle. Keep you engine intact, follow Frenctown flyers method of making custom longer pushrods. I'd forget better pistons, and try the thicker gasket approach. The gaskets are cooler than the cylinder block.

I'm certain making effectivly, a thicker gasket via this method is the best idea. A compact chamber, and a close to o-deck, isn't ruined by having a thick gasket.

Up to you.

 

[Anonymous]

You also lose all quench when you layer the HG's. Go with pistons and/or chamber mods.


-=Whittey=-

 

[Anonymous]

How does layered head gaskets destroy the quench area?

 

[Anonymous]

My old turbo 250 had a decompression plate and two head gaskets
Worked a treat

 

[CobraSix]

so...basically put a piece of metal between 2 gaskets?

Slade

 

[xctasy]

Affirmative. If it was more complicated, you just know I'd tell you E-V-E-R-Y-T-H-I-N-G!



With a stock compression of 9.1:1 with a 7 cc dish and 30 thou under piston register, I'd say a 62.5 thou plate with two 41 thou gaskets would take the compression down to a super safe 7.4:1

If you just search out a 62 to 79 thou gasket, you get 8.4:1 right away.

Whittey is right though, on a non-cross flow, the quench (or squench as Mark P puts it) may be upset a little.

 

[DYNOED250]

here in oz im running one on my na 250 with two acl head gaskets, it certenly didnt kill power, still made close enuf to 300 hp, no turbs no noz and no boost, so it might have a slight neggative effect but deff worth it to lower compression. I braught mine from ACL australia, of the shelf there avaliable in a few thicknesess, this was for a cross flow, not sure if they sell em for the pre crossflow but they might.

 

[Anonymous]

You can also have a machine shop turn the dish in the piston a bit deeper. Dont go too much, no more than .050, but it won't take too much to drop your c/r a bit. This will work with other mods to get it down where it needs to be.

 

[CobraSix]

What about shaving the top deck of the piston, say .050 or so? THat would effectively counter the millin go of the head, right? Good/bad idea?

Slade

 

[Anonymous]

You could do that too. Just cant get too aggressive or your pistons will be so thin they will burn through. In reality, you should measure the total thickness of the piston head and prcoeed carefully with this info. One other thing you could do is use rods with bigger pins and cut the pin bores in the pistons offset to lower the total commpresion hight. At that point, though, you might as well just buy custom pistons unless you can get a gonga deal on machine work. Find a machinist that will do side jobs for beer. The rods would also be a bit tricky, and I have no clue what rods you could use.

 

[xctasy]

You'll then find the car is more likely to detonate. The hot, hard ege of the cyilnder wall will be exposed at top dead centre. This is what causes detonatation...hard edges. A couple of stacked gaskets don't hurt as much. With the reduced compression height from a, say 1.49" tall piston pin to top, verse the stock 1.53" top, it will expose more of the hot cylinder wall. It will reduce the compression ratio, but based on the evidence from smaller engines like ohv Brtish engines, getting the deal O deck is the best option. Even if you have to have awfully tall gaskets to get it.

At days end, what ever works, works. You don't have to do anything crazy like stripping the engine down, but a good Tempo piston shaved 30 thou, even if it stops 50 thou before the top of the bore, well, its better to have it doing that than blowing up the engine. If that seams better than new longer pushrods and just a gasket change, then that's possibly a safer way to do things.

I've been checking my posts, and they have been getting very theoricital lately.

Live in the real world, follow your instincts. Your call, Mister Gasket!

 

[Anonymous]

Back to Whittey's comment about losing quench: ideal quench height is something in the .030-.045" area. less than that and you risk hitting the head with the piston. more than that and you lose the effect. If you're running a head with closed chambers (deck surface visible in the cylinder bore) then it's best to lose compression with a dished piston, so you maintain quench.
What's quench do? it causes turbulence in the fuel/air charge, increases octane tolerance, and promotes a rapid, efficient, clean burn.

Airflow's a waste if you're not going to burn right.
Ern