Spacer for lowering compression ?'s

[CAT3]

Ok, so looking around I hear of ppl using a .090" thick spacer b/w the case and cylinders. I understand this lowers compression, however... Doesn't adding the spacer increase the quench so much, .090", that it would leave the engine severly prone to detonation?

In the car engine building world I come from, and my experience with SBC, and recently LSx engines a tighter quench height is preferred to quell detonation. I built my Camaro with a quench height of only .035".

For those wondering, quench height is the distance from the top of the piston in TDC to the cylinder head... depending on if the piston is out of the hole (+) or in the hole (-) deck height, the thickness of the head gasket. In auto applications, having a quench of less than .045" is preferred and dramatically helps FI cars run boost with less chance of detonation. On top of this, it of course adds a miniscule amount of hp, but mostly its set to keep the engine from grenading. Of course you can run more aggressive timing with a tighter quench height as well, so there are many advantages. Only disadvantage is if using rods that stretch easily, weaker rod bolts, etc.. that when cold, before the operational expansion is finalized you can smack pistons with valves...again, not an issue if your using say Aluminum block, heads and quality rods and bolts. But who really romps on their stuff when cold anyway, thats just careless.

I am thinking a nice stock bore, maybe slight stroke, using new rods, bolts, dished pistons, and the turbo in the end. However, until then, thinking add turbo, and save for the engine build. Just looking for experienced users of or against the spacer issue to lower compression. Yes I know its not needed, just keep the boost lower, but I also know boost (or any power adder) is addictive.
Thanks for the reading time.

Charlie

 

[speed750]

The spacer lowers compression which lowers your chance of detonation. It does NOT increase your quench area.

 

[CAT3]

(speed750 @ Dec. 27 2006,18:02) The spacer lowers compression which lowers your chance of detonation. It does NOT increase your quench area.

Not sure I understand how both are possible? If you space up the cylinder, thus making the piston not travel all the way up by said spacer thickness, be it .090", then the distance b/w the piston top and cyl head have changed? Or am I not understanding the spacer issue that well?

 

[speed750]

You understand the spacer issue. I think the quench issue is a lot more complicated than you realize. I don't know where to start on this one. Try Google. The short version is 2 motors running 11:1 compression. Motor A detonates, motor B doesn't. Motor B has a quench area that forces all the fuel and air around the sparkplug thus giving optimuim flame travel.

 

[CAT3]

(speed750 @ Dec. 28 2006,20:35) You understand the spacer issue. I think the quench issue is a lot more complicated than you realize. I don't know where to start on this one. Try Google. The short version is 2 motors running 11:1 compression. Motor A detonates, motor B doesn't. Motor B has a quench area that forces all the fuel and air around the sparkplug thus giving optimuim flame travel.

Piston deisgn, chamber quench pad (design) do play into it as I'm aware. Which is what brought me to question the validity of using a spacer. Wondering, if the lower compression is allowing more boost by itself, or if it helps the quench area, by virtue of design and how luck would have it. Naturally my thought is the quench would be increased, as it is using a spacer, but also messing with the compression of the charge, and flame propagation. But, sounds like the spacer is a safe temp solution. Thanks.

 

[Jay]

What you call quench area or squish area is indeed increased when you run a spacer. The distance between the top of the piston and the cylinder head is increased the amount of the spacer.

Jay

 

[CAT3]

(Jay @ Dec. 31 2006,19:02) What you call quench area or squish area is indeed increased when you run a spacer. The distance between the top of the piston and the cylinder head is increased the amount of the spacer.

Jay

Jay, roger on that. Is the Busa chamber designed in a way which the quench doesn't lose its efficiency, i.e. make it more prone to detonation? OR, is the lowering of compression (not sure what it lowers to using the .090" spacer) enough combined with tuning that the turbo bikes aren't pushing it? Thanks.

Charlie

 

[2busa]

CAT, May I toss a few, "off the wall ideas" past you? Stay with a stock head gasket. Why add a huge carbon ring to the wall. Low compression will build even more carbon; once you add a Pcom. Flame front ends at the wall. Heat transfer begins at the wall = A Glow Plug in the making ~ detonation/pre-ignition. What squish.... From shop vacuum to car (cig-lighter) vacuum?... (analogy for turbulent pressure)
Here are a few advantages/loopholes I see staying stock, (head gasket wise). The mileage on your bike has lowered the compression somewhat, (I would assume). You could file the compression ring (only) close to it's, "serviceable limit" and still be within book spec. You can set the valve clearance loose (sill within book spec), so you pack less air ~ lower compression.

 

[BA BUSA]

(2busa @ Jan. 01 2007,22:20) CAT, May I toss a few, "off the wall ideas" past you? Stay with a stock head gasket. Why add a huge carbon ring to the wall. Low compression will build even more carbon; once you add a Pcom. Flame front ends at the wall. Heat transfer begins at the wall = A Glow Plug in the making ~ detonation/pre-ignition. What squish.... From shop vacuum to car (cig-lighter) vacuum?... (analogy for turbulent pressure)
Here are a few advantages/loopholes I see staying stock, (head gasket wise). The mileage on your bike has lowered the compression somewhat, (I would assume). You could file the compression ring (only) close to it's, "serviceable limit" and still be within book spec. You can set the valve clearance loose (sill within book spec), so you pack less air ~ lower compression.

Ring gaps and valve lash has ZERO effect on a engines compression
ratio...he wants to run a turbo. The only way to do this...lower the
static compression ratio is to run lower compression pistons and or
a thicker head gasket/spacer plate.

 

[2busa]

If I have a door 1/4 open or 1/2 open, how much faster could I warm the room... statically speaking.
If I set the valves tight (w/in specs) I will gain (approx) 2 extra mph on the top end, (1/4 mile time slips ~ Experiment 1985). The "static" air had more time to stay in motion. More "static" air in the cylinder chamber will create a resistance, (compress more air). If less air is pulled, that is less static air to compress. This is the most, "subtle" of static adjustments. The air loss through 4 (closed) valves~ring gap = static loss.
"ZERO effect?"

I'd chase, cylinder studs, Carrillo rods, then play with piston dome design. I still have "squish" for the fuel/air turbulence and the limited carbon band. Statically, my ring gap, valve lash, and 6~7% leak down is (statically) draining away. Here you are with a well sealed, 2% (or less) leak-down, no matter how thick the gasket.

Still think compression ratio matters? The thicker gasket will not only pull less air (gap), it will compress less on the return stroke.
I'd say we achieved similar results.

 

[dadofthree]

Search turbo. I think everyone installs a spacer. Don't know if that is in reference to a thicker head gasket.

 

[cougar694u]

(CAT3 @ Jan. 01 2007,17:45) not sure what it lowers to using the .090" spacer

I think the 0.090" spacer will drop you to around 8.3~8.8:1. Do a compression check on all 4 cylinders before and after. If before you're averaging 200 PSI and after you're averaging 160 PSI, you're at about 8.8:1.

General rule of thumb on general automotive engines is 0.1 points per 0.005" thickness. That would put you around 9.2:1, but the smaller bore of a motorcycle engine affects it more. It probably won't be more than 0.15 points per 0.005" thickness (that'd put you around 8.3:1).

 

[mountainmotor]

The very small combustion chamber volume helps effect static compression ratio in a big way with small numerical deck height changes .

STD bore flat top with :

.015 negative deck = 12.47-1

.019 negative deck = 12.26-1

.025 negative deck = 11.96 -1

Carefull on manufaturers advertised compression ratio for aftermarket pistons and assume nothing .

 

[CAT3]

2Busa, as BA pointed out, I am asking about spacers, which reside b/w the case and jug, not the jug and head. Thanks though.

dadofthree, I know ppl use them and do so safely. I am wanting to know what the limits to it are, etc, more intimage knowledge of the effects I guess. Like on LSx engines, you can sleeve them all day long and go from 346ci to well over 454ci out of a new small block chevy LS engine. However, one huge disadvantage is almost any KR (knock retard from detonation etc) will separate the sleeve from block and toast a lot of $.

Cougar and Mountain, thanks for the tips and info. Guess I need to search the stock bore, stroke, chamber size, head gasket thickness and stock deck - or + and then hit the calculators, then of course back it all up with measuring and checking. Thanks again.

Charlie

 

[cougar694u]

Bore: 3.189" or 81 mm
Stroke: 2.480" or 63 mm

I think I read somewhere the combustion chamber is something like 18.8 cc, but I don't recall. I had a factory head gasket a few months ago I should have mic'd to get its specs.

 

[2busa]

CAT, sorry, I meant the base gsk. I did mention the larger carbon ring (cylinder area) to avoid. I also tried to communicate that static compression could be dropped by the ring/lash.
There are a few 13.5's turbo'd. With this engine family, your cylinder barrel IS the upper case. You cannot shim an integrated case. As far as I know, no one is changing to thicker head gaskets. They seem to be surviving in stock configuration.
Something to think about.