Topic: Best way to REDUCE compression ratio?

[Ricky_Racer]

I have a set of NOS Action Four 836 pistons I'd like to use in a '72-76 K motor...  Problem is that they are 12:1 CR and I don't fancy spending for, or hassling with, fuel enhancers unless I absolutely have to.  So, what's the preferred method of reducing the CR?  '77-78 F cylinder and head?  Cylinder base plate?  Thick head gasket? 

Anybody got an answer to this dilemma?  I'd really like to use these slugs!  Thanks in advance. RR

[mlinder]

Not a lot you can do, outside of machining the pistons down :/
Or raising deck height for the head, which throws cam chain off.

[ProTeal55]

Compression is a good thing...

[DammitDan]

Run it with the spark plugs out? 

[mack808]

Cometic gasket makes copper head gaskets in varying thickness. You can order from them direct with headgasket thickness, even call them and they may be able to advise you with the standard kits gasket thickness and than with what they have available....   I did this playing with a Yoshimura 590 kit for a 500f.  their number is 1-800-752-9850... hope this can help you

[jabbadeznuts]

Thicker head gasket is the only way I know of without doing machine work.

[heffay]

we are talking about reducing right?  then a thinner gasket would be what you are after, right?
example:
12:1 w/ stock gaskets
11.99:1 w/ thinner gaskets

correct me if i'm wrong guys... i drank a lot this weekend 

i'd say the best way aside from the plug comment might also be run it w/out oil for a while    should do the trick

[mlinder]

we are talking about reducing right?  then a thinner gasket would be what you are after, right?
example:
12:1 w/ stock gaskets
11.99:1 w/ thinner gaskets

correct me if i'm wrong guys... i drank a lot this weekend 

i'd say the best way aside from the plug comment might also be run it w/out oil for a while    should do the trick

Correcting you.

Thinner gasket makes more compression, thicker, less.

[holysmokes]

double up your head gasket, use 2 instead of one

[DammitDan]

Thicker gasket = more space between top of piston and compression chamber = less pressure on compression chamber (less compression on atomized fuel)

Thinner gasket = less space between top of piston and compression chamber = more pressure on compression chamber (more compression on atomized fuel).

So the relationship is inversely proportional.  Now the trick is to find the correct equation to get the right ratio.

[rhinoracer]

Why not double the base gasket instead of the head gasket, that would raise the deck w/o compromising the head gasket?

[heffay]

you're right   ... i actually had to draw it out

carry on...   

[mlinder]

you're right   ... i actually had to draw it out

carry on...   

Why does your piston with the smaller gasket (and presented graphically with less volume) have a larger volume written above the piston?
O_o

[Ricky_Racer]

Wow!  We've had some usable inputs so far - and some that were, well...    

It is not uncommon in building blower motors to build a spacer under the cylinders to reduce compression, or even in a non-aspirated high-RPM motor to reduce rod angularity.  I just wasn't sure if an SOHC Honda had any design-specific pitfalls with this treatments.  I suppose that the stud length and available timing chain length (or slack) will be the limiting factors.  

I also wonder how much thicker a head gasket could be without causing problems... and the difficulty of getting solid copper head gaskets to seal without repeated retorqueing is well known...  

Guess Mack808 had the best suggestion so far.  I'll try to give Cometic a call when they reopen tomorrow.  But if anyone else has been down this road, please let us know!  Thanks.  RR

[Sam Green Racing]

RR, if you try to reduce it by raising the block or head, you're going to have to correct the valve timing and may have to run a worn chain....is it worth it.

Sam.

[heffay]

you're right   ... i actually had to draw it out

carry on...   

Why does your piston with the smaller gasket (and presented graphically with less volume) have a larger volume written above the piston?
O_o

compression ratio is a ratio not an exact value...  those numbers should be thought of as "parts"

like at top dead center... cyl. 1 has one part while at bottom dead center it has 2.2 parts

the grid helps to visualize... you can count the boxes (vertically only) to decipher the drawing.

what is so confusing about this is that w/ different gaskets the volume in the combustion chamber changes for bdc AND tdc.  compression ratio is somewhat counterintuitive (at least for me) because of this very reason.

[Ricky_Racer]

RR, if you try to reduce it by raising the block or head, you're going to have to correct the valve timing and may have to run a worn chain....is it worth it.

Sam.

Good points and good question, Sam.  I'm not overly concerned about the valve timing but... Do you have any idea how much adjustability the chain would have with a manual adjuster?  Do you feel it would be enough to compensate for the taller cylinder? RR

[mwohlenhaus]

If you put the hottest cam you can buy for a sohc motor, the overlap is so great that it actually bleeds off compression in lower rpm's.  one problem among many is you would have to run above 8g rpm's to get alot of power from it. or just run an oil cooler and 100 octane gas.

[Sam Green Racing]

I did it some years ago with what you guys over there know as the CA95 Benly twin.

I bored it out to 175 and used CD175 pistons that had a very high crown.

I took a lot off the top of the pistons but still could not get enough clearance.

I put 2 extra gaskets under the cylinder block and had to use a semi worn cam chain just to get the split link in.

This pulled the timing out by about 2/3 of a tooth. The motor would not rev above 9000 rpm and made 12.5 bhp.

I modified the cam sprocket to realign the timing and bingo....14000.

The motor sounded real nice but wouldn't make more than 13 bhp at 10500 rpm.

The compresion is still to high and needs to be run on methanol,...I'll get round to it one day

Sam.

[Ricky_Racer]

Okay, Guys...  This is turning out to be a PITA...  I only wanted to use these slugs because they're part of a complete NOS Action Fours 836 kit:  pistons, rods, springs, retainers and cam.  It just seemed like a sacrilege to toss in a set of Wiseco's... 

So, is ANYBODY out there successfully running 12:1 on the street???  If so, how are you doing it?  Bear in mind that there's a real serious shortage of 100-102 out here in sunny SoCal!     Thanks.   RR

[Sam Green Racing]

To put it in a nut shell RR, they were not designed for street use or street fuel.

Sam.

[heffay]

ricky... why are your avatar and your username spelled differently... just curious

there has been talk of ricky rockett from poison on here... any relation or want to be relation?

[mlinder]

I don't think 12:1 is really practical on pump gas. 11:1 is pushing it.
Retard timing, run very cold plugs, oil cooler, and don't stay anywhere under 5 or 6 k rpm at any time.
I'd have the pistons milled, personally, or throw in wisecos until you make it into a retro racer where you can get race gas..

[physician]

you can also grind in the combustion chamber. chose where, like unschrouding the valve. you will reduce the chamber volume and the ratio at the same time

[MRieck]

A .060 copper base gasket combined with the .043 head gasket would work OK.