Best way to reduce compression ratio?

-
M

MOPARoldtimer

Banned
Joined
Nov 9, 2012
Messages
263
Reaction score
20
Location
California
I apologize because I know I've told this story a million times. When I built my 408, 93 octane gas was available and I naively thought it would remain available, so I built the engine with 10:5.1 compression. I used forged pistons and had the reciprocating assembly balanced. The pistons are either at zero deck at TDC or maybe a little above, I can't remember. I'm running some ported "J" heads on the engine.

It ran pretty well on the 93 octane, but it detonates like crazy now that I can only get 91 octane, and I am afraid I am going to damage the pistons, so I would like to reduce the compression to 9.5:1 somehow without replacing the pistons.

I'm using a stock Fel-Pro head gasket which is already pretty thick, I believe, so I doubt I'd get much of a C.R. reduction from a thicker gasket, assuming one existed.

Any suggestions for me, or am I "SOL"? Thanks, FABO buddies.
 
Ink, thanks for the info! Not to be an ingrate, but what do you think of those who say that installing a thicker head gasket decreases quench, which only leads to more detonation?

Has anybody reduced their compression this way? Please give the details.

Are there any other cylinder heads that I should consider that might reduce the C.R. while still being efficient?
 
MOPARoldtimer said:
Ink, thanks for the info! Not to be an ingrate, but what do you think of those who say that installing a thicker head gasket decreases quench, which only leads to more detonation?

Has anybody reduced their compression this way? Please give the details.

Are there any other cylinder heads that I should consider that might reduce the C.R. while still being efficient?
Click to expand...

Very good probability....perhaps a cam swap to bleed some of the compression is more in order....
 
You can either try and get a thicker gasket and lose quench or slap on a set of aluminum heads and you won't ping anymore even with you current gasket thickness.
Or you can do what Ink said and swap in a cam to help bleed off compression.
 
A set of heads with 6 more cc's should put you at or just below 9.5. If you know what your combustion chamber volume is on your current heads. Heads are easier to swap than pistons....
 
krazykuda said:
A set of heads with 6 more cc's should put you at or just below 9.5. If you know what your combustion chamber volume is on your current heads. Heads are easier to swap than pistons....
Click to expand...

And alot more $$ than a camshaft...
 
Camshafts don't bleed off compression. You can choose a camshaft with a later intake closing point. That makes the cylinders begin the compression later. That means since the compression is not begun as early, there will be less cylinder pressure. You lose nothing. You simply begin compressing the mixture in a different place. What camshaft do you have now? A cam change would be the easiest and cheapest way.
 
You can blend the chambers a bit, if you're saavy on a carbide bit (round and cone do well, combined).

Lowering compression does help with detonation. Ultimately, changing valve timing, combustion temp, getting good quench and lowering overall cylinder pressure will help.

Aluminum heads help, because they have better heat/ quench properties.

Inkjunkie's suggestion is a good one, too. Retarding the cam or running a camshaft with more overlap will lose it.

Running colder fuel can help (ice box, air gap intake, rerouting fuel lines away from the engine, electric pump, resin bodied carbs drop temp about 20F, etc.)

Retarding the cam's advance cycle can lower compression, because the intake valve closes after the piston is already on it's way up.

I wouldn't suggest retarding ignition timing, as that can cause more heat issues.

All of this is going to cost power, but it can save your engine.

Water injection may allow you to keep the power you have and use it at WOT or under pinging conditions.

If it were me, I'd get another set of heads, keep the quench, go with the same gaskets and dial the cam in nice with a good chain.
 
Before you go tearing stuff apart and throwing money at it, try taking some timing out of it first. A couple degrees off the top might be the easy fix you need. Make note of your current initial and total timing, then go from there.
 
UOP said:
Before you go tearing stuff apart and throwing money at it, try taking some timing out of it first. A couple degrees off the top might fix it.
Click to expand...


Or get some racing gas... :cheers:
 
A lot of good suggestions here I think. If me, I would maybe do what I could to make sure I'm running as cool as I could.....coolant, fuel, intake manifold and engine cold air intake. Then I think I would probably have my distributor closely checked out and then maybe even do a wide-band 02 install to monitor my carb for any lean spots.

But first, I think I'd go out and buy a water/methanol injection setup. I don't see any downside to running it, other than they're about $400 bones. I like compression!
 
Like others have mentioned. Cam with a later intake closing point. Retard the ignition timing a few degrees. Get a wide band O2 sensor kit on the car and get rid of any lean out spots. Try to get the motor to run cooler.
 
Octane boost additives are crooked. They raise Octane rating (the octane rating of fuel is not actual octane content, as it used to be, octane is a component of gasoline, mixed with heptane and ethanol) up a few points. And by points, I mean going from a 91 Octane rating to 91.3... 91.4...

It would take more than an 8-12 oz can to do anything to boost fuel to the levels you are talking about.
 
Another suggestion you may want to give a try;

-cooler temp. rated thermostat
-coolant additive that helps transfer in the radiator with new coolant
-colder rated spark plugs with a couple (literally 2) degrees of timing retardation
-shroud your radiator fan (if you don't already have one) go to a direct drive fan, vs. clutch or higher CFM electric. I would recommend an OE fan that fits your radiator dimensions, over an aftermarket. They just work better, from my experiences. Junkyard and a tape measure can do a lot for you.

One thing that I have seen done is reflective coating on pistons and cylinder head chambers. Even if you were to just polish the cylinder heads' chamber cast surface up a bit with a ball sanding stone bit and some sand paper, you can help heat reflection ( which is also one of the reasons aluminum works well) and reduces carbon build up.

You may also consider some seafoam to decarb. O2 sensors, even if you tune and cap it off, are awesome for a lot of reasons.

If you are running headers, you may consider wrapping them. There is a silicone spray that is on the market, that helps with deflecting oil soaking into the wrap, if you have any leaking issues.

Basically, anything you can do to reduce heat is what you are after. Start cheap.
 
MoPar used to offer a shim @'.060. That'll kill that ratio dead! LOL!

The aluminum head idea is almost good. Here's why it. Won't be.
Chamber sizes available and cost.

Different cam timing is a great idea and a few degrees off the distributor is where you should start first. Between those two you'll get where you need to be I believe.
 
You could always convert to E-85. Don't know if you can get it everywhere. Have to change carb or valve bodies on carb. Have to pump a lot more fuel to it but no more pinging.
 
Look, you're talking about it pinging because of only TWO octane points. It's GOTTA be close to not pinging. Surely maybe you can tune it out.
 
I would think so too, is there a particular rpm or throttle setting/load it pings at? In any case, just because its only 91 v 93, it may actually be a totally different blend of fuel with equivalent octane rating, as Dave alluded to. Octane boost is OK as long as its only to keep it from pinging until you get to a better gas station, but it doesn't raise octane really, it alters the properties of the fuel to resist detonation by chemistry...MMT.

Example: my Mustang ran 9.2:1 compression with a Vortech supercharger...best gas around me was 91 octane, and I could get 12 psi boost with iron GT-40 heads and NO detonation anywhere...but boost wasn't in until above about 2500rpm.

I'd probably back off a couple degrees ignition lead or at least alter the curve to where there's less timing under low rpm/increased load and swap to a colder plug, if that doesn't work, time to play with cam timing or changing heads.

You mentioned running J heads, do they still have the big chamber relief? That's surely doing you no favors for quench.
 
txstang84 said:
I would think so too
Click to expand...

x2

I'd start with a re-jet on an O2 readout and ignition timing a li'l.

If nothing else, richen up the mix and throw a lower thermostat in it, if not some coolant additive as well.

E-30 up to E-95 all have issues with availability and require coated float bowls, gas tanks and replacing rubber seals with synthetic ones to run on a regular basis. It works, but I think it would cause the same issue with not finding 93. supply/ demand issue.

The easiest fix would be premixing fuel with 100+ octane rated, but that can be a hassle for some uses.
 
...if by hassle you mean, extra cost, that's no lie. I don't know what 100+ octane rated unleaded goes for anymore, but I know it's more than pump premium.
 
the difference between 91 & 93 is not that huge... those are all estimated measurements anyways.

second clue is 10:1 is not too high. you should not be "detonating" at all.

my guess is you have an ignition problem. :D

misfiring / backfiring could be caused by several things...
so the real fix is to get the car to someone with the tools to check timing/dwell/etc.

also I like this chart. I know people don't like champion plugs but this is a nice little guide ;) it says carb but it includes ign stuff

[ame]http://www.championparts.com/carb_troubleshooting_guide.pdf[/ame]
 
You must log in or register to reply here.
-

Back
Top