How much compression have you guys got away with on pump gas?

[Byots]

I'm in the final stages of an engine assembly where I'm tweaking things to reduce a greater than 12:1 compression ratio resulting from small chamber heads. My pistons are currently flat top, which alot of guys seem to consider optimal for flame travel, etc., so I'm leary of putting too big of a dish in them. I will be ceramic coating the chambers and piston tops. Any "data points" out there for max compression utilized would be appreciated. Thanks

[CSX 4027]

Cut heads are great for compression.
Dished pistons and valves are not the way to go.

If it were me, and I have had both street compression and 12:1 in a Cobra....
Just go the 12:1.
You will be happy.
For the amount of time you will use the car and the fact that you will have a 42 gal tank, don't even thing about pump gas, not worth the issue.

But if you need..
Pump gas
Iron heads 10:1 or less
Aluminum 10.5:1 max

A lot depends on other build issues such as static compression and other things I can not explain.
But, my engine builder partner can if you need.

[Jack21]

First off, you may not want to ceramic coat the pistons or chambers. Aluminum absorbs heat faster than iron which is why you can go up on CR by as much as a point or so. Ceramic coating pistons and chamber retains heat, and in this case would work against you.

Next, dishing pistons OK, except under the quench or flat area in head. Dish, in this case would be a "D" shape in piston.

Open up chambers a little. Unshroud valves and plug. Tough part is getting all 8 chambers the same.

When grinding valves, seat valves a little deeper in head.

Thick head gasket.

Add these small changes up and dial in your CR.

[Excaliber]

12.5 to 1, iron heads, freakin' nightmare on the street. Out with old pistons, in with new dished ones so I can keep my 1964 high riser iron heads (I like 'em). Shooting for 9.5 or 9.8

[Eljaro]

I blew the headgaskets twice with my iron heads with 11.5 compression. Pump gas is all I can get around here, and that at 97 RON with added octane booster. I also blew a lot of money with the two engine repairs needed, because in both cases the metal rim of the gasket found its way to the crankshaft and ruined it.
Now I have a set of DOVE F5 aluminum heads with 9.6 compression and TRW flat pistons. I think that with the better flow and better heat characteristics of the aluminum heads, I did not lose much power with the switch. What I have now is peace of mind, a motor that runs like a clock and a decent idle. 95 unleaded is what I use with no problem.

[jdean]

In order to run 10:8 with premium gas (93), i decked the block so that the pistons are .005 out of the hole. This gives a very tight quench area with flat top pistons, which helps control detonation.

[CSX 4027]

I don't understand blowing head gaskets with 11.5 to one with a good build.

For the .005 out of the hole guy, I also don't agree. I would never have a piston out of the hole because...........

Piston rock will cause an unequal compression in a cylinder and the potential for hitting the valve.

[decooney]

10.8:1, aluminum heads.

[My427stang]

I run 11.5:1 in my heavier Mustang on 89 Octane. The trick is polish everything for no hotspots (valve reliefs, edges of valve, chamber, etc, and in your case I'd probably have the chambers opened slightly, unshoruding the valves for more airflow and even bore matching while cheating a few cc's out.

With a timing curve of a 26 degrees in no sooner that 2700 or so, and an initial of 12-14, you should be fine on 91 pump gas in the lighter car

The last thing is, make sure you are running a cam with some overlap, if you go with a mild cam, it will ping

[Ron61]

I am running 11:1 in both my small block with aluminum heads and my big block with the factory iron heads with no real problems. The aluminum heads dissipate heat much better and I don't have to watch the timing quite so close on hot days. And believe me, I do notice a big difference when I use pump gas and my racing gas.

Ron

[Byots]

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[Byots]

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[jdean]

How can .005 out of the hole cause piston rock? Better check your facts. I would not say this is common practice, but it is certainly not unusual, esp. for racing engines.

Keep in mind that between rods, crankshafts and compression heights there is enough room to combine for a difference of .005 off or more in many engines.

[CSX 4027]

I was trying to say that since there is going to be piston rocking, it would be safer to be .005 in the hole than out.

[Byots]

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[bremillard]

First, let me assure you that .005" out of the deck will not in the least make the piston rock or in any other way harm the performance. There are a lot of motors that blueprint with the piston out of the hole to get factory advertised compression ratio. It will, however exacerbate your problem of too much compression and or cylinder pressure (which is the problem here). A long overlap cam will reduce cylinder pressure. Do not sink the valves into your head as one poster advised as this will negatively affect airflow. Cometic gaskets makes thick head gaskets purposely to reduce compression ratio. I think they have applications for all engine makes. Lastly, if all else fails just retard your ignition timing. This is the easiest way to reduce excess cylinder pressure. -Bob

[Anthony]

Byots, From my reading and understanding, I think the difference you're referring to actually is secondary to the use of closed chamber vs' open chamber heads (read more carefully) , the former having a smaller combustion chamber with better (quicker) combustion (smaller area/volume for the flame of combustion), as opposed to the difference with running either flat top or dished pistons. I think the efficiency is pretty close between flat top and dished pistons, but drops with domed pistons, with the dome impeding flame travel during combustion.

You want a small combustion chamber for good combustion, but yet you also want unshrouding of the valves for high rpm breathing, contradicting aspects somewhat.

At high rpm's, under full load, you may not hear detonation before your engine fails. I'd be careful.

Regarding piston height, there is an optimum quench distance, I can't remember, but I think it is about .030-.040". Knowing your head gasket thickness, you can then determine what piston height you will need to get the targeted quench distance.

[Byots]

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[Byots]

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[Byots]

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