Too Much Compression!


Nov 6, 2006
Outer Banks, NC
You guys have helped me out time and again and could use you expertise again. I have been trying to diagnose and fix a leaning out problem without any luck and decided to pull my heads to see what is going on. I have a 289 bored 0.030 with 351W heads that are stamped C90E G(?) as near as I can tell. I can't find anything with a G ending and I'm assuming must actually be a B or D with 60.4cc chamber volume. The valves are Ford stamped and believe them to be stock. The pistons are dome topped TRW L2249 N 030. When I bought the car in '06, I had the engine rebuilt and was told the block and headed were decked 0.010". I was told that the compression ratio was 10.5:1 but similar set ups seem to be closer to 11.5-12:1 from what I'm reading.

Since the car is a street car that needs to run on 93 octane pump gas, I need to lower the compression. Here is where I need help.

1) What is the highest compression that I can have to run pump gas?
2) Is there a way to bring the compression down using the current pistons? Different heads with larger combustion chambers, head gasket shims, machining domes down? Or am I best to replace pistons with flat tops?

I appreciate any advise, bearing in mind a limited budget.
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Thanks for your advise Horse Sense. I have been talking to a couple of friends who, like you, have far more experience than I do here. One of them, who is a Ford guy, agrees with you that I need to go flat top. The other, who is a Chevy guy but also has Fords and knows how tight my budget is, suggested that I could leave the domes and my heads, assuming they check out ok, and change my cam to one with more duration. Specifically a Comps Big Mutha Thumpa. He lost me on the technical explanation but what I understood was that the longer duration and slower valve opening rates would allow the engine to "bleed off" a little compression and run fine on pump gas. He did say it would make it idle roughly though. What are your thoughts about this suggestion?
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The 351 firing order majes for less stress on the crank for sure. You could drop the compression a good amount by using thicker head gaskets and maybe while the heads are off take some of the valve shrouding off the chambers in the heads, best to have a machine shop do that part though. Even a few cc difference will make a pretty good drop in compression. I'm cheap though and so is my advice... lol
The tighter the quench (that's the piston-to-head distance), the less the chance for detonation to occur at a given compression ratio.

A thicker head gasket is the wrong way to drop compression in your case as it opens up the door for detonation. Lowering the compression by using a dish or flat top piston with a .040-.050" head gasket with the right cam is the proper way to do it without risking a costly motor melt down.
I would rather not have to pull the engine and replace my pistons but it does seem to be the smart approach. If i do get rid of the domed pistons, what should I look for in new pistons if I keep my heads and cam as is? I would like to keep as much performance as I can but easily run on pump gas.
Probably want a flat top on the 289 to keep the compression up with the 351w heads. Its hard to get to much compression with that combo unless you have the pop-ups. Many piston catalogs give an expected compression based upon a certain CC head (and you say yours are 60cc.) I suggest looking at the Probe piston catalog online (or even Summit racing) to see what I am talking about an then look for a comparable piston in your price range.
Speed Pro is OK for a budget build. Hypers are OK too for a mild build without a power adder-just don't run the motor on the extreme lean side. In most cases, 9.5-10:1 will work fine, but remember that cranking compression is also based upon the camshaft events. With careful matching one can go higher (11:1+) in compression with pump gas, but the performance difference will be negligible on the street.

Balancing is normally in the $200 range. Will be more if heavy metal is needed in the crank.
Are you really sure you have a problem with high compression? Have you used a tester to check it? Before you pull the unknown engine apart it might be worth some simple tests.

If you did already pull the heads then I would check out the condition of the bottom end. If it's in good shape I'd consider buying aluminum heads with the right chamber size for the CR you want to run. If you get pistons you need honing, possibly boring, rings, assembly of the pistons to rods, balancing.. not a huge deal but heads are easier to swap out. Those 351W heads are better for holding paper down on your work bench than for making power. I've heard of people running up to 11.5:1 CR with aluminum heads and premium pump gas. 10.25 is pretty safe with the right cam. What you need to determine is the dynamic compression ratio based on the valve events from your cam shaft. You will need the cam card. As alluded to earlier it's also helpful to know the quench distance (pistons want to come up even with the deck at TDC for good quench). If the heads are off you can measure this. If making the most power is important you really want to work out all the details of the engine build before buying anything. There are dynamic CR calculators online.
That would be my preference. Right now the engine is still in the car, I've just removed the intake and heads. Initially, the engine ran real rich and I'm concerned about ring wash out from that. If I could buy aluminum heads that would work with my dome tops, I would then just freshen up the rings and bearings. What combustion chamber would alum heads need to run pump gas with the domed pistons and my .512/292 cam? They would also need to clear the pistons.
In order to get the right chamber to match the pistons you will need to determine the part number of the pistons. I would expect if you don't have the information it may be located on the underside of the pistons (sorry, not easy) so you would have to pull the oil pan to check. Otherwise, possibly someone who knows much more than I may be able to tell you what pistons you have from a photo. You could try here, or your machinist might know.

I understand too much fuel can wash out the oil and cause excessive wear. If you can catch your fingernail on the lip of the cylinder bore at the highest point where the rings reach, you might consider going the piston/machining, etc. route. The top of the bore is the highest wear area.
The current pistons are TRW L2249N 030s.

A friend of mine has a set of heads he used on a similar 289 build with the exact pistons. They are from an '86 5.0 T-bird. They have been exhaust ported, have hardened valve seats, good HP guide seals, Crane springs and retainers, have Crane roller rockers and have been decked 0.010". He says they will clear the piston domes, work with my cam and drop my compression to 9.5-9.8. Less than 2500 miles for $400. Sounds good if they truly will help the engine run well but still pondering if I'm better to bite the bullet and switch to flat top pistons and keep my heads.