Reduce compression ratio

Good day,
This was my first V8 engine I've built for my AC Cobra, so I'm not an expert. I based my decision from information gathered from forums and articles from the web. If you see anything wrong with my setup please feel free to comment.

I need some advice to decrease the compression ratio. Initially I build the engine for street/strip use but it's such a mission to get race fuel and I'm predominantly using the car as a daily driver.
Below is the specifications:

1. Block with 4.390" bore, 10.322 deck height
2. Flat top Diamond Racing pistons P/N: 40501, 4.390B/3.850S/6.605R/1.765CD/Effective volume -3.25
3. Stock crankshaft
4. Ford Performance Parts Super Cobra Jet Cylinder Heads, M-6049-SCJA, 72cc
5. Hydraulic Flat Cam for Ford 429,460, Xtreme Energy 230/236 - I selected this cam to suit the intake manifold. Low torque.
6. Edelbrock Performer RPM Intake Manifolds 7166
7. Ford Performance Parts Timing Chain Sets M-6268-A46
8. Ford Performance Parts M-6003-A429 - Ford Performance Parts High Performance Engine Gasket Sets, I assumed standard size with a 0.038 compressed thickness
9. Crower Enduro Stainless Steel Stud Mount Roller Rocker Arms 73615-16
10. Holley Ultra Double Pumper Carburetors 0-76750RD
11. 2" Headers, 3'" side pipes

With the above specifications and piston to deck clearance of 0.027, I calculate the compression ratio at 11.446 to 1. For pump gas I assume a ratio of 9.5:1 should be ideal?

With all of that above said, what's the best option to reduce the compression ratio to use pump gas:
1. Cometic has a head gasket that's 0.120" compressed height - MLS Head Gaskets C5666-120. If I use these gaskets won't I have a problem with the intake being too high from the block and possible leak? This option will decrease the ratio to 9.538 to 1.
2. Should I just rebuild the engine with new dish pistons? What can you recommend with the above setup?
3. What other alternatives do I have?

Thank you in advance,
Jurie Venter (South Africa)

The proper way to reduce the static compression ratio is with a piston with an appropriately sized dish or swapping to larger chamber heads.

I figured that will be the best option. I was trying to avoid a full rebuild, Do you have any suggestion for a suitable piston?

Thanks,

Not specifically but you want a piston with a 15cc to 22cc dish that will accommodate SCJ valve angles to get in the neighborhood of 9.5:1. I am sure Diamond or any of the other major manufacturers can help.

You might want to consider having the block cut to reduce deck clearance after you have selected your piston.

You can do better on the cam too.

Your math looks close if not correct. I think there's nothing wrong with using thick Cometic gaskets but I think you can go 10 to 1 on the compression if the octane is 87, similar to fuel in the USA. If you have 93 octane then 10.5 to 1 is best. Either of these will make the head gasket thickness be less.
Other options to help hold off detonation...
Keep the water temp below 170f
A slow timing ramp curve
Keep the timing 2 degree retarded from the optimum

The head gaskets in the M-6003-A429 kit are more like .042" ...not .038"

Jurie.private@gmail.com wrote:Good day,
This was my first V8 engine I've built for my AC Cobra, so I'm not an expert. I based my decision from information gathered from forums and articles from the web. If you see anything wrong with my setup please feel free to comment.

I need some advice to decrease the compression ratio. Initially I build the engine for street/strip use but it's such a mission to get race fuel and I'm predominantly using the car as a daily driver.
Below is the specifications:

1. Block with 4.390" bore, 10.322 deck height
2. Flat top Diamond Racing pistons P/N: 40501, 4.390B/3.850S/6.605R/1.765CD/Effective volume -3.25
3. Stock crankshaft
4. Ford Performance Parts Super Cobra Jet Cylinder Heads, M-6049-SCJA, 72cc
5. Hydraulic Flat Cam for Ford 429,460, Xtreme Energy 230/236 - I selected this cam to suit the intake manifold. Low torque.
6. Edelbrock Performer RPM Intake Manifolds 7166
7. Ford Performance Parts Timing Chain Sets M-6268-A46
8. Ford Performance Parts M-6003-A429 - Ford Performance Parts High Performance Engine Gasket Sets, I assumed standard size with a 0.038 compressed thickness
9. Crower Enduro Stainless Steel Stud Mount Roller Rocker Arms 73615-16
10. Holley Ultra Double Pumper Carburetors 0-76750RD
11. 2" Headers, 3'" side pipes

With the above specifications and piston to deck clearance of 0.027, I calculate the compression ratio at 11.446 to 1. For pump gas I assume a ratio of 9.5:1 should be ideal?

With all of that above said, what's the best option to reduce the compression ratio to use pump gas:
1. Cometic has a head gasket that's 0.120" compressed height - MLS Head Gaskets C5666-120. If I use these gaskets won't I have a problem with the intake being too high from the block and possible leak? This option will decrease the ratio to 9.538 to 1.
2. Should I just rebuild the engine with new dish pistons? What can you recommend with the above setup?
3. What other alternatives do I have?

Thank you in advance,
Jurie Venter (South Africa)

change the head gasket, call up cometic and have them make you one, I wouldn't go any less than 10. to 1

I have done this quite a few times for guys, then years later when they want more power they have went back to a thinner gasket when freshened

easiest way is find e85 and send out your carb. their is also the swapping heads out as the next simplest, but expense, less than new pistons, but still costly. problem with oversized head gasket is you remove your quench area and become more prone to detonation. When switching to mls gaskets the block and heads need to be prepped because the surface finish is different. much finer finish for mls to seal. As for the intake gaskets, can just run double gaskets as needed.

If you have access to 95 octane you're probably ok. 93 octane probably good for 11:1 you're not as far off as you think.

Race gas additive or some other octane booster would do the trick as well.

Jurie.private@gmail.com wrote:Good day,
This was my first V8 engine I've built for my AC Cobra, so I'm not an expert. I based my decision from information gathered from forums and articles from the web. If you see anything wrong with my setup please feel free to comment.

I need some advice to decrease the compression ratio. Initially I build the engine for street/strip use but it's such a mission to get race fuel and I'm predominantly using the car as a daily driver.
Below is the specifications:

1. Block with 4.390" bore, 10.322 deck height
2. Flat top Diamond Racing pistons P/N: 40501, 4.390B/3.850S/6.605R/1.765CD/Effective volume -3.25
3. Stock crankshaft
4. Ford Performance Parts Super Cobra Jet Cylinder Heads, M-6049-SCJA, 72cc
5. Hydraulic Flat Cam for Ford 429,460, Xtreme Energy 230/236 - I selected this cam to suit the intake manifold. Low torque.
6. Edelbrock Performer RPM Intake Manifolds 7166
7. Ford Performance Parts Timing Chain Sets M-6268-A46
8. Ford Performance Parts M-6003-A429 - Ford Performance Parts High Performance Engine Gasket Sets, I assumed standard size with a 0.038 compressed thickness
9. Crower Enduro Stainless Steel Stud Mount Roller Rocker Arms 73615-16
10. Holley Ultra Double Pumper Carburetors 0-76750RD
11. 2" Headers, 3'" side pipes

With the above specifications and piston to deck clearance of 0.027, I calculate the compression ratio at 11.446 to 1. For pump gas I assume a ratio of 9.5:1 should be ideal?

With all of that above said, what's the best option to reduce the compression ratio to use pump gas:
1. Cometic has a head gasket that's 0.120" compressed height - MLS Head Gaskets C5666-120. If I use these gaskets won't I have a problem with the intake being too high from the block and possible leak? This option will decrease the ratio to 9.538 to 1.
2. Should I just rebuild the engine with new dish pistons? What can you recommend with the above setup?
3. What other alternatives do I have?

Thank you in advance,
Jurie Venter (South Africa)

Without changing pistons ... the only way is using thick Cometic head gaskets.
That will present some issues of it's own,(intake manifold fitment and possible detonation), but, that is the only way to arrive at that wanted compression ratio without changing major parts.

I have done the thicker head gaskets over 20 times for people, NO ISSUES EVER. In my own, we have gone way up with timing to test,


We have also sprayed the crap out of a few of these engines, NOTHING, NOT ONE PROBLEM.

BOSS 429 wrote:I have done the thicker head gaskets over 20 times for people, NO ISSUES EVER.  In my own, we have gone way up with timing to test,


We have also sprayed the crap out of a few of these engines, NOTHING, NOT ONE PROBLEM.

Have you gone .080 thicker on the gasket. I have a .059 and saw an .140 gasket to bring it a little nicer level. its a460 head.

litshoot wrote:

BOSS 429 wrote:I have done the thicker head gaskets over 20 times for people, NO ISSUES EVER.  In my own, we have gone way up with timing to test,


We have also sprayed the crap out of a few of these engines, NOTHING, NOT ONE PROBLEM.

Have you gone .080 thicker on the gasket. I have a .059 and saw a .140 gasket to bring it a little nicer level. its a460 head.

2 I have done in the past 3 years have .140 in them. I rem guys using 2 head gaskets per side in the late 1970s, that worked for them. saw them make many passes at the track, and drive them home.

Might have to elongate intakes bolt holes on some engines. people have been doing this for years with no issues.

By my calculator , you are at 11-1 now and a .060 thick gasket will get you to 10.5 and an .080 will get you to 10-1 , "I" would go for the .060 as aluminum heads can tolerate one point more compression than iron heads.

Get proper pistons, thicker head gaskets are a halfass hillbilly fix that make the engine less knock tolerant.

The cam you have is meant to work with lower compression too, cam timing can also help in bleeding off some of the cylinder pressure when done properly.

I've been digging and it seems that these heads are really intended for a stroker engine. So I'm considering rebuilding it with a stroker kit. One thing I'm not clear is what cam and intake manifold to use for a street/strip application. What's the recommendation? Does anyone have a build recommendation they can share with me with performance numbers?

Thank you for everyone's advice and responses.

Good luck with the US elections.

Those heads are fine to use on stock stroke as well. bigger is always better, but your should be able to get easily to around 500 hp and torque with those heads and a 2 plane intake. For street use hit it with e85 as is, or thicker gasket. If you want grab another block(200-400) and then build a stroker for later use at your leisure. That's also a sound route, then when your ready to change it up, can swap over heads and be on the street again quickly. can rebuild your short block for a spare again or sell it

You people are all aware that this is coming from SOUTH AFRICA so, be aware, a lot of things are not just readily available there.