How much dome to effect burn rate/timing?

[The Iron Icon]

Like the title says, how much of a dome can we use until it starts to effect timing and burn of charge? I'd imagine a small dome is not bad but eventually it's got to be diminished returns compression vs timing and burn.

[mag2555]

Nothing is carved in stone because of all the veriables involved, but I have seen a drop in a motors compression from 12.8 to 11 to 1 produce a 75 hp gain, and others have even seen more!

Remember, where not just talking about the rate of burn or how complete the burn is , but the limiting of air flow also!

[PackardV8]

Like the title says, how much of a dome can we use until it starts to effect timing and burn of charge? I'd imagine a small dome is not bad but eventually it's got to be diminished returns compression vs timing and burn.

How high is up? Some assume every non-specific question here is about a SBC; mostly that'd be correct. However, we have a regular member who's about old Triumphs and with their true hemispheric chamber domes are very different from a 23-degree wedge. The bathtub chamber in older Pontiac, Packard, Volvo is a different deal again.

Nothing is carved in stone because of all the veriables involved, but I have seen a drop in a motors compression from 12.8 to 11 to 1 produce a 75 hp gain, and others have even seen more! Remember, where not just talking about the rate of burn or how complete the burn is , but the limiting of air flow also!

Back in the bad old days, we spent money for Jahns forged 13:1 popups for the 301"s and wondered why they caused more problems than power.

[David Redszus]

The combustion chamber shape, when viewed in three dimensions, is a very weird beast.

The upper chamber surface is formed by the cylinder head while the lower surface is
shaped by the piston dome.

Using a molding material, a casting can be made of the entire solid shape to help visualize the
flame front path, niches, and corners of the chamber. And of the squish jet paths.

The actual flame front is not a perfect sphere, but rather an ever changing amorphous,
wrinkled, irregular shape that changes directions and velocity.

Any surface that block the path of the flame from spark plug to distant corners is
not desirable since it will lengthen and delay the completion of burn.

Absolute pressure matters little, but local pressures that affect charge motion are very
important and difficult to understand.

[panic]

Even rod ratio is a variable.

[BILL-C]

It is a mistake to assume all domes act the same in all engine designs. With a little thought and craftsmanship you can make big domes effective. Spark plug location has alot to do with where the point of dimished returns occurs.

[Racing68]

Went from a 44cc JE BB Chevy dome to their 49cc dome on a freshen job and had to add 4 degrees timing to pick up 10 HP. Original timing was a loss of 80 HP.

[rustbucket79]

+1 for spark plug location.

2 565 bbc’s, one with Dart 355’s /custom forged 14:1 domes, you could see the top of the dome through the spark plug hole. 32 to 34 degrees for best power on the dyno.

Other, AFR 357’s/custom forged 14:1 domes, these had a much taller dome, you only saw the face of the dome, 38 degrees to max out power.

406 SBC, Iron Bowtie heads, 2 separate builds, one as a flat top with 11.4:1, other with catalog 14:1 Wiseco 10 cc domes, 34 to 35 degrees for best power for both. Once again, the dome top was below the spark plug holes.

[Truckedup]

Like the title says, how much of a dome can we use until it starts to effect timing and burn of charge? I'd imagine a small dome is not bad but eventually it's got to be diminished returns compression vs timing and burn.

How high is up? Some assume every non-specific question here is about a SBC; mostly that'd be correct. However, we have a regular member who's about old Triumphs and with their true hemispheric chamber domes are very different from a 23-degree wedge. The bathtub chamber in older Pontiac, Packard, Volvo is a different deal again.

Nothing is carved in stone because of all the veriables involved, but I have seen a drop in a motors compression from 12.8 to 11 to 1 produce a 75 hp gain, and others have even seen more! Remember, where not just talking about the rate of burn or how complete the burn is , but the limiting of air flow also!

In the case of the true Hemi vintage Triumph with a side entry plug , I use a flatter top 10.5 piston instead of the competitors usual high dome 12 to one...My heaps go just as fast or faster with less cam timing. There's theories about flame travel interference with a high dome and the dome may have an effect as the cylinder is filled on intake.....And the other variables...dual plugs changes the rules as does a central plug

[panic]

Quick rule of thumb for total spark with added plug: deduct 5 degrees. If either plug fouls the cylinder will run hot.

Bad high static CR scenario: big bore + wide chamber + short stroke + long rod (BBM 383): the piston motion is very small & slow around TDC, any dome at all is a problem (also P-V).

[Truckedup]

The spark retard is variable depending on the engine. Air cooled Hemis can often be retarded as much as 10-15 degrees wth no loss in performance, maybe even slightly better performance. So an engine that needed 93 octane will now run detonation free on 89 or even 87 octane...