Hemi engine information. Trying not to get screwed by a shop

Squirrely · May 15, 2012

Does the modern hemi still use a hemispherical head?

BlackZ71Silverado · May 15, 2012

well i dont think these have ever been done and theres like 120k on the truck

They probably weren't done since he doesn't know how many spark plugs his truck has.

@ jro909,

The second plug in the Hemi actually fires after initial combustion to ignite the unburned fuel. Makes for a cleaner burn that way.

Daryl Z71 · May 15, 2012

well i dont think these have ever been done and theres like 120k on the truck

They probably weren't done since he doesn't know how many spark plugs his truck has.

@ jro909,

The second plug in the Hemi actually fires after initial combustion to ignite the unburned fuel. Makes for a cleaner burn that way.

Im probably wrong here, but isn't this kind of the same principle with Direct Injection? I know it's set up different, but kind of sounds like there going for the same thing? Maybe not though Im not sure?...

Chris · May 15, 2012

He didnt have it done. He was unsure and wanted me to find out for him. So I was hoping you guys would know. Thanks a lot for the info

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MikeNH · May 15, 2012

Does the modern hemi still use a hemispherical head?

Sort of. Nowhere near the the original but close-ish. There is a flattened out area in the combustion chamber that improves emissions so the chamber is no longer hemispherical.

Prokopchuk_9 · May 15, 2012

Yes they have two plugs per cylinder. I beleive it an emissions thing, don't really know. Helped change them on a buddies truck. Major PITA...

truckofwinandawesome · May 20, 2012

well i dont think these have ever been done and theres like 120k on the truck

They probably weren't done since he doesn't know how many spark plugs his truck has.

@ jro909,

The second plug in the Hemi actually fires after initial combustion to ignite the unburned fuel. Makes for a cleaner burn that way.

yep,its called a Wasted Spark Ignition System, my Plymouth had a Wasted Spark System, due to the Construction of the Coil Pack, where for example, if Cylinder 1 is fired, Cylinder 4 is also Fired because they share windings.

Doug_Scott · May 20, 2012

well i dont think these have ever been done and theres like 120k on the truck

They probably weren't done since he doesn't know how many spark plugs his truck has.

@ jro909,

The second plug in the Hemi actually fires after initial combustion to ignite the unburned fuel. Makes for a cleaner burn that way.

yep,its called a Wasted Spark Ignition System, my Plymouth had a Wasted Spark System, due to the Construction of the Coil Pack, where for example, if Cylinder 1 is fired, Cylinder 4 is also Fired because they share windings.

Chrysler is doing something a bit more involved than that. They basically have one plug per cylinder that fires at x degrees BTDC, they have a second plug that fires part way through the power stroke.

Engines have pairs of cylinders that have their pistons in the exact same location at all times, generally you can take the firing order and split it in half, with the second half of the order pairing up with the first half of the order. So, 18436572 firing order would have cylinders 1 and 6, 8 and 5, 4 and 7, 3 and 2 being pairs of cylinders. On the wasted spark system, those are the cylinder pairs that fire at the same time, one of power stroke, the other at the end of exhaust stoke (likely in valve overlap as well)

The Chrysler system is more complicated, In this example, the first plug is the one that was always there, and the second plug is the additional plug they have used.

The coil for cylinder 1 fires the first plug in cylinder 1 and fires the second plug in cylinder 2, then cylinder 8 coil fires the first plug in cylinder 8 and will also fire cylinder 1 second plug. There are 8 coils, firing 16 plugs, in a cross firing arrangement.

Nitro375 · June 8, 2012

The "Hemi" does indeed have two per cylinder. They should have informed him before doing the work of any cost differences and gotten an ok... or a no.

Chris · June 8, 2012

they did. but it did not sound righ thats why i asked

Nitro375 · June 12, 2012

Does the modern hemi still use a hemispherical head?

Sort of. Nowhere near the the original but close-ish. There is a flattened out area in the combustion chamber that improves emissions so the chamber is no longer hemispherical.

Yep, more of what they call a "Pent Roof" design. Honestly though so many engines have a "Hemipheriscal" combustion chamber anymore its really not all that big a deal. Motorcycles including Harley Davidsons have it, most import engines etc. It's mainly marketing the Legend of the "Hemi" more than anything. Good motor, my buddy's runs plenty strong, but its not as rare and unique as when it came out in the 50's.