What a difference 93 to 110 octane

[Marquis_Rex]

Al, never run methanol in your car. That crap will poison you, and eat every component it touches. Use high-octane gasoline, or E-85. If we are lucky, more E-85 will become available soon, nationwide. Marquis, it sounds like you have a fun ride, too. I am a fan of high cylinder pressures, and compression is our friend. It has taken the manufacturers a long time to grasp this particular concept, and it seems to have taken the performance aftermarket even longer to get on board. Guys, it is also important to remember that aluminum heads aren't as thermally efficient as iron, and thus require at least an additional point of compression to run with comparable iron head engines.

Thanks mate.
I've worked in engine Ressearch and Development for Jaguar, Aston Martin and for a consultancy in Germany. You're spot on when it comes to the thermal efficiency comments.
The biggest issues our old RBs and LA block engines have compared to modern engines is the offset spark plug- which is worse for knock limit, combined with massive bores-which increases flame paths (again worse for knock limit). On the other hand, I've done alot of flow bench testing and looked at burn rates of various engines and have found modern pent roof 4 valve chambers particularly poor when it comes to in cylinder motion: Swirl and tumble, where as our Wedge chambers have quite good swirl and alot of squish- this is good for knock limit and also for lean running at part load without running into misfire.
As regards our Mopar Hemi engine- this design isnt the best in terms of motion and probably worse than a Wedge (this is why Hemis typically need more all out ignition advance than the Wedge-due to the slower burn- resulting from less motion. Thats why Im interested-at least from an accademic perspective- into the D5 twin plug- that should quicken up the burn and allow a more relilient combustion process.

[The Cuda Guy]

Thanks mate.
I've worked in engine Ressearch and Development for Jaguar, Aston Martin and for a consultancy in Germany. You're spot on when it comes to the thermal efficiency comments.
The biggest issues our old RBs and LA block engines have compared to modern engines is the offset spark plug- which is worse for knock limit, combined with massive bores-which increases flame paths (again worse for knock limit). On the other hand, I've done alot of flow bench testing and looked at burn rates of various engines and have found modern pent roof 4 valve chambers particularly poor when it comes to in cylinder motion: Swirl and tumble, where as our Wedge chambers have quite good swirl and alot of squish- this is good for knock limit and also for lean running at part load without running into misfire.
As regards our Mopar Hemi engine- this design isnt the best in terms of motion and probably worse than a Wedge (this is why Hemis typically need more all out ignition advance than the Wedge-due to the slower burn- resulting from less motion. Thats why Im interested-at least from an accademic perspective- into the D5 twin plug- that should quicken up the burn and allow a more relilient combustion process.

What?   

Don

[NoMope Greg]

What?  

Don

In a nutshell:

Modern multi-valve engines have the spark plugs in or near the center of the head, so the flame front spreads out evenly, like the Hemi.  To demonstrate, take a bucket of water and drop a pebble into the middle and watch the ripples spread out from the center.  Older wedges (all V8's except the Hemi) have the spark plug off to one side so that there's room for the big valves needed to get decent flow in and out of the cylinder.  This puts the flame off to one side and makes it travel all the way across the cylinder to reach the mixture on the other side, which takes a long time.  Same bucket, but this time drop your pebble off to one side.  As with a bomb, the further you get from the ignition source, the less power the "blast" has.  In this case, it's not an explosion in the cylinder (unless you have detonation, aka "knock"), it's ideally the flame front of an even burn, albeit one that happens very rapidly.

The disadvantage with the modern heads and Hemis is that the mixture doesn't swirl around inside the combustion chamber as much as it does in the Wedge head.  More swirl and squish promotes more efficient burning of the mixture when the engine is running lean.  To make up for it, the Hemi has to run more ignition advance (start the mixture burning sooner). 

In addition, this is where modern computer-controlled electronics - both ignition and fuel injection - help; the computer can control the mixture and ignition points much more efficiently under all conditions than older mechanical systems.   Mechanical systems usually have to be optimized for one condition or another.  Want all out power?  Emissions, fuel economy, idle quality and part-throttle driveability suffers.  Want to be more usable in every day driving - gotta give up some power.  That's why we now have a 425 hp 6.1L Hemi that's far more livable  than the old 426 ever was and gives twice the gas mileage.

Rex, is that about right?

[Changin Gears]

1 gallon of leaded race gas in a tank of 93 goes a long way to raising octane.  Race gas has a lot of lead and a little goes a long way.

[Supercuda]

Greg, bravo for putting that into English for the masses. I understood Marquis just fine, but the regular people often wind up scratching their heads over this kind of stuff. Excellent analogy with the pebble into the water. A partial solution to the lack of turbulence, and the attendant poor mixture distribution in the chamber of a hemispherical combustion chamber, is building a piston and a combustion chamber, to provide a quench area around the perimeter of the chamber. It provides some turbulence, and also gives some detonation protection. The Ford Escort 1.9L engine was a good example of this kind of design. Several times, it has been tried with the traditional 426 Hemi chambers, with varying degrees of success. I want to do a radical Hemi engine for high-rpm, but I like my wedge just fine for the power I want throughout the rev range.

[Marquis_Rex]

In a nutshell:

Modern multi-valve engines have the spark plugs in or near the center of the head, so the flame front spreads out evenly, like the Hemi.  To demonstrate, take a bucket of water and drop a pebble into the middle of the glass and watch the ripples spread out from the center.  Older wedges (all V8's except the Hemi) have the spark plug off to one side so that there's room for the big valves needed to get decent flow in and out of the cylinder.  This puts the flame off to one side and makes it travel all the way across the cylinder to reach the mixture on the other side, which takes a long time.  Same bucket, but this time drop your pebble off to one side.  As with a bomb, the further you get from the ignition source, the less power the "blast" has.  In this case, it's not an explosion in the cylinder (unless you have detonation, aka "knock"), it's ideally a the flame front of an even burn, albeit one that happens very rapidly.

The disadvantage with the modern heads and Hemis is that the mixture doesn't swirl around inside the combustion chamber as much as it does in the Wedge head.  More swirl and squish promotes more efficient burning of the mixture when the engine is running lean.  To make up for it, the Hemi has to run more ignition advance (start the mixture burning sooner). 

In addition, this is where modern computer-controlled electronics - both ignition and fuel injection - help; the computer can control the mixture and ignition points much more efficiently under all conditions than older mechanical systems.   Mechanical systems usually have to be optimized for one condition or another.  Want all out power?  Emissions, fuel economy, idle quality and part-throttle driveability suffers.  Want to be more usable in every day driving - gotta give up some power.  That's why we now have a 425 hp 6.1L Hemi that's far more livable  than the old 426 ever was and gives twice the gas mileage.

Rex, is that about right?

Absolutely spot on!
Amazes me the number of knowledgeable people on this board, I mean even in terms of engine theory!
I'm willing to hazard a guess that a few people from Michigan (perhaps former or current employees within the car industry) are present-especially seeing as how you visit Michigan you see how many Muscle cars (esp Mopars) come out the woodwork come summer time
This is TOTALLY different to my usual automotive forums haunts- The Porsche forums, Pistonheads and others and a real pleasure to visit!
I will say this though, regarding Hemis IMO, BMW made the land mark hemi engine in the 1960s and 1970s, thanks mainly to a very gifted engineer called Apfelbeck. he offset the valves slightly to endow the BMW Hemis with alot of swirl motion along with a shallow valve angle (BMW m20=22 degrees, BMW M30/M10=23 degrees)-which gives a very low surface area to volume ratio. My heavily modifed BMW M20 only needs about 28-29 degrees all out ignition advance. Compare this to a traditional hemi, such as used in old Jaguar XK engines, Dodge 426s and old style Porsche 911s. Ford mimicked BMWs good design sense with the 1.9 litre in this country (America) and the 1.6 to 1.8 litre CVH in Europe (CVH=compound Valve Hemi) but perhaps went Toward TOO much motion which lead to a harsh combustion process. As with most things, its a fine balancing act.

[NoMope Greg]

Supercuda, Rex, thanks.  I'm no engineer, just a motorhead bus driver who reads a lot and has pretty decent command of the language.  I took a few engineering courses in college 25 years ago and I train bus drivers, including teaching them about the vehicle, so the language and concepts weren't foreign to me.  I don't really know anything about engine design, beyond what I've read in magazines and on the Internet, so I was a little nervous about posting that, but figured I understood Rex's explanation well enough to offer a "translation."   

[71chally416]

Did somebody on here say that the LA Wedge head DOESN'T have a good spark plug location??? If that's true, then why did GM adopt the same position in the combustion chambers for it "Revolutionary" LS motors?? The newer Magnum might have a more compact closed chamber, but the basic plug location and valve placement is right from the 1960's 273. The motor the 273 replaced was the Poly 318, which has a valve and chamber layout that bears more of a resemblance to a modern ProStock head valve layout than anything else from it's era. 

[NoMope Greg]

Ummm...I don't think anyone said the plug location in those engines wasn't good.  My understanding of Rex's post is that, because of the limitations inherent in the head design, the plug isn't in the best place for flame travel and overall burn of the mixture.  It's not as ideal as the centrally-located plug of a Hemi or pent-roof multi-valve chamber.   The LA engine may have the best plug location for a wedge headed engine, maybe better than an RB, SBC or SBF (I honestly don't know), but not as good as a Hemi and certainly not as good as a four-valve engine.

I know nothing about current NHRA Pro-Stock rules and very little about engine design, so I respect your superior knowledge  (honest), but I imagine one of the reasons they use that head is because the rules dictate a two-valve "wedge" type head.  If there were an advantage to that type of head and valve layout versus a Hemi or pent-roof design, I'm sure they'd be running them in Top Fuel and all cars would continue to use that head.  Certainly, the manufacturers wouldn't go to the added complexity and expense of overhead cams and all that extra valvetrain gear if it didn't offer some added benefit.

[71chally416]

The LA motor has had the best plug location for a wedge type head for over 4 decades. While the 1st gen SBC used over 40* of advance in their race motors the LA had no more than the 426 Hemi. 

Here's a Alan Johnson Pro stock head compared to a 1960's 318 poly head. See any similarity in the valve placement???