Larry's Soft Head

[lil289]

Larry, Others,

I'm trying to understand the combustion characteristics of the soft head. In this link http://www.theoldone.com/articles/Hot_R ... Head_3.jpg the article mentions the following:

"Because the mixture is more homogenous, it gives a faster burn that lasts longer - the ignited mixture expands more slowly":?:

How is it that the mixture burns faster yet expands slower?

[SWB]

A little contradictory yes,

You need to read the part where Larry states that he is spreading the event out over a longer number of crank degrees than "standard". Remember that typically as the compression goes up, the engine starts to make peak cylinder pressure sooner in the cycle than with a lower compression engine. What Widmer is claiming, is that by increasing the burn rate, the ignition lead can be reduced, thus allowing the peak pressure point to happen later in the cycle, closer to peak piston velocity and therefor the point of maximum leverage on the crank throw.

Remember also that leaner mixtures tend to burn more slowly than do richer ones, I think there has to be some kind of offset between the very high compression and the leaner, more burnable mixture.

Maybe Larry can chime in and set me straight?

The talking heads article says a lot if you read it objectively and look between the lines...

SWB

[Keith Morganstein]

Larry, Others,

I'm trying to understand the combustion characteristics of the soft head. In this link http://www.theoldone.com/articles/Hot_R ... Head_3.jpg the article mentions the following:

"Because the mixture is more homogenous, it gives a faster burn that lasts longer - the ignited mixture expands more slowly":?:

How is it that the mixture burns faster yet expands slower?

wish I could read the article, but that .jpeg is impossible for me. Do have a better link??

[Gary Blair]

http://www.theoldone.com/articles/ I think the info is here. The 1986 Hot Rod article.

[Cobra]

Do you want to weigh in on this Larry?

[kid vishus]

I hope he does chime in; I am running a set of 4 barrel clevaland heads that look just like the chamber pics in that article.

[Justin Jones]

I emailed him a link to this discussion...

[Larry Widmer]

I've been in GrandAm head "heaven" for the last couple weeks, so spare time's been non-existant.

Try reading this...it might answer a few questions: http://theoldone.com/articles/The_Soft_Head_1999/

I should have some spare time in another week or so and I'll get back to you.
Larry

[DavidNJ]

Reading Larry's Soft Head article, it seems that the suggested combustion chamber for a 2v head would have the piston dome within .100" or so of the closed intake valve at TDC, the spark plug (or plugs over the exhaust valve, and an approximately spherical shape around the exhaust valve and plug. Intake and to a lesser extent cam timing would be adjusted to allow that combustion chamber. The resulting improvements in combustion and the timing of combustion versus connecting rod geometry would result increased performance and efficency.

It seems that the piston, cam, and chamber modifications necessary to achieve this would be managable by many (most? all?) of the engine builders on this forum. Has anyone tried it? Is my understanding correct?

[lil289]

It seems that the piston, cam, and chamber modifications necessary to achieve this would be managable by many (most? all?) of the engine builders on this forum. Has anyone tried it? Is my understanding correct?

Well...custom pistons with the ideal dome shape on the intake side to match the chamber seems like it might be beyond the average engine builder. I was wondering if there was a way to come close to this without having to make custom pistons. It may not be as effective but if it's better than the usual, hey... In the article, it shows how Larry tilted the floor of the intake ports to induce some swirl - I think before the mixture was released into the cylinder. It mentions that the mixing motion helps to keep the AF mixture homogenized. As the mixture flows past the valve and enters tangentially into the cylinder it continues to swirl as usual down the bore. In the other link Larry posted to above he mentions that they later learned that the mixture was not really homogenized but rather layered with rich on top, lean on bottom. Perhaps these benefits can be had regardless of dome configuration. I see what he is doing with the one sided dome, it squeezes the mixture into a very small space around the exhaust valve and spark plug, allowing for a very quick burn.

Here's the weird part. I was wondering if instead of using that custom piston, one could try that infamous groove that we keep hearing about. You know, Somender Singh's groove. Some of the guys have done some testing on it and I think it's fair to say that the groove is causing a faster burn? Or is causing fuel wet-out? Air changes direction immediately but the fuel just keeps going straight and slams into walls and such? I'm just trying to figure this stuff out.

By the way, which way should the floor of the intake port be tilted? Towards the center of the bore or toward the cylinder wall? How far in should you go? Start from the intake flange and taper down to nothing at the short turn apex?

[Ape]

Well great that larry finally joined in on the thread.
Maybe (hopefuly pretty pretty pleaaazze) he could also elaborate a lil on michael mays fast burn techniques, as he seemed to have some insight in his research, and its almost impoosible to get a hold of Prof. may.

cheers

[automotive breath]

Using adobe Photoshop I was able to enlarge the magazine article big enough to read it. I haven’t had time to read it but an interesting graph on the last page of the Hot Rod article displays cylinder pressure generated by a typical head and the soft head. Interesting stuff.

The cylinder pressure vs. crank rotation shows the advantage of the soft head over a typical cylinder head. The soft head yields more cylinder pressure during the time of maximum piston velocity on the power stroke.

Larry, Do you mind if I post clips from the article?

[Ape]

Quote:

The cylinder pressure vs. crank rotation shows the advantage of the soft head over a typical cylinder head. The soft head yields more cylinder pressure during the time of maximum piston velocity on the power stroke.



Thats also pretty much what michael may discovered during his research whom widmer also "correctly" cites in his articles. Unfortunately there is hardly anything to be found from his R&D work.

cheers

[SWB]

By the way, which way should the floor of the intake port be tilted? Towards the center of the bore or toward the cylinder wall? How far in should you go? Start from the intake flange and taper down to nothing at the short turn apex?

Canting the floors is an attempt to equalize port wall lengths or at least set them more to your liking. Remember that air always tries to take the shortest route. If you cant the short side turn you'll effectively be changing where that shortest route now occurs and therefore where the flow goes. Keep in mind also, your short side turn velocities, etc. Where Larry talks about the jet fighter thing, that's also relevant to the topic. Look at the pictures of the ford heads (super swirl) and you'll see where he added the bump on the short side turn...What do you think that does? (see above)

SWB

[automotive breath]

The entire time I read the Hot Rod Soft Head article I picture the late 90’s GM LT1 head. It’s obvious the designer followed the same concept. The LT1 engine is a very impressive performer in street cars as well as at the race track.

Reading the article generates many questions, one being the ability too run ultra lean air/fuel mixtures (18:1). Is this typical in today's race engines?

…racing engines may run 12:1 or 12.5:1 air/fuel ratio and see excessive exhaust temperatures if leaned down to 15:1. Larry’s engines can run up to 18:1 air/fuel ratios and record significantly lower exhaust temperatures....