Vizard on 106* LSA vs 112* LSA in a BBC

[Erland Cox]

How do you get 15:1+ compression on a big bore short stroke P/S engine with an LSA under 112 degrees?

Erland

I don't think you'd want to, would you?

Why not?

Erland

[RevTheory]

I guess it depends on what kind of through-flow you have at TDC along with inlet and exhaust tuning pressures. I think we're still painting with a broad brush.

[teamrude]

I have been reading this entire thread for a while. IMO it all revolves around each particular combo and usage. NO exact LSA will always be best even on same size and brand motor due to different needs or usage.

[4banger]

NO exact LSA will always be best even on same size and brand motor due to different needs or usage.

BINGO.....DING DING DING.....WE HAVE A WINNER!!

[buddy rawls]

... NO exact LSA will always be best even on same size and brand motor due to different needs or usage.

Actually, size, brand, needs have very little to do with LSA.
LSA really doesn't do much. Its terms used for manufacturing or checking a camshaft. The valve events, themselves, are what defines the real relationship; the openings and closings and ramp rates.

To me, LSA is not a design criteria, its a calculation from the timing events, and a manufacturing criteria. But I dont want to start that discussion, for fear of missing out on all the conversations, if I cant check the site.

So, I will just simply talk in terms of the relationship of the intake and exhaust lobe (LSA). LSA is the relationship of the intake lobe to the exhaust lobe, and that is not a design criteria, brand criteria, or usage criteria. It is based on the inlet and outlet parameters and their relationship to the displaced cyl volume within the intended rpm criteria.

In general, a restrictive motor (defined as a limited flow capability in comparison to the displaced cylinder volume and very high port velocities) do not require early exhaust valve activity. Therefor LSA will trend narrower. The real answer will be driven by E/I system relationships.

An unrestricted motor (defined as a LESS restrictive flow capability in compariosn to the cylinder volume and decreased port velocities) will trend towards earlier exhaust valve activity. Therefore the LSA will trend wider. Once again, the actual answer is a function of the inlet and exh systems and the cylinder geometry.

A motor that unrestricted in lower rpms, can become restricted at higher rpms.

ITs all relative, narrow for one combination, may be wide for another.

[vortecpro]

Here is the way I see it-- The better the head, the more it will like wide centers. the WEAKER the head, the more it will want tighter centers-- It works this way on every engine I have ever seen

JOWE SHERMAN RACING ENGINES

I have to agree with this statement.

[bigjoe1]

Thanks Mark


JOE SHERMAN RACING

[digger]

wont a bigger cube bottom end on a given head make it "weaker" ?

[hysteric]

In general, a restrictive motor (defined as a limited flow capability in comparison to the displaced cylinder volume and very high port velocities) do not require early exhaust valve activity. Therefor LSA will trend narrower. The real answer will be driven by E/I system relationships.

And:

Here is the way I see it-- The better the head, the more it will like wide centers. the WEAKER the head, the more it will want tighter centers-- It works this way on every engine I have ever seen

JOWE SHERMAN RACING ENGINES

Any chance we can get an explanation on why tighter lobe centres benefit the weaker/flow limited headed engine?

Hysteric

[bigjoe1]

I have been building engines for 60 years now, and every time I try the wider centers , the engine does not produce like I expected. I do not know exactly why, and I can not explain it, but this is what I have seen in hundreds of engines


JOE SHERMAN RACING

[vortecpro]

I have been building engines for 60 years now, and every time I try the wider centers , the engine does not produce like I expected. I do not know exactly why, and I can not explain it, but this is what I have seen in hundreds of engines


JOE SHERMAN RACING

Because your typical build uses conventional heads.

[Erland Cox]

As Buddy said, if you don't get so much in you don't have to let so much out.
If the heads intake side is a restriction and the exhaust is good you want to open the exhaust later.
To much exhaust duration or to early exhaust opening wrecks the power curve below maximum hp.

Erland

[pdq67]

I finally read this whole thread!

Great thread too!

What's the deal that I once read where the roundy-round guys were buying a lot of say 98 to 102 cams and then installing them on say something like 104 to 108??

The author of the article said that nobody was talking about why they did this??

I may be way off on the numbers, but I hope somebody here read the same article and might be able to explain what is going on?

pdq67

[Tom Walker]

I just read through the whole thing also. Interesting too, that the thread has died twice for years only to return to life.

[statsystems]

I just read through the whole thing also. Interesting too, that the thread has died twice for years only to return to life.

That happens. Shit stirrers don't take holidays off.