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Minimum Cross Sectional Area: Question

General engine tech -- Drag Racing to Circle Track

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MadBill
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Re: Minimum Cross Sectional Area: Question

Post by MadBill »

raynorshine wrote: Sun Feb 09, 2020 10:47 pm...
-my basic physics/ math sez it's hard to build compression when the valves are open, off the seats... :lol:
BUT, with a well-matched cam in its power band, that's what's happening all the way to IVC, which might be 90° ABDC.
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Re: Minimum Cross Sectional Area: Question

Post by Stan Weiss »

Torque Master - hydraulic cam duration @ 0.006" - solid cam duration @ 0.020"

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Re: Minimum Cross Sectional Area: Question

Post by gmrocket »

raynorshine wrote: Sun Feb 09, 2020 10:47 pm
GARY C wrote: Sun Feb 09, 2020 9:00 pm Another thought on duration, compression and rpm is that Pro Stock now uses 10 + less total degrees of duration but turns more rpm than what they were doing in the past.
-my basic physics/ math sez it's hard to build compression when the valves are open, off the seats... :lol:
apparently with less compression you need to keep them open longer/close later to build TQ?

if the comp ratio was raised even more on this build, does the cam get into the 215 at .050" range? im still not understanding it
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

Stan Weiss wrote: Mon Feb 10, 2020 12:47 am Torque Master - hydraulic cam duration @ 0.006" - solid cam duration @ 0.020"

Stan
Thanks.
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Re: Minimum Cross Sectional Area: Question

Post by gmrocket »

skinny z wrote: Mon Feb 10, 2020 12:12 pm
Stan Weiss wrote: Mon Feb 10, 2020 12:47 am Torque Master - hydraulic cam duration @ 0.006" - solid cam duration @ 0.020"

Stan
Thanks.
Did you already post the spread sheet of the program recommending a shorter cam when you upped the comp ratio?

Would like to see that
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

gmrocket wrote: Mon Feb 10, 2020 5:48 pm Did you already post the spread sheet of the program recommending a shorter cam when you upped the comp ratio?
Would like to see that
Yes. Back on page 4.
There are four screen shots. Each compression ratio (9.8 and 10.4) gets either a 5500 RPM limit or a max available port CFM of 250. 5500 RPM was selected as it appears to be the max RPM achievable with the given MCA and CID.
With those constraints and given the rest of the data I can see why the program does what it does. It's not a simple comparison as it would be if you were simply trying to spec duration relative to the CR with heads that can meet the demand (CID and RPM). The 250 CFM limit really skews the result. As does the peak HP RPM limit.
We all know, that with enough breathing capacity (as it isn't in this case) and no specified max RPM the higher CR always gets the bigger cam (or almost always as I don't wish to make a blanket statement like that and then hear about it later!).
I'm not arguing that point.
I think the program illustrates that if there isn't enough available flow, then the higher CR isn't needed or wanted. Just like you wouldn't take your 350 with stock heads and expect 12.5:1 and a 300 degree cam to do anything special.
Similarly, if the RPM ceiling is low because the heads can't supply the CID at higher RPM, the same thing holds true.
So what I see is the small heads, bigger cubes prefer the lower compression ratio so as to work with the cam spec'd.
Simple as that.
Does this not sound reasonable?
Other than that I can't think of why the program does what it does.
I will say that I'm liking the looks of 9.8:1. a 5500 RPM peak and the 282/286, 106/102 suggestion.
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

Now I see where the misinterpretation is.
If you raise the CR and expect a cam spec to follow what we've all come to expect, you have to raise the target peak RPM also. As I said before, keeping the RPM target the same and raising the CR doesn't make sense. So then, neither do the results.
The same holds true with peak port flow. If you're at the limit, then cam and compression aren't going to have the same effect.
I've inquired about this with the developers but I think that explains it.
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Re: Minimum Cross Sectional Area: Question

Post by Orr89rocz »

skinny z wrote: Mon Feb 10, 2020 9:28 pm Now I see where the misinterpretation is.
If you raise the CR and expect a cam spec to follow what we've all come to expect, you have to raise the target peak RPM also. As I said before, keeping the RPM target the same and raising the CR doesn't make sense. So then, neither do the results.
The same holds true with peak port flow. If you're at the limit, then cam and compression aren't going to have the same effect.
I've inquired about this with the developers but I think that explains it.
I just dont see how compression correlates with more rpm?
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

Orr89rocz wrote: Tue Feb 11, 2020 7:41 am I just dont see how compression correlates with more rpm?
Doesn't more compression call for more duration? I think that's the whole point of a comment posted earlier.
Put in that kind of context then doesn't more duration move the peak HP RPM up? Pretty sure that's the thinking being thrown about here. Following that, when using the program and the peak HP RPM is fixed (at 5500 for example) and add more compression then there's a conflict as the added duration wants more RPM to deliver the goods. It can't (program-wise) so there's this conflict and results are what you see.
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Re: Minimum Cross Sectional Area: Question

Post by CGT »

skinny z wrote: Tue Feb 11, 2020 9:42 am Doesn't more compression call for more duration?
Not really...or necessarily unless chasing a DCR that makes you feel good, or in reverse..trying to offset low end losses from the extra duration with compression.
skinny z wrote: Tue Feb 11, 2020 9:42 am Put in that kind of context then doesn't more duration move the peak HP RPM up?
It could, but it also could not.

What is the relevance of intake duration to these small changes in static compression ratio when talking about WOT?. Doesn't make a lot of sense. Compression vs Exhaust duration could be a valid discussion, but I don't see exhaust duration being discussed, although exhaust duration also would be very unlikely to get smaller with an increase in compression.
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

CGT wrote: Tue Feb 11, 2020 10:11 am
skinny z wrote: Tue Feb 11, 2020 9:42 am Doesn't more compression call for more duration?
Not really...or necessarily unless chasing a DCR that makes you feel good, or in reverse..trying to offset low end losses from the extra duration with compression.
skinny z wrote: Tue Feb 11, 2020 9:42 am Put in that kind of context then doesn't more duration move the peak HP RPM up?
It could, but it also could not.

What is the relevance of intake duration to these small changes in static compression ratio when talking about WOT?. Doesn't make a lot of sense. Compression vs Exhaust duration could be a valid discussion, but I don't see exhaust duration being discussed, although exhaust duration also would be very unlikely to get smaller with an increase in compression.

The original comment regarding CR vs duration was posed by another poster. I was at a loss to describe how the program was realizing the results it does. (See the 4 screenshots on page four of this thread).
It seems though you have a better insight than I do (I'm strictly an old timer enthusiast) and perhaps your replies to my questions above provide the real answers.
Not sure what to make of it really although when all of the dots line up in the data points in TM , that is to say a compression ratio, port flow and RPM ceiling are all within what one would typically build, then the results IMHO are something worth considering.
And yes, all WOT (for the purposes of making these conversations less complex) .
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Re: Minimum Cross Sectional Area: Question

Post by SpeierRacingHeads »

IMO, the key isn't a cylinder head or a manifold, it's the camshaft.

A Super Stock 358 with a 1.6in² pinch peaks at 7400.
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Re: Minimum Cross Sectional Area: Question

Post by Orr89rocz »

SpeierRacingHeads wrote: Tue Feb 11, 2020 10:40 am IMO, the key isn't a cylinder head or a manifold, it's the camshaft.

A Super Stock 358 with a 1.6in² pinch peaks at 7400.
Right, but that valvetrain isnt gonna last on the street tho right? Or can something work to achieve both the rpm and the longevity of parts?

I always thought what they do out of stock heads was impressive but never would last long. Aka square lobe cams type stuff
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Re: Minimum Cross Sectional Area: Question

Post by SpeierRacingHeads »

Orr89rocz wrote: Tue Feb 11, 2020 11:18 am
SpeierRacingHeads wrote: Tue Feb 11, 2020 10:40 am IMO, the key isn't a cylinder head or a manifold, it's the camshaft.

A Super Stock 358 with a 1.6in² pinch peaks at 7400.
Right, but that valvetrain isnt gonna last on the street tho right? Or can something work to achieve both the rpm and the longevity of parts?

I always thought what they do out of stock heads was impressive but never would last long. Aka square lobe cams type stuff
The Super Stock stuff isn't square, but the Stocker stuff is. The Super Stocker runs lots of duration and lift for sure.
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Re: Minimum Cross Sectional Area: Question

Post by skinny z »

SpeierRacingHeads wrote: Tue Feb 11, 2020 10:40 am IMO, the key isn't a cylinder head or a manifold, it's the camshaft.
A Super Stock 358 with a 1.6in² pinch peaks at 7400.
An interesting fact and right on topic. For the record, what kind of CR does a Super Stock engine have? The 1.6in² would reflect the "stock" nature of the heads correct?
I'd to be interested to see how this program would respond to the input of an SS engine. I'll have to research the details. (Love the Stockers by the way. Well worth the price of admission.)
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