POST ENDED & GONE

 

 

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I look at the data and the first thing that jumps out is the CFM vs the cu in displacement? Because CFM calculated based on displacement, shouldn't their be conversion here? inches to feet or vice versa/ I can see how CFM and RPM can relate, cancel the M's and you get CF per Revolution. What am I missing?

Looking at the numbers, it would then be no surprise that a typical MAF sensor could feed a large engine at reasonable revs. Since even a 427 at 6500 rpm can only suck in less than 804cfm at any given moment.

"at any given moment."
Is that what this chart represents? I'm trying to also understand how to determine the maximum amount of air an engine needs available.

 

Here is a guess on my part...no facts to back it up, just a lot of reading. The induction and exhaust system must be viewed as a whole. That is, you can "cam up", bigger throttle body, etc, til the cows come home, but if the additional intake capacity cannot get out of the engine through the exhaust system, all that happens is we create a sluggish engine because we lost intake flow velocity (compare a big diameter water hose with a small one, both with the same volume of water). Flow velocity is a big deal because it tends to "pack" the cylinder on the intake cycle and "evacuate" the combustion chamber during the exhaust cycle. Larger diameters on both the intake and exhaust slow down the flow and so these two elements get compromised if they are not all in sync.

Same on the exhaust side:, if induction is restricted somewhere (throttle body, intake runners, or cylinder heads), putting on headers does not change performance by much. The induction capacity simply max's out and there are no further gains. So a 58mm TB without upgrading the rest of the system does not change much and maybe can even become a negative. The induction flow will be restricted by its biggest bottle neck.

But I am in over my head when we start throwing CFM numbers around. NO facts to back up opinions.

While I am at it, I'd like to know what you guys think about putting ported AFR 195's (flow about the same as AFR 210's, unported) on a 415-427 engine. Assume LPE 219 cam and SR induction. Are these heads too small to produce 500 HP at 5,500-6,000 rpm's? I want to build a big inch/low RPM/hi-torque/smooth running engine for the street. I am not trying to steal this thread, it sort of fits the general question.

 

 

That table presents CFM of air & fuel required for each engine listed vs RPM of the engine.

This table shows that the faster an engine is turning the more charge is required per minute and that a larger displacement engine needs more than a smaller one. Since air flow is the paremeter of interest here, CFM is the proper unit.

There is an adage in engine building that 'bigger is not necessarily better". This is because air flow velocity is very important in a high performance street/strip engine. If the air passeges are too large, velocity will be too low at low RPM and the engine will not have good low end torque nor performance.

So, for a street engine, we must size intake & exhaust runner passages so as to yield high flow velocity. This means designing ports with minimum volume but high CFM numbers.

For example, if your MAF is sized too big for the engine displacement, at the hit of the throttle the enigne will likely lag or bog due to low air flow velocity. So we recommend not going bigger than a 52mm TB on a nearly stock C4 engine.

So, as in other aspects of life, with engine building size does matter but it is not necessarily the most important aspect in assuring a satisfactory result.

 

 

The table you're looking at is the end result of the calculations, I omitted the actual calculations where the conversions are done. I have the spreadsheet if you would like, it just takes your bore & stroke in inches and converts it over to feet, then converts the cubic foot displacement before it calculates anything.

For display purposes on that graph I left the displacements labeled as cubic inches because that's what everybody knows. If I said a .2 cubic foot displacement would anybody really know what it meant?

Also understand that these values are based on 100% efficiency which we all know is not the case.

 

There are so many variables in this that its nearly pointless for me or anyone else to attempt a good explanation.

CFM only means the amount of flow that can come in, under STP laboratory conditions, which is only a good approximation to real conditions. The engine is still bottlenecked by whichever component flows the worst, but the port velocity changes when going to a part with a huge opening are detrimental to power at a given rpm. Say, for example, putting on a 58mm would lower the port velocity at 1000rpm compared to the stocker, and that will drop the low-end torque some...but its not eliminating that power, its just moving the power to a higher rpm range, so the power you had at 1000, would be at 1500...for example.

I'm sure there are books on the subject up the wazoo, but most builders just play with stuff until they find characteristics they want from a motor. Few people outside of GM Powertrain even know the equations that go into all of it.

 

One variable that always seems to be ignored in stating CFM figures is "at what pressure/vacuum"? (just like in a garden hose, the more pressure - the faster the fluid goes through even with a same-size hose)
Something the table leaves out is "at what throttle opening?"
i.e. 500rpm; is that at idle or under a load at WOT?
same for all the rpm points....under full load or not? It does make a difference.
Don't take anything as the truth, the whole truth, and nothing but the truth.
Larry
code5coupe

 

This is a good table to compare throttle bodies vs. engine capacities, and maybe get a satisfactory answer to what should work.

What is the flow rate in CFM of the 48mm throttle body? Isn't the 52mm TB equivalent to a 750CFM?

The reason I ask is, my engine was falling off at 6200 rpm, 350 cu in with the heads and exhaust flowing well. Is the 48mm throttle body flowing only something like 625 CFM (or less)? That would explain what I see with my engine.

 

A stock 48mm L98 TB flows 668cfm (Re. TPIS "Insider Hints") and the 52mm TB flows 750cfm (Re. BBK spec sheet), the same as a gutted MAF.

 

 

That table is meant to represent the absolute maximum possible. Pure and simple it is calculating the physical volume required to completely fill the cylinder, hence 100% efficiency. I think you may be trying to read too much into what it is trying to say because it's somewhat taken out of context.

I originally posted that table in a thread on header size. The statement was made that "1 5/8 headers are adequate for a 383 because people have run bigger engines on 1 5/8 headers without a performance loss." I created that table to illustrate the point that such a statement can be dangerously misleading. The larger displacement engines that the person was referring to make are operated in the sub 6000 RPM range. Meanwhile a 350 or 383 can easily be made to operate above the 6000 RPM range. I made the table to illustrate the point that a smaller displacement engine operated at a higher RPM can actually flow more air than a larger displacement engine operated at a higher RPM.

In this kind of comparison, pressure doesn't matter, think of it as a non-compressable fluid instead of air. The table simply measures what is the maximum volume that can be processed through a given displacement engine per unit time.

 

Thanks for clearing up the units, I knew there had to be something, it's not like you post a chart with such a gross error, I just didn't know how you got around it. Thanks for sparing us the gory details.

And yes, the chart does need to be kept in context. I think does show well how to roughly calculate what the max cfm your engine could use given size and where you want to make your power.

People ask alot about what TB size and I think this is a good demonstration as to why chriswtx's 377 needs a bigger TB than 85vet's 383. That is not to say who would be faster.

need to change a diaper....hold please...

 

that was fun....

As far as flow velocity, I have been looking at this for a while and going to AFR's site, Motown, DART, and Brodix I have found that Brodix M2 227 have probably the highest flow per cc. Now, they max out lower than others and even lower than some of their own other models, but those are heads with 245+ cc runners.

I think it's also a helpful chart to determine at least what will or won't work. I'm in the planning stages of a 427 and one concern is the TB, I have a 58 do I need to budget for a bigger? The answer actually lies in where I want to make the power.

PS I hope you don't catch a bunch of flack for the chart, it really is meant not to be more than what it is, a calculation of max possible meant to illustrate the how the air needs of diff't engines compare at diff't rpm.

 

That chart, which has grown, is what it is. The only flak I'll take from it will be from people who don't read enough to understand what it is trying to say.

The numbers are facts, you cannot argue them without looking like an idiot (unless of course I made a compuational error.) As far as what it really means for selecting engine components, it may well be useless. That's a debate I'm gonna stay away from!

 

Nathan, your chart is absolutely correct......I compliment you on creating a very useful reference.....

forgive me for hijacking it from the other post, but I intended it to be a FACTUAL reference base for a discussion of how to interpret the CFM ratings of various devices and how to select the proper size....you should be flattered I hijacked it....

One thing it is NOT & that is useless.......

 

I thought meant that the member was hanging out and watching with interest...? I was!

I think vader86 is right about it being too complex to determine what a change in CFM rating to one component of a system will have on overall output. You'd have to have very accurate structural models of each component.

And sometimes you have some surprising results.

On the 48mm vs. 52mm TB issue...there's a series of LT1 dyno results in the back of HPBooks' "Small Block Chevy Performance" that shows progressive modification of an LT1. On pg 190, Test L reflects a 48mm to 52mm TB change. Added 12 HP, and 6 ft/lbs torque. peak...but what is interesting is that even at 3000 rpm, the 52mm was producing an extra 2hp and 3 ft lbs. over the 48mm.

We're not remotely approaching the max CFM for a 48mm at 3000rpm on a 350, so this would seem to indicate that some (most? all?) components introduce flow resistance well before their max, and going a little bigger is justified.