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Old 02-02-2013, 01:42 AM
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Check my math, Searching for airflow needs

Ok as some of you know I'm building a 350 sbc. Going to be over boring it to 4.03. I called around to summit, brodix, howards, local machine shops.

Long story short was suggested the ik180's by brodix and now am worried. Here are my parts since It would take a while to type it out. First post.
: 355 build check up

Anyways trying to make sure I'm doing everything right and I decided to look at it from a math perspective since it seems to make more sense for me. While these guys are professionals and I trust their input I need to know the why's etc. So suggestions are appreciated especially answers to the questions I'm looking for.

Anyways on to the math. Trying ot make sure I'm doing this right.

First volume swept using the Pi*Radius^2*height(used stroke?) (3.14159x2.015^2*3.484)
Volume swept 44.4404164 Cubic Inches
Converted to Cubic Feet
0.02571783 Cubic feet

One cubic inch =.0005787037 (7037 repeats)

Engine Math for .03 overbore 350 SBC
since only one out of every revolutions of the crank will be the intake
Swept volume/2 (
0.01285891679491172395506460070265 cfm per Engine RPM per cylinder

12.858916794911723955064600702654 CFM per 1000 rpm per cylinder

51.435667179646895820258402810616 CFM per 1000 rpm Per deck

102.87133435929379164051680562123 CFM per 1000 rpm For engine

So basically you see at 6000 rpm you wind up needing about 600+ cfm of flow for the engine or
668.66367333540964566335923653801 CFM for 6500 RPM. Well this helps with carb size at least I think :V.

Notes: Use of static dimensions. Not accounting for Air/Fuel Inertia or other variables. Like I said Just trying to get a grasp on this. This is the best way for me to attempt that and get more in depth and make the best decision I potentially can.
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Old 02-02-2013, 03:02 AM
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I like the rule of thumb on whether to use static or dynamic formulas! I use both but to each his own!

Dynamic is a constant it never changes the formula works on the notion that there is absolutely no compression till the valves are completely seated!

but I don't believe there is no compression in the cylinders until both valves are seated!!

I believe in using static compress in that on the upstroke as valves are almost closed there is some compression in the cylinders and the closer they get to seating, the less air can pass through the smaller the gap gets creating some more compression! The faster the rpm the more it builds compression as the valves are almost closed!!! And scavenging with headers also helps pack the
cylinder and ram effect adds more

Static compression is always changing with RPM! and mods! dynamic doesn't change at all through the whole RPM range!

Its True that The only way to accurately maximize compression ratio is to obtain dynamic cylinder pressure. Unfortunately, this is not very practical because the engine must be assembled and the rings should be seated to limit blow by. any discrepancy in cylinder sealing will effect the DCR .The reason different company's or manufacturers DCR tables have different totals for DCR is some try to include cylinder loss in their formulas some figure in piston expansion, and dome rise as cylinders heat up! Kb pistons don't expand the same as TRW ETC, and cast, forged and hyp. pistons don't have the same either!

So I go by "rule of thumb" static compression numbers are used!

I don't know who came up with the rule of thumb?
That's how I leaned and have always used it! and its always worked for me.

I'm not telling any one their wrong I'm just saying do it the way your comfortable with. and the way you learned I do use DCR in some instances and SCR in others! I use one formula for choosing cams and one for the octane I need ETC,ETC.

Only my view: Jester
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Old 02-02-2013, 05:13 AM
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Volumetric efficiency has to be multiplied to the equations to get a good estimation.

Your engines are not the perfect heat engines like the Carnot heat cycle

in thermodynamics ,energy going in can be changed but some of the heat (energy)will be lost.


bore x bore x stroke x .5678 x 8(number of cylinders) =displacment.

Take into account your head flow for the cfm of the carb also.

I have an auto engineering book with all the info in it it gets interesting.
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Old 02-02-2013, 09:51 AM
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I posted this this morning and see I have a couple of spelling errors LOL Correcting now



Quote:
Originally Posted by painted jester View Post
I like the rule of thumb on whether to use static or dynamic formulas! I use both but to each his own!

Dynamic C.R. is a constant it never changes the formula works on the notion that there is absolutely no compression till the valves are completely seated!

but I don't believe there is no compression in the cylinders until both valves are seated!!

I believe in using Static C.S. in that on the upstroke as valves are almost closed there is some compression in the cylinders and the closer they get to seating, the less air can pass through the smaller gap creating some compression! The faster the rpm the more it builds compression as the valves are almost closed!!! And scavenging with headers also helps pack the
cylinder and ram effect adds more

Static compression is always changing with RPM! and mods! Dynamic C.R. doesn't change at all through the whole RPM range!

Its True that The only way to accurately maximize compression ratio is to obtain dynamic cylinder pressure. But that's Maximum under perfect conditions! Unfortunately, this is not very practical because the engine must be assembled and the rings should be seated to limit blow by. any discrepancy in cylinder sealing will effect the DCR .The reason different company's or manufacturers DCR tables have different totals for DCR some tables and charts try to include cylinder loss in their formulas some figure in piston expansion, and dome rise as cylinders heat up! Kb pistons don't expand the same as TRW ETC, and cast, forged and hyp. pistons don't have the same either!

So I go by "rule of thumb" "Static compression numbers are used"!

I don't know who came up with the rule of thumb?
That's how I leaned and have always used it! and its always worked for me.

I'm not telling any one their wrong I'm just saying do it the way your comfortable with. and the way you learned, I do use DCR in some instances and SCR in others! I use one formula for choosing cams and one for the octane I need ETC,ETC.

Only my view: Jester
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Old 02-02-2013, 09:58 AM
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Your math is full of holes and faulty assumptions that make it pretty irrelevant, its a good attempt to understand things but a oversimplification.

If your sbc is peaking at 6500 rpm first of all you'll need a little bigger head, 200cc's or so, and secondly you should be around 500hp which will need a 750-850 carb. This can vary a lot according to who's carb you use as none of them flow as advertised and while most are under a few are slightly over.

For a 6000 RPM peak a 180cc runner is just about right (I'd prefer 190ish but its pretty close) and you'll probably be able to use a 750cfm carb pretty effectively.

A 600cfm carb is good for a very mild or near stock engine.
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Old 02-02-2013, 10:25 AM
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Why didn't you keep this on your other thread you had very good answers and are now on a new thread leading into the same answers! AP72 is giving you good info you already have been told on your other thread???? Have this joined on the other thread!!!

Jester
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Old 02-02-2013, 12:37 PM
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Thanks guys thats actually what I was looking for. The suggestion for the 180's is just what made me worried. I also did read through my previous thread and didn't see anyone saying that the ik200's were too big but I didn't ask as well.

Also yes it is an extreme over simplification. I wouldn't know where to start other than right there and work my way up :/.

short block is in the shop, the rest is just me waiting on that getting done and money. I'm reading as much as I can and trying to make sure I am making as close to the best decisions possible. I really appreciate the answers and corrections thanks!
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Old 02-02-2013, 04:52 PM
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Jester
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