Exhaust / intake flow ratios on LS heads

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Newold1
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Exhaust / intake flow ratios on LS heads

Post by Newold1 » Mon Jan 09, 2017 6:31 pm

What is the general concensus on why the best flowing LS3, LS7 or LSX factory, CNC, aftermarket or aftermarket CNC heads cannot seem to achieve 70% plus E/I flows even on the bigger bore versions for the 454+ cubic inch LS engines. Obviously not as big a factor on supercharged or turbocharged engines but I wonder hoe a better ratio would benefit the N/A LS engines. ??

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Re: Exhaust / intake flow ratios on LS heads

Post by Warp Speed » Mon Jan 09, 2017 7:39 pm

Most modern, 2 valve na race engines don't have very good "flow ratio" numbers.
Does flow ratio really mean anything?

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Re: Exhaust / intake flow ratios on LS heads

Post by bigjoe1 » Mon Jan 09, 2017 8:41 pm

I have never seen exhaust flow be important



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Re: Exhaust / intake flow ratios on LS heads

Post by randy331 » Mon Jan 09, 2017 9:26 pm

If the engine would run better with a higher ex to in flowz ratio, just lower the intake flowz till it has the flowz ratio you desire, then it will run better.

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Re: Exhaust / intake flow ratios on LS heads

Post by pamotorman » Mon Jan 09, 2017 9:37 pm

there is only 14.7 PSI moving the air into a NA engine but there are 100s of pounds pushing out the exhaust. TF dragsters use it to increase down force and increase top speed depending on which way the exhaust outlet is positioned

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Re: Exhaust / intake flow ratios on LS heads

Post by phat68 » Mon Jan 09, 2017 10:05 pm

Around 60% of the exhaust pressure has bled off before bottom dead center and the piston will force most of whats left out.

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Re: Exhaust / intake flow ratios on LS heads

Post by cgarb » Mon Jan 09, 2017 10:16 pm

I was talking with a Pontiac guy at the strip one day who was pissed off at his head porter because he found an additional 20cfm's on the intake side and couldn't improve the exhaust any. Makes sense to me...lol. I wonder if he would fill the intake ports with expanding foam to choke down the intake if it would go faster? He was complaining the car didn't pick up any and that it was because the exhaust flowed the same. I didn't hear him say he had ported the intake any...maybe there was something to that.

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Re: Exhaust / intake flow ratios on LS heads

Post by Larry Salisbury » Mon Jan 09, 2017 10:29 pm

Most builders are not concerned with E/I ratios unless a tuned Intake/Exhaust system is required. It's only one piece of the puzzle, but an important one. When I order collectors from Jack Burns at least 81% average is desired for a properly designed functioning system. The hard part is to find a head that is at least close to this percentage; otherwise, a very expensive custom port job will be required to produce desirable E/I ratios. I was able to get in the "ballpark" with the Arias LS Hemi Heads when I was building the EFI AH427LSX for Nick Arias a few years back. We got roughly 79% E/I out of the box with the CNC ported versions.
Arias LS Hemi Cyl Head Flow Data.pdf
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Re: Exhaust / intake flow ratios on LS heads

Post by gvx » Mon Jan 09, 2017 10:34 pm

I understand that if the more biased the exhaust ratio is the smaller exhaust duration you can run to get a broader torque curve and a cylinder fill efficiency
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Re: Exhaust / intake flow ratios on LS heads

Post by swampbuggy » Mon Jan 09, 2017 11:14 pm

Larry: You mentioning the word Hemi heads reminds me of my late fathers stockcar from the mid fifties. 1934 Ford Coupe with a 298" Desoto, 3 deuces, straight ALKY, biggest cam Isky had, a wopping .450 lift, Vertex Mag. on 1/4 mile dirt. He always said you dont need to worry about the exhaust side, just work at getting it in. Mark.

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Re: Exhaust / intake flow ratios on LS heads

Post by goldmember63 » Mon Jan 09, 2017 11:32 pm

I'm no expert but getting in in seems the hardest part.

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Re: Exhaust / intake flow ratios on LS heads

Post by bigjoe1 » Mon Jan 09, 2017 11:33 pm

I have always felt the intake was much more important than the exhaust



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Re: Exhaust / intake flow ratios on LS heads

Post by swampbuggy » Mon Jan 09, 2017 11:53 pm

Goldmember63, you are exactly correct, it is the hardest part------------thats why the cylinder head design is SOOOOO important, and the most important single part of the engine or maybe i should say has the greatest effect on making H.P.

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Re: Exhaust / intake flow ratios on LS heads

Post by cpmotors » Tue Jan 10, 2017 12:03 am

randy331 wrote:If the engine would run better with a higher ex to in flowz ratio, just lower the intake flowz till it has the flowz ratio you desire, then it will run better.

Randy
Thanks for the chuckle Randy! Quote of the year! :D
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Re: Exhaust / intake flow ratios on LS heads

Post by digger » Tue Jan 10, 2017 12:47 am

does the in/ex flow ratio have any meaningful bearing on the in/ex duration split/backsplit/no split you might choose?

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