exhaust length calculation

Any form of normally aspirated power mods.
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350matt
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exhaust length calculation

Post by 350matt » Fri Sep 22, 2017 8:35 pm

Wotcha

so with the recent Dyno Might results that show PeteH's manifold to be the tweak of the week I thought I'd open the discussion
I've attached a spreadsheet with a rough calc for exhaust length and the key part of the calc is the exhaust cam timing BTDC so has anyone got any figure for an the exhaust port timing ?

I've found a PDF that looks quite official and that states its 50 degrees BBDC?

http://www.rx8help.com/doc/RenesisTechnical.pdf
so this comes out at a manifold length of 508mm? which seems a bit short

I open to the floor
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exhaust_length.xls
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warpc0il
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Re: exhaust length calculation

Post by warpc0il » Fri Sep 22, 2017 9:33 pm

Trying to tune the exhaust length, just to hit the highest reading on a dyno sounds like a hiding to nothing, or did I miss something?

Phil had a much better idea in tuning the exhaust length to remove the resonant drone when driving on the road.
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350matt
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Re: exhaust length calculation

Post by 350matt » Fri Sep 22, 2017 9:39 pm

Well the standard manifold is a not a tuned device at all so usually the gains won't just be at peak power you'll add area under the curve throughout, you may lose out here and there with some negative tuning effects but overall a tuned length is a desirable thing
you don't tend to see them on modern cars as its all gone a bit cheating air movers with log manifolds and water cooled manifolds built into the head

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Re: exhaust length calculation

Post by Goobie » Fri Sep 22, 2017 9:46 pm


350matt
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Re: exhaust length calculation

Post by 350matt » Fri Sep 22, 2017 10:26 pm

has anyone on this side of the pond invested in one of these? And verified the result?

plus thats more than I paid for the car!

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Re: exhaust length calculation

Post by PeteH » Fri Sep 22, 2017 10:40 pm

We have a member who has one. You can do a search. IIRC he loves the car, but certainly couldn't verify the ridiculous BHR claim when it went on the dyno.

The BHR is badly flawed in at least one important respect.

You are right that port timing is important. The standard port timings are well known. It didn't take much effort to find them. They are listed in "Street Rotary" for instance.

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Re: exhaust length calculation

Post by Goobie » Fri Sep 22, 2017 10:44 pm

Around 5bhp gain a member on here got on RR hub dyno.

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Re: exhaust length calculation

Post by PeteH » Fri Sep 22, 2017 10:49 pm

Dave, I beg to differ. I aimed for a peak between 6.5k and 9k. I was hoping to see "something" on the dyno. I was surprised by the level of the change, but it's there on record now.

If someone wanted to do a road version then I think it should be possible to get close to the same gains.

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