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Re: Carb Identification Help
Posted: Wed Aug 20, 2014 5:47 pm
by Johnny Roadkill
haha, it's fun to nerd it up occasionally, for me anyway !
fwiw, I highly recommend HP books "How to Hot Rod your Datsun" series of publications. particularly the original one that deals with Datsun 510s and 240Zs. There's a chapter by Racer Brown which is purportedly about camshafts, but the guy can't help himself and explains the complete workings of the internal combustion engine...and I mean all of it ! I learnt a lot from that book, and especially that chapter - but I've had to reread it countless times to get my head around most of it, there's still some sentences I read but just don't take in.
A copy of that chapter can be found here :
http://www.datsport.com/racer-brown.html
To keep this loosely on-topic, somewhere in the books I mentioned the methodology for exceeding 100% VE ( within a very narrow rpm band ) is explained - it's to do with the pulsing nature of the airflow in an engine.
cheers,
D
Re: Carb Identification Help
Posted: Wed Aug 20, 2014 6:16 pm
by ylopez
moore_rb wrote:A very simplified CFM equation (that does not take air density or temperature into account) :
((RPM x Cubic inch Engine Displacement) / 3456) * VE (volumetric efficiency*) = CFM (air flow) that the engine will pull at that particular RPM
So, for the Patrol: 4.0 liter = 244 cubic inches, and so the formula becomes:
(4000 max RPM x 244 cubic inches) / 3456 = 282 MAXIMUM CFM that the Patrol engine could pull when it is running at 4000 RPM
* Now, more about that VE adjustment - the result above demonstrates what the Patrol engine
could achieve if the VE = 100%. But, no internal combustion engine can generate 100% VE - high RPM Formula One race cars can actaully EXCEED 100%, but since the Nissan P engine is a low compression, low RPM "undersquare" engine (meaning the stroke is much larger than the bore), my guess is that itprobably generates about 75% VE at best.
so, we take the max theoretical CFM calculated above (282) x .75 VE =
212 CFM as a real world example of what the Patrol engine can pull when it is screaming at its 4000 RPM redline
So, a carb that flows at least 212CFM is mandatory- anything smaller would hurt performance.
Going with a carb that is bigger than 200CFM would not hurt, but it also won't help - if you put a 700CFM carb on an engine that can only pull 250 CFM, then the carb is only a 250CFM carb by default
WOW
clear and clear... more simple impossible
Thanks all of you
Re: Carb Identification Help
Posted: Wed Aug 20, 2014 11:43 pm
by faux40
moore_rb wrote:A very simplified CFM equation (that does not take air density or temperature into account) :
((RPM x Cubic inch Engine Displacement) / 3456) * VE (volumetric efficiency*) = CFM (air flow) that the engine will pull at that particular RPM
So, for the Patrol: 4.0 liter = 244 cubic inches, and so the formula becomes:
(4000 max RPM x 244 cubic inches) / 3456 = 282 MAXIMUM CFM that the Patrol engine could pull when it is running at 4000 RPM
* Now, more about that VE adjustment - the result above demonstrates what the Patrol engine
could achieve if the VE = 100%. But, no internal combustion engine can generate 100% VE - high RPM Formula One race cars can actaully EXCEED 100%, but since the Nissan P engine is a low compression, low RPM "undersquare" engine (meaning the stroke is much larger than the bore), my guess is that itprobably generates about 75% VE at best.
so, we take the max theoretical CFM calculated above (282) x .75 VE =
212 CFM as a real world example of what the Patrol engine can pull when it is screaming at its 4000 RPM redline
So, a carb that flows at least 212CFM is mandatory- anything smaller would hurt performance.
Going with a carb that is bigger than 200CFM would not hurt, but it also won't help - if you put a 700CFM carb on an engine that can only pull 250 CFM, then the carb is only a 250CFM carb by default
I love applied math! This is cool! Thanks.
John
Re: Carb Identification Help
Posted: Thu Aug 21, 2014 9:28 am
by moore_rb
Johnny Roadkill wrote:
somewhere in the books I mentioned the methodology for exceeding 100% VE ( within a very narrow rpm band ) is explained - it's to do with the pulsing nature of the airflow in an engine.
cheers,
D
Yes, achieving 100%+ VE is definitely possible - long duration cams, and turbos both tend to really boost volumetric efficiency, especially in higher RPM bands, as do "ram air" style air intake systems where vehicle speed actually contributes to static air flow into the induction system.
but, I'd be willing to wager that the P engine is never going to join the 100% VE club
Now one thing that I am guessing will certainly improve the P engine's VE are headers/extractors- The scavenging effect of extractors usually helps boost VE to a small degree
You guys are dragging me back into my custom engine and race car days...
Re: Carb Identification Help
Posted: Thu Aug 21, 2014 10:30 am
by jwgreen
moore_rb wrote:Johnny Roadkill wrote:
somewhere in the books I mentioned the methodology for exceeding 100% VE ( within a very narrow rpm band ) is explained - it's to do with the pulsing nature of the airflow in an engine.
cheers,
D
Now one thing that I am guessing will certainly improve the P engine's VE are headers/extractors- The scavenging effect of extractors usually helps boost VE to a small degree
I happen to know a couple folks that have these!!
Re: Carb Identification Help
Posted: Thu Aug 21, 2014 10:37 am
by RiverPatrol
Being arithmaphobic myself, I find the formulas difficult to follow. That's just me. I get the gist of it though, and it's remarkable stuff.
I can add that headers do indeed improve the performance of the Patrol and/or P engine. My Oz pickup was the first Patrol I ever drove that had headers, and I have been very impressed with it's highway abilities. It also has an electronic ignition and a Stromberg carb, which help to enhance the performance. Sorry, I am not able to provide you all with formulas explaining precisely why the Stromberg, electronic ignition and headers all improve the performance, you'll just have to take my word for it as one who has driven many different Patrols.
This is getting off topic,
but I wonder what kind of effort would be required to get someone to produce a few sets of headers. I know one member recently went through some pain bringing a set over form Oz. I have a spare set that could be used as a template. The one's on the Oz pickup are water cooled, the other set isn't.
Re: Carb Identification Help
Posted: Thu Aug 21, 2014 9:08 pm
by darren
interesting reading even though maths was not my strong point i also get the jist just i also have headers on my shorty but are getting a little old small cracks appearing and such a muffler bloke in adelaide quoted me about 800.00 AUD to custom up a set but found recently another forum member got some from oz from a place called mufflers to the max in queensland i did some more research and found the manufacturer called mercury mufflers in NSW who sell them for about 400 a set part No HU071ST for the P40 motor not sure of postage to us but may be cheaper than having a set made
Re: Carb Identification Help
Posted: Thu Aug 21, 2014 10:31 pm
by RiverPatrol
Good info!
