Interesting thead for all you engine builders out there

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Charlie Boy

Well-Known Forum User
Humm most of this stuff is on my engine just that my exhaust manifold is to small at the moment 42.5mm hence only 322bhp just need to change it over to 45mm :thumbs:
 
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SKiddell

Well-Known Forum User
Not meaning to be a pedant :thumbs: but
Bouncing hp figures around is worthless without some context of the torque, power profile and purpose or the engine as HP is just a function of torque x RPM so if you can make power higher up the rev range for a set amount of torque then you will have a higher resultant BHP........ but still the same torque....makes more sense when you look at an F1 car making 750HP at 19000 rpm but their torque figure (rarely discussed) appear low in comparison (sub 300 ft/lbs)

Also for a better "measure" of an engine, look at a number called BMEP or Brake Mean Effective Pressure.
This is a "reasonable" but well respected method of quantifying torque and BHP numbers of similarly aspirated engines into some form of reality...it works on the premise that a given engine configuration has a finite ability to produce power (can't go on forever it doesnt work like that).......and a BMEP of 212 psi is F1/Nascar territory, 200psi is considered best in class for most proffesional engine builders and Ferrari and Porche top out at around 205psi for their top of the range multivalve variable whatnot exotica so if an engine appears to yield greater than that then why isnt the builder working for Ferrari or Porche.

Now don't take my word for it as Sean knows I just make it up as I go along. Go do some research on BMEP, port, valve and stroke ratios, mach numbers and volumetric efficiency etc, it will change the way you think your engine works.

Finally consider the power band and compression ratios that super high hp engines work within and ask "do I really want to drive a car that needs 4000rpm just to get off at the traffic lights"

And Charlie, not wishing to upset any apple carts:thumbs: but on back to back testing on very well respected rolling road dynos. I have put larger exhaust manifolds on (that were identical in all other respects) and ......LOST power:eek:, due to loss of gas speed and lack of cam overlap killing the scavenging process, meaning that if the engine doesnt need it then it wont use it, not saying that it will happen, you just can't take it as a given.

Anyway, just my 2 cents worth
 
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SeanDezart

SeanDezart

Well-Known Forum User
I'm lost - I even tried to use an internet conversion page and didn't twig it.......

How to convert both ways from French cv to hp to bhp to ps (which I think is the same as cv.....) ?

And the typical % loss from flywheel bhp to rw bhp ? Alright, I'll be specifique, new 200SX box, R200 Nismo plate-type LSD but I'd like to know for an average Z in good nick please ?
 
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SKiddell

Well-Known Forum User
18% is a good starting point to go from flywheel to RWHP, going the other way of course its greater, around 22%.

Transmission and parasitic losses are a source of great debate, you often get quotes such as "it can be that much, otherwise the engine would melt due to heat" or "my gearbox oil would boil"

Every time the power has to go through a gear ratio or get modified from a straight line force (piston) to a rotational force (crank) or something rubs against something else then power is lost due to friction.
Most of the losses are at the tyres themselves, due to deformation, ancillary components such as pumps, alternators etc all drag precious power from the engine.

So if your running smaller width tyres, straight cut gears, light oil, no alternator etc your "parasitic" losses will be less than a car running wide tyres, a poorly balanced engine and a dry diff.

One of the dichotomies I see on the 1/4 mile is that you need low tyre pressures to get launch grip but this leads to rolling resistance increase at higher speed :confused:

A much overlooked source of loss is balance and not just engine balance, combustion chanber balance is just as important, why spend hundreds blue printing the bottom end when you have CC variation in the head which leads to burn speed and pressure variations in each cylinder.

Anyway back to BMEP..... the engine sanity checker, here is usefull site,
Brake Mean Effective Pressure (BMEP): The Performance Yardstick
mainly an aero engine site but the basic principles of the internal combustion engine is the same, good quote is
For a long-life, naturally-aspirated, gasoline-fueled, two-valve-per-cylinder, pushrod engine, a BMEP over 200 PSI is difficult to achieve and requires a serious development program and very specialized components.
Click to expand...
, OK so we don't use a pushrod in the L series but it matters yeah not, a BMEP above 200 is possible but is directly proportional to the size of the owners wallet and that needs to be big (see the F1 data on the page).
 
grolls

grolls

Well-Known Forum User
SKiddell said:
Anyway back to BMEP..... the engine sanity checker, here is usefull site,
Brake Mean Effective Pressure (BMEP): The Performance Yardstick
mainly an aero engine site but the basic principles of the internal combustion engine is the same, good quote is , OK so we don't use a pushrod in the L series but it matters yeah not, a BMEP above 200 is possible but is directly proportional to the size of the owners wallet and that needs to be big (see the F1 data on the page).
Click to expand...



Skiddy's been eating peanuts again....:rolleyes: great article btw...;)
 
SeanDezart

SeanDezart

Well-Known Forum User
Er, yeah, right - thanks for that. Still doesn't answer my question :

How to convert both ways from French cv to hp to bhp to ps (which I think is the same as cv.....) ?

I'm looking through loads of old test reports of Zs and also comparisons to other cars from various countries and I'd like to do a table of comparative power (torque too would be nice) so lb-ft and nm and kg-m comparisons too ................... please, anybody ?
 
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SKiddell

Well-Known Forum User
I am sure there is a way to compensate for forced induction on BMEP, I will have to look at my automaths book a little closer:unsure:

Simon you must run your car up the strip, if only to prompt the lesser spotted long haired Ellicot to come out or to spank the young RB engined upstarts backside
 
C

Charlie Boy

Well-Known Forum User
SKiddell said:
Also for a better "measure" of an engine, look at a number called BMEP or Brake Mean Effective Pressure.
This is a "reasonable" but well respected method of quantifying torque and BHP numbers of similarly aspirated engines into some form of reality...it works on the premise that a given engine configuration has a finite ability to produce power (can't go on forever it doesnt work like that).......and a BMEP of 212 psi is F1/Nascar territory, 200psi is considered best in class for most proffesional engine builders and Ferrari and Porche top out at around 205psi for their top of the range multivalve variable whatnot exotica so if an engine appears to yield greater than that then why isnt the builder working for Ferrari or Porche.

And Charlie, not wishing to upset any apple carts:thumbs: but on back to back testing on very well respected rolling road dynos. I have put larger exhaust manifolds on (that were identical in all other respects) and ......LOST power:eek:, due to loss of gas speed and lack of cam overlap killing the scavenging process, meaning that if the engine doesnt need it then it wont use it, not saying that it will happen, you just can't take it as a given.

Anyway, just my 2 cents worth
Click to expand...

150.8x260/189=207psi:thumbs::thumbs::thumbs::thumbs:

Sorry I know dyno figures can never be trusted and most over read to make the customer happy but I don't have any other figures to go with at this time.

Re manifold 45mm really is about right for my engine 48mm would be over the top and is mostly for drag cars, 42mm is ideal for 2.8L which was what I was building until I got carried away and went 3.1L. I would ideally go with a YS Garage manifold ysgarage but ££££ so looking to get something in the uk.

C
 
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SKiddell

Well-Known Forum User
Personally I think your bang on with the 45 mm Charlie (1" 3/4 to us oldies) Its what I use.....the volume of air your moving is significant, my caution is over how sensitive these L6's are to cam overlap lift and exhaust gas speed (Wako 75S and some Kamerai cams have narrowed lobe centres and large lift on overlap maximising scavenging)

Talk to Mr Jarman (DJ road and race), top bloke, knows these engines better than anyone and "may" be able to help you on the exhaust front;)
 
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