Cams vs. bore/stroke inquiry

[Mikester]

Hey all,

I have sort of a physics question:

I have a pretty good general working knowledge of my SR, but I am wondering if increasing cam size and increasing bore/stroke can yield similar results.

To the best of my understanding, the following is generally true (please correct me if I am wrong):

Increase in bore/stroke = more air/fuel = more power thru entire rpm range

Increase in cam lift/duration = more air/fuel = more power thru specific rpm range

For instance, my motor is currently at the builder. I elected to stay w/stock bore/stroke (honed to 86.5mm), and went with 260/12mm Procams... What would the difference have been if I had stuck to stock cam lift/duration but increased bore/stroke?

- If it matters, I have a full compliment of supporting mods, and a reputable tuner; which I also understand the importance of. To make it easier, let's say hypothectically that my turbine choice was the 2871, .86 a/r (not my actual turbine, but one I'm sure a lot of you have experience with).

Thanks in advance.

--Mike

[army240]

Here's a thread with a lot of information :

http://zilvia.net/f/engine-tech/308456-my-02-cams-boosted-motors.html

Have fun reading!

Frank

[Mikester]

Thanks for the link man- interesting read for sure! I am even more happy that my choice of cams was a good one for the setup I decided to go with... But I still wonder what the disparity in the power curves would be say between a bore/stroke-up SR22 w/256 cams and a standard bore SR20 w/260's on the same turbocharger/boost setting. If I understood that thread correctly, the end result would be similar, but there would be more "usable" power throughout rpm- most notably the mid-high range on the SR22. Yea?

[jr_ss]

A larger bore/stroke increases your overall power potential. Cams do the same, however they shift your powerband to the right on graph. Having more cubic inches will help to spool the turbo, where the cams allows you to make more peak power and a larger/broader curve.

[Kingtal0n]

Hey all,
(please correct me if I am wrong):

Increase in bore/stroke = more air/fuel = more power thru entire rpm range


Increase bore/stroke means an increase in displacement... and thats it. You might get more fuel into the engine... if the fuel injectors inject more. but that can be said for any engine, and any fuel injector, and it has nothing to do with the displacement.
similarly, you might get more air into the engine. In fact, you should, because that is the generic goal of increasing the displacement. However, an increase in displacement does not guarantee an increase in the number of molecules of atmospheric constituents that enter the cylinders.


Increase in cam lift/duration = more air/fuel = more power thru specific rpm range


Similarly, altering camshaft characteristics do not guarantee anything.
generally, as you said, more airflow through the engine during a specific rpm range is the idea behind camshaft swaps. Somebody taking advantage of the rpm capability and flow capability of an engine would want to maximize the volumetric efficiency of an engine through such a range. There is much more to it than just a camshaft swap, though.



For instance, my motor is currently at the builder. I elected to stay w/stock bore/stroke (honed to 86.5mm), and went with 260/12mm Procams... What would the difference have been if I had stuck to stock cam lift/duration but increased bore/stroke?

specifically, because all engines are different, specifically for the sr20det, the oem camshafts are fairly "restrictive" and "tame" when compared to the overall natural capability of the engine to ingest air. The camshaft change would have a much greater effect, specifically, on the sr20det engine, because of the fact that so much engineering and design went into that engine from the point of airflow- that is, the head, which is the major/central important detail of such an engine and determines overall capability and requirement for airflow regardless of all other configurations. Any bore/stroke you choose with OEM camshafts will be limited as far as volumetric efficiency because it directly affects the ability of the head to allow the passage of air.


- If it matters, I have a full compliment of supporting mods, and a reputable tuner; which I also understand the importance of. To make it easier, let's say hypothectically that my turbine choice was the 2871, .86 a/r (not my actual turbine, but one I'm sure a lot of you have experience with).


I wrote in haste and will elaborate but I have to run. Think your questions and responses through and give some example engines and I will use actual physics math to help explain if you are willing

[Frank_Jaeger]

Hey all,

I have sort of a physics question:

I have a pretty good general working knowledge of my SR, but I am wondering if increasing cam size and increasing bore/stroke can yield similar results.

To the best of my understanding, the following is generally true (please correct me if I am wrong):

Increase in bore/stroke = more air/fuel = more power thru entire rpm range

Increase in cam lift/duration = more air/fuel = more power thru specific rpm range

For instance, my motor is currently at the builder. I elected to stay w/stock bore/stroke (honed to 86.5mm), and went with 260/12mm Procams... What would the difference have been if I had stuck to stock cam lift/duration but increased bore/stroke?

- If it matters, I have a full compliment of supporting mods, and a reputable tuner; which I also understand the importance of. To make it easier, let's say hypothectically that my turbine choice was the 2871, .86 a/r (not my actual turbine, but one I'm sure a lot of you have experience with).

Thanks in advance.

--Mike
Dear lord. What body kit is that on the s13 in your sig?

[Mikester]

Increase bore/stroke means an increase in displacement... and thats it. You might get more fuel into the engine... if the fuel injectors inject more. but that can be said for any engine, and any fuel injector, and it has nothing to do with the displacement.
similarly, you might get more air into the engine. In fact, you should, because that is the generic goal of increasing the displacement. However, an increase in displacement does not guarantee an increase in the number of molecules of atmospheric constituents that enter the cylinders.



Similarly, altering camshaft characteristics do not guarantee anything.
generally, as you said, more airflow through the engine during a specific rpm range is the idea behind camshaft swaps. Somebody taking advantage of the rpm capability and flow capability of an engine would want to maximize the volumetric efficiency of an engine through such a range. There is much more to it than just a camshaft swap, though.




specifically, because all engines are different, specifically for the sr20det, the oem camshafts are fairly "restrictive" and "tame" when compared to the overall natural capability of the engine to ingest air. The camshaft change would have a much greater effect, specifically, on the sr20det engine, because of the fact that so much engineering and design went into that engine from the point of airflow- that is, the head, which is the major/central important detail of such an engine and determines overall capability and requirement for airflow regardless of all other configurations. Any bore/stroke you choose with OEM camshafts will be limited as far as volumetric efficiency because it directly affects the ability of the head to allow the passage of air.



I wrote in haste and will elaborate but I have to run. Think your questions and responses through and give some example engines and I will use actual physics math to help explain if you are willing

I don't need anyone to go to kinds of trouble with a bunch of math lol, but I do appreciate it=)

To keep it simple, let's say the average difference between SR22 with 256/11.5mm lift cams vs. SR20 with 260/12mm lift cams; same turbo, same supporting mods, same tuner... Just wondering what the average difference would be

Dear lord. What body kit is that on the s13 in your sig?

Vanquish front, BoMex sides, TBO rear.

[AdamR]

LOL at Kingtalon. Nice pseudo-intelligent post. Basically you said increased bore doesn't always increase power because of other factors and the stock cams arent the best.

[Kingtal0n]

let's say the average difference between SR22 with 256/11.5mm lift cams vs. SR20 with 260/12mm lift cams; same turbo, same supporting mods, same tuner... Just wondering what the average difference would be


This is a loaded question. you are asking for a short answer to something that requires a book.

Lets arrange some data first;
Engine A: SR22 with 256/11.5mm lift cams
Engine B: SR20 with 260/12mm lift cams
-assume 7,000rpm redline


assume T-25 turbocharger
VE maths: ((CID*7000/3456)*.069=Y)
Y= lb/min after turbocharger constant is applied, before boost is accounted for.


Due to the larger displacement, the 2.2L engine should make more torque through the engine rpm range. The main limiting factor on this engine is the turbocharger; once the compressor is flowing around 27lb/min the air temperature will begin to increase dramatically and diminishing returns will prevent more power from being made. Both engines will make the same peak power, which is limited by the compressor at this point, but the larger displacement engine will have:
lower boost pressure, better throttle response, more idle vacuum, more low rpm torque, quicker spool characteristics, and more, due to it's displacement.

Increasing the camshaft duration while using a T-25 is nearly pointless on either engine. You will hurt low rpm VE; that means lower idle vacuum, less throttle response, worse fuel economy, etc... for the sake of having an engine that can breath more at higher rpms... but the T-25 cannot support that additional flow! So you lose down low... and you gain nothing up top. You will have a lower average boost pressure across the board for the same power production as before; this is one benefit, the less boost you need to make the same power is always a good thing. But you do not actually gain any power.


change the turbocharger from a T-25 and it's a different story.

[Croustibat]

Engineering motors is a full time job, and that requires tons of work. You just cant "get a short answer" . You have to know there is a relation to keep between lift, valve diameter, valve angle (which you cant really change), and cam profile (valve acceleration, early/late opening, duration and so on). Boosted applications brings even more variables in there.

Basically, going at it blindly and hoping for the best is not going to give your wallet a nice time. You either know these things - which often means you work for a car manufacturer, are a race engineer, or own a successful race business - or you dont, which means you pay someone to get things sorted. You can very easily end with less performance than you had before, so dont just mess with that.

What makes power really is the turbo anyway. Unless you have already reached the limit of engine strength (which you lower by reboring it, as thinner cylinder walls dont like high boost), just stick to putting a bigger turbo on your engine with the supporting mods ( fuel system delivery, air metering system, adequate charge cooler, capable ECU, capable mapper) and you should be all right. This goes for every engine...

Not saying there is no way to get power from cams, or head modding. Porting head is usually a very good thing to do. Just that to REALLY get improvement, there is a lot of trial and error, luck and maths involved, and anyone who went through that very expensive process will NEVER give you the answer for free.

BTW procams need solid lifters. You may want to forget them and use poncams. Or stick with oem ones ... 260/12 is way too much imho on the SR engines, unless you plan to go very high in RPMs (which requires a lot of other work too). Oem cams are 240 duration w/9.2 lift... the cams you selected are a good upgrade for the SR20DE engine, unfortunately many people think that the bigger the better ... and as you can put non turbo cams on turbos, this happens.

If you fancy getting no power under 4000 rpm, go on and put these cams.

[TheRealSy90]

I say run JWT S4's on everything cause I don't give a fuck and they are rad.

[codyace]

I say run JWT S4's on everything cause I don't give a fuck and they are rad.

Or S3's :D