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2.5 head vs 2.2 cams head swap ?'s

Posted: Sat Oct 06, 2001 8:32 pm
by ssspoon@aol.com
Hello knowledgeable list members, this will take around 5 or 6 minutes to read

I have been playing with the idea of a cylinder head swap for a while, and have just purchased a set of heads from a 2001 Imprezza RS 2.5L
engine with side impact damage.

Since I have started buying parts to upgrade my current stock 1992 factory turbocharged 2.2L Subaru Legacy, I have come upon a few questions I would like input and help with at your convenience please.

When conducting cylinder head flow tests, measurements are usually taken at predetermined static values of valve lift and air pressure.

It has occurred to me that dynamic volumetric efficiency during different engine operating conditions, is not only related to static flow results, but may also be dependent on other things controlled by cam design, such as valve acceleration, duration, overlap, etc.

QUESTION;
Are there some generalizations I can depend on? For example, would it be a safe assumption to say an increase or decrease in static flow, generally indicates an increased or decreased area under the power and/or torque curves? Likewise would it be a safe assumption to say cam design controlled features cause the power and/or torque peak to move up and down the rpm range without much affecting the area under the torque/power curves much?

The reason for my curiosity is because information seems to point to the probability that a swap of my stock 2.2L Turbo Legacy heads for a set of late stock 2.5L heads would result in a flow improvement equal to or better than a 2.2 port and polish. However the Subaru 1991 Model Year Update Course Reference Booklet says "...The intake valves have been reduced to 27 mm for the turbo and the camshaft has a new profile designed to match the characteristics of the turbo engine...". But no info is given to indicate what the new profile does. It may well be that Subaru Engineering felt they could get away with decreasing valve lift and or duration in order to reduce valve train component stress, and promote longevity, since the engine is force fed. Why else would they reduce intake valve size?

Although the above valve size comparison was written in reference to the 1991 2.2L turbo engine in comparison to the 1991 2.2L normally aspirated engine, still to me it raises the

QUESTION;

whether to use the 2.5 heads AND cams, or whether to use the 2.5 heads with the 2.2 turbo cams?

As stated earlier, I suspect the stock 2.2L turbo cam actually "detunes" the engine, so the 2.5 cam would be preferable. To support my theory I offer these numbers giving relationships between the 2.5 N/A engine, 2.5 turbo engine (2.5T), 2.2 N/A engine and 2.2 turbo (2.2T).

Typical 2.5T = 39.4% more power (230) than 2.5 N/A (165) @ 5 psi,
Stock 2.2T = only 23.1% more power (160) than 2.2 N/A (130) @ 8.7 psi
2.5T = 16% more power increase on 42.5% less boost.

Why is this? Partially due to 21.2% more compression (9.5 vs 8.0), but we're still talking about 42.5% less boost.

Using the Ray Hall Turbocharging website turbo calculators shows the turbo compression for the 2.5T at 5 psi to be virtually the same as the 2.2T so they cancel for compression considerations. That leaves us wondering about the 16% additional increase, and remember, we are talking about PERCENTAGE increases here so we are looking at proportional numbers, not absolute numbers. What that means is I'm trying to take the engine size difference out of the "equations" so we're comparing apples to apples. I think again, that 16% additional is gained by superior head design.

If we look at the difference in engine size the questions remain. The 2.5 N/A only has 13.6% greater displacement than 2.2 N/A, but generates 26.9% more power (165) than 2.2 N/A (130). If we subtract the percentage size increase that leaves us with 13.3% unaccounted for power increase.

All things being equal other than displacement, one would expect the power increase to be proportional to size increase, yet the PERCENTAGE POWER INCREASE IS 2X THE PERCENTAGE SIZE INCREASE. Why is this? I say it's all in the heads (or almost all) But is it all due to head design or cam profile or some of each? I think most, if not all, is due to head design because the 2.5 head static flow is so much superior to the 2.2.

Cobb Tuning head flow studies show the 2.5 N/A head to average 25% greater static flow than the 2.2T, with figures ranging from 20% to 28% better.

So with a 25% static flow advantage with the 2.5 N/A heads over the 2.2T heads, and power increases of nearly equal 16% and 13% unaccounted for by compression and size differences for the 2.5T vs 2.2T and 2.5 N/A vs the 2.2 N/A, it looks to me like head design is the dominant factor, not cams.

I would appreciate any experience or input anyone might have, to shed more light on this issue, in favor of one cam over the other please.

It is interesting to note that while the 2.5 NA generates 3.1% more power (165) than 2.2T (160), with 13.6% greater displacement, the 2.2T delivers 9.0% more torque (181) than 2.5 N/A (166) and 32.1% more torque than 2.2 N/A (137)

My primary objective is to increase low end torque. Any power increase would be a bonus


PLEASE CONNECT ME WITH ANYONE THAT HAS ACTUALLY DONE THE 2.5 N/A TO 2.2T HEAD SWAP

Thank you very much

Larry Witherspoon
Torrance (Los Angeles)
ssspoon@aol.com
larry.d.witherspoon@boeing.com





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2.5 head vs 2.2 cams head swap ?'s

Posted: Sat Oct 06, 2001 9:48 pm
by acroxford@hypermall.net
WOW! that is a question Larry I don't know if knowedgeable applies but
what I did was find a set of mechanical (non-hyd.) rocker arms with
rollers in a yard and like you not being able to resist a a "bargain"
$15 scooped them up. I installed them set the clearance to the 2.5
spec's and started it up. It ran very poorly and on checking I found
that the rollers were contacting the cams at a different angle and I
had retarded the cam timing quite a bit. Someone told me that a some
'96 n/a engines had these roller mechanical arms and on checking I
found that the timing was almost the same as the turbo motor (within
one degree intake and exhaust) but the lift was a bit higher So maybe
my hot setup would be '96 n/a cams and the arms at least the friction
would be lower. When we had a 2.0l engine apart I noticed that one of
the exhaust valves for a cyl. would open and close sooner than the
other one and the lift was low ??? Something tells me that they are
playing tricks for some reason ?? Possibly for VE , emmisions,
velocity to get a better fuel mix ?? Let me know when you figure this
out it give me a headache as it really does't make much sense the way
I see it. If I can find that SAE paper they might say something about
this.
AL(CO)





--- In BC-BFLegacyWorks@y..., ssspoon@a... wrote:
> Hello knowledgeable list members, this will take around 5 or 6
minutes to
> read
>
> I have been playing with the idea of a cylinder head swap for a
while, and
> have just purchased a set of heads from a 2001 Imprezza RS 2.5L
> engine with side impact damage.
>
> Since I have started buying parts to upgrade my current stock 1992
factory
> turbocharged 2.2L Subaru Legacy, I have come upon a few questions I
would
> like input and help with at your convenience please.   
>
> When conducting cylinder head flow tests, measurements are usually
taken at
> predetermined static values of valve lift and air pressure.  
>
> It has occurred to me that dynamic volumetric efficiency during
different
> engine operating conditions, is not only related to static flow
results, but
> may also be dependent on other things controlled by cam design, such
as valve
> acceleration, duration, overlap, etc.
>
> QUESTION; 
> Are there some generalizations I can depend on?  For example, would
it be a
> safe assumption to say an increase or decrease in static flow,
generally
> indicates an increased or decreased area under the power and/or
torque
> curves?   Likewise would it be a safe assumption to say cam design
controlled
> features cause the power and/or torque peak to move up and down the
rpm range
> without much affecting the area under the torque/power curves much?
   
>
> The reason for my curiosity is because information seems to point to
the
> probability that a swap of my stock 2.2L Turbo Legacy heads for a
set of late
> stock 2.5L heads would result in a flow improvement equal to or
better than a
> 2.2 port and polish.   However the Subaru 1991 Model Year Update
Course
> Reference Booklet says "...The intake valves have been reduced to 27
mm for
> the turbo and the camshaft has a new profile designed to match the
> characteristics of the turbo engine...".  But no info is given to
indicate
> what the new profile does.   It may well be that Subaru Engineering
felt they
> could get away with decreasing valve lift and or duration in order
to reduce
> valve train component stress, and promote longevity, since the
engine is
> force fed.   Why else would they reduce intake valve size?
>
> Although the above valve size comparison was written in reference to
the 1991
> 2.2L turbo engine in comparison to the 1991 2.2L normally aspirated
engine,
> still to me it raises the
>
> QUESTION;
>
> whether to use the 2.5 heads AND cams, or whether to use the 2.5
heads with
> the 2.2 turbo cams?  
>
> As stated earlier, I suspect the stock 2.2L turbo cam actually
"detunes" the
> engine, so the 2.5 cam would be preferable. To support my theory I
offer
> these numbers giving relationships between the 2.5 N/A engine, 2.5
turbo
> engine (2.5T), 2.2 N/A engine and 2.2 turbo (2.2T).
>
> Typical 2.5T = 39.4% more power (230) than 2.5 N/A (165) @ 5 psi,
> Stock 2.2T =  only 23.1% more power (160) than 2.2 N/A (130)  @ 8.7
psi
> 2.5T = 16% more power increase on 42.5% less boost.
>
> Why is this? Partially due to 21.2% more compression (9.5 vs 8.0),
but we're
> still talking about 42.5% less boost.
>
> Using the Ray Hall Turbocharging website turbo calculators shows the
turbo
> compression for the 2.5T at 5 psi to be virtually the same as the
2.2T so
> they cancel for compression considerations. That leaves us
wondering about
> the 16% additional increase, and remember, we are talking about
PERCENTAGE
> increases here so we are looking at proportional numbers, not
absolute
> numbers. What that means is I'm trying to take the engine size
difference
> out of the "equations" so we're comparing apples to apples. I think
again,
> that 16% additional is gained by superior head design.
>
> If we look at the difference in engine size the questions remain.
The 2.5
> N/A only has 13.6% greater displacement than 2.2 N/A, but generates
26.9%
> more power (165) than 2.2 N/A (130). If we subtract the percentage
size
> increase that leaves us with 13.3% unaccounted for power increase.
>
> All things being equal other than displacement, one would expect the
power
> increase to be proportional to size increase, yet the PERCENTAGE
POWER
> INCREASE IS 2X THE PERCENTAGE SIZE INCREASE. Why is this? I say
it's all in
> the heads (or almost all) But is it all due to head design or cam
profile or
> some of each? I think most, if not all, is due to head design
because the
> 2.5 head static flow is so much superior to the 2.2.
>
> Cobb Tuning head flow studies show the 2.5 N/A head to average 25%
greater
> static flow than the 2.2T, with figures ranging from 20% to 28%
better.
>
> So with a 25% static flow advantage with the 2.5 N/A heads over the
2.2T
> heads, and power increases of nearly equal 16% and 13% unaccounted
for by
> compression and size differences for the 2.5T vs 2.2T and 2.5 N/A vs
the 2.2
> N/A, it looks to me like head design is the dominant factor, not
cams.
>
> I would appreciate any experience or input anyone might have, to
shed more
> light on this issue, in favor of one cam over the other please.
>
> It is interesting to note that while the 2.5 NA generates 3.1% more
power
> (165) than 2.2T (160), with 13.6% greater displacement, the 2.2T
delivers
> 9.0% more torque (181) than 2.5 N/A (166) and 32.1% more torque than
2.2 N/A
> (137)
>
> My primary objective is to increase low end torque.   Any power
increase
> would be a bonus   
>
> PLEASE CONNECT ME WITH ANYONE THAT HAS ACTUALLY DONE THE 2.5 N/A TO
2.2T HEAD
> SWAP
>
> Thank you very much
>
> Larry Witherspoon
> Torrance (Los Angeles)
> ssspoon@a...
> larry.d.witherspoon@b...


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2.5 head vs 2.2 cams head swap ?'s

Posted: Sat Oct 06, 2001 10:08 pm
by Josh Colombo
Larry,

Not sure I can help explain your cam/head questions, but I am goin to pose another question. Looking at your numbers. 160hp for 2.2T and 165hp for 2.5 N/A. Keeping those two numbers in mind. The 2.5 has more hp, yet I think I would be fairly safe to say the *STOCK* 2.2T Legacy would take a *STOCK* 2.5 N/A Impreza outfitted with manual trannies same gearing, etc. That is even with quite a bit of weight hindrance in the Turbo Legacy's corner.

As you said, the 2.2T motor has more torque then the 2.5, which would give you faster acceleration, hence faster car, which it sounds like that is what you want.

Now for a couple things to think about or what not. Do you or anyone else for that matter have hp/torque curves for these two engines, or for the 2.2 N/A engine? I think if we had the curves we might be able to make some inferences about things.

Other thing I just thought of, as you said the valve's are smaller on the 2.2T compared to the 2.5 heads. If we look at both heads, 2.2T and 2.5, but ONLY look at the valve sizing. The 2.5 has larger valves, I'm assuming for intake and exhaust. And taking the knowledge that bigger the valve the more "stuff" can fit through at the same volumetric flow rate and density. So with that knowledge the 2.5 heads just looking at the valves ONLY, looks like it would make more HP.

Next let's look at the valve train, are you looking at getting DOHC or SOHC 2.5 Heads? In theory the DOHC setup is better, but I question why they went back to the SOHC setup for the latest motors. So I'm not sure if there is any real major pros or cons in favor of either.

Ok now the cams. Now this is my understanding of what the different setups of the cams do and stuff. The two main things you look into with a cam is valve duration and cam profile (ie. how far does it open) Yes like you said you also have overlap and probably some other stuff, but I think duration and profile are the two biggies. Looking back at the valve comparison I said you can fit more "stuff" through a larger opening. The same holds true to duration and profile. If you have a large profile the valve will open more and more "stuff" can enter combustion chamber, same goes for duration, more stuff will enter if it is open longer. So why don't you get a cam with the largest duration and profile you can get? Because you sacrifice drivability, low-end/rpm performance.

I guess the way I understand it is using a comparison you hear when talking to any of the muscle car buffs. "They always say I got a wicked cam in this thing." When they start it up, the car is sorta idling a little rough, taking big gasps of air, just a very throaty, mean sound. I'll also add, they get alot of unburned fuel bein spit out of exhaust, however I guess you'd have less of that with EFI instead of carburetor....anyway back to what I was talkin about. These engines with these wicked cams run so so at low rpm but when the engine rpms get high performance and hp climbs considerably. Other thing I just thought of, is timing advance, not sure if that is set in the cam or if that is controlled by the computer.

Ok now that I've talked about that analogy, here's the bummer, I really am not sure myself whether duration, profile, overlap, advance, etc causes more low end power or high end power. I'm sure there are other factors in it as well, but I'm just not sure. I do think the 2.5 heads just for there valve size is probably better, (using the bigger is better idea), but not sure what to tell you about the cams.

As you can see choosing a cam design for an engine isn't just an eeny meenie miny moe. It takes a little research. It would be helpful to know what sort of thinking subaru engineers were using when they made the cam profile, and what sort of goals they were trying to achieve. They may have had problems with larger valves or something and went with a smaller setup on the 2.2T, or just were like you said detunin the engine.....it's hard to know.

Now as far as what I would do next. I would try and get a hold of a muscle car buddy, tuning shop, or someone that knows shit loads about cams, and what characteristics of the cam do what, what gives more low-end, high end, etc. Once you know about that, and digest it, then what I would do, (this is where the hp/torque curves come into play), look at the hp/torque curves for the two engines, and try and see if you can match up some of the characteristics of the two, to the knowledge you learned about the cams. Other thing I just thought of, if you had the cams out of the heads, you could try and get them profiled to see what the setup was on them, same thing with the 2.2T heads.

Ok, well I've talked way too much now. This is alot of info, I hope what I've said is all correct, I'm far from a expert in this subject, if I'm wrong anywhere just let me know.

Larry, looks like you got your homework cut out for you. Do let us know what you find out.

Josh "should be studying for the FE exam, but studying on engine design" Colombo


************************************
Josh Colombo
jcc189@psu.edu

"Life, an ever-changing melody
of beats and rhythm" - ME
************************************


-----Original Message-----
From: ssspoon@aol.com [mailto:ssspoon@aol.com]
Sent: Saturday, October 06, 2001 7:23 PM
To: BC-BFLegacyWorks@yahoogroups.com; STi_Mlist@imprezawrx.org
Subject: [BC-BFLegacyWorks] 2.5 head vs 2.2 cams head swap ?'s


Hello knowledgeable list members, this will take around 5 or 6 minutes to read

I have been playing with the idea of a cylinder head swap for a while, and have just purchased a set of heads from a 2001 Imprezza RS 2.5L
engine with side impact damage.

Since I have started buying parts to upgrade my current stock 1992 factory turbocharged 2.2L Subaru Legacy, I have come upon a few questions I would like input and help with at your convenience please.

When conducting cylinder head flow tests, measurements are usually taken at predetermined static values of valve lift and air pressure.

It has occurred to me that dynamic volumetric efficiency during different engine operating conditions, is not only related to static flow results, but may also be dependent on other things controlled by cam design, such as valve acceleration, duration, overlap, etc.

QUESTION;
Are there some generalizations I can depend on? For example, would it be a safe assumption to say an increase or decrease in static flow, generally indicates an increased or decreased area under the power and/or torque curves? Likewise would it be a safe assumption to say cam design controlled features cause the power and/or torque peak to move up and down the rpm range without much affecting the area under the torque/power curves much?

The reason for my curiosity is because information seems to point to the probability that a swap of my stock 2.2L Turbo Legacy heads for a set of late stock 2.5L heads would result in a flow improvement equal to or better than a 2.2 port and polish. However the Subaru 1991 Model Year Update Course Reference Booklet says "...The intake valves have been reduced to 27 mm for the turbo and the camshaft has a new profile designed to match the characteristics of the turbo engine...". But no info is given to indicate what the new profile does. It may well be that Subaru Engineering felt they could get away with decreasing valve lift and or duration in order to reduce valve train component stress, and promote longevity, since the engine is force fed. Why else would they reduce intake valve size?

Although the above valve size comparison was written in reference to the 1991 2.2L turbo engine in comparison to the 1991 2.2L normally aspirated engine, still to me it raises the

QUESTION;

whether to use the 2.5 heads AND cams, or whether to use the 2.5 heads with the 2.2 turbo cams?

As stated earlier, I suspect the stock 2.2L turbo cam actually "detunes" the engine, so the 2.5 cam would be preferable. To support my theory I offer these numbers giving relationships between the 2.5 N/A engine, 2.5 turbo engine (2.5T), 2.2 N/A engine and 2.2 turbo (2.2T).

Typical 2.5T = 39.4% more power (230) than 2.5 N/A (165) @ 5 psi,
Stock 2.2T = only 23.1% more power (160) than 2.2 N/A (130) @ 8.7 psi
2.5T = 16% more power increase on 42.5% less boost.

Why is this? Partially due to 21.2% more compression (9.5 vs 8.0), but we're still talking about 42.5% less boost.

Using the Ray Hall Turbocharging website turbo calculators shows the turbo compression for the 2.5T at 5 psi to be virtually the same as the 2.2T so they cancel for compression considerations. That leaves us wondering about the 16% additional increase, and remember, we are talking about PERCENTAGE increases here so we are looking at proportional numbers, not absolute numbers. What that means is I'm trying to take the engine size difference out of the "equations" so we're comparing apples to apples. I think again, that 16% additional is gained by superior head design.

If we look at the difference in engine size the questions remain. The 2.5 N/A only has 13.6% greater displacement than 2.2 N/A, but generates 26.9% more power (165) than 2.2 N/A (130). If we subtract the percentage size increase that leaves us with 13.3% unaccounted for power increase.

All things being equal other than displacement, one would expect the power increase to be proportional to size increase, yet the PERCENTAGE POWER INCREASE IS 2X THE PERCENTAGE SIZE INCREASE. Why is this? I say it's all in the heads (or almost all) But is it all due to head design or cam profile or some of each? I think most, if not all, is due to head design because the 2.5 head static flow is so much superior to the 2.2.

Cobb Tuning head flow studies show the 2.5 N/A head to average 25% greater static flow than the 2.2T, with figures ranging from 20% to 28% better.

So with a 25% static flow advantage with the 2.5 N/A heads over the 2.2T heads, and power increases of nearly equal 16% and 13% unaccounted for by compression and size differences for the 2.5T vs 2.2T and 2.5 N/A vs the 2.2 N/A, it looks to me like head design is the dominant factor, not cams.

I would appreciate any experience or input anyone might have, to shed more light on this issue, in favor of one cam over the other please.

It is interesting to note that while the 2.5 NA generates 3.1% more power (165) than 2.2T (160), with 13.6% greater displacement, the 2.2T delivers 9.0% more torque (181) than 2.5 N/A (166) and 32.1% more torque than 2.2 N/A (137)

My primary objective is to increase low end torque. Any power increase would be a bonus



PLEASE CONNECT ME WITH ANYONE THAT HAS ACTUALLY DONE THE 2.5 N/A TO 2.2T HEAD SWAP

Thank you very much

Larry Witherspoon
Torrance (Los Angeles)
ssspoon@aol.com
larry.d.witherspoon@boeing.com



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