2.5 head vs 2.2 cams head swap ?'s
Posted: Sat Oct 06, 2001 8:32 pm
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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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
ADVERTISEMENT
<http://rd.yahoo.com/M=168643.1620686.31 ... Fcid=12715>
<http://us.adserver.yahoo.com/l?M=168643 ... =918695605>
To unsubscribe from this group, please send an email to:
BC-BFLegacyWorks-unsubscribe@egroups.com
zZz <http://docs.yahoo.com/info/terms/> .