Marginal Compression Loss = Lost Hp?
02-11-2005, 08:58 AM
#1
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Join Date: Jan 2003
Posts: 93
I'm on a goose chase, and having read many of the posts in this area, there's a bunch of rotary experience here- I'd love to hear your thoughts.
Last year, I'd cracked an apex seal on my '93 twin turbo. The motor was rebuilt (with a streetport) by a very reputable builder on the east coast, and found that I needed a new housing, but with a small amount of work, the rotor was ok. Also, the turbos (amazingly) survived. The corner of the rotor had some small damage that intruded into the apex seal groove. The builder cleaned up the groove, checked specs and clearances with seal installed, and it was proclaimed ok.
The car runs absolutely fine, but when I brought the car to KDR to have him tune the PFC, he claimed that the rear rotor's compression between chambers was beyond spec (checked with strip chart recorder), and would only tune it with a disclaimer should the motor break. I declined.
I brought the car back to the builder, and he ran a series of compression checks- cold, hot, etc.with an older style, non-strip chart recorder- and found that the variance between chambers on that rotor was 4-5%. Mazda's limit between chambers is 10% (?), and he called around to a number of other builders, and universally agreed that the motor is fine.
Okay, now here's the kicker. I had Steve Kan come to NY and tune my car. The motor has all the upgrades: dp, hi-flow cat, RB cat back, PFS intercooler and intake, fuel system upgrades, y-pipe, modified twins, Apexi PFC. Motors with this amount of work should produce, at least, 350 rwhp at 14-15 psi. My car produced 290rwhp at 14 psi, and as boost was increased- each pound of boost only increased hp by 8hp, and Steve Kan feels that you should see at least a 15hp increase. Steve and the motor builder feel that there is a restriction somewhere that is accounting for the 60 hp loss.
So, I changed the HKS DP to a mandrel bent ATR unit. I just checked the hi-flow cat (less than 1,000 miles) to ensure it's not clogged (it's not). The RB cat back is a new piece with a couple of thousand miles on it, too.
I'm feel that the compression loss is the culprit. What do all of you think???
Last year, I'd cracked an apex seal on my '93 twin turbo. The motor was rebuilt (with a streetport) by a very reputable builder on the east coast, and found that I needed a new housing, but with a small amount of work, the rotor was ok. Also, the turbos (amazingly) survived. The corner of the rotor had some small damage that intruded into the apex seal groove. The builder cleaned up the groove, checked specs and clearances with seal installed, and it was proclaimed ok.
The car runs absolutely fine, but when I brought the car to KDR to have him tune the PFC, he claimed that the rear rotor's compression between chambers was beyond spec (checked with strip chart recorder), and would only tune it with a disclaimer should the motor break. I declined.
I brought the car back to the builder, and he ran a series of compression checks- cold, hot, etc.with an older style, non-strip chart recorder- and found that the variance between chambers on that rotor was 4-5%. Mazda's limit between chambers is 10% (?), and he called around to a number of other builders, and universally agreed that the motor is fine.
Okay, now here's the kicker. I had Steve Kan come to NY and tune my car. The motor has all the upgrades: dp, hi-flow cat, RB cat back, PFS intercooler and intake, fuel system upgrades, y-pipe, modified twins, Apexi PFC. Motors with this amount of work should produce, at least, 350 rwhp at 14-15 psi. My car produced 290rwhp at 14 psi, and as boost was increased- each pound of boost only increased hp by 8hp, and Steve Kan feels that you should see at least a 15hp increase. Steve and the motor builder feel that there is a restriction somewhere that is accounting for the 60 hp loss.
So, I changed the HKS DP to a mandrel bent ATR unit. I just checked the hi-flow cat (less than 1,000 miles) to ensure it's not clogged (it's not). The RB cat back is a new piece with a couple of thousand miles on it, too.
I'm feel that the compression loss is the culprit. What do all of you think???
02-11-2005, 08:19 PM
#3
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Join Date: Jan 2003
Posts: 93
I don't have the figures available, but as it was a fresh rebuild, and the compression was perfect on the front rotor, and the one chamber on the rear was equal to the front. The other two chambers were 4-6% lower than the 3rd chamber.
I should add that Dave at KDR felt that the problem was an apex seal that was sticking. Dave's feeling was that no matter how well the apex seal groove is repaired, there is always the chance that the seal will 'stick', resulting the compression variance between chambers. When viewed on the strip chart generated on Dave's machine, the variance looked dramatic, but when my builder checked the compression, the results were as I've quoted.
The question is, will lower compression in 2 chambers, on a turbo'd motor, result in a noticeably lower power production?
And, oh, I'm not trying to fault anyone here at all, but I'd like to find that hp
I should add that Dave at KDR felt that the problem was an apex seal that was sticking. Dave's feeling was that no matter how well the apex seal groove is repaired, there is always the chance that the seal will 'stick', resulting the compression variance between chambers. When viewed on the strip chart generated on Dave's machine, the variance looked dramatic, but when my builder checked the compression, the results were as I've quoted.
The question is, will lower compression in 2 chambers, on a turbo'd motor, result in a noticeably lower power production?
And, oh, I'm not trying to fault anyone here at all, but I'd like to find that hp
Originally Posted by rfreeman27' date='Feb 11 2005, 01:48 PM
what was the actual compression #'s?
It can be 30psi, 30psi, 30psi with no variance between chambers but it's still low...
It can be 30psi, 30psi, 30psi with no variance between chambers but it's still low...
02-11-2005, 09:14 PM
#5
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Join Date: Jan 2003
Posts: 93
I'm grasping for straws at this point, so all theories are valid. I happen to agree with you- if 2 of your 6 'cylinders' (33%) are slightly down on compression, it has to have an effect of power production, especially when it's pressurized.
Originally Posted by allied35' date='Feb 11 2005, 06:35 PM
I would think that a small amount of compression loss at cranking speed would be much worse under boost. I'm no expert though.
02-12-2005, 07:53 AM
#7
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Join Date: Jan 2004
Location: Central Ohio (Hebron) Zephyrhills Fla.
Posts: 1,322
Originally Posted by gcthree' date='Feb 11 2005, 06:58 AM
I'm on a goose chase, and having read many of the posts in this area, there's a bunch of rotary experience here- I'd love to hear your thoughts.
Last year, I'd cracked an apex seal on my '93 twin turbo. The motor was rebuilt (with a streetport) by a very reputable builder on the east coast, and found that I needed a new housing, but with a small amount of work, the rotor was ok. Also, the turbos (amazingly) survived. The corner of the rotor had some small damage that intruded into the apex seal groove. The builder cleaned up the groove, checked specs and clearances with seal installed, and it was proclaimed ok.
The car runs absolutely fine, but when I brought the car to KDR to have him tune the PFC, he claimed that the rear rotor's compression between chambers was beyond spec (checked with strip chart recorder), and would only tune it with a disclaimer should the motor break. I declined.
I brought the car back to the builder, and he ran a series of compression checks- cold, hot, etc.with an older style, non-strip chart recorder- and found that the variance between chambers on that rotor was 4-5%. Mazda's limit between chambers is 10% (?), and he called around to a number of other builders, and universally agreed that the motor is fine.
Okay, now here's the kicker. I had Steve Kan come to NY and tune my car. The motor has all the upgrades: dp, hi-flow cat, RB cat back, PFS intercooler and intake, fuel system upgrades, y-pipe, modified twins, Apexi PFC. Motors with this amount of work should produce, at least, 350 rwhp at 14-15 psi. My car produced 290rwhp at 14 psi, and as boost was increased- each pound of boost only increased hp by 8hp, and Steve Kan feels that you should see at least a 15hp increase. Steve and the motor builder feel that there is a restriction somewhere that is accounting for the 60 hp loss.
So, I changed the HKS DP to a mandrel bent ATR unit. I just checked the hi-flow cat (less than 1,000 miles) to ensure it's not clogged (it's not). The RB cat back is a new piece with a couple of thousand miles on it, too.
I'm feel that the compression loss is the culprit. What do all of you think???
Last year, I'd cracked an apex seal on my '93 twin turbo. The motor was rebuilt (with a streetport) by a very reputable builder on the east coast, and found that I needed a new housing, but with a small amount of work, the rotor was ok. Also, the turbos (amazingly) survived. The corner of the rotor had some small damage that intruded into the apex seal groove. The builder cleaned up the groove, checked specs and clearances with seal installed, and it was proclaimed ok.
The car runs absolutely fine, but when I brought the car to KDR to have him tune the PFC, he claimed that the rear rotor's compression between chambers was beyond spec (checked with strip chart recorder), and would only tune it with a disclaimer should the motor break. I declined.
I brought the car back to the builder, and he ran a series of compression checks- cold, hot, etc.with an older style, non-strip chart recorder- and found that the variance between chambers on that rotor was 4-5%. Mazda's limit between chambers is 10% (?), and he called around to a number of other builders, and universally agreed that the motor is fine.
Okay, now here's the kicker. I had Steve Kan come to NY and tune my car. The motor has all the upgrades: dp, hi-flow cat, RB cat back, PFS intercooler and intake, fuel system upgrades, y-pipe, modified twins, Apexi PFC. Motors with this amount of work should produce, at least, 350 rwhp at 14-15 psi. My car produced 290rwhp at 14 psi, and as boost was increased- each pound of boost only increased hp by 8hp, and Steve Kan feels that you should see at least a 15hp increase. Steve and the motor builder feel that there is a restriction somewhere that is accounting for the 60 hp loss.
So, I changed the HKS DP to a mandrel bent ATR unit. I just checked the hi-flow cat (less than 1,000 miles) to ensure it's not clogged (it's not). The RB cat back is a new piece with a couple of thousand miles on it, too.
I'm feel that the compression loss is the culprit. What do all of you think???
Unless there is a huge difference in cranking compression, I think it is not a factor in your problem. At cranking speed there is ample time for a small leak to deliver a very low compression number.
So you have apature size (the damage allowing the leak) Time and pressure that determine how much can get passed the damaged area. When the engine is screaming the presure is higher, but aperture time drops to near zero. From maybe one millisecond to 0.0075 milliseconds at 8,000 RPM.
The porting makes exhaust tuning more important. Make a pass with nothing after the turbo but the down pipe, and a long tube to keep from setting it on fire. My guess it poor tune and, or, an exhaust problem of restriction or a tuning incompatibility amongst the components.
A stuck apex seal will just about kill the engine not just make it down on power.
Lynn E. Hanover
02-12-2005, 02:04 PM
#8
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Join Date: May 2002
Location: California
Posts: 22,465
i would be thinking about removing the cat. if you remove the cat and or the muffler and find the hp then you're done. next stop would be the intake. id try the pfs with and without the box, make sure the double throttle is opening all the way etc etc.
if all that is still ok, its time to look at the turbos
if all that is still ok, its time to look at the turbos
02-16-2005, 10:01 PM
#9
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Join Date: Oct 2004
Location: North Carolina, Pittsboro
Posts: 45
You may need to take a very hard look at the turbo. Check the inter cooler for restrictions. turbo just about to go cause very similar symptoms. The turbo went on my 87 and just before I noticed a lack of power for a week or so. Rowever I continued to pound the bajeesus out of it, instead of babying it, and am now rebuilding my engine because of it. I know I've never posted before but ive got a semi decent idea of what I'm talking about. Good luck!
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