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<blockquote data-quote="MalcolmV8" data-source="post: 15700694" data-attributes="member: 8854"><p>Yes that keeps the bypass valve open at idle with low vacuum but it doesn't fix the surging or jerking that happens when the bypass valve closes.</p><p></p><p>In my testing a low pressure bypass valve takes about 3 ~ 4" of vacuum to hold it open.</p><p></p><p><img src="https://lh3.googleusercontent.com/80O5Tk0NPM7JlvDj8D7gKsU-anFU_42JOfniV9zfZ-Lx5P81NObExd2KWCigi2lXHyJdwnH2W0ywgJ1q2yKtj3n4J7cthdJIRJvpPoW6KuaG9rsBKaNScZb-LxMFnVNjku1ue8cqFaAX_w6Pl5F5cXLv_iMMa7JGvXOK_MMGd1CY2aiUxecEFgPL89QB78Bz17iQwqbrSMHeN5fkUEj2ed4RGBOe8bjshLqoQUH0VUzaJYn-U2ZeDq_Dd7Wwr4gkNh2huddtXoWGEXs1h_LWmEfGUH0oZOdoSuzfafeAQ61MZGwoGMmZa7SBiS02oHgIY6vVjQcfMogv6x5Ib6nd1a8nJTcx8ze2NK7ZmHAyOpARBjY-zMbbA35_8muqnRlHuiR-oP9D8Waf6IJMRHn1KMM6J5mEcSMCl02YwrBTiAk4NEwtiUTPsXZyPdMiekerZqbHHD2F_G7iyq_uXiTFEkSFCRHy8sqk9BOFPgS1FLZtVte5toABjiMy4HKPcEcdYTohdOF-SSJs6Z1H4r2Svi7wx8fU8o3hN5WZgZBb535VLrghBnaJiKmtOQ481OecMht5UoiHN2V196TQHnOmGx_2XwLdAL1K76OOLFWIwjzDzGeAdIWjmrwdb_4d9Y26MILwHyMuCgBKoa3Y9ngBFO1vEejzIa4cyjQ=w800-h600-no" alt="" class="fr-fic fr-dii fr-draggable " style="" /> </p><p></p><p>The problem is when you're down to 3" of vacuum that's not very much of a vacuum at all and the rotors are filled with air. So when that valve closes there's a sudden surge or spike of air that's slammed into the motor as that happens. It can be observed on the wideband as well as felt by the driver. In normal cruise, even highway at times as you're slightly on and off the gas pedal and that valve is transitioning between open/closed the car surges and jerks.</p><p></p><p>I even tried slowing down the transition of the valve. I used nitrous jets in the vacuum / boost line going to the bypass valve actuator and that helped a lot. It smoothed out the driving a fair amount. It did not cure it but it made a big difference. However it had a massive side affect. When cruising along and then suddenly stabbing the throttle wide open there was a momentary delay for the power to come because the valve couldn't snap shut instantly due to the nitrous jet. It felt like the sort of momentary lag you can feel on a turbo car. I hated it. One of the things I love about the PD blowers is instant torque and power.</p><p></p><p>A stock bypass valve on the other hand is closer to 11" of vacuum to hold it open.</p><p></p><p><img src="https://lh3.googleusercontent.com/zvTAb2rWATRsUvO0sXGeIpLWOAkGPZTDoMHUNAQAYN-x3jp8D6BcdgeF7San4Fj_zk0pr6n9wrz4cH-RrTHf0xehTFNToAY4lbCaXZsL5Kz7zmwowa9g6aZnUOr3Xw9EJ2dXkSgQd2eLr_FwUAE82LL_M_7RhFZ4_JR-rhcOgPdQivSQxKzsc3h4K1SAZNKOVTUuUAghnEz_Gk6jiQwUqbmrgTc8Qs6jWawZw7E6ZU7QW-BaEhp7PDhZyOs-k6oXB3NpG6GCb3vHuQkOzFyVyCOliwBs7jZIjLI0eyb7ASHyCHAbTJG5YDZAWq8gCpq9S0NU_5hJ5u8XklwXRbVNSW-ttEw2TGjE-vppAE7yYoIUsXFdzZnlLcf2J9u-woiECUorAtw6STMI5NIuwgmhx7cK8LH2nA73V1i8UYJcjiF1dGNXmVTbGyrGSp3o8yH8In_6tSCKqRphEjAyhJTwQ4JHFCWDLgYS90EpoevSpAN7YLzrGoJ08PUxaR9ezduv83_KLlHeMzrKMueSP8CtzXYmmf0pLYnu9yUIjf98HyBT5FlrOStpACgpSuf-bSIRkySAArC0ozpizS-I-ksshhFRn_TvNjIegrgh6ptWSs98zS27TwVMN87O1c5sA-zRpnCIZ2hANVPzhdrs27RFSpxsa7amO_g3WDg=w800-h600-no" alt="" class="fr-fic fr-dii fr-draggable " style="" /> </p><p></p><p>That's important because when the valve closes deeper in vacuum there's not much air in the rotors and you don't get a surge into the motor and its a smooth transition.</p><p></p><p>When I first started out I was running 8 degrees of overlap on my cams with a 110 ICL. It had a nasty idle and low idle / midrange vacuum and drove like crap.</p><p></p><p>I moved to smaller duration cams with not much luck. I then re-degreed them per the "expert" at 109 ICL with his LSA of 115 which gave me 0.5 degrees of overlap. Doesn't sound like much overlap but car still had poor idle vacuum and while it drove a tad better it seemed, it still drove horrible.</p><p></p><p>After talking to many people and doing my own research and info gathering and understanding I came up with my own center lines and LSA and ended up with just -4.5 degrees of negative overlap and wow what a massive improvement. I now had about 14" of vacuum at idle and was able to swap back in the stock bypass valve actuator instead of the low pressure one and the car drove like OEM. No bucking, jerking, surging or any of that junk. It idles solid and reliable and drives properly.</p><p></p><p>If you want to go with aftermarket cams I'd highly suggest getting in touch with someone that specializes in cams and knows these cars very well. Its not something you want your average joe moron spec'ing for you.</p></blockquote><p></p>
[QUOTE="MalcolmV8, post: 15700694, member: 8854"] Yes that keeps the bypass valve open at idle with low vacuum but it doesn't fix the surging or jerking that happens when the bypass valve closes. In my testing a low pressure bypass valve takes about 3 ~ 4" of vacuum to hold it open. [IMG]https://lh3.googleusercontent.com/80O5Tk0NPM7JlvDj8D7gKsU-anFU_42JOfniV9zfZ-Lx5P81NObExd2KWCigi2lXHyJdwnH2W0ywgJ1q2yKtj3n4J7cthdJIRJvpPoW6KuaG9rsBKaNScZb-LxMFnVNjku1ue8cqFaAX_w6Pl5F5cXLv_iMMa7JGvXOK_MMGd1CY2aiUxecEFgPL89QB78Bz17iQwqbrSMHeN5fkUEj2ed4RGBOe8bjshLqoQUH0VUzaJYn-U2ZeDq_Dd7Wwr4gkNh2huddtXoWGEXs1h_LWmEfGUH0oZOdoSuzfafeAQ61MZGwoGMmZa7SBiS02oHgIY6vVjQcfMogv6x5Ib6nd1a8nJTcx8ze2NK7ZmHAyOpARBjY-zMbbA35_8muqnRlHuiR-oP9D8Waf6IJMRHn1KMM6J5mEcSMCl02YwrBTiAk4NEwtiUTPsXZyPdMiekerZqbHHD2F_G7iyq_uXiTFEkSFCRHy8sqk9BOFPgS1FLZtVte5toABjiMy4HKPcEcdYTohdOF-SSJs6Z1H4r2Svi7wx8fU8o3hN5WZgZBb535VLrghBnaJiKmtOQ481OecMht5UoiHN2V196TQHnOmGx_2XwLdAL1K76OOLFWIwjzDzGeAdIWjmrwdb_4d9Y26MILwHyMuCgBKoa3Y9ngBFO1vEejzIa4cyjQ=w800-h600-no[/IMG] The problem is when you're down to 3" of vacuum that's not very much of a vacuum at all and the rotors are filled with air. So when that valve closes there's a sudden surge or spike of air that's slammed into the motor as that happens. It can be observed on the wideband as well as felt by the driver. In normal cruise, even highway at times as you're slightly on and off the gas pedal and that valve is transitioning between open/closed the car surges and jerks. I even tried slowing down the transition of the valve. I used nitrous jets in the vacuum / boost line going to the bypass valve actuator and that helped a lot. It smoothed out the driving a fair amount. It did not cure it but it made a big difference. However it had a massive side affect. When cruising along and then suddenly stabbing the throttle wide open there was a momentary delay for the power to come because the valve couldn't snap shut instantly due to the nitrous jet. It felt like the sort of momentary lag you can feel on a turbo car. I hated it. One of the things I love about the PD blowers is instant torque and power. A stock bypass valve on the other hand is closer to 11" of vacuum to hold it open. [IMG]https://lh3.googleusercontent.com/zvTAb2rWATRsUvO0sXGeIpLWOAkGPZTDoMHUNAQAYN-x3jp8D6BcdgeF7San4Fj_zk0pr6n9wrz4cH-RrTHf0xehTFNToAY4lbCaXZsL5Kz7zmwowa9g6aZnUOr3Xw9EJ2dXkSgQd2eLr_FwUAE82LL_M_7RhFZ4_JR-rhcOgPdQivSQxKzsc3h4K1SAZNKOVTUuUAghnEz_Gk6jiQwUqbmrgTc8Qs6jWawZw7E6ZU7QW-BaEhp7PDhZyOs-k6oXB3NpG6GCb3vHuQkOzFyVyCOliwBs7jZIjLI0eyb7ASHyCHAbTJG5YDZAWq8gCpq9S0NU_5hJ5u8XklwXRbVNSW-ttEw2TGjE-vppAE7yYoIUsXFdzZnlLcf2J9u-woiECUorAtw6STMI5NIuwgmhx7cK8LH2nA73V1i8UYJcjiF1dGNXmVTbGyrGSp3o8yH8In_6tSCKqRphEjAyhJTwQ4JHFCWDLgYS90EpoevSpAN7YLzrGoJ08PUxaR9ezduv83_KLlHeMzrKMueSP8CtzXYmmf0pLYnu9yUIjf98HyBT5FlrOStpACgpSuf-bSIRkySAArC0ozpizS-I-ksshhFRn_TvNjIegrgh6ptWSs98zS27TwVMN87O1c5sA-zRpnCIZ2hANVPzhdrs27RFSpxsa7amO_g3WDg=w800-h600-no[/IMG] That's important because when the valve closes deeper in vacuum there's not much air in the rotors and you don't get a surge into the motor and its a smooth transition. When I first started out I was running 8 degrees of overlap on my cams with a 110 ICL. It had a nasty idle and low idle / midrange vacuum and drove like crap. I moved to smaller duration cams with not much luck. I then re-degreed them per the "expert" at 109 ICL with his LSA of 115 which gave me 0.5 degrees of overlap. Doesn't sound like much overlap but car still had poor idle vacuum and while it drove a tad better it seemed, it still drove horrible. After talking to many people and doing my own research and info gathering and understanding I came up with my own center lines and LSA and ended up with just -4.5 degrees of negative overlap and wow what a massive improvement. I now had about 14" of vacuum at idle and was able to swap back in the stock bypass valve actuator instead of the low pressure one and the car drove like OEM. No bucking, jerking, surging or any of that junk. It idles solid and reliable and drives properly. If you want to go with aftermarket cams I'd highly suggest getting in touch with someone that specializes in cams and knows these cars very well. Its not something you want your average joe moron spec'ing for you. [/QUOTE]
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