What size BOV and wastegate
Thread Starter
Teching In
Joined: May 2025
Posts: 20
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What size BOV and wastegate Have an LQ4 bored 10 over and 4" stroke with -7.8cc dished piston I have both 243 and 317 heads (was going to run 317 but open to hear opposing opinions) but my main question is what size BOV and wastegate i should get i have a g42 want to run low boost for now but do have plans to go more bananas in the future tyia
2 44mm wastegates and 1 50mm bov. With my 6.2 I got some boost creep with 2 44 wastegates, so I now have 2 50s. I use ebay wastegates and bovs exclusively but I know to check them periodically. Your cylinder head choice would come down to fuel type to me and power goals of course. I like the extra compression, so with e85 I would use the 799s. Pump gas I would use the 317s.
I'm going to give a pair of gen 2 VS racing 44mm waste gates a try and a pair of VS Racing 50mm bovs. Took them apart to loctite all the set screws and they pressure/vac tested great. Diaphragms can be swapped with the Tial MVS ones.
Gen 2's, so still diaphragm but the gen 3 pistons are better. One guy buys the cheap summit 44mm diaphragm waste gates and just replaces them each year.
Gen 2's, so still diaphragm but the gen 3 pistons are better. One guy buys the cheap summit 44mm diaphragm waste gates and just replaces them each year.
Depends on the WG priority positioning more than anything. 2 cheap diaphragm 38mm gates is more than enough if placed well. And if you service them before installing and maintain them they will last for many years. Had the same $60 38mm china gates on my rx-7 for 10+years.
I’m not a fan of BOV’s. You don’t need to run any. Mainly just for sound. I’ve yet to see any data showing turbo damage from not running one. Or any data that shows improved response with one. Definitely don’t see the need to run 2. I don’t hate the sound, just one more thing to go wrong and added weight/$ for no purpose.
I’ve actually seen data from some road race guys that did laps with and without the BOV and monitored turbo speed. The turbo RPM went up without a BOV between shifts and down with a BOV. Lap times and boost response was actually slower with a BOV FWIW. Opposite of what I though was going on when the turbo “surged/fluttered” between shifts.
I’m not a fan of BOV’s. You don’t need to run any. Mainly just for sound. I’ve yet to see any data showing turbo damage from not running one. Or any data that shows improved response with one. Definitely don’t see the need to run 2. I don’t hate the sound, just one more thing to go wrong and added weight/$ for no purpose.
I’ve actually seen data from some road race guys that did laps with and without the BOV and monitored turbo speed. The turbo RPM went up without a BOV between shifts and down with a BOV. Lap times and boost response was actually slower with a BOV FWIW. Opposite of what I though was going on when the turbo “surged/fluttered” between shifts.
I would be a bit concerned with my throttle body blade with no BOV. They were not really designed for several psi boost spikes to hit them when fully closed. I know factory cars were made with no BOVs, but they were all auto trans cars at low boost levels. Not sure what kind of premature wear or damage high boost and a manual trans might do to a TB blade and shaft with no BOV.
I would be a bit concerned with my throttle body blade with no BOV. They were not really designed for several psi boost spikes to hit them when fully closed. I know factory cars were made with no BOVs, but they were all auto trans cars at low boost levels. Not sure what kind of premature wear or damage high boost and a manual trans might do to a TB blade and shaft with no BOV.
I’ve run them on a few of my setups thinking it was the right thing to do in the past. But I keep coming back to the “why bother attitude”. If in the end its just additional time/money for no real purpose.
I believe in the typical street/strip auto drag stuff I build they don’t seem to be necessary. I don’t appear to do damage to any parts without one or suffer any performance loss. Never hurt a TB or turbo (to the extent I could tell anyway.) in the past 20+ years of playing with turbo cars anyway.
Ran 34lbs regularly on my DSM with factory TB. The blade on those are thinner than the LS TB. 30+ on 2jz stuff, 25 or so on GN stuff... etc etc... Hell my 1500cc wave runner sees 16lbs or so at 8600rpm and has no BOV from the factory. And its a centri blower motor! TB on it is tiny thin stuff.
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So, when I first built my last turbo LS I got used Tial WG's and a BOV.
The BOV unbeknownst to me was stuck shut and it took a friend with turbo experience to ride with me to point it out.
After I fixed it, I noticed a definite lull in the power between shifts, it was a T56 car and I assume this is to the boost loss whereas when the BOV was stuck shut it seemed way more responsive on the shift.
I toyed with blocking off the BOV for the entire rest of the time I had the car, it was that noticeable.
The BOV unbeknownst to me was stuck shut and it took a friend with turbo experience to ride with me to point it out.
After I fixed it, I noticed a definite lull in the power between shifts, it was a T56 car and I assume this is to the boost loss whereas when the BOV was stuck shut it seemed way more responsive on the shift.
I toyed with blocking off the BOV for the entire rest of the time I had the car, it was that noticeable.
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Verdad Gallardo I hear ya... Not saying I 100% think a BOV is a waste of time in all applications. They make total sense in my head. Just haven’t seen the data to backing up their purpose in my application.
I’ve run them on a few of my setups thinking it was the right thing to do in the past. But I keep coming back to the “why bother attitude”. If in the end its just additional time/money for no real purpose.
I believe in the typical street/strip auto drag stuff I build they don’t seem to be necessary. I don’t appear to do damage to any parts without one or suffer any performance loss. Never hurt a TB or turbo (to the extent I could tell anyway.) in the past 20+ years of playing with turbo cars anyway.
Ran 34lbs regularly on my DSM with factory TB. The blade on those are thinner than the LS TB. 30+ on 2jz stuff, 25 or so on GN stuff... etc etc... Hell my 1500cc wave runner sees 16lbs or so at 8600rpm and has no BOV from the factory. And its a centri blower motor! TB on it is tiny thin stuff.
I’ve run them on a few of my setups thinking it was the right thing to do in the past. But I keep coming back to the “why bother attitude”. If in the end its just additional time/money for no real purpose.
I believe in the typical street/strip auto drag stuff I build they don’t seem to be necessary. I don’t appear to do damage to any parts without one or suffer any performance loss. Never hurt a TB or turbo (to the extent I could tell anyway.) in the past 20+ years of playing with turbo cars anyway.
Ran 34lbs regularly on my DSM with factory TB. The blade on those are thinner than the LS TB. 30+ on 2jz stuff, 25 or so on GN stuff... etc etc... Hell my 1500cc wave runner sees 16lbs or so at 8600rpm and has no BOV from the factory. And its a centri blower motor! TB on it is tiny thin stuff.
Thread Starter
Teching In
Joined: May 2025
Posts: 20
Likes: 6
I hear ya... Not saying I 100% think a BOV is a waste of time in all applications. They make total sense in my head. Just haven’t seen the data to backing up their purpose in my application.
I’ve run them on a few of my setups thinking it was the right thing to do in the past. But I keep coming back to the “why bother attitude”. If in the end its just additional time/money for no real purpose.
I believe in the typical street/strip auto drag stuff I build they don’t seem to be necessary. I don’t appear to do damage to any parts without one or suffer any performance loss. Never hurt a TB or turbo (to the extent I could tell anyway.) in the past 20+ years of playing with turbo cars anyway.
Ran 34lbs regularly on my DSM with factory TB. The blade on those are thinner than the LS TB. 30+ on 2jz stuff, 25 or so on GN stuff... etc etc... Hell my 1500cc wave runner sees 16lbs or so at 8600rpm and has no BOV from the factory. And its a centri blower motor! TB on it is tiny thin stuff.
I’ve run them on a few of my setups thinking it was the right thing to do in the past. But I keep coming back to the “why bother attitude”. If in the end its just additional time/money for no real purpose.
I believe in the typical street/strip auto drag stuff I build they don’t seem to be necessary. I don’t appear to do damage to any parts without one or suffer any performance loss. Never hurt a TB or turbo (to the extent I could tell anyway.) in the past 20+ years of playing with turbo cars anyway.
Ran 34lbs regularly on my DSM with factory TB. The blade on those are thinner than the LS TB. 30+ on 2jz stuff, 25 or so on GN stuff... etc etc... Hell my 1500cc wave runner sees 16lbs or so at 8600rpm and has no BOV from the factory. And its a centri blower motor! TB on it is tiny thin stuff.
Going with an aftermarket 102mm TB is just asking for tuning problems without getting any benefit in return on a turbo setup. A stock TB will flow enough under boost to make 4 digit power numbers. The BOV discussion is sort of a side issue that is unlikely to make a lot of difference to you either way.
Going with an aftermarket 102mm TB is just asking for tuning problems without getting any benefit in return on a turbo setup. A stock TB will flow enough under boost to make 4 digit power numbers. The BOV discussion is sort of a side issue that is unlikely to make a lot of difference to you either way.
I'm running a 105mm Holley TB now only because the car came with it and fortunately, Pat G. has it pretty dialed in.
Every now and then I'll get a dip/bog in the idle when the second cooling fan comes on but that's about all I've noticed.
I don't think my RAF is totally dialed in for the cooler spring/summer mornings but that's my fault.
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Joined: Nov 2014
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Going with an aftermarket 102mm TB is just asking for tuning problems without getting any benefit in return on a turbo setup. A stock TB will flow enough under boost to make 4 digit power numbers. The BOV discussion is sort of a side issue that is unlikely to make a lot of difference to you either way.
For the record, I do have a small BOV even though my setup is automatic and a small TB. Just peace of mind.
I don't run a wastegate.... boost is controlled by the TB.
Want to size the TB to the motors "NA" needs. Going with less will make less NA power. So it will make less power per pound of boost. So if ya pick up power with a 102mm NA, then it will pick up more for each pound of boost.
The issue is drivability at low throttle inputs. A mild 5.3 sure doesn't need a 100mm+ TB. I have a 102mm on my 5.3 as it came with the elbow I run. It does have a pretty large "cam" in the throttle cable mount. That opens the blade really slowly at first. So its not horrible to cruise around on IMO. But the factory 78mm def drove better.
The issue is drivability at low throttle inputs. A mild 5.3 sure doesn't need a 100mm+ TB. I have a 102mm on my 5.3 as it came with the elbow I run. It does have a pretty large "cam" in the throttle cable mount. That opens the blade really slowly at first. So its not horrible to cruise around on IMO. But the factory 78mm def drove better.
What sorts of tuning problems do people encounter with those larger 100+mm TB's?
I'm running a 105mm Holley TB now only because the car came with it and fortunately, Pat G. has it pretty dialed in.
Every now and then I'll get a dip/bog in the idle when the second cooling fan comes on but that's about all I've noticed.
I don't think my RAF is totally dialed in for the cooler spring/summer mornings but that's my fault.
I'm running a 105mm Holley TB now only because the car came with it and fortunately, Pat G. has it pretty dialed in.
Every now and then I'll get a dip/bog in the idle when the second cooling fan comes on but that's about all I've noticed.
I don't think my RAF is totally dialed in for the cooler spring/summer mornings but that's my fault.