alritzer

Does the RX7 come from the factory with the BOV installed? If not, do I really need a BOV? If yes, are the more expensive ones a lot better than the $50 eBay ones?
thanks, ashley

alritzer

I found it by using google search.

Evil Aviator

Just in case your Google search came up with some bogus internet info:

The car comes stock with a compressor bypass valve, which is like a BOV but when it vents it feeds back into the system to keep the engine running the correct air/fuel ratio. Were it to vent to the atmosphere, the engine would run rich between shifts, causing the engine to bog a bit and reduce acceleration performance.

No, you don't need a BOV or a compressor bypass valve.

Yes, the more expensive ones are better. The cheap ones tend to leak and suck unfiltered air into the system under vacuum.

HoustonMS3

"No, you don't need a BOV or a compressor bypass valve."
Compressor surge can seriously damage your turbo, or is if different some how for a turbo on a rotary?

1SWEET7

iTrader: (1)

There is lots of conflicting information on this subject. Arghx has done some testing running without a BOV, I currently run without a BOV and have had no ill effects. The Australians have been running some serious HP cars without BOV's, a lot of race cars including F1 cars of the past have run without BOV's. Search RiceRacing and you will find some of his old posts. From what I could tell by researching the stock BOV is there purely to reduce noise. I'm sure someone will say that damage to the turbo will occur, but we will see. I will definitely post the results of my NO BOV experiment if anything unexpected happens.

HoustonMS3

I am not knowledgeable enough to really comment on the lower psi and other contributing factors of a stock rx7 turbo, but I would imagine most the same principles apply.

There is definitely a reason whey they come on turbo street cars. If you are pushing 20 lbs boost and the throttle snaps shut that pressure can double (if not more) between the throttle body and turbo instantly, and it has the capability of just snapping the shaft on the turbine in the short term and wearing your bearings susbstantially in the long term.

I would imagine that F1 and serious drag cars (a lot of drag cars don't have intercoolers also...) are not as concerned about durability because of their budgets and relatively limited miles driven on it, and possibly because they are always under throttle unlike a street car.

Spirit-RE

iTrader: (25)

On a stock ecu FC with the afm, I would run a bov just because when it surges, its going to slam the door on the afm shut...

Evil Aviator

If you look in the FSM you will see that the bypass valve is just there to reduce noise. Most people who buy aftermarket BOV's just want the noise amplified, lol. Most turbocharged aircraft engines don't have blowoff valves, and this certainly would not be the case if it caused excessive damage. A BOV or bypass valve may help the turbo last longer on sports cars, although this depends on the boost level and the construction of the turbocharger. On the good side, a BOV or bypass valve will not hurt anything as long as it doesn't leak, so if you like it then use it. However, the answer to the original question, is no, it is not needed.

Forum discussion on the subject:
https://www.rx7club.com/forum/showthread.php?t=884695

It is for noise reduction, just like intake baffling, body sound deadening material, and exhaust mufflers.

Look at the factory diagram, and you will see that in the stock configuration 100% of the bypass air shoots back between the turbo and AFM. This is by design, and it works just fine.

arghx

iTrader: (3)

I consider BOV a personal preference issue. The stock air bypass valve is fine. If you have the stock intercooler and you don't care about sound, there's usually no reason to replace it. I'm sure somebody somewhere on the internet will claim that it leaks. I have bench tested an FD stock air bypass valve, which from what I can tell is pretty much the same as an FC, and found it to hold upwards of 30+ psi. In fact it blew off my rigged-up tester before it leaked.

And yes, my used turbo is still holding up fine to no BOV (I do not have the stock ECU). But that doesn't mean I am encouraging everyone to throw their BOV in the trash. If you have the stock ECU/Rtek and want a fully atmospheric BOV for the sound, plus you are willing to accept the engine running rich between shifts, then by all means go ahead and get one.

Black Knight RX7 FC3S

iTrader: (7)

Im not sure about the stock turbo, but on the turbos of todays, especially ball bearing ones, and ones that has a thicker shaft, you dont need a BOV, especially if you are running at low boost.

Yes it stalls the impellers when it surges, but so far from what I know, it will not damage the turbo but its good to just have one if you want to be safe, or just like the noise :P

HoustonMS3

Interesting, I'm reading about people running 100K miles at 15 psi with not BOV or BPV still on the stock turbo on other cars. Give it a whirl

Frostycrowd

iTrader: (9)

I don't run a BOV just to make everyone argue with me how I need one, which as Evil said, is not the case.. Its a great conversation at car meets.

Really though, I seem to have better response from the turbo on shifts without a BOV.

arghx

iTrader: (3)

Haha, this is so true. Most people are just confused though. They will see you pull in and compliment you on your "BOV" but then then when you pop the hood they're like uhhh...

Some people will claim better response without a BOV. Some will claim better response with [insert brand here] BOV, especially Synapse nutswingers. I can't tell much of a difference really.

SirCygnus

iTrader: (1)

with an airflow meter would run a bov if you need to pass emissions and it will cause a rich spike when you close the throttle.

if you dont need emissions, yank the leaky ****** and enjoy better responce.

i dont run one cus they tend to be dumb, and idiots love to argue about how it will make the compressor surge and or stall. but if i recall, stalling a compressor isnt a bad thing at all.

sharingan 19

iTrader: (16)

Exactly what I was thinking...

Lmao! I would LOVE to see that one...

My BOV flange didn't ship on time so when I got my v-mount welded up I had no BOV. I did notice slightly better response, but the turbo (stock S5) started leaking oil about a month later, it was perfectly dry prior to that.

No BOV might be just fine on aftermarket turbos (maybe even hybrids) but it is not a good idea on the stock units.

HoustonMS3

Not on an rx-7, I was reading some other car forums to see how many others don't use a bov.

Evil Aviator

Depends on the application.

Surge is also a function of the turbo design and the airflow of the engine, so it does not always relate to the relief valve. Stalling a compressor may or may not be a problem, as this depends on the construction of the turbocharger and the conditions in which it stalls. It may not cause any damage, or it may ruin the thrust bearings.

Overboosting is hard on the stock turbos, regardless of the BOV configuration. However, a relief valve would reduce the stress on the turbo, which could make a difference if the turbo is operating past its intended boost range.

The stock RX-7 turbo is a standard Hitachi unit that is similar to other turbos of the same era. The engine type doesn't make any difference in this case.

alritzer

Thanks everybody. When I first asked the question nobody immediately replied so I thought it must have been a really dumb question.

I glad everybody responded, it is more info than I found using Google.

ash

SirCygnus

iTrader: (1)

dont worry about getting an instant response. sometimes it may take a day or two sometimes. if you dont get a reply, then thats usually better than getting another flame war filled thread.

there is a large thread about it in the single turbo section of the forum.

https://www.rx7club.com/single-turbo-rx-7s-23/8-months-counting-no-blowoff-valve-884695/

alritzer

Thanks for the link. I'll check it out.

ash

arghx

iTrader: (3)

1st gen turbo engines (12A turbo) did NOT have a blowoff valve, or an intercooler for that matter. Here is the turbo on the 12AT



It is labeled as an HT18S. The 2nd gens had an HT18S-2S. Here are pics of the piece that connects the turbo directly to the downdraft-style throttlebody:





You will notice that there is a pop-off valve for overboost protection. This is similar to what you would see on the turbo Z cars of that era.

HoustonMS3

Amongst many other design features they got rid of for a reason...

1SWEET7

iTrader: (1)

Which the reason seems to be... Noise reduction. Good info Arghx, as usual.

SirCygnus

iTrader: (1)

only when they sent it to the usa. we got a crap version and everyone else got permanent and only turbo rotary goodness.

HoustonMS3

Yes, I've noticed stock JDM S5's with no BPV.

And the 12AT produced 6psi, maybe not enough heat is generated from the compression of 6 lbs to require and intercooler, and a properly sized turbocharger does not require a valve.

I think "It is for noise reduction" attitude is patently false. It is not a safe assumption across the board for all turbos, boost levels, and applications.

I work around turbo Diesel motors for John Deere, and certain smaller turbos will destroy themselves when the machine is running full throttle and turned off without idling down. The back pressure of the turbo spooling into a closed block will almost instantaneously result in shaft play. But I think I've gone way off tangent of the original post.