silver86

I don't know a great deal about blow off valve design. But, is it possible to utilize a blow off valve in a wastegate fashion so to speak?

My understanding of the main function of a BOV is to prevent compressor surge. But it seems that an adjustable BOV could be used to limit the upper boost range.

As an example, my boost levels climb too high for my application around 5200-5300 rpm. At that level, I'm pushing 11-12 psi, but want to limit to 9-10 psi total.

Maybe this is really academic, but I don't have much experience in this area.

89onlyZ51

Yeah, you could certainly do that with an adj blow off valve. A BOV is a little different from a wastegate. A wastegate reroutes exhaust gases in a turbo setup to bypass the exhaust turbine when the boost limit is reached. A BOV bleeds off excessive boost in the intake path. The only thing to be aware of is that if you're using a MAF system, you can't put the BOV anywhere after the MAF or the ECU will think the engine is getting way more air than it is when the BOV trips.

tequilaboy

Sounds like you want a boost limiter valve, or you could always slow the blower down.

Here's one:

http://www.928m.com/parts/boostlimitervalve.php

BAM92

I use these in some of the medical equipement I work on. Keeps the air compressors from going over a preset amount of back pressure if there is a restriction. Ours are usually preset at 35psi. Some can be adjusted. They usually only cost less than 20 bucks though.

BLWN BUX

Shane,

If your concerned about making too much boost the you need to change to a larger pully.

This will also help your belt slip problem as well.
Larger pullies have more contact area.

Are you using the stock tensioner?
How is it loaded?

I couldn't find the correct lenth belt for my set-up.
One was almost too long and the next was to short to install.

I had to shim out the A/C unit to load the tensioner right

silver86

The Blowerworks system utilizes the factory serpentine system, so I am using the stock tensioner.

My only thought about running the smaller pulley, is I'd like to have close to my max boost setting (utilizing the BOV as a limiter) pretty much through the entire rpm band. When I was back on the stock engine, I could get my max boost of 10psi at ~5,000, but it was a gradual climb, such is the nature of centrifugal chargers I guess.

mn_vette

Black Bart did something very similar to this and he said it worked great.

He got the smallest pulley for his blower that he could without having belt slippage. Then he welded a wastegate to his intake piping. He ran the boost reference to the wastegate from the IC piping just before the TB.

This gave him maximum boost at a much lower RPM and boost adjustability just like with a turbo setup.

silver86

That's exactly the effect that I want.

I was looking at a Greddy universal style BOV. Anyone have any thoughts or experience with these?

mn_vette

You'll want to use an actual wastegate, not a BOV.

Wastegates are designed to modulate airflow. BOV's just pop open dump air and then close, they are on/off valves.


You should be able to pick up a deltagate off of ebay for about $100 or so. Actually I think I may have one laying around my garage somewhere if you want to buy it.

silver86

That's a possibility. What spring rate does it have? I'm looking for a 10psi max.

qwiketz

that seems really simple. I've seen some fancy ways to do it electronically over the years but the setups are really expensive.

Can someone post more pics of these limiter valves? I may want to pick one up just in case my belt really decides to grab sometime.

I bet you can find it on mcmaster carr. I tried looking briefly but didn't see anything though. I'll try again later.

silver86

McMaster-Car has them listed under Pressure Relief Valves as:

Brass Adjustable Vacuum/Pressure Relief Valves

This is the exact same part as referenced in the 928m.com, but for only $31.

lead foot 85 vet

sounds like a great idea.........buy that baby....install it.......and if I can get the boost without belt slip, I'll follow your lead It would be like having a roots style blower.

qwiketz

I'm going to buy one but I'm not sure how to pressure test this thing to set it. I guess I could probably mock it up to a can of compressed air somehow... we'll see. I'll try to post some feedback once I get it an test it out.

It certainly is cost effective but do you think it'll work as good as a waste gate?

89onlyZ51

Personally, I wouldn't use one of those in an FI application. It looks a large schrader valve sized to flow well enough to bleed excess pressure from a 20 CFM air compressor, but a 400-500 CFM engine???

BOV's are more expensive because they usually have a vacuum signal line that pulls a diaphram that with leverage opens a big flapper allowing a huge amount of air to escape.

BLWN BUX

I wouldn't add another componet that could devolop problems for a couple pounds of boost.

If your concerned then I would lower you boost and add a little timing. You will probably make more power.

To belt up the blower and add a waist gate seams crazy.
If it was that simple then I would think that one of the Blower Manufacturers would be doing this?

The blower puts a drag on the engine.
To belt the blower to make boost faster is an interesting concept
But would there be additional drag on top rpm's without any added power?

qwiketz

I'm doing it to limit potential max boost, not regularly limit max boost. Turbo People in new york have a kit that limits max boost but it's a little pricey but top notch equipment. With my setup there's a bunch of belt slip but if it ever does grab, 5 extra lbs of boost wouldn't surprise me. I just want to safeguard my engine a bit.

JoBy

You should try a restrictor on the supercharger inlet instead.

* A passive unit with no moving parts.

* It does not restrict flow during WOT at lower engine speeds.

* WOT at higher engine speeds it creates a slight vaccum at the supercharger inlet. That vaccum is mutliplied by the supercharger pressure ratio. Let's say that you had 14.7 psi max boost before the restrictor with atmospheric pressure at the supercharger inlet ( Pressure ratio = 2 ). A slight vaccum, 2.5 psi, at the supercharger inlet will result in ((14.7-2.5)/14.7) x 2 = 1.66 total pressure ratio. 0.66 * 14.7 = 9.7 psi boost.

* With less dense air the supercharger will rob less hp from the crank. This will reduce belt slip at high RPM as the supercharger only has to compress the air used by the engine and only to the boost level required.

* If you dump air after the supercharger to reduce boost then the air flow thru the supercharger will increase and more hp will be robbed from the crank. This is not a good idea.


Look at this Vortech compressor map.



One example.
You have the pulley ratio set so the supercharger has 40.000 rpm when the engine is at 6000 rpm. The supercharger working point is somewhere along the red '40.000' line depending on the air flow thru the supercharger.

The starting point is the blue dot in the middle. Pressure ratio is 1.8 ( = 12 psi boost ). The supercharger flows 850 CFM.

You want to reduce boost to 10 psi at 6000 rpm on the engine.

If you add a Pressure Relief Valve to reduce boost you will move to the right along the red '40000' line. A pressure ratio of 1.7 will give you 10 psi boost. As you see the air flow thru the supercharger has increased to 1050 CFM. At the same time the engine uses less air because of the lower boost, about 800 CFM. About 250 CFM goes out thru the Pressure Relief Valve.

With a restrictor on the inlet inlet you move to the left along the red '40.000' rpm line instead. You reduce the air flow to 800 CFM, same as above and that reduces boost at the engine to 10 psi. The vaccum before the supercharger will be about 0.8 psi = 1.6 InHg. Not much.

The supercharger will require a lot more HP from the crank to compress 1050 CFM at 65% efficiency compared to 800 CFM at 70% efficiency.
You need those hp more at the wheels.

Information about restrictors: http://www.raetech.com/Restrictors/R...r_Function.php

silver86

Grrrr! Stupid web filter. I'll see if I can't access it wirelessly here shortly.

89onlyZ51

I like the idea of an inlet restrictor because of the simplicity and it accomplishes the goals, but I have a question about the benefit to parasitic loss. It seems to me this would only be a very minor consideration compared to a BOV because only a small % of the loss is due to the air friction (denser v. less-dense air going through the blower). The vast majority is due to compressing large volumes of air and the heat generated (lost energy) as a result. Bleeding off boost (or, more accurately, not allowing the boost - and heat - to build) would prevent the majority of parasitic loss over max boost, wouldn't it?