Dual 850 dp,s atop a weiand 8-71 with a 4" cowl hood.Enderle style,Hilborn?I am concerned with intake restriction and the small air filters that fit in these type scoops.Any suggestions for looks vs hp loss?

George,
I can't answer your question in regards to the air filters in the blower scoop but a couple of guys here can. rat3j396 Jerry has a blown 427

prostreet69camaro
Mike has a blown 502

Either of them should be able to answer your question. BTW,do you have any pics of your ride? We sure would like to see it!!!!!

I dont run air cleaners in my scoop. I tried it a couple of times and it made the motor run richer and I did not want to start messing with the jets. It took me a while to get it dialed in.

David..I must say that is one fine pro street 69.:thumbsup:
My 68 should be up and running in 30 days.I am close to finishing and getting some thoughts and suggestions from the Pro Street crowd.By all means..I will post some pics when I figure out how.I can boost referance carbs and correct rocker geometry..but my fat old fingers turn on and off several features with a digital camera.lol.Aside from the hp loss.what do you think about blower scoops.Some say the enderle style is plain wrong unless fuel injected(looks like ET from the movie).The Hilborn is classic,I like them both.As you can imagine..I did not opt for dual 850,s just to choke off the 8-71 with restrctive filters.Just to add,this pro street forumn is long overdue.Big engines and steam rollers never go out of style imho.:hurray:

I dont run air cleaners in my scoop. I tried it a couple of times and it made the motor run richer and I did not want to start messing with the jets. It took me a while to get it dialed in.

I sometimes get into dusty conditions in my commute.Always seems to be some type of road construction going on with concrete saws billowing concrete dust.I can not see how this would benefit my rotor to case clearances.That shotgun scoop is a mean look for sure...but i will filter the air on the street.The rotors and case are not hard anodized...just a out of the box Weiand 8-71.

I've had this motor in another pro-street Camaro (1981) and a '69 Chevelle. It has a 671 Weiand and 2 Holley 750 DP. I use two 6 inch x 2 inch high ( i think) round filters. I've never had a problem. However, I've also had a 6000 RPM pill in the MSD at all times. I've used the "paper' filters, but K & N makes filters that size, I believe. That should improve air flow.

I'm having the same problem just hight of hood clearence I can only get a 14x2 filter and my buddy says a 14x3 would be better and I'm only at 500hp.Another words I'm no help either.Sorry

If the filters can't handle the cfm, they will probably collapse from the vacuum. The screen type gasket between the blower casing and the carb mounting plate provides extra insurance from objects being sucked down into the motor. I have a friend who left the foam cover in a dragster scoop (fiberglass) once, and the vacuum created collapsed the scoop. And that was on a naturally aspirated motor.


...as for the Shotgun scoops...
ET?! I was thinking they looked more like "ALIEN"

I had the older style but opted for this style...just like anything on a custom car...it's about what YOU like, not so much what others think.

Good luck with your car George, can't wait to see it!

George,
Speedway Motors sells the shotgun scoop. They sell them polished or unpolished / blemished. I would not buy the polished one because it still has blemishes and when you polish it you will not be happy. I bought the blemished one and had it chromed.

http://i268.photobucket.com/albums/jj16/prostreet69camaro/DSC09525.jpg

http://i268.photobucket.com/albums/jj16/prostreet69camaro/DSC09519.jpg

David..I must say that is one fine pro street 69.:thumbsup:
My 68 should be up and running in 30 days.I am close to finishing and getting some thoughts and suggestions from the Pro Street crowd.By all means..I will post some pics when I figure out how.I can boost referance carbs and correct rocker geometry..but my fat old fingers turn on and off several features with a digital camera.lol.Aside from the hp loss.what do you think about blower scoops.Some say the enderle style is plain wrong unless fuel injected(looks like ET from the movie).The Hilborn is classic,I like them both.As you can imagine..I did not opt for dual 850,s just to choke off the 8-71 with restrctive filters.Just to add,this pro street forumn is long overdue.Big engines and steam rollers never go out of style imho.:hurray:

Thanks for the compliment George:yes:. As for my preference on a blower scoop,the Hilborn looks better to me than that scoop that Mr. Gasket sells that "looks" like an Enderle injector hat. I just about puke when someone pulls up next to me in their "beater" and the first thing you notice about their car is that thing that they had to cut a hole in their hood just to put it on and the linkage to open the butterflies isn't even hooked up:rolleyes:l:).Now if we're talking fuel injection,the "bug" or "bird catcher" is the only way to roll:D:cool:.About posting pictures,you are welcome to contact me via a PM(private message) and you either send them to me that way or I can give you my email address to send them to me.

Thanks guys.First off,i would like to say how impressed i am with the qaulity and workmanship that shows in your builds.:thumbsup:.
FWIW..I am leaning towards the Hilborn style and thanks jerry,I will fill it with K@N,s and wartch for collapse.
Mike..is that a 6" cowl hood?Mine is a 4".Curious if my upper pulley will be exposed.
Thanks David for the offer to post pics.If my dumba$$ can not figure out how to post pics..I will hire the 3rd grader next door..or take you up on the offer.:D.
Can i pick your brains?This may belong in performance or engine,but I am looking for a baseline start up with the Holleys.
I purchased 2 850 dp,s new out the box.They are fitted with #80 main jets front and rear and no numbers on the pv,s front and rear.My Holley referance book state they are 6.5,s.I will test them with a vac gauge to be certain.
I plan on blocking off the secondary pv and upping to #88,s.I plan on leaning out the primary to #78 and keeping the primary 6.5 pv.I blocked off the vac source in the main body for both pv chamber and port vacuum.
I drilled on an angle into the pv chamber and will use the ported vac nipple and source it under the blower.
The carbs have 4 corner idle mixture screws..will this set up ballpark me until I can get some running vac/boost #,s?The .030 over 460 will have around 7.5/1 static..cam will be dual pattern with mid .500 lift and 240ish @.050 with a 114 lsa.heads flow I350/E275cfm @.600" lift.I will test fire and break in n/a,then install the 8-71 12% underdriven and tune from there.Am i close with the carbs for baseline??:)

George,

I joined this forum a short time ago, and these guys are VERY skilled and knowledgeable, not to mention helpful!

Here are some pics with a 4" cowl hood with a 671. The members of the forum even gave me guidance on how to measure and cut it! As you will see, I opted to cut the hole a bit close, which causes me to remove the scoop before opening the hood. A body shop had cut the hood on my Chevelle like this a while back, and I just liked the look. I may end up trimming the rear of the opening a bit, but hopefully this will give you a general idea of what to expect. If you're racing, it may be a bit of a hassle.

You can get by with not cutting for the pulleys if you like. You can use smaller diameter pulleys and still maintain underdrive/overdrive ratios. There is a grid in the back of the Holley/Weiand catalog, or I believe it is available online on their website tech area. My blower uses the 1/2 pitch pulleys. I have a '34' on top, and the hood clears it. The 871 may be longer, which may make this info irrelevant, so i apologize if that is the case.

I wish I would have spoken with Mike earlier about the scoop. You can see how much better his looks chromed versus the polished one I have.

Jerry

George,
Since I don't have a big ol' blower on my car I can't help you with your tuning issue but I know either Jerry or Mike will know how to help you :yes:. Kinda wish now that I had put a blower on my car:(.

Thank you for the pics Jerry.It helps me visualize.My 8-71 uses the 8mm belt and a 61 upper/54 lower for now.:D.i was looking at Mikes side views of his pro street and it appears to be a 6 inch cowl..maybe a 4".Hopefully Mike chimes back in.I would like the upper pulley exposed on this little street deal only.A little bit of show.a little bit of go.:hurray:Nothing fancy..just an honest 700 hp blown big block in a first gen.:).

David, your '69 is Badass just like it is!

George,

I have the 4" cowl hood and I had to cut the hood past the front pulley. I cant remember what size pulley is on top but it is like 54 on top and 52 on the bottom which is almost 1 to 1. It puts out 8 psi boost. I have a 502 and 2 -750 carbs. I had 2-850 that flowed 930 cfm each which was to much for the street. It ran real rich and fowled plugs. I tried different jets and it still ran rich at idle. I ended up with the 750's and started leaning it out and it is better now. I had to much cfm for a 502 and you might run into the same situation. If it is a race motor then you wont have a problem. The problem is running it on the street. Once you start leaning it out it will start running hotter.

I forget which blower site has it but it tells you how much cfm you need with the cubic inch motors. It was either Holley or Weiand that I got the info from. Another good source is Big Als Toy box @ www.bigalstoybox.com. I talked to Al and he was very helpful.

The carbs are blower referenced and the motor puts out about 14/16 inches of vacuum. The way you did your reference should be fine. Also post this in the performance section and there is a guy name Royce that is real good on blower motors. Maybe he will chime in.

As far as prostreet cars are they are all badass to me and I ran mine with a 406 small block for 10 years. All my buddies said once you go to a blower motor you wont go back. They are right and my wife said the same thing. When you pull into a show with the blower whining it will get everyones attention.

David, your '69 is Badass just like it is!

Thanks Jerry :beers:

Thank you all for your feedback.I am patiently awaiting the short block..should be only days out.Once i recieve it..I can start the mock up.Once the pushrod length is established..I hope to fire up before attending a wedding May 17 in Vancouver BC.In a perfect world..I would be seated beside Don Hampton,Larry Payne or Ed Pink for the 5 hr flight.:yes:

George,
You may want to check out

www.carsofthetimes.com

Go to the "garage sale" area. Joe is a friend of mine, and he has a scoop for $35 but it is missing the base plate...I believe those are available or easily made. Anyway, it would be a cheap way to see if that is what you want and how it looks on your Camaro. If you don't like it, you could probably sell it and at least get your money back. I sold mine like that at a swap meet for $100. I've bought parts from him before, and they've always been good deals.

I believe he'll ship it, but if he won't, I can get it and ship it to you if you want it.

I went with the Garlits Scoop set up on my car. The quality is great. Not like the cheaper looking Mr. Gasket version. Overall I'm happy with it and I don't have any flow issues with a pair of 3 inch tall K&N filters. The only thing I'd do different is I like the "Barn Door" version better tha the round that I chose. I may sell this one and switch the the Barn Doors in the future. They're not cheap, but the quality speaks for itself.

Here's info on the scoop:

http://www.alsblowers.com/sc520.html

They make different versions for different applications.

Here it is on my car.

http://img180.imageshack.us/img180/7226/c3ua0.png (http://imageshack.us)
http://img131.imageshack.us/img131/2998/p8120002ly5.jpg (http://imageshack.us)
http://img180.imageshack.us/img180/8505/p8120004ja1.jpg (http://imageshack.us)
http://img404.imageshack.us/img404/7982/p8120005zo4.jpg (http://imageshack.us)
http://img245.imageshack.us/img245/6420/p8120003sb5.jpg (http://imageshack.us)

That Garlits style scoop looks very close to an actual injector hat :yes:. I like it.There is no comparison to the Mr. Gasket scoop:noway:

Also check out the Accel Cool Blue filters, much like the K&N's cotton, oiled filters that flow real well. If you are going to drive the car on the street air filters are a must. I have the Accel filters and haven't had any issues at all. You can buy a brand new scoop from www.dragparts.com or www.goodvibrations.com (same place) for right around $100 brand new. This is a very good source for reasonably priced blower parts (pulleys, belts, gaskets, linkage, etc...). I have done business with them before and they have treated me well.

... You can buy a brand new scoop from www.dragparts.com or www.goodvibrations.com (same place)....

Royce,,
the Good Vibrations link took me to an interesting site..... not really what I was expecting, but nice :yes:

I think you meant to type goodvibesracing.com (http://www.goodvibesracing.com)

lol, ooops!! I was going on the top of my head with the web addresses. Glad you figured it out.

Great links guys to cool and affordable blower accys.Royce..can you read back to my post regarding the 850 dp,s and baseline tune and give me your thoughts/suggestions?

Fatblock,
I couldn't find the post you are talking about. If you don't mind put a link in your next post and I will take a look. Heading out to work in 30 minutes so, I might not be able to respond until tomorrow.

Thanks guys.First off,i would like to say how impressed i am with the qaulity and workmanship that shows in your builds.:thumbsup:.
FWIW..I am leaning towards the Hilborn style and thanks jerry,I will fill it with K@N,s and wartch for collapse.
Mike..is that a 6" cowl hood?Mine is a 4".Curious if my upper pulley will be exposed.
Thanks David for the offer to post pics.If my dumba$$ can not figure out how to post pics..I will hire the 3rd grader next door..or take you up on the offer.:D.
Can i pick your brains?This may belong in performance or engine,but I am looking for a baseline start up with the Holleys.
I purchased 2 850 dp,s new out the box.They are fitted with #80 main jets front and rear and no numbers on the pv,s front and rear.My Holley referance book state they are 6.5,s.I will test them with a vac gauge to be certain.
I plan on blocking off the secondary pv and upping to #88,s.I plan on leaning out the primary to #78 and keeping the primary 6.5 pv.I blocked off the vac source in the main body for both pv chamber and port vacuum.
I drilled on an angle into the pv chamber and will use the ported vac nipple and source it under the blower.
The carbs have 4 corner idle mixture screws..will this set up ballpark me until I can get some running vac/boost #,s?The .030 over 460 will have around 7.5/1 static..cam will be dual pattern with mid .500 lift and 240ish @.050 with a 114 lsa.heads flow I350/E275cfm @.600" lift.I will test fire and break in n/a,then install the 8-71 12% underdriven and tune from there.Am i close with the carbs for baseline??:)

Your thoughts Royce.

George,
Your tune up plan seems fine to me. Usually when you remove/block the PV you go up 10 jet sizes, I see you are thinking 8. Mine seconday PV is blocked and the jet size is 10 larger and works perfect. I don't know if you really need to break in or tune it N/A first but, that is up to you. The blower really doesn't hurt anything as long as you aren't beating on it until you get it tuned. The tune up will have to change from N/A to blown anyway. Timing makes a huge difference with a blower as well. They love initial timing and will run much cooler that way. Not sure what you have planned for a distributor or ignition but, make sure your distributor is easily adjusted (meaning initial advance and total timing can be limited).

With 7.5:1 compression and the cubic inches I think you will find that 12% under is not going to produce that much boost. That is a good place to start and you can always increase it as you feel the need.

George,
Your tune up plan seems fine to me. Usually when you remove/block the PV you go up 10 jet sizes, I see you are thinking 8. Mine seconday PV is blocked and the jet size is 10 larger and works perfect. I don't know if you really need to break in or tune it N/A first but, that is up to you. The blower really doesn't hurt anything as long as you aren't beating on it until you get it tuned. The tune up will have to change from N/A to blown anyway. Timing makes a huge difference with a blower as well. They love initial timing and will run much cooler that way. Not sure what you have planned for a distributor or ignition but, make sure your distributor is easily adjusted (meaning initial advance and total timing can be limited).

With 7.5:1 compression and the cubic inches I think you will find that 12% under is not going to produce that much boost. That is a good place to start and you can always increase it as you feel the need.

Thanks for your input Royce.I will up the blocked off secondarys to#90 as suggested and retain the boost referanced primary 6.5 pv and #78,s.I initially had 750 dp,s in mind..but got both 850,s new with a swap for $300 out of pocket.These carbs appear on paper to be large by huge for a 460..but I believe I can make them behave atop the 8-71.
You got me thinking..it would be a waste of time breaking in the flat tappet cam n/a.As long as it is not pig rich at 2k break in and lead to fuel washed cyl walls,the cam lobes will not give a rats a$$ what induction package is atop.
I plan on reusing my msd dizzy with the crane hi6r cd box.I currently have the 18* stop bushing installed with light springs.I am thinking 16* initial + 18* mechanical for a total of 34* all in at 2800 rpm.No vac advance.I have the manual retard knob right now and can purchase the map/baro sensor if needed.I will spend the $200 for a vac dizzy if you feel I could use some more advancs at low rpm/cruise speed.
As for boost..the Weiand kit comes with the interchangeable 61/54 pulleys.I think firing up and tuning underdriven is the safest bet right now.Am i far off,thinking the 11% underdriven 8-71 will supply 7-8 lbs boost at 6k?I am thinking 1 to 1 may be might sweet spot,but will not know until road tested and evaluated in the little f-body street machine.
Hearing from folks that have real world experiance with roots type blowers can save a ton of money and headaches.I appreciate all the suggestions and look forward to sharing my end results/what worked/what did not.
Sincerely George.:)

Thanks for your input Royce.I will up the blocked off secondarys to#90 as suggested and retain the boost referanced primary 6.5 pv and #78,s.I initially had 750 dp,s in mind..but got both 850,s new with a swap for $300 out of pocket.These carbs appear on paper to be large by huge for a 460..but I believe I can make them behave atop the 8-71.
You got me thinking..it would be a waste of time breaking in the flat tappet cam n/a.As long as it is not pig rich at 2k break in and lead to fuel washed cyl walls,the cam lobes will not give a rats a$$ what induction package is atop.
I plan on reusing my msd dizzy with the crane hi6r cd box.I currently have the 18* stop bushing installed with light springs.I am thinking 16* initial + 18* mechanical for a total of 34* all in at 2800 rpm.No vac advance.I have the manual retard knob right now and can purchase the map/baro sensor if needed.I will spend the $200 for a vac dizzy if you feel I could use some more advancs at low rpm/cruise speed.
As for boost..the Weiand kit comes with the interchangeable 61/54 pulleys.I think firing up and tuning underdriven is the safest bet right now.Am i far off,thinking the 11% underdriven 8-71 will supply 7-8 lbs boost at 6k?I am thinking 1 to 1 may be might sweet spot,but will not know until road tested and evaluated in the little f-body street machine.
Hearing from folks that have real world experiance with roots type blowers can save a ton of money and headaches.I appreciate all the suggestions and look forward to sharing my end results/what worked/what did not.
Sincerely George.:)


The two 850's will be fine on a 460ci. You don't want to under carb with a blower, that's where the air has to come from (through the carbs). I thought two 750's would be too much for a 388 ci SBC but, after talking to All State Carbs, they said they don't recommend anything smaller for any blown engine. I trusted them and they were right on the money.

No, I do not recommend a vac advance dist. for a blown engine. I use a Mallory magnetic pick up dist with no vac advance I have had it for years. I would buy it again or I would look at the Crane dist, I believe it is real easy to adjust the curve. You may find that you will want your initial in the 18-22* range and your total around 30-32*. The engine will be more responcive and run cooler. You Crane HI6R is a great ignition (that's the same one I have). I know some guys have had problems trying to get the MSD dist. to run that much initial and to limit the advance. I don't think they make a bushing (if yours is the type that uses bushings) to accomplish this. You might want to PM TokyoTorquer (Mike) and see what size bushing he had machined for his MSD dist.

Starting with the blower under driven is a good and safe idea. The amount of boost will depend on a few things but, I would think you would be in the 6-8lbs range with an engine of that size. I run mine (8-71) 10% under on a 388 and it makes 12-14lbs of boost. Your car will be a beast for sure, you're in for a treat.

Thanks fellas for the great advice.I appreciate it.I am off to the machine shop tomorrow and hopefully pick up the short block and then mock up the valve train.I will let you know how the build goes and will take some pics.:beers:

George,
Sorry, I'm no help on the carb settings. This set-up was already together/running when I bought it.

Not a problem Jerry.Royce had a boo at my baseline settings for start up and he sees no issues.I boost referanced the second carb today.Fitted it,with #80 primarys and 6.5 pv.I blocked off the rear pv and installed #89 main jets.(I did not have 4 #90,s).
FWIW..I did get up to Booth/Aarons machine to view the 460 short block.All done,real nice.5140 nitrided forged crank,balanced with a double key way and ati steel dampner and sfi flexplate.Lunati billet rods(1300 hp-9000 rpm).Speed pro dished forged slugs.We placed an order for custom grind crane cam.Flat tappet mechanical with edm lifter.I will post the cam card when I get it.Mid .500" lift.250ish @ .050,116 lsa.I was concerned with my 135lb seat and 375lb open springs...but Denny at booth/aaron settled me down,says I am fine at that.No need for 6k shifts on the street with this combo.:hurray:

You won't have enough room to see 6K RPM on the street. You are going to be amazed at the low end torque and HP from idle to redline. 116 LSA is pretty wide but, should produce a lot of low end. The cam is pretty mild so be careful with your boost level until you get it dialed in.

You won't have enough room to see 6K RPM on the street. You are going to be amazed at the low end torque and HP from idle to redline. 116 LSA is pretty wide but, should produce a lot of low end. The cam is pretty mild so be careful with your boost level until you get it dialed in.

Thanks Royce for the advice.It sure helps.I will start off at 11.5 % underdriven.61 upper/54 lower.This should keep the boost down while i tune.Have you had to alter your tune up after a pulley swap?I usually start off with idle mixture screws 1.5 turn out.Do you recommend 2 full turns off seat for initial fire up.I plan on hitting the pedal twice and then counting to ten before hitting the starter with the throttle closed .

As for washable K&N style filters, I looked long and hard for two that would fit under my hilborne style scoop with dual 600's atop an 871 sbc in a 68 pro street. I finally found just the right ones from Good Vibrations in CA, they are special made by K&N for Good Vibrations, they fit perfect. I have sold the car rolling but I believe I may still have the filters in as new condition if you are interested. Motor produced in the 650 hp range at 14% underdriven and filters did not seem to effect flow or mixture at all. Driving any distance on the street without filters scares me some.

Thanks Royce for the advice.It sure helps.I will start off at 11.5 % underdriven.61 upper/54 lower.This should keep the boost down while i tune.Have you had to alter your tune up after a pulley swap?I usually start off with idle mixture screws 1.5 turn out.Do you recommend 2 full turns off seat for initial fire up.I plan on hitting the pedal twice and then counting to ten before hitting the starter with the throttle closed .


I did not swap pulleys, I was shooting for 12lbs of boost and Don Hampton (Hampton Blowers) put me right where I wanted to be. As far as your start-up you have it right DO NOT open the throttle when trying to start a roots blown engine. If you do you will get a backfire back through the carbs. 1 or 2 pumps and leave the throttle alone. Don't change to much in your tuning style for the blower (other than timing) as far as the carbs go you tune them the same as a single carb on a N/A engine, you just do everything twice (once for each carb). 1.5 turns out should get it fired up, get the RPM's up and break in the cam, once that's done bring it down to an idle and start tuning the air mixture screws until you get the highest idle, you will have to make several rounds to get it right. From your posts I can tell you know how to tune a car so, you will be fine.

Thanks Royce..again.:thumbsup:.Quick question if you do not mind.How do you protect your power valves with an under the blower hiccup that does not open the pop off plate?This referance line has to be bi-directional..vac/boost.The pv is in the open position until vacuum is applied to the diaphram side and pulls in the seat.What do you think of a one way check valve in favor of vacuum between the manifold and carbs with two small drilled orifices in the line between the check valve and pvs to dump vacuum when boost closes up the check valve.My thinking is any amount of boost will close the one way check valve and the drilled vac orifices will introduce atmospheric pressure and the pv spring pressure will open the pv and enrich the main ckt.Obviousy..the drilled orifices will remain small enough to as to not make much differance in the vac #,s ,but still bleed off vac from the pv chamber when boost shuts the check valve.I have done this mod to some older vacuum regulated egr,s to ramp down the opening speed and glue in place a 1/4x1/4"foam filter atop the orifice.

Thanks Royce..again.:thumbsup:.Quick question if you do not mind.How do you protect your power valves with an under the blower hiccup that does not open the pop off plate?This referance line has to be bi-directional..vac/boost.The pv is in the open position until vacuum is applied to the diaphram side and pulls in the seat.What do you think of a one way check valve in favor of vacuum between the manifold and carbs with two small drilled orifices in the line between the check valve and pvs to dump vacuum when boost closes up the check valve.My thinking is any amount of boost will close the one way check valve and the drilled vac orifices will introduce atmospheric pressure and the pv spring pressure will open the pv and enrich the main ckt.Obviousy..the drilled orifices will remain small enough to as to not make much differance in the vac #,s ,but still bleed off vac from the pv chamber when boost shuts the check valve.I have done this mod to some older vacuum regulated egr,s to ramp down the opening speed and glue in place a 1/4x1/4"foam filter atop the orifice.


In theory your idea should work. My carbs are blower carbs but, not boost referenced. Meaning I do not run a vacuum line to any ports on the carb (there is not a place on my carbs). They are "blower calibrated". You might want to do a little research but, my understanding is you don't need to boost reference the carbs on a roots blower. You also do not need to boost reference your fuel regulator if you have that type. Let me know what you find out.

In theory your idea should work. My carbs are blower carbs but, not boost referenced. Meaning I do not run a vacuum line to any ports on the carb (there is not a place on my carbs). They are "blower calibrated". You might want to do a little research but, my understanding is you don't need to boost reference the carbs on a roots blower. You also do not need to boost reference your fuel regulator if you have that type. Let me know what you find out.

It is my understanding that the pv,s should respond to manifold vacuum vs vacuum above the rotors.It is possible to get into boost while still having sufficient vacuum under the carbs to keep the pv,s closed and lead to a lean surge at part throttle/load.My thought also.is a high winding roots with less than optimal cfm flow thru the carbs can maintain under carb vacuum and close the pv when enrichment is reqd when under boost.I do believe alot of folks get by with an earlier opening pv to eliminate surge..but may be flushing some mpg down the drain and also risk the pv opening up at idle speed with radical camshafts.I feel a pv is reqd on the primarys for street usage and boost referanced will repond immediately when transitioning from vac to pressure below the blower.I think it is a good mod Royce.Ensures enrichment with low boost and can easily be done on a pair of holleys in 1 hr.A different approach than mine,but some have simply blocked off the pv port in the baseplate and drill and install a a small tube.I opted to use the port vac nipple already installed in the primary metering block.I would be interested to hear how your carbs are set up internally.jet sizes.pv,s/reo pumps etc?:beers: