FI modified as a tunnel ram on flea. Never saw one like this.

[John D.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Jerry, The dual air meter thumbtack style spill valve modification purpose is your question I believe.
I have wondered about this for an eon now. The valve used the stock nail but a smaller thumbtack. Smaller in OD. A restrictor was pressed into the bottom section of the spill valve body. The thumbtack now rested on this restrictor.
When the vacuum drops the thumbtack rests on the seat and you get full fuel flow. With the restrictor in place the flow of the fuel was restricted.

Problem with the old FIs is they actually need more air and less fuel. So the dual air meter accomplishes this in a rather crude fashion though.
So with all these increased air flow the spill valve hole must be too big and hence the need for this modified spill valve.
The first time I ever saw one of these spill valves was when I got a ton of the Grady Davis mechanical parts from a Gulf Research car. I remember looking at the weird spill valve and didn't have a clue what I was looking at. One of my pals here got that spill valve.

Seems that lately the dual meter craze has hit here. Yes they are neat to look at but do they actually work like a charm????
Take a stock 63 to 65 cone (also a 62 cone) and have it bored out so that the back edge of it is paper thin-or a tad thicker.
Not that difficult although I don't get involved with modifications.
The drawing I had/have around here was drawn by John Eyestone. John was into dual air meter units.
At the NCRS National in Warren, MI in the old days John had a Gulf Research dual air unit for sale for $3750.00 That was a lot of bucks in 1990 or so and I t hink he had a hard time selling it.
At the NCRS National at Seven Springs there was a Gulf dual air meter FI unit and maybe the engine (I forget) on display next to Rich Mason's Gulf car.
Gulf research must have made a lot of those setups. None of them set the world on fire in the old days. JD

[Rick B.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

John, as you know, the Rochester FI unit works through a delicate balance of pressure signals (both fuel & vacuum) from various systems in the unit. By adding a second air meter, the strength of the vacuum signal to the main control diaphragm was significantly reduced. Without changing the effective area of the spill valve, the main control diaphragm signal is now too weak to properly control fuel flow. Without modification of the spill valve, the fuel pressure signal will "over power" the main control diaphragm signal, bypassing too much fuel and the unit will go lean. By reducing the metering area of the spill valve, the control "balance" is restored between the main control diaphragm signal and the operating fuel pressure. This information was shared with me by Lou Cuttitta (Chevrolet Engineering) and Frank Sciabica (Rochester Products Engineering) many years ago. Both of these men were more than happy to answer any & all of my FI questions back in the day and I thank them for their generosity.

I think many of the dual air meter systems found today probably came from Bill Thomas in California. Plus, I found several print ads from old magazines advertising the Bill Thomas dual air meter conversion service for a nominal fee. Apparently, the nominal fee was more than sufficient to pay for a Holley carb & Edelbrock intake as many FI units were replaced with carburetors as more performance was desired.

Thomas Ad - Dual Air Meter FI Unit.jpg

[Tom P.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Rick,
I completely agree that VERY PROBABLY a Holley and aftermarket intake would be cheaper than the expense to modify an FI unit for 2 air meters. But the bottom line is that I just want one for the SB400 (like Rich Mason's) in the 56! And have a dual air duct connected to the Airbox.

Anybody got a spare early finned top plenum for dual meters?

[Jerry G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Rick, Thank-you. Good explanation. I'm familiar with this fix and I now have a very stable A/F ratio up to around 700CFM. The problem is breaking cables. I'm wondering two things, why lengthen the thumbtack and does this modification have any effect on the back pressure the gear pump sees?

[Jerry G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Thanks John. You didn't happen to get any of those spiffy distributor mounted fuel pumps did you? Or do you have any information on them? Jerry

Jerry, The dual air meter thumbtack style spill valve modification purpose is your question I believe.
I have wondered about this for an eon now. The valve used the stock nail but a smaller thumbtack. Smaller in OD. A restrictor was pressed into the bottom section of the spill valve body. The thumbtack now rested on this restrictor.
When the vacuum drops the thumbtack rests on the seat and you get full fuel flow. With the restrictor in place the flow of the fuel was restricted.

Problem with the old FIs is they actually need more air and less fuel. So the dual air meter accomplishes this in a rather crude fashion though.
So with all these increased air flow the spill valve hole must be too big and hence the need for this modified spill valve.
The first time I ever saw one of these spill valves was when I got a ton of the Grady Davis mechanical parts from a Gulf Research car. I remember looking at the weird spill valve and didn't have a clue what I was looking at. One of my pals here got that spill valve.

Seems that lately the dual meter craze has hit here. Yes they are neat to look at but do they actually work like a charm????
Take a stock 63 to 65 cone (also a 62 cone) and have it bored out so that the back edge of it is paper thin-or a tad thicker.
Not that difficult although I don't get involved with modifications.
The drawing I had/have around here was drawn by John Eyestone. John was into dual air meter units.
At the NCRS National in Warren, MI in the old days John had a Gulf Research dual air unit for sale for $3750.00 That was a lot of bucks in 1990 or so and I t hink he had a hard time selling it.
At the NCRS National at Seven Springs there was a Gulf dual air meter FI unit and maybe the engine (I forget) on display next to Rich Mason's Gulf car.
Gulf research must have made a lot of those setups. None of them set the world on fire in the old days. JD

[Clem Z.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

hey jerry give me a call 724-468-5692

[George J.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

California hot rodder Bill Thomas (builder of the Cheetah) used to advertise the dual air meter conversion in Hot Rod and other magazines during the sixties. Several of his Cheetahs were raced with these conversions. One of the original Grand Sports (prior to the wide tire & wheel well flare package) was also raced during the 1963 competition season with a dual air meter FI system. Some good photos of this Grand Sport can be found in the "Star Spangled Sports Car" coffee table book.

As I live and breathe! Did you just get a computer, Mr. Boyd? Good to see you here, and great shot of the Bill Thomas ad.

George

[John D.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

HI Rick, THanks for the good explanation. I didn't know that Sciabica was into the dual air meters in the old days.
As mentioned Bill THomas was just one of the players in the old days with the dual air meter units. Gulf Research/Grady Davis' crew build a lot of those units.
So did Yenko Chevrolet as the same mechanic that worked for Grady worked for Yenko.

[John D.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Jerry, When you say lengthen the thumbtack I am not sure what you mean. You mean the OD of the round part of the tack? Actually the thumbtack is quite smaller in OD.
Maybe Tom Parsons can post some pics of the real deal spill valve. Now maybe he already has as I didn't read all the posts.

Jerry, I don't know anything about a distributor mounted fuel pump for FI's. Never saw such a thing. John

[Wayne G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Jerry, I had a Bill Thomas version of a dual air meter FI unit that I bought in the 70's. I did change it back to original condition, but kept all of the dual air meter components. I'll look around for the modified spill valve and post a photo for anyone that is interested. If I remember correctly the plenum runners were extended about 1" inside the plenum as part of the modification.The workmanship on this FI unit was quite well done for it's time.

Wayne

[Jerry G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Thanks, that would be helpful. Jerry

[Rick B.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

These are photos of a modified spill valve similar to the drawing posted earlier. I believe they came from Jerry Bramlett's website. If you enlarge the inside diameter of the diffuser cone as John DeGregory described earlier, you will likely have to modify the spill valve as well. However, not to the extent of the dual air meter spill valve modification. I wish I had the recipe for you, but some experimentation will have to be done to match the reduced "mass air flow" signal from a bored out diffuser cone to a compatible spill valve cross sectional area.

Jerry, your avatar reminds me of a '63 Corvette roadster raced by Ken Searles of Columbus, Ohio. Is it Ken's former car? Ken had the diffuser cone bored out and the injectors modified for more flow, but it just wouldn't run well at low speeds. Using a stock diffuser cone gave back the idle and low speed performance, but now it was too rich due to the drilled out injectors. I would have liked to have tried matching the spill valve to the modified diffuser cone, but didn't get the opportunity.

Dual Air Meter Spill Valve 001.jpgDual Air Meter Spill Valve 002.jpg

[Jerry G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

This is East Coast racer Ed Lowthers race car. He was a factory racer for Corvettes and had local connections to Penske. This car is full of unusual modifications to lighten or strengthen factory components.. The Air meter flows approx. 700CFM based on flow bench testing, it has been modified . The nozzles are custom made to match air flow and fuel mass. There are internal modifications to the fuel meter similar to the ones in your photos. This unit unlike stock units does not go lean. The A/F ratio is stable up to 7300 RPM(limit of stock stamped rockers). It idles stable down to about 1000RPM.The only issue I have had is breaking of fuel pump cable. This is a BIG one. i'm working on a solution for this in January. Gotta be ready for Monterey. Jerry

[Wayne G.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

Here a couple photos of the spill valve with a measurment for your information. I added a picture of the air meter plate also.bill thomas fuel injection1.jpgbill thomas fuel injection2.jpgbill thomas fuel injection5.jpgbill thomas fuel injection3.jpg

[Clem Z.]

Re: FI modified as a tunnel ram on flea. Never saw one like this.

can someone post a good website where i can learn the operation of this spill valve ???