Twin Impco

[BattleTruck]

Simple question guys - Has anyone run a pair of Impco 425 mixers (single Holley 950cfm baseplate) from a single converter?

Any info appreciated!

Nick

[franz]

Dont bother with it. A single 425 is worth about 295 bhp or so, depending on what fuel is used and several other factors. A single model E vaporizer is worth about 350 bhp, when fully at operating temp of about 180 deg F. Doubling the mixers wont double the horsepower, it doubles the amount of capability. I remember one gent who put on two 425's and complained that his car wouldn't go any faster. He had a stock 5.7L Chevy, single exhaust.

Franz

[BattleTruck]

The motor the single 425 is currently on was previously running an 830cfm gasoline carb and spinning to around 6800rpm... Made a bit over 300hp at the wheels. More capability is definitely required - my question was more related to the capability of a single converter to feed enough fuel for the extra cfm of the second mixer.

Thanks!

Nick

[storm]

In my opinion the amount of air and fuel that can conceivably flow through a single (naturally aspirated) 425 wont feed your engine to 300 rwhp. You will require more fuel AND air so another 425 or something comparable will be required to achieve the 300 rwhp that your engine is capable of. If you do this you are better off having another Impco E with a balance tube to feed both mixers.

[BattleTruck]

I'm inclined to put the second 425 on with the single converter and see what the mixtures look like on the dyno... My understanding is that there's no issue getting a 425 to run rich, the problem is airflow, which the second one should help with. I don't intend to buzz it to 11 every day of the week, but the car has lost 25mph down the 1/4 mile from gasoline, which is pretty lame!

[storm]

I can understand you have lost mph down the 1/4, you have gone from 830cfm to approx 425-450 cfm. A single 425 will be a restriction at high revs for a decent Cleveland. Two 425s should go along way to fixing the issue IF you also use a regulator for each one. Then a dedicated LPG tune will help alot more.

[Steptoe]

I have a single 425 on a 350 ci when on the dyno she leans out around the 4200 rpms and starts to seriously lean out around 5000 rpms.

[Hal]

Hello,
i drive since 2 month two Imco 425 mixer with two Impco E converter feed by two 8mm pipes.
With the right camshaft my 460 cui Chevy BB revs to about 5600 rpm and the air/fuel mixture is good without tuning.



Greetings Hal

[Steptoe]

Thats looks so cool with 2 'carbs'
With single carbs on a V8 generally the cnr cylinders are not as efficient due to length of runners....having 2 would reduce or even elimate this..picking up even further efficiency.

Shouldnt there be a balance tube between the carbs????
And having the converters leaning back like that they could collect oil/sludge ...rather than having them more vertical.

I like it.. well laid out and tidy

[jono]

Looks very impressive. The converters do ahve a good downward slope for the junk to at least collect at the bottom away from diaphragms and when level gets high enough it will surely spill over into vapour tube.

QUESTION does/can/will the junk actually flow uphill intp converters or does a converter vapour outlet always have to be above mixer for the natural draining to occur?
I have been running a water hose inline thermostat designed to theoretically prevent oil forming, but oil has still made it into the conveter. It may be engine oil from the balance line and smoke it blows going downhill

[Steptoe]

QUESTION does/can/will the junk actually flow uphill intp converters or does a converter vapour outlet always have to be above mixer for the natural draining to occur?

Now thats a thought
Hang on just going to shoot out to the garage, I cant rem if mine is level or not

Cool back lol
Mine goes up from the bottom of the convertor around a 36 to 45 degree angle...in over 200.000 miles it has never been an issue, I cant ever rem taking the hose off and even have oil or crap come out or had to even clean the pipe.
I have never had a build up because I mounted vertically 25 yrs ago.....but have seen a heap of convertors so full of crap hand to dig out with a screwdriver...using the same supplies.

[Hal]

Hi Steptoe,
there is no need for a balance tube between the carbs, the edelbrock dual quad intake is internally connect between all runners.
Thank you for your comments and it is really true that the two mixer work more efficiently than the one mixer (OHG 450) that i drove before, about 1-2 liter on 100 kilometers less.
Greetings,
Hal

[franz]

Due to the fact that every OHG X-450 was hand built and there were numerous mis-matched parts, a balance line is often used to ensure the air-valve lifts evenly when using two mixers. The Impco 425 was mass produced with matched parts, and what I find as more consistent manufacturing. A balance line is not that helpful on the 425.

Franz

[KILLERB468]

Hal is that an open plenum dual quad manifold? Awesome set up... I will be running dual x-450 on a 468 ci BBC with 10.5:1 cr.

Franz what is this balance tube you speak of and where would I find one? Thanks.

[franz]

The balance line would be connected to the two black plugs on the side of the Impco 425, just visible on a couple of the excellent photos. Install a fitting and fit a flexible tube. It doesnt have to be very large, 8mm is fine.

On the X-450, I would connect the two vacuum lift pistons together with a tee fitting.

Franz