Outer Limits 4x4 Board

hi,

I have a impco 425 blow through setup on my turbo tb42.

It is hesitating badly during high throttle gear changes, or high to low boost changes, like when you boot it hard, then back off and return to cruise throttle.
It feels like the diaphram is being negatively affected by the boost, which then causes a delay in its fuelling.

I have a very soft blow off valve, that is venting externally.
When it had a turbosmart bov, whenever it would make the 'turbo flutter' noise from the bov being tighter, it would create the same hesitation until the flutter stopped, then cruise fine.
I havnt tried recirculating the bov, but I thought that might solve it.

I'm not asking much out of the car these days, so I just need it running right.

If anyone has a good suggestion for an experienced tuner near the gold coast, let me know.

Otherwise, if you have any suggestions that might solve my problem, that would be better, as the car runs perfectly other than this.

cheers

I have an impco 425 on TB42 but non turbo. I have the same problem. If I rush it through the gears it will stammer. I found advancing the timing and richen the idle has made it a little better. Apart from that I have just tried to live with it. I would love to hear from others if they have cured it.
My 425 is running sideways and I have always put it down to that.

I hope its not due to being mounted on its side.

Anyone else got a similar setup?

I asked a question recently about a 425 on a V8 that wasnt running right- apparently theree's 3 different pressure diaphram springs available- maybe thats the issue. Perhaps you need a harder spring to offset positive boost pressure/sudden drop in boost via BOV affecting the diaphram?

if you change the spring, does it need to be fully retuned?



cheers

Does it run the balance tube from converter to pre turbo?
Running the gas carb sidewards won't effect it at all.

it has a small hose running from the top side of the mixer, to the front convertor. 1/4 or 5/16.

The only other hose off the convertor is the gas feed hose - 1 inch.

If you pm me your email, I can send you a pic

If that balance hose is running from the wrong place, does that mean it will need to be retuned?

cheers

Here is one end of the hose



Here is the other end

You need a hose that runs from the convertor to pre turbo.

From memory it is mounted in the centre of the convertor or the primer (has been awhile) & runs to between the air filter & turbo. pretty sure there is a diagram on the gas research website that may help...

ive now figured out that you need to drill and tap the convertor to take a 1/2 inch barb fitting, which you run to the pressure feed

Only problem is, the convertor has a plastic lid.

Any ideas?

I take it you have a B2 convertor?
You can buy replacement metal lids for them.

I'd go look for an Impco model E convertor though if it was me.
It will probably cost you less to buy one of these 2nd hand then what it would a new lid for your current convertor and it will allow alot more power and further development out of your set-up. They are also less prone to freezing over...

ok this will fix your problems, i spent a long time getting mine running nicely on the 425 before i blew it up, i now use a 225. but do these things:

swap that damn plastic lid, it will flex under boost pressure and fk with your mixture settings.

swap the std spring for a blue one (may need another dyno run, but will only take one or two goes to get it right)

that balance line GET RID OF IT NOW and replace it with atleast 10mm ID line, thats whats starving your gear changes, as the reaction time isnt going to be quick enough, i use 19mm id pipe

after all that it should be fairly spot on

oh and its not in the wrong place i run mine like that

I dont have a balance line! My 425 has a blanking bolt and the convertor has a gauze cover.
I will have to get some fittings and fix it up.

I have a similar problem to you Quadcamshorty, but not as bad by the sounds of it.
When the boost pressure rises fast in low gears it can stumble as it leans out. I noticed that it changed markedly after fitting the intercooler, as it was ok before. I might also try increasing the reference line diameter so the converter sees the same pressure as the mixer.

_

cooki, thanks for the input.

Just to clarify a couple of things.

Do you drill out and tap the metal lid in the same place as the smaller barb is now?


Where is the ideal place for the pressure feed end of the hose?
I would assume tapping the side of the mixer would be a no go


cheers.

create wrote:I dont have a balance line! My 425 has a blanking bolt and the convertor has a gauze cover.
I will have to get some fittings and fix it up.

create, you shouldnt really need one as your mixer isnt seeing positive pressure, its the only reason why i run one.

raptorthumper wrote:I have a similar problem to you Quadcamshorty, but not as bad by the sounds of it.
When the boost pressure rises fast in low gears it can stumble as it leans out. I noticed that it changed markedly after fitting the intercooler, as it was ok before. I might also try increasing the reference line diameter so the converter sees the same pressure as the mixer.

_

also check your spring rate, if its leaning out at the top end it might be easiest to lean out your power valve but put a heavier (blue coloured) spring in, but id be going with 3/8" balance line or bigger (10mm)

cooki_monsta wrote:

create wrote:I dont have a balance line! My 425 has a blanking bolt and the convertor has a gauze cover.
I will have to get some fittings and fix it up.

create, you shouldnt really need one as your mixer isnt seeing positive pressure, its the only reason why i run one.

Yes I just tried it and it runs real lean. Then I re-read this thread and another and found you dont need it for a non turbo.

yep cos if u connect it, your engines vaccum will close up the diaphram in the converter and shut your gas supply down, hence why its running lean