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Old 07-08-2004, 22:16   #1 (permalink)
351 XE on LPG
 
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Tuning an Impco 425

The tuning characteristics of adjusting the idle screw have changed. I can only go about one full turn in either direction before the RPM starts to drop. Any idea why this could be?

The 425 is a little over 2 years old and was mounted on a new marine version of the Holley 650 square bore throttle body. No obvious vacuum leaks detected.

I've also had another try at tuning the 425 with my K&N filter replacing the standard paper element. It appears to be going thru more gas now and I've noticed the torque for the off idle to 2000 rev range feels like she's dropped quite a bit. The K&N is definitely better above 2000. The problem is that on the street with a 2.77 diff ratio I rarely get above 2000. I think I'll end up putting the K&N back into storage and put in another new paper element.
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Old 07-09-2004, 22:37   #2 (permalink)
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Re: Tuning an Impco 425

Rhett,

I'd be looking at the Carb as your issue before the Air filter.
I've also mucked around with different Air Filters to see if there's a difference.

I can honestly say that I couldn't really detect a difference in idle quality between the the paper or the K & N. I did however notice a seat of the pants difference when the car was hooting along @ 3000 - 5000+ RPM.

Your base plate may also be adjusted wrongly. I've got the primary butterflies opening first with secondaries opening at quarter throttle. I found that by having them open together under mild throttle the car did bog down a little at the start before the revs got higher.

Also get your converter checked out. Sometimes the spring can go weak which will allow the motor to draw in too much gas on idle and mild throttle. This can really bog the car down and it can become difficult to tune as you have to lean the idle mixture off and possibly the cruise mixture as well which will then lose you power at the top end.

I'm thinking of replacing my converter also with a new ELKO E420 unit.
Click HERE

I'm then going to try and hook up with xdc351 to try out his modified 425 to see if I can get my car to boogie a little more.

A new mandrel bent 2 1/4 twin system with x pipe and then hopefully a 14.0 or 13.9 quarter at Calder. ( If I can get any traction that is )

Currently running 14.3.
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Old 07-10-2004, 03:29   #3 (permalink)
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Re: Tuning an Impco 425

what did xdc351 do to his 425? i have to run 2 425's on an adaptor, as 1 wasnt enough at around 5000rpm. (aspirated). my engine was reasonably stock, and still needed more than 1 could fix.
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Old 07-10-2004, 05:43   #4 (permalink)
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Re: Tuning an Impco 425

Thanks ESP.
Good idea about the baseplate. I've had all 4 butterfies opening at the same time for the last 2 years. I'll try your config out once I sort this problem out and see how it goes, but I haven't changed anything recently on the carby, so my butterfly setup couldn't be the cause of the change in tune.

Yes, you are definitely correct about the advantages of a K&N after 3000 RPM. Today I changed back to a new paper element for low RPM cruising and it made a very noticable improvement over the K&N from standing to 2000 RPM.

I also noticed that when the motor was idling around 900 RPM, there was a loud whistle from the top of the 425 that isn't normally there. Maybe I need to put a kit thru it and change the top rubber ring. I'll sus out a supplier here in Perth on Monday.

I am also very interested in acquiring xdc351's tips re: 425 modifications to enhance flow, as my son and I are both planning 408 cube buildups and would rather not have to muck around with fabricating a manifold to handle two 425s. Blake is going to stroke his 302C to 408 and I'm very interested in building up a hi compression yanky 400 (small/big block?) using 302C heads.

Come on xdc351, we are keen grasshoppers who want to be enlightened!
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Old 07-10-2004, 06:02   #5 (permalink)
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Re: Tuning an Impco 425

the adaptor is a bought one. impco make them. you put a holley base plate on the bottom, and the 425's on top. the only drama, is they almost touch, so you have to booty fab up an aircleaner.
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Old 07-10-2004, 07:21   #6 (permalink)
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Re: Tuning an Impco 425

I think the adapter is also very ugly and sits the carbs too high. I know there is a tunnel ram available for the Cleveland, but it would kill lowend torque on the street, and they are even higher. I'd like to find a nice low profile dual setup. I think I've seen some on Windsor powered Mustangs, but nothing made for the Clevo, (and definitely nothing for the 400). Back to fabricating. Thanks Crushu.
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Old 07-10-2004, 15:35   #7 (permalink)
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Re: Tuning an Impco 425

I also noticed that when the motor was idling around 900 RPM, there was a loud whistle from the top of the 425 that isn't normally there. Maybe I need to put a kit thru it and change the top rubber ring. I'll sus out a supplier here in Perth on Monday.




Rhett,

The whistle on the 425 means that it is running correctly.
Impco will confirm this if you ring them and so should many a good LPG tuning centre.
From what I understand xdc351 spent a lot of time de dagging the accesible internals of the 425 to make it flow a little better.

When and if we do the back to back dyno i'll let you know the results.
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Old 07-11-2004, 20:05   #8 (permalink)
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Re: Tuning an Impco 425

Sorry I havn't replied... didn't get the compter all weekend!

Rhett, the whistling is normal.... my K&N with the filter lid always turns heads from the loud whistle and 'whoosh' noise from the air filter when I give it a bootfull! I've searched for years and never found a low rise cleveland 2x4bbl manifold, but you would want one serious engine to warrant 2 impco's or else it'd be an absoloute pig to drive around town.

I don't have my impco modification email with me at work, i'll email it to you when i get home if you PM me your email addy.

There is no way i'd want to run any ening bigger than 351 cubes on a single 425 no matter how many mods you do to it. My modifications netted me an extra 800RPM out of the carb before it ran lean but its still not running to the 6300RPM max power of my cam. A dual impco might supply enough at the top end but you'd have to restrict them to use it around town!

Thats whay i'm going for a dual GRA carb setup. Far more expensive but the only system capable (in my opinion) of supplying big CFM rates at WOT and a big cam and still maintaning decent around town.

But on the same note i'm on the edge of my seat waiting for the dyno day with ESP's car
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Old 07-14-2004, 20:40   #9 (permalink)
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Re: Tuning an Impco 425

i have just bought a 400 donk, just last weekend
2V closed chamber heads, stainless valves, hardened seats, heavy duty springs, mild porting
Yella terra roller rockers,
Edlebrock performer manifold,
bigger sump.
etc etc etc,
pics are here,
http://www.tmaauto.com/tma-auto/show...=5821#post5821

it has a crow cam/double row timing chain etc with it, but i will get new ones, the cam isnt what i want, its model # 21771
http://www.crowcams.com.au/templates...leveland.shtml
2nd on on this page, so if anyone wants it, make an offer, comes with lifters, done between 5000-8000km in the 400.

Now i was planning to run a single impco 425 on it, should i be looking at twin impco setup, or dual GRA?? it will be mainly a cruiser anyway, and is going into my 81' LTD,

I currently have an Impco 425 with a Impco model E convertor, actually in the gas place today getting gas line changed from 1/4 to 5/16 and another tune, its a fairly close to stock 351 ,
with a stage 2 cam (previous owner couldnt tell me specs, but it was bigger than standard)
2V closed chamber heads that were put on last weekend when i cooked the bloody thing for the 2nd time...lost water and did a headgasket.
story on that one here
http://www.tmaauto.com/tma-auto/showthread.php?p=5828
Weiand X-cellerator manifold,
dual 2 1/2 exhaust, tri-Y motorvator extractors...
picked up alot since going straight LPG from dual fuel,
with dual fuel on LPG (had a small LPF brand mixer and convertor) it made 100rwhp, bit sad...
then after bolting the impco 425 and Model E convertor it went to 140rwhp leaning out really bad anywhere after 3800rpm, so it had more in it.....

and well i will find out this afternoon some time what difference the bigger feed line and a better tune does, probably nothing spectacular, getting replaced with the 400 in the coming months anyway, thought i might as well squeeze what i can out of the 351 in the meantime.
cheers
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Old 07-14-2004, 21:09   #10 (permalink)
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Re: Tuning an Impco 425

Make sure you pull the restrictor out of the 425.

If your unsure how to do this PM me.
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