IMPCO 425 Improvement

Propane, Butane, LPG, GPL, C3H8, C4H10
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Marc
Posts: 73
Joined: Mon Apr 23, 2012 8:54 am

IMPCO 425 Improvement

Post by Marc »

Hello to everybody !

I still have my Camaro 68 with SBC 327 vortec L31 heads, that is fed with propane through a IMPCO 425 an model E converter. The cam is a Comp Cam 268h, 218/218 @ 0,050" . The engine is not high performance however good for 320 HP around 5500 rpm (that is on pump gas). With the Impco I do not get all the engine is capable.
I use the car on good weather days mostly to cruise and I like to pull it hard on some occasions.

I know one can improve CFM figures on holley carbs by grinding angles, adding velocity stacks, adding circular to oval bore base plates,....making the air flow more smoothly therefore reducing pressure drop.
I also read that Sjak Brak modified his IMPCO 425 that was then tested by Franz Hofman delivering 340 HP.
I contacted him however he seems to be out of the propane stuffs by now.

I am not willing to dismantle my mixer and discover things on my own. I'd like to have guidelines to start the tuning following good practice, knowing the ultimate results I could achieve.
I am also not willing to change completely my set up to vapor injection, I am very satisfied of my current little and I would like to spent some time if possible to tune this carb and get improved cfm.
I hope you can help me or re-direct me. Searching the internet did not help me find anything on that subject.

Thanks!

Marc

storm
Posts: 796
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: IMPCO 425 Improvement

Post by storm »

G'day mate, gotta love the SBC in the F body. I miss my old T/A with the Pontiac 400. But I digress.

As far as I know there are 3 things that limit the amount of air that a 425 can flow at any given pressure drop. The 1st is the Holley baseplate, the 2nd is the air valve assembly, and the 3rd is the actual body of the 425 between the air valve and the baseplate.

The best way to check this would be to get an assembled 425 and check it on a flowbench. Test it assembled, then test it with out the air valve, then test it without the baseplate.

Holley produce a few different baseplates that will fit the 425. I have mentioned before that I would like to test the airflow of a 425 with the different baseplates that are available to see if using one with a bigger throttle valves would increase airflow. If airflow doesn't change from the smallest to the largest baseplate you know the airflow restriction is above the baseplate (i.e. the 425 body or the restriction through the air valve).
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

Marc
Posts: 73
Joined: Mon Apr 23, 2012 8:54 am

Re: IMPCO 425 Improvement

Post by Marc »

Hi Storm

Thank you for your indications.
I think you are right it needs to be tested and this is the way to do. I am not equiped with a flow Bench and I quickly found this page as an example http://performancetrends.com/ez_flow_system.htm
I do not have the parts to assemble such a bench and at least we need reliable figures out of it. I should better find a place where they have such a flow Bench. I will start to inquire.

As holley's are not at all commun in my area (Belgium), and as in our society more and more the individual worker cannot sustain its activity because of the cost of manpower, I think I need to also use such internet networks to find the right guys to help, or I need to do the work myself.

To your knowledge such air flow data as you mentioned are not available? This should make things more easy of course. There are so many guys with the same dream of getting more cfm out of their impco 425 mixer...


Thanks again,

Marc

storm
Posts: 796
Joined: Wed Jan 10, 2007 10:10 pm
Location: NSW, Australia

Re: IMPCO 425 Improvement

Post by storm »

Hi Marc
I will be building a flowbench,based on the PTS Flowbench Technology design, in the future where I will test my theories on carbs/mixers/throttle bodies/heads/manifold/etc etc etc but they are not cheap to build. The bench I want to build will cost me at least $2000AU (approx 1200 Euro) just for vacuum motors alone. It will also run Performance Trends Port Flow Analyser to make sure the testing is repeatable. Until then all we can do is search the net to see what others have found or ask questions here.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

Marc
Posts: 73
Joined: Mon Apr 23, 2012 8:54 am

Re: IMPCO 425 Improvement

Post by Marc »

Thanks for your interest Storm.
I called two classic car race preparation companies in my area (the only ones) and they cannot help me, I then search more generally on flow bench testing and as I feared there are big laboratories which will ask more than the cost to build a flow bench my self...
So it is a good news you are planning to make some tests and measurements, and in the meantime as you said we can keep contact here on the forum. Let's hope Sjak Brak can come back to us on what he experienced.
With regards,

Marc

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