Puckering a velocity stack to lower cfm.

[coaster]

Hello, i picked up a set of 50mm itb's for a duratec 2 litre for a good price, i want to put them on a street car (locost) but dont want the huge cam to make them flow.

Could i 3d print a set of puckered velocity stacks and get the performance to match a mid range camshaft?

[63l8qrrfy6]

Is this a manufacturing question or an engine performance question ?

[coaster]

Performance, manufacturing is a simply a parabolic curve leading into an 11 degree taper, choked to 45mm diameter, not super critical as most porting shops use plastacene shaped by hand so i wont lose sleep on the perfect answer.

[63l8qrrfy6]

The pragmatic approach is to keep the stock cams and set the trumpets to give you the same length from the open end to the valve as the original inlet runner. If I remember correctly these engines came with switchable length intake so you could just replicate the short runner length to achieve a similar pulse tuning effect at high engine speed.

An OEM would use computational methods (GT Power, Ricardo Wave, AVL Boost etc) to simulate the unsteady gas dynamics in the inlet and exhaust and calculate a preliminary area/length profile which would then be verified on an engine with pressure transducers in the intake and exhaust ports.

In practice the most you can do on a homebrewed project is to try several lengths (can even build simple manually adjusted trumpets) and see what makes the most power but there is no guarantee you will find the optimum.

In terms of lowering the flowrate - most of the pressure drop is probably dictated by the valve diameter and the 45mm diameter throat so I struggle to see how you can lower the flowrate significantly without unusually long trumpets.

[godlameroso]

Your answer is in the ports, not the trumpets. Make them smaller, they will flow less, but will flow faster. Better cylinder filling, despite the shorter duration and lift cam.

[63l8qrrfy6]

He may struggle to put material back in

[J.A.W.]

If you are limited to a fixed bell-mouth set-up, perhaps it may be
worth considering an emulation of this Yoshimura dual-system?

[gruntguru]

I think the question is about making 50mm ITBs work on an enegine with 45mm runners.

What you can do is make tapered runners (common practice and they do work well) and position the ITBs far enough from the head to match the runner diameter.

[J.A.W.]

I think the question is about making 50mm ITBs work on an enegine with 45mm runners.

What you can do is make tapered runners (common practice and they do work well) and position the ITBs far enough from the head to match the runner diameter.

That's fine gg, if you want to accept both - ITB to inner bodywork & ideal rpm/power - limitations..

[coaster]

Seems it was too good to be true, ebay deemed my purchase restricted for import and a full refund, strangely, the seller wont receive the item returned.
Stolen property maybe?
Anyhoo, back to the drawing board.

[flynfrog]

pretty good video on what goes into the tuning

[godlameroso]

He may struggle to put material back in

If the head is off, it's cheap and easy to slap some JB weld and then contour it with a dremel. I learned a lot this way about airflow.

[coaster]

Fuel tank putty works also, I smoothed the inside of a 2 barrel stromberg and removed its choke, added a homemade nylon 25mm open chamber carb spacer on a worn out V8. The cam timing was already retarded a good amount by way of a stretched timing chain which i did not touch. None of my car buddies could believe how hard that motor went, i kept secrets like a 6mm taller air filter which bypassed the snorkel, ball bearing in the vacuum line for vacuum advance delete, bigger fuel filter and fittings, drilled the originals which could not be upsized.
That was in 1997, a 1979 VB Holden Commodore which was basically a German Opel Senator with an American iron V8 jammed in, from factory!
I hate that I grew up in an auto industry city because in doing so I was obsessed by cars, still am to my detriment. My career scope had a narrow focus to this which i now regret in middle age.
I should have switched on that Sinclair computer more often and learnt something other than spanner work.

[J.A.W.]

Fuel tank putty works also, I smoothed the inside of a 2 barrel stromberg and removed its choke, added a homemade nylon 25mm open chamber carb spacer on a worn out V8. The cam timing was already retarded a good amount by way of a stretched timing chain which i did not touch. None of my car buddies could believe how hard that motor went, i kept secrets like a 6mm taller air filter which bypassed the snorkel, ball bearing in the vacuum line for vacuum advance delete, bigger fuel filter and fittings, drilled the originals which could not be upsized.
That was in 1997, a 1979 VB Holden Commodore which was basically a German Opel Senator with an American iron V8 jammed in, from factory!
I hate that I grew up in an auto industry city because in doing so I was obsessed by cars, still am to my detriment. My career scope had a narrow focus to this which i now regret in middle age.
I should have switched on that Sinclair computer more often and learnt something other than spanner work.

Hey wait on, that VB Commode was reinforced locally to handle the 'rough as guts' local
roads & not fall apart too quickly, compared to the 'Detroit' mandated Kingswood/Torana
which it was replacing, & that "American iron V8" - was also much lighter (& better sounding
with more grunt) than the actual 'Detroit' 5 litre GM V8's of the era - to be fair, wasn't it?

[coaster]

Yes you are correct, it was just a watered down version for overseas readers to digest.
Reinforced front structure, pbr brakes, bosch electrics, locally designed v8.
It was a 253ci with a 2 barrel, my brother and his mate used to sneak it out at night and do burnouts i am told, i never caught them.
308ci was rare even back then.