The V8 Owners Forum -

Can anyone shine any light on the pros and cons of phenolic carb spacers ?
Assuming there is enough room would two inch be better than one inch ?
Is an open plenum better or worse than four holes ? I know it all probably depends on the type of use the engine will see and what other bits it has inside etc. but any advice would be gratefully received.

Koopsterman

Hi
what is the engine? ford rover, or chevy. . . . or oldsmobile, pontiac, daimler??
capacity?
what state of tune ?
what use will it be put to?
what sort of manifole dual plane or single open plenum?
With open type spacers generally speaking they are used for two reasons,
On a dual plane manifold where the carb is on the small side for the engine you can space off the carb from the inletso it sees all 4 barrels at both halfs of the manifold and it can "stretch the" small carb to flow enough for the, this is a bodge but will asve you buying a new bigger carb, you want at least a 1" spacer, preferably a 2" one the throttle response will be much less precise at low engine revs and it will be difficult to get good idle fueling but the transition circuite will work again as intended.
On an open plenum carb they are used to tune the plenum volume to a particular part of the rev range, also because the manifold manufacturers know you will be doing this any way they tend to cast the top of the plenum short, this saves them manufacturing cost. Adding an open spacer will nearly always as a result improve flow through the plenum as you end up with the carb where it should be reative to the plenum floor.
4 hole phenolic, used on a dual or single plane to isolate the carb from heat of the engine.
4 hole tapered generally used on an open plenum to reduce turbulance on the exit of the carb, until you have done everything listed in Mr Vizards carb book to your 4 hole leaky bucket don't bother with one, along with a selection of open spacers they are used to tweak the last couple of bhp out of one bitof the rev range on your 850 plus BHP Winston cup motor. You can put one on because it looks prettyand you can brag about it in the pub!
Best regards
Mike

Thanks for the reply kiwicar, the engine is a 3.9 Rover with a Huricane cam, Edelbrock inlet and carb, 1.5" headers and EDIS. When the truck is done it will be a frequent driven hotrod with maybe the odd trip to North Weald airfield.
From the info you gave me I`m still a little unsure what is what.
Regards
koopsterman

Hi
I would start with making sure butterflys open fully especially the scondarys when you fully depress the throttle, this can take some settting up. Next get the carb jetted and set right for it as it is now. Stick a vac gauge on each half of the inlet manifold in turn and see what happens to manifold depression as you accelerate up a long hill in third gear gear all the way through the rev range, is the carb restricting the flow at the top fthe rev range? does it go very richat the top? if no leave alone, if yes them maybe a 1" carb spacer could help butyouwill need to rejet it with the spacer. As a general rule 4 barrel carbs are too big for 90% of normally aspirated rover engines, they are designed for domestic market US cars with engines of around 240 260 bhp output, less than that and they stuck a 2 barrel on there. By the sound of your spec yousrelooking at an output of about 190 to 200 bhp tops so I wouldn't bother with an open one. you could stick a 4 hole parralel bore one on there it may improve part throttle cruise and throttle response orit may not, but a taller airfilter could help in the same way if you have the space then you could fit both.
Best Regards
Mike

Thanks again Kiwicar - advice taken on board and appreciated, I have re-jetted the carb allready - follwed the advice from Sidecar in his article on down jetting the Eddy for smaller engines than five litres.
Regards
Koopsterman

I fitted a 4 hole one onto my dual plane performer RPM manifold initially to help with heatsoak, but it did seem to improve the low down responsiveness too (350 chevy, edelbrock 500 at the time)
I made all mine myself;



This is an earlier one i made when i was running the Victor JNR manifold, which is crap on a 4x4:

I've messed about with open and 4 hole spacers on my Eddy manifold fitted to my 4.6 lump, in theory the open spacer might give you a bit more top end but did not notice any difference, the manifold vacuum at WOT was 1.5" even with a 4 hole spacer which indicates that the carb is not too small for my engine even when it is 'split in half' by the dual plane manifold. I did notice that the engine was VERY slightly less responsive at VERY small throttle settings. I think the best reason for fitting a spacer is to stop heat soak, you only need a thin 10mm spacer to sort this out. Like Kiwi stated you do need to re-calibrate an Eddy carb when fitted to a Rover engine, if you don't you are likely to suffer bore wash which will trash to piston rings if left unchecked.

Thanks for the replies guys - now I have some direction !
Koopsterman