Rejetting Edelbrock For a 4.35 Engine.

[CastleMGBV8]

I'm going to be using my existing Edelbrock 500 and Performer manifold as an interim measure on my new 4.35 engine with the Buick 300 Heads and Crower 50232 cam.

Any suggestions for a suitable rod and jet combination for the primaries.

I would have thought that jetting something similar to a warm 4.6 would be pretty close, Perry what are you using and anyone else with a similar spec engine what have you tried?

Kevin

[mgbloke]

Kevin
According to my innovate wideband the best setup for my 4.6 is 67/55 rods in 086 primary with 092 secondary using the strongest springs and the accelerator pump in the middle.
Hope this helps
Mark

[CastleMGBV8]

Mark,

Thanks for that, i've put the numbers in my edelbrock handbook so I don't lose them by the time were setting up the engine.

I've got the primary jets as they came with the carb.

Your engine is I would think in a higher stae of tune than mine will be so may need to be a little weaker.

I'm fitting a couple of bungs for a wide band so should be able to see what it's doing, just wanted a decent starting point.

Kevin.

[sidecar]

I was running a 3.5 but I'm building a 4.6 at the moment.

My 3.5 ran 67x55 and 86 mains, everything else was standard.

If you use the subtraction method of taking the needle size awy from the jet size this ends up 19 on cruise 31 on the power step.

I spoke to RPI about setting up the carb for the 4.6 and they reckon standard needles (65x52) but 83 mains. This would be 18 cruise and 31 on the step. In otherwords slighly leaner than my setup. I'm going to leave the carb as it is setup now.

The 4.6 that I'm building will be in a good state of tune, StageIII heads 285 cam, etc, etc.

By the way I used to "bang" on about the "subtraction" method of comparing needle and jet combinations but I now realize that it must be used with care for the following reason:-

If you had a 100 jet and 80 needle the gap for fuel to flow would be 20" but this is not the same as a 50 jet and 30 needle even though the number would still be 20".

The reason is that the gap is a ring shaped, the cirumference of the big jet and big needle combination would be bigger even if the thickness of the ring is still 20"


Regards,

Pete

[CastleMGBV8]

Pete.

Thanks for the reply, interesting that your original 3.5 jets and rods are the same as Marks pretty hot 4.6 so should work well in your new engine.

I am expecting to get in the region of 1 BHP per cubic inch from the new engine with the Buick heads and Crower 50232 cam say 260BHP so will need enough fuelling to support that power.

In the long term I will probably go Megasquirt and Edis ignition to have full control of the engine with full tunability, but I've got a bit to learn yet, and the carb will probably be good enough for the time being.

Kevin.

[sidecar]

Pete.

Thanks for the reply, interesting that your original 3.5 jets and rods are the same as Marks pretty hot 4.6 so should work well in your new engine.

I am expecting to get in the region of 1 BHP per cubic inch from the new engine with the Buick heads and Crower 50232 cam say 260BHP so will need enough fuelling to support that power.

In the long term I will probably go Megasquirt and Edis ignition to have full control of the engine with full tunability, but I've got a bit to learn yet, and the carb will probably be good enough for the time being.

Kevin.

Hi Kevin,

I think that a lot of people just assume that a bigger engine must require bigger jets (or smaller rods) in order to supply the fuel that the new, bigger engine will require. They do this without understanding how the carb works. Basically a bigger engine can create more vacuum in the carb and it's the vacuum that pulls the fuel through the jets. (Actually it's atmospheric pressure in the float bowl that pushes the fuel but you get what I mean!)

Therefore it is just as likely that the carb could need either small jets or the same jets as before. If the cam on the new engine is more wild then it could require bigger jets.

All of the above is just my opinion on the subject!


Cheers,

Pete

[kiwicar]

True for any fixed venturi carb.
It is also why mechanical secondary carbs can be so dificult to get right on transition.
Mike

[CastleMGBV8]

Thanks for the info guys, I should be able to get it fairly close from what's been suggested, it's only a temporary measure during the running in period until I get the EFI sorted.

Kevin.

[Robrover]

The Edelbrocks are intended for sale mainly in their homemarket in the States and are jetted accordingly, i.e. in the case of the 500 for a small block engine of around 4.9 litres / 300 ci.. A lot find their way onto 289 Mustangs. So if you're running a 3.5, 3.9 even 4.2 RV8 you probably need to downsize the main primary jets from the standard .086 to .083 or .080.

My 4.6 SD1 with Crane cam, fully flowed big valve Buick heads and MSD ignition has .086 primaries, .095 secondaries, 063 x 047 metering rods and a silver step up spring. I've experimented with different combinations using the proprietry Edelbrock jet kit, but this seems about the optimum for my engine. Idle mixture screws are set about 3 turns anti-clockwise, and the accelerator pump link is on the bottom of the three holes.

This is using the Edelbrock Performer inlet manifold and carb to manifold insulating spacer.

[CastleMGBV8]

Rob,

Thanks for the response, the new engine spec is 4.35 Buick 300 heads unported, but with bulleted guides new 1.63 Inlet valves 1.4 exhaust valves approx 9.8/1 compression and Crower 50232 cam with .488 In.
.490 Ex Lift at .50 and advertised duration of 276 and 281 degrees.

A the moment the 3.5 has primary jets of .80 and 062 x 052 Rods.

I still have the original stock rods and jets so may stick those back in and see how it runs, and jet down from there if necessary.

You didn't mention which cam your using, what level of power is your engine producing if you've had it tested?

Kevin.