looking at replacement jets for a Holley carb. On Holley's site they list Holley, QFT and Demon jets. For a #78 jet they list different size orifice for the same jet number. I understand jets are rated for flow but the Holley is .089 and the others are listed as .078.
Is there really that big of a difference or is it a mistake? Is there a difference in the flow or quality between these jets? There is a price difference which seems odd, are the cheap ones outsourced?, or of lower quality? Are the genuine Holley actually any better? or better quality as closer matched?

Only Holley jets that are accurate are the air bleeds. For mains, buy someone else's. I like the BLP jets especially the ones that have the jet extension built in. Whoever's you buy should have the hole size, not the flow numbers. Holley quit using their flow machine many many years and you will chase your tail. Buy them in .002 increments and never look back.

Holley jets are rated by flow. The flow is determined by the length of the orifice and the diameter of that orifice (L/D ratio). While two different numbered jets can have the same orifice diameter the length can be different. Thus different flow amounts. A chamfer on the leading and trailing edge of the orifice alters flow also. This was all worked out in 1919 by NASA, then NACA. in TR-49 available on line.

However, I use BLP jets. The are flow rated also. I just go up or down 2 numbers at a time as needed. Plus, as noted they are more accurate. The Holley air correctors are sh?t. Pin one to find out. Buy BLP correctors.

I pin everything when making a air bleed or jet change. I have found the Holley air bleeds very accurate when pinning.

So what about the screw in style jet extensions that place the jet out into the bowl? Do these change the flow of the jet?
I pinned a couple of Holley main jets and did find a slight difference between them.

Found BLP products accurate on the mains.I pin everything. Cheap investment is a pin kit. Holley
AB's found accurate also.

Can you recommend a pin gage and drill set.

Two company sell them, but they sure do look like the same product...

https://www.ezbore.net/Pin-Gau...to-250-p/nog2061.htm

https://blp.com/cart/index.php..._205&products_id=953

Well BLP offers 2 sets on the pins.
.011-.060
.061-.250
The EZ bore pins area "minus" tolerance pin kit.
.0000-.0002 of which is undersized. They were used back in my karting days>> Spec Check. When you hope you never get checked. lol. I know nothing.
I will have to look at the set i have in the cabinet at the shop and get you a name.

This message has been edited. Last edited by: TD3550,

quote:

Originally posted by Eman:
So what about the screw in style jet extensions that place the jet out into the bowl? Do these change the flow of the jet?

Don't use them. Sometimes the orifices in the extensions are smaller than the jet. This is another reason to use the extended BLP jets. Or you can make your own over the jet extensions if you have to use Holley jets.

My experience with the Holley correctors has been they don't meet my standards. .001 variance in a hole that small is more critical than the same variance in a hole larger than .100 I either drill my own or buy the BLP correctors.

Buy both sets of pin gauges and get the metric to SAE conversion chart from McMaster-Carr.
they have the metric drills that with get you those sizes that are in between the SAE sizes.

This message has been edited. Last edited by: nomad,

For pin gauges get the minus set, so it fits the hole you are putting it in.

As far as jets I use BLP jets almost exclusively. They have the widest selection of sizes and they have three different thread pitches to be able to go up to .260. I keep Holley jets for those that don't want to switch or just need a set of jets. The different brands are all made differently and you should not mix and match, Holley jets are square edge orifice and some of the others are chamfered. While they may flow the same at a fixed pressure the flow characteristics over the range a carb will see will be different. Stick with the Holley if you want to stick with that style jet. As nomad pointed out avoid threaded jet extensions. If they are reamed as big as they will tolerate you can use up to about an 80 jet, over that the passage starts becoming a restriction. You can make your own with a lathe and 11/32 .014 wall brass tubing, one end cut cleanly and the other end swedged to form a seat for the jet. The jet then slips thru the tube so it is not a restriction to fuel flow.

I'll scrap the screw in extensions and go back to the tubes.
QF and Holley are the same company aren't they? Are the jets the same between the 2? I've got a Holley jet box and that's all I've been using. Went to help someone at the track and I didn't have any 78's so I wanted to restock. I've donated a lot of jets!

I don't know what Holley is selling under the QF line now, Holley branded is your best bet.

I have a QF 1050 and have used it for 4+ years with the screw in, 5/16 hex jet extensions that came with that carb.

Jets in the low to mid 90's ( Holley numbers) and have zero problems using those extensions that move the jet back with a notched float...

Carbs I had with slip on brass or steel tubes for secondary jet extensions, the tube fell off occasionally.

That QF carb has been better than any Holley I ever owned......right out of the box...

Thanks for the info on pin gauge sets. Just ordered a both sets from ezbore.

David

quote:

Originally posted by SCDIV1:
I have a QF 1050 and have used it for 4+ years with the screw in, 5/16 hex jet extensions that came with that carb.

Jets in the low to mid 90's ( Holley numbers) and have zero problems using those extensions that move the jet back with a notched float...

Carbs I had with slip on brass or steel tubes for secondary jet extensions, the tube fell off occasionally.

That QF carb has been better than any Holley I ever owned......right out of the box...

Slip on extensions are not what I suggest, the ones we are taking about are retained by the jet. Holley sees a machined brass version, 122-5002. The tubes we make have a flange on one end, the jet slips in it and seats on the flange. Low 90's jet can be tolerant of the screw in, ideally the extension needs to be reamed in the low .150 range. The problem arises more when you see an extension in the .136 range and the main jet larger...

I googled that part number and see 2 different items shown with the same part number.

1 is a brass sleeve and 1 is a screw in extension like I have in my QF carb.


I assume the brass sleeve has the flange at one end that the jet goes thru into the metering block. That does make the most sense for not being a restriction....


I don't recall if I ever had or saw those brass flanged tube types....

The 122-5002 is the brass flanged type. They were originally in some of the Demon carbs, which Holley absorbed. You will find them in some if not all of the Gen 3 Dominators.

I've got a set of those brass sleeve style extensions. My only complaint with them is you can't use a jet tool, only a screwdriver so you better have one that fits the jet good.