Author Topic: Is there a difference performance wise?  (Read 1434 times)

Offline 55 Tony

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Is there a difference performance wise?
« on: August 12, 2017, 07:31:26 AM »
Which one of these q-jets are better performance wise or is there no real difference?  No need to match numbers or anything like that.
7042240 or a 17084266  (both 800cfm)

Engine is a 454 ~10.3:1 compression
Cam:
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=436&sb=0

I have always had trouble with fuel starvation when it's hot and going WOT.  I'm guessing I get fuel vapor in my lines and the pump can't keep up pushing the vapor out and pumping fuel again?

The 17084266 is new to me.  It has the large void shown on page 12 of Cliffs book, but it didn't come with the plastic cup.  It looks to me as if I could drill a hole down near the bottom of that void that it would basically make the bowl larger/more capacity.  Anyone try that?

Offline carmantx

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Re: Is there a difference performance wise?
« Reply #1 on: August 12, 2017, 04:00:46 PM »
I can't find any info on the 4266, but it is probably electric choke. 
So choke would be a difference to consider.
Also consider the internal APT for tuning on one, not other
2240 will have more parts selection for rods.

Just some thoughts to consider.
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Offline Cliff Ruggles

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Re: Is there a difference performance wise?
« Reply #2 on: August 13, 2017, 03:35:25 AM »
17084266?

Not familiar with that part number, can you post a pic of it?

Offline 55 Tony

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Re: Is there a difference performance wise?
« Reply #3 on: August 13, 2017, 05:41:29 AM »
I goofed, it's a 17084226.

Offline Cliff Ruggles

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Re: Is there a difference performance wise?
« Reply #4 on: August 14, 2017, 01:09:42 AM »
Late model truck Q-jet, nice unit and same capabilities as the early Buick but a much better carb all the way around....IMHO

"I have always had trouble with fuel starvation when it's hot and going WOT.  I'm guessing I get fuel vapor in my lines and the pump can't keep up pushing the vapor out and pumping fuel again?"

Start by putting the correct high flow N/S assembly in the carburetor to make sure it isn't simply a fuel delivery issue.  I would install our HP pump at the same time, the big block application needs both of those items...

Offline 55 Tony

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Re: Is there a difference performance wise?
« Reply #5 on: August 14, 2017, 10:02:06 AM »
Re: 17084226

Thanks, good to know I got a good model.  My old one was only a 750 and the 800 with the larger primaries seem to suit my engine quite well!

I already have a .145" needle and seat, a 110 gallon/hour fuel pump, 3/8" fuel line.  When I mentioned the fuel starvation "always", I meant in the previous q-jet and this one I just rebuilt last week.

Yesterday I did some re-routing of the fuel lines and regulator and it didn't have a problem going WOT through first and second and into third, but it wasn't real hot out either.  Without a doubt better, not sure if it's fixed.

It's not great yet at idle, I still have to read your book better to enlarge the idle circuits.

Offline 55 Tony

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Re: Is there a difference performance wise?
« Reply #6 on: August 14, 2017, 03:46:24 PM »
Oh, by the HP pump I guess you meant the accelerator pump.  That does have an HP pump in it also.  Never a problem there even in the old q-jet after grinding the shaft just a little shorter.  And with both, when the fuel starvation acts up, if I pump the throttle I get short bursts of power, so the accelerator pump is working fine.  With this one I tried just for the heck of it to move the linkage to the outer hole and it had a little hesitation but not much.  No hesitation at all with the linkage in the proper hole.  I can go from a stop at idle and floor it and it doesn't skip a beat.  Actually left the  vacuum canister slow at about 2 seconds and it seems to help me from spinning the tires till it hits 5500rpm and shifting.  Old one I had close to 1 second and still had no hesitation but it just spun the tires.  So the slower pull off is sort of like a torque limiter of sorts.  A good thing for me to have without slicks on a track.