Q-jet and Holley
 

Chevy small block 350,make 597 hp with holley 750, and drop to 562 with Q-Jet .What can I do?

Any input Thank,

Re: Q-jet and Holley
 

Whos q jet is it why did you take the 750 off??

Re: Q-jet and Holley
 

The air valve was probably not working properly.--- open all the way.
Or most likely you were trying to dyno it with the turbine installed on the carb.
I have experienced problems with some q-jets and the turbine. I think ours causes some air currents that the q-jet boosters/bleeds dont like. Look at your EGT's. I bet they were haywire from the Holley pulls. You might also check the fuel flow from the q-jet to the Holley. It could have been the N/S in the Q-jet not flowing enough fuel. Had that problem before, too. There are some hi-flow N/S available. Good luck

Re: Q-jet and Holley
 

I agree with Adger. I just did a test on dyno 2 weeks ago and we removed a Holley and installed a Qjet with the dyno top hat and saw a significant drop in power. Air flow way down. Removed hat and power much closer.
reed

Re: Q-jet and Holley
 

Thank every body for the input!

Adger,What do mean by NS?

Ex. temp cyl . 1 2 3 4 5 6 7 8
Holley 1319,1295,1335,1315,1406,1263,1360,1314,

Q-JET 1318,1247,1346,1259,1359,1288,1261,1253,

Re: Q-jet and Holley
 

4-7 swap cam?

Re: Q-jet and Holley
 

I think it is an abbreavation for needle & seat valve. EXCUSE, I dont mean to steal any ones thunder. ed:)

Re: Q-jet and Holley
 

Cam is Bullet Standart firing order 18436572

Thank!

Re: Q-jet and Holley
 

Yes, N/S = Needle and Seat
Put a stock curved bottom Air filter on the Q-jet to test it. Use o2's instead of the hat to tune. Don't run the fuel pressure too high. The q-jets I've delt with don't like high fuel pressure because of the float. The bigger the N/S the harder it is for the float to overcome pressure (W/added N/S area)and maintain proper float level. Thanks, No thunder here, Ed :~0
Sorry didn't see the temps. Look how the numbers changed. You had cool get hotter and hot get cooler and I looked at it the temps a little differently. Actual firing order.
Cyl # 1 #8 #4 #3
-1 -60 -55 +11

Cyl #6 #5 #7 #2
+23 -18 -55 -48
Notice 3 & 6 got hotter. Is it a coincidence they are in the center of the engine and on opposite banks? Look at the manifold design.
With just a carb change you were down on power and the temp changes were significant
leads me to believe there is an air / fuel problems. It is either the hat distrubing the air or the carb is way off.

Re: Q-jet and Holley
 

Thank for your help!
Adger, With the turbine on the Q-jet,the engine pif,paf,like loosing cylinder,Engine won"t pull . Q-jet is hedwork with DA metering rod.

Re: Q-jet and Holley
 

Get the turbine off!!!, put in a fresh set of plugs and tune by reading the plugs.
Remember: It would be correct to assume the Holley told you the engine was in mechanical sound condition. Those Holley EGT's were not out of the ballpark. Don't just chase anything & everything. Don't do needless changes.

Re: Q-jet and Holley
 

What is a top hat or turbine? Is this something used on the dyno only?

Re: Q-jet and Holley
 

Darren,Yes the turbine is only dyno supply air from outside
Thank!

Re: Q-jet and Holley
 

why did you take the 750 off??

Re: Q-jet and Holley
 

Mickey, Just dyno test ,on my ss engine.

Re: Q-jet and Holley
 

Q jets are funny get a good one it really will be a surprise the difference!

Re: Q-jet and Holley
 

Yes ,mine need some upgrade or tuning !
Thank,

Re: Q-jet and Holley
 

Turbine is used to mearsure air flow (cfm) engine is using,usaully has cat (carb air temp) sensor mounted in it also.
My engine does'nt make near that power, pure stocker,my Q jet will match HP all way through and have about 3lbs. torque less @ peak than 750 holley. I compared on 351C on time w/ my q,jet 750&850 dbl. pumps and had same results.
I leave turbine off and mount cat sensor on carb to where it picks air temp going in carb so correction is repeatble.

Mike Taylor 3601

Re: Q-jet and Holley
 

This is very interesting subjet?
Thank a lot everybody!

Re: Q-jet and Holley
 

i removed a 780 holley and
i put a q-jet on my 429 ford
same mph but et was .2 slower
carb when very rich in the first 100'
need help with that

Re: Q-jet and Holley
 

Vic, does your carburetor have the little air deflector on the top behind the choke horn? Often you find that the combination of a slightly high float level and that little sheet metal deflector not being there will allow fuel to splash out of the bowl vent and into the secondaries.

If you do not have it, the best book for the Q-Jet racers may possibly be the book Doug Roe wrote, it's available from HP Books.

http://www.amazon.com/Rochester-Carb...4497285&sr=1-2

Re: Q-jet and Holley
 

The two holes above the rear upper throttle blades may need restricted. On larger engines they can often be plugged. They act similar to an accelerator pump for the secondaries. Might try tape over those holes for a pass to see how it acts. If it stumbles with them plugged, you may need to restrict them.

Re: Q-jet and Holley
 

I have used toothpicks in the holes.

Re: Q-jet and Holley
 

My apologies for going a little outside of this thread, but I have always
wondered why some QJs perform much better than others. I realize there could be many theories on the subject, what makes one QJ fly and the other a dog! tks ed

Re: Q-jet and Holley
 

For myself I have a bone in my engine ,it is the Q-jet, or a distribution problem from the spacer and intake.

I hope , I am not the first one with an drop of performance when we switch the Holley for the Q-jet?
Thank,

Re: Q-jet and Holley
 

Sounds like maybe you need to see about borrowing another carburetor, and maybe talk to someone about intakes and spacers. I think a 35HP loss is far too big a difference to be acceptable from just a carburetor change, if the two carburetors are close to the same size.