Holley Main Body Upgrade.

[Yellow Truck]

My neighbour and friend (and Dr. Carburetor here on Vancouver Island) suggested to me today that if I was willing to give up the choke on my QFT SS 830 I could switch to one of the new style main bodies and pick up close to 50 hp at 5,500 rpm.

Pretty cheap power if it is true. Not that I don't trust Percy, but I don't know how well the new style body compares to the main body QFT used on the Super Street series carbs.

Does anyone have any insights? He said before he made any changes he'd want to put a calliper on the venturi on my carb. Giving up the choke is not a big deal.

[GerryP]

If you are making around 650HP now, he might be right.  Fifty horsepressures is quite a bit for removing the choke horn and profiling the venturi threshold.  Most of the time, until you start getting to that 1.5 HP/CI and above, you don't see big gains on the intake side unless you are really screwed up. 

[My427stang]

I would be amazed if you saw those gains with a QFT, they do pretty well. A 3310 with a choke and straight boosters maybe, but I wouldn’t expect it with what you have

[blykins]

I agree, I don't see those kind of gains being made, but if the engine is crying for it, a choke delete can bump a few ponies and I've seen 20 hp by switching to the billet metering block/base plate QFT carbs from the die cast QFT carbs.  50 hp seems like quite a stretch. 

[TJ]

I wonder if the choke has any benefit on these higher performing engines combos.  I have a quick fuel HR 780 on my mild 482 and the choke plate does nothing to help cold starts.  I removed the electric choke control and replaced it with a modified holley manual choke so that I have better control over the fast idle.  The choke plate itself does nothing (as far as I can tell) but controlling the fast idle via the choke cable is great for warming my engine from cold starts...keeps the motor running until it's warm enough to idle right.

Chokes are a must on smaller, tamer engines like the 352 and 390 I once had and of course chain saws, riding mowers, etc.  But to start my 482 after a long rest, I put a little gas in the carb bowls, press the gas pedal to pump a few squirts of gas into the intake and fire it up.

It's possible I'm doing something wrong.  I figure someone likes the electric chokes on these quick fuels but they're the only part I don't like with my carb.   They don't help cold starts and they're configuration makes it difficult to control fast idle.

I guess I'm saying, I wouldn't miss the choke if it was gone.

[GerryP]

You "gassing it up" method is just substituting for a choke.  A properly functioning choke comes off after a very brief on time.  If you have an electric choke, maybe 20 seconds.  Most carbs have a choke pull off that opens the choke by vacuum right after the engine starts.  It's interesting reading on the Internet people's perceptions of how it was and is living with a carburetor.  Like carbureted engines are hard to start in the cold.  Can you imagine how hard it would have been to sell cars up north in the 1960s if they didn't start in winter?  Back then, it was common practice to do the 12k mile tune up just ahead of winter and that was plugs, points, condenser, cleaning and lubing the carb choke mechanism.  Maybe replace the rotor, cap, and plugs depending upon what was showing on the scope.  People who took care of their cars could actually start them and drive them in the winter.  Those who did nothing had a hard time.

[TJ]

It's a mystery to me.  I can fire up my 482 right away at any outdoor temp.  I just plum gave up on the quick fuel choke...can't find a setting where it does any good electric or manual.

I've never had problems with using chokes...original carb on the 352, 600 ebrock on the 390, every push mower, farm tractor, 4 wheeler, riding mower, etc.  On push mowers or roto tiller where the choke cable was messed up I remove the air filter and choke by hand.  Only with the quick fuel have I not been able to get the choke to help start the engine.  No amount of partial choke will keep the engine running when it's cold so I set up the fast idle for manual control.  Otherwise the carb is great in all driving conditions and scenarios.     

If Yellow Truck's quick fuel choke acts like mine, it wouldn't be missed by removing it.

[Yellow Truck]

Thanks. Percy is old school and the go-to guy here for carbs. He does Rochesters, Quadrajets, and Webers, but mostly Holley's. It is kind of a retirement hobby with a little income attached. I often hear him firing up his SBF test mule on an engine stand behind his garage.

He is thinking about doing this with a stroked 302 (347 now) in his Cobra replica. He has it pretty much at the limit so he will probably see a jump. He didn't have a good look at the main body when we had the QFT off to adjust it, and he hasn't seen many of them, so he said he would want to measure mine before he would go ahead. I think the replies here have saved us the effort.

BTW regarding the choke - I like it to let the engine warm up, but I start it with the choke open. I deleted the electric choke and converted it to a manual. I have never liked automatic chokes. It wouldn't be an issue to lose the choke, but it would be a fair bit of work to convert the main body for little or no gain.

[Thumperbird]

True, false, or just stupid?

I suppose one reason total gain may not be much as one subjectively might think is that the secondaries are not shrouded much anyways so during pulls at high RPM airflow is less restricted overall and at low RPM on primaries only the horn just becomes less of an issue anyways?  Also, dyno's always seem to have perfect virtually unrestricted air feed from above so it again is a non-issue there.  In the real world with a lower profile filter and or housing where the air has to squirrel around to get in the hole I wonder if the real gain would be more?

[Yellow Truck]

Percy has been building race cars since the 60s, and has been a carb guy most of that time. He is not generally subject to marketing noise. I think the new main body is probably a good bit better than the original - he has one on the bench and it is a good bit different than the stock. It also comes with smaller jets, which I didn't understand and didn't have time to ask him about - as a non-expert I would have assumed more air would call for a bigger jet. He had some dyno results from Proform, who make a competitive main body.

What I was wonder was how different the QFT main body is compared to the standard Holley, and would there be a gain in the basic venturi design. We do have access to a chassis dyno here on Vancouver Island, but it would be hard to do an A/B test on the QFT since we would have to do a baseline, then take off the carb, swap the main body, put it back and run it again. It would cost $400 in dyno time alone.

[Barry_R]

I have done as many as six carbs on an engine chasing advantages.  As long as the carbs are reasonably comparable in airflow, the differences are incremental in the vast majority of cases, and seem to be entirely application dependent.  Something that makes a big difference in one engine will do almost nothing in another.  And in almost every case, the differences are in the single digits, with it rarely getting to 20 horsepower, let alone 50. 

One notable - and entertaining - exception was when I used a two inch adapter spacer to install a 1050 Dominator on a P.I. intake.  That unusual looking concoction made about 40 more than a traditional 750, and would leave you laughing at it until the throttle went to WOT.

[Yellow Truck]

As always, excellent advice here for the asking. I love Percy but this seemed like one of those old fairground deals where you put a catalyst in the fuel line and get 100 mpg.

I do expect that venturi design has improved over 50 years, but my QFT is pretty new. I assume what has been learned is incorporated into it.

[plovett]

I put a Proform mainbody on my Holley 4781.  That got rid of the choke, had contoured entrance, and different boosters.   Nothing empirical or quantified, but I could feel the difference.   To me that meant at least 10, maybe 15, or 20 hp at the very most.   

paul

This was on a 428 in the 535-545 hp range.

[Falcon67]

I've done a couple of "proform" body conversions and once dialed in they work pretty well.  I prefer the QFT metering blocks, rather than the ProForm blocks. But not 50 HP.  The gain is going to really be combination dependent. The 351C in the Falcon runs 7.5x at 88~89 MPH in the 1/8 mile with a 650 DP.  With a ProForm'ed 750 (about 840 is typical for those @ 1.5 in/hg drop) it runs....7.5x at 88~89 MPH. Your mileage may vary LOL. 

In the way back, had a friend running a Firebird drag car with a 454 and 8.5:1 compression.  He was using a 850 DP and kept having to screw with it.  AT one of the Friday TnTs, I lent him a 750 vacuum "3310" that had a updated ProForm main body for a comparison test.  The car picked up like 5 MPH and over 1 full second in the 1/8 mile.  Had to ask a few time later to get the carb back LOL.

[Drew Pojedinec]

Venturi design has not really changed in over 100 years.  An optimal tube is still an optimal tube.

I will say, that booster design and science has improved a considerable amount.
Maybe not on shelf carbs or parts, but many booster designs will simply work better, seeing more signal while taking up less room, thus the carb flows more.