marky mark

I'm still researching / considering all options possible.

One was to get the GM 502/502 and use my low-rise factory oval intake mated with the Quadrajet to clear my stock big block hood ('69, 427/390 car). I know this combo would hurt power somewhat, but how much?

Has anyone tried this or similar build ups with large cube engine and Quad carb?

Also, in looking at other options I noticed that the H.O. 454 & H.O. 502 both have rectangular port heads & 8.75 to 1 compression and a .211/.230 cam. But the 502/502 has the bigger cam, .224/.234 with the higher compression alum ovalports. Now GM must have a reason for this cam difference. Would you think that the larger cam would not work as well in the lower comp / rectangular port head 454/502 (H.O.)? Is it a question of compression ratio or head flow or perhaps both? Anyone tried the larger cam in the H.O. motor?

I ask because another option if I stayed with a GM crate motor would be to acquire the LS-6 intake and use the Holley carb instead. Either 454 or 502 H.O. would then work I believe. This would seem to be a better combo than the 502/502 with a quadrajet, unless I'm mising something (which I probably am)???

Overall summary: Can a Quadrajet work well on a 502?

Any help appreciated!!
Sorry for so many questions!!
Never knew hood clearance issues could take so much time! But I do want to retain the stock bb hood.

Mark :flag :chevy

RATT7

I have purchased Doug Roe's book on Q Jets and with Lars technical advice I dont see why not. I was very skeptical about the Q Jet at the begining, but now I believe its a very good carburator to use. I am learning more about these carbs as the weeks go buy and have also learned that there are 10 second super stock cars that are using the Q jet. Lars paper regarding these carbs should provide you with all the technical trouble shooting and performance upgrade info to run this carb. Well, thats my 2 cents :)

Corey_68

While the Q-jet is a great carb, why limit yourself? To me if your spending all this money on a engine why not get a Holley, Edelbrock of Demon carb?

The_Dude

If you want to stick with a stock looking air filter assembly, you might need to use a quadrajet.

Gator81

For performance, especially if you're going to limit yourself somewhat with the stock-type intake and air cleaner for hood clearance, the Q-jet makes sense to me. Even Ford used it, back in 70-71, on the 429 Cobra Jet. They needed to feed the big block, but the Holley 780 they had produced to much emissions. The low-speed drivability of the Q-jet has always been the thing that impressed me most about it. I suppose there is probably some small tradeoff in top end power, but with the stock intake and air cleaner, you've already made that decision.... just my $.02... :seeya

marky mark

Dude & Gator,

Is it safe to assume that switching to the Holley will require a non-stock air cleaner? I have the typical drop base unit that comes on the low rise 427/390 cars. I haven't tried to fit it on a Holley for comparison yet.

Ratt, Lars is the man. He's helped me with two quads already and I can trust he will be able to help in the future if needed. I was just wondering if the large cube motor would starve at high end.

Thanks for the opinions. Any other comments?

Regards, Mark

The_Dude

I don't know from experience, but I've been told that you can't use some aftermarket carbs with the stock air cleaner assembly without a spacer, which can make it difficult to use the stock hood as well.

marky mark

Dude, good point. Forgot about the Holleys as tend to use a spacer in there also. As it is with my Quadrajet set-up I can't use one as there isn't any spare hood clearance.

I gotta believe that, unless the heads are set up higher somehow that this will fit. May not be the greatest idea but it should work. Only one way to find out. If I have to get the other hood then at least I could put on a higher perf intake and carb. I just really like the stock bb hood.

Thanks, Mark

The Money Pit

I've run Q-Jets most of my life and had great throttle response,decent mileage and very little trouble,but my current 406 was too much for it once the tach tipped 6000.The 3310 will easily maintain 6500 rpms without fuel starvation,as supplied out of the box.
I talked to several Q-Jet guru's and they seemed to agree that fuel delivery was the weak point in the design.If your 502 will mostly lumber down the boulavard,the Q-Jet would be THE choice.If you plan on trouncin on the peddle,go Holley.

marky mark

Thanks Money. That's what I was afraid of as I had some fuel delivery issues on the 427 but didn't trace it down before the cam lobe wiped. I was going to upgrade the fuel pump first to eliminate that possibilty

I know there were a few 1971 454 LS6 'Vettes made that I believe had the standard big block hood. I was looking for a picture of one to confirm it but I can't find one. That would confirm a low rise LS6 style intake would fit on a rebuilt 427/454 w/ rect port heads instead of the 502.

I looked up the specs on the LS7 and even with alum heads and solid cam it still comes up short to the 502. WOW!!!

Thanks,

Mark

Ganey

GM put Q-Js on all V8s including 454, 455 & 500 C.I. Cad. engines.
The 800 has larger pri. & is rec. if you want a Q-J which should be fine for you esp. since you have said you don't drive hard.

WA 2 FST

Mark,

I just pulled this combo off my 454 HO. I was running a Holley 770cfm carb on top of an LS6 low-rise intake with the crate 454 HO engine (iron heads). Fit fine with a 3" aircleaner with the typicaly drop-base. No problems at all.

FWIW, I would not run the bigger 502 cam in the 454 engine. Lower CR does make a difference. The 211/230 cam is small, but mine still pulled to 5500rpm.

marky mark

Ganey, thanks for the input. I forgot about those big cube motors!

W A 2 Fast, that's just the info I needed. So, the standard drop base open element 3" air cleaner w/ chrome top will fit with no problems, thats great!

I don't know a whole lot about the Holley's to be honest. Was yours the vacuum secondaries or the mechanical? And, would either style be ok to use with the air cleaner I described?

Also, since you did your upgrades I don't know if you had a chance to compare the LS-6 set-up with your new set up before the other work. If you did, was there a big difference? In general, with the LS6 set-up, how was the performance/throttle response, top-end, etc?

Thanks again.

Mark

WA 2 FST

Yes, it will fit under the stock BB hood. :)

I don't know anything about Q-jets. :lol: This is my first carb'd car, FYI. I had a 770cfm Holley 4bbl with mech. secondaries.

Well, there was a huge difference, but that's primarily due to the heads and cam. The stock 454 HO with LS6 low-rise, 770cfm carb, and Hooker side-mount headers made 334rwhp and 384rwtq. Strong runner, but nothing like what it is now. Throttle response was very snappy. It was done at 5500rpm, and that's where GMPP says the redline of the 454 HO is anyway.

But the new combo makes 100rwhp more than the old one. That is obviously a substantial difference. I do not know what my combo would make with the LS6 intake now. I was told I would probably lose 20-25hp, which is why I went with the L88 hood and AirGap intake.

marky mark

W A 2 Fast,

Thanks so much for those answers!!! It's tough to figure out stuff without seeing it yourself. This is why this is such a great Forum.

I owe you a beer or three!!!

Regards, Mark

marky mark

Oh I knew I forgot something. On the 454 does it run on 92 octane with no detonation?

Thanks again!

Mark

GDaina

this won't help you much, but what the heck...I'm running the factory tripower unit, flat intake, and air cleaner....and it pulls like a bear all the way past 7,000 R's.

Tominator

I guy on the forum bought the 454 ho motor and put the 502/502 cam in it and seemed happy with it, think it was orvette. Hopefully he can chime in.

By the way the ZZ502 base kit cost not much more than the 502 HO

WA 2 FST

Marky mark, yes the 454 HO runs on high octane unleaded. It's only 8.75:1 CR. Mine now has 9.5:1 CR, but also aluminum heads...again, no issue with high octane unleaded (we have 93 here in TX). I'd say depending upon cam selection, you could easily run 10.0-10.25:1 CR on 93 octane.

And Tominator is right... ORVette put the 502 cam in his 454 HO, and did seem happy with it. I am not sure if he ever dyno'd it. I'd be interested to see if it made more average HP across the rev-range than the stock 454 HO cam.

If you can afford the 502/502, that's what I'd get.

marky mark

Thanks again for all the good info and insight.

I am leaning towards going with a rectangular port head, low rise intake and Holley set-up. But, nothing is definite.

The 502 short block does come with the 505/502 cam. The GM advice is to switch to the 454/502 HO cam if it is used as a replacement for either. I was guessing this was due to the lower comp ratio and the iron rect. port head not flowing as well as the oval port alum.

I'll try and find out how he liked the 454 w/502/502 cam.

Money is of course, the limiting factor in any decision such as this. I've got a pretty good idea that after all is said & done this conversion is going to be in the $10,000 range +/-. That's enough to make you blink, at least it is for me. So, I'll have to consider is going this route worth the price.

Thanks again for all the input.

Regards, Mark


[Modified by marky mark, 3:39 AM 7/31/2003]