MANIFOLDS

Monte Lauer

Monte Lauer

Active member
I have both 4bbl manifolds, square and spread bore, for my 345. I am dropping in another 345 and I don't know which manifold I want to use. The new motor already has the square bore on it. Since this Scout is used for just driving around, no wheeling or towing, would the spread bore benefit me? Will I see a noticable difference in gas milage? I am ready to drop in the new motor and I don't want to change out manifolds unless it would greatly benefit me. Also. If this helps : 345, edelbrock 600 cfm, auto trans, 3.07 gears, 33" tires, highway driver.

Sorry if I put this in the wrong section, but a carb does bolt to the manifold.
 
In your case, the spreadbore would be worthless! And it really limits carb options, though I do have two near new Holley spreadbores, but they will be real pricey!

We do have several thermoquad spreadbore cores, but those were never considered a "performance" carb and can be problematic to deal with, out of the 6 cores I have, only two are worthwhile for reconditioning.

"potential" of maybe slightly better highway milage (1>1.5) but doubtful when used in normal driving or towing app, but from my perspective since ya don't have a spreadbore carb to use now, and it would involve swapping a manifold, there is no payback on that deal! Squarebore holleys and carter/edelbrocks are a dimea dozen.
 
Thank you Mike. I will stick with the square bore. Fyi I do have a spread bore adaptor plate for my edelbrock carb.
 
If the "spread bore" carbs aren't better for "typical" driving, why did GM use so! Many and Ford a few, quadrajet's?, one of my buddies put one on his Scout and it drive and wheels like never before!?
 
The "spread bore" concept was used by nearly all fuel systems manufacturers in one form or another over the years. It's primary reason for being was an "emissions" design. Out of all the offerings, the rochester quadrajet became the best known/used simply because of the GM applications, sales numbers of GM products back in the day simply mandated dominance when spread through the entire GM family.

And...ya have to understand the "supply side" relationship between vehicle manufacturers and the component suppliers. Rochester was dominant for GM apps (a subsidiary), Holley was dominant for Ford apps (along with manufacturing the joint venture motorcraft mixers), and carter was dominant for chrysler apps. But GM also used many Holley carbs, as did chrysler. Most especially in the high performance offerings and later the muscle car stuff.

The Holley version of a spread bore throttle body (4165/4175 series) was simply too expensive to be a player in the oem world.

The carter thermoquad had the most potential (both drivability and performance) of all the spreadbore apps in my opinion, but it was plagued with production problems, mostly related to the extremely unique (for the era) thermoplastic fuel bowl used to combat fuel percolation resulting from the extremely high engine and underhood temps generated as part of the design of third tier emissions vehicles beginning in about 1972 and going forward. Those issues were ultimately cured but not before the mixer received a bad rap in the industry.

The concept of the spreadbore regarding "improved" fuel economy is basically a marketing myth. Any comparable carbs of that era can be set up to provide excellent drivability and economy if "emissions numbers" don't matter.

The current reiteration of the carter avs carb, known as the edelbrock "thunder" series is not a true spreadbore design, but is somewhere between a square bore and a spreadbore.
 
I have a new carter thermoquad I just put on my Scout II. I have a square bore mani, on it, but also have a spreadbore manifold. Looking for the best performance for the truck, it's not a daily driver, so looking for power. Oh yeah the engine is a 345, cammed, with 304 heads I think for the compression bump.
Is it worth changing the manifold for the spreadbore of should I stick with the squarebore installed?
 
Way more carburetor choices on the square bore manifold. Very few spread bore carbs availible.

For performance stay with the square bore manifold. Square bore manifold runners are better matched for length.

The only reason I would consider swapping on the spread bore mani. Would be to pass emissions and run the factory carb.
 
Thanks, I'll stick with the square bore. Trying to set the adjustments on this new carb is a pain. Trying to find all the measurements needed, the carter manual on carb doctors goes thru the adjustments but reference measurements I can't find.
Need a shop manual to get those I guess, still looking for a clean one like a pdf download, I could get by this weekend, to get it right.

Thanks again for the info.
 
ruper said:
thanks, I'll stick with the square bore. Trying to set the adjustments on this new carb is a pain. Trying to find all the measurements needed, the carter manual on carb doctors goes thru the adjustments but reference measurements I can't find.
Need a shop manual to get those I guess, still looking for a clean one like a pdf download, I could get by this weekend, to get it right.

Thanks again for the info.
Click to expand...

We need complete information/documentation regarding the thermoquad in order to be able to assist, there were hundreds of variations of this carb...when you say "new", that means to me it was a new carb out of the box and correct for the application. It should need nothing other than verification of idle mixture adjustment with the engine running. And if new, then it will have idle limiter caps in place that preclude much change unless they are removed/bypassed in some manner.

And running a spreadbore carb of any source on a square bore manifold is not the best thing to do...I'm fighting one of those now and the adapters for doing so leave much to be desired along with creating vacuum leakage.

So I'm confused...exactly which carb on which manifold are you trying to work with now that you need specs for?

If you are going to play with a true ihc thermoquad carb on a dedicated spreadbore manifold, then you do need to use the "specs" in the IH service manual, specs for a chrysler version of a tq are not correct and are for somewhat different variations of the tq.

The thermoquad carb was introduced to IH field service personnel in July of 1973 through fleet service letter slf73-22 and subsequently that information was include as an added section to the service manual sets. It includes all information related to the tq along with all emissions-related info needed to approach vehicle service as an integrated system...that bulletin consist of 88 pages so it's not going to be made available here as a .pdf!
 
I will need to do some research. I had a IH thermoquad on the sii, but it began to run very " inconsistently". I found another tq the my brother had bought about a year ago, he said it was new. It was in to mylar sealed bag. No limiter caps however as that is what I was expecting. To my knowledge it was bought at a discount from somewhere maybe ebay, and I believe it was for a chrysler product.

It does not have the tin tag on it, so I assume it is not an IH spec carb. I was thinking that the square bore was possibly having vacuum issues, seems like the timing is all over the place then will settle out, found cracked bowl on old carb that's what led to this "new" carb.
I will do some searching, but if I read you right the spreadbore would be with changing to in this application? I have one sitting at my dads garage so just putting it on would be the only issue
 
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