Thornton exhaust manifolds (SBO) on a G-body

[olds307 and 403]

https://www.summitracing.com/parts/uem-1639hc-020
I see both Speedpro and KB has hyper pistons for cheap for the 307. The KB pistons are a better choice, a slightly taller compression height. It will help keep the 8 to 1 compression, make sure they mill the head and block to retain that or you may end up with 7 to 1 compression. Either way a set of coated hyper pistons coated for under $200 is good. Too bad they didn't upgrade the ring pack to the thinner rings, would have helped power and mileage.

 

[kustomkyle]

https://www.summitracing.com/parts/uem-1639hc-020
I see both Speedpro and KB has hyper pistons for cheap for the 307. The KB pistons are a better choice, a slightly taller compression height. It will help keep the 8 to 1 compression, make sure they mill the head and block to retain that or you may end up with 7 to 1 compression. Either way a set of coated hyper pistons coated for under $200 is good. Too bad they didn't upgrade the ring pack to the thinner rings, would have helped power and mileage.

I'll have to check, but I believe those are the exact pistons I have. I ended up getting a standard bore and .030-over set because they were very cheap. Might not be able to use either, but I'll just have to see. Years ago I just randomly bought some internal engine parts, no bearings or anything, but pistons and rings.

And I was just thinking that about pistons with thinner rings. Overkill on an old non-performance engine I guess, but it would be interesting to see how much gain in power and mileage there would be.

Always heard stories of 307-equipped cars getting 30-MPG highway, but every time I heard it, it was in reference to a heavier B-body wagon. Including a family member who lived in the D.C. area.

 

[olds307 and 403]

Remember my 30 mpg is imperial, since that us what we use up here. Our 81 Delta 88 got32 mpg imperial once, then high 20's, then mid 20's. Non CCC Canadian 307/TH250C/2.41. For comparison our 2010 Challenger got 32 mpg imperial on a long trip. That is a 3.5/5spd auto/ optional 3.07 combo. New changes will be an igniter 3 module and 225/70R14 tires. They will lower rpm by 100 rpm on the highway, less rolling resistance than the 235/60R14 tires and put my speedo bang on. I calculated mpg off the GPS, speedometer was 10 to 11 km slow and that made odometer slow as well. Your motor should clean up to .030" overbore but who knows for sure. The thin rings are worth 10 hp, hard to say what the mpg gain would be.

 

[kustomkyle]

Remember my 30 mpg is imperial, since that us what we use up here. Our 81 Delta 88 got32 mpg imperial once, then high 20's, then mid 20's. Non CCC Canadian 307/TH250C/2.41. For comparison our 2010 Challenger got 32 mpg imperial on a long trip. That is a 3.5/5spd auto/ optional 3.07 combo. New changes will be an igniter 3 module and 225/70R14 tires. They will lower rpm by 100 rpm on the highway, less rolling resistance than the 235/60R14 tires and put my speedo bang on. I calculated mpg off the GPS, speedometer was 10 to 11 km slow and that made odometer slow as well. Your motor should clean up to .030" overbore but who knows for sure. The thin rings are worth 10 hp, hard to say what the mpg gain would be.

32 Imperial is still nearly 27MPG US. That's very impressive for a big heavy "brick" of a car, using a 4bbl carburetor (and a 3-speed auto to boot).

I do appreciate all the good info.

I'll have to wait a few months, but Thornton is supposed to come out with the stainless tubular small block headers in January 2018. Best choice would still be the manifolds though.

 

[kustomkyle]

I had contacted Thornton by email the same day after I created this thread, and have yet to hear back.

Doesn't matter I suppose, appears they will in fact work on the later heads, they just won't be as good as a match compared to 5A 307 heads or 350/403 heads. And if they work for swirl port 307's, then I would assume they would work on the 260 if anyone was so inclined.

 

[olds307 and 403]

The 260 has basically the same exhaust port as the early 307 heads. I had Sanderson shorty headers and 2.5" X pipe duals on my 260 for a bit. It did help but the peanut cam and mighty Dualjet limited the gain. I would say 5hp over 260 iron manifolds and 2" dual exhaust.

 

[kustomkyle]

Thornton headers are finally out. Look better than the ones they had for the big blocks:

http://rover.ebay.com/rover/1/711-5...0001&campid=5335822911&icep_item=382403976943

 

[kustomkyle]

And about a year later, the headers are on special until January 4, 2019 on eBay. Manifolds are as well, for even less money. Have to watch though, there are several listings, and not all are at the sale price.

A heads up to those interested in picking up a set, they're about $40 off....

 

[69hurstolds]

Could have got them for 216 + shipping with the 15% discount ebay had until 9 pm ET last night along with their own 15% off for the sale.

 

[kustomkyle]

Could have got them for 216 + shipping with the 15% discount ebay had until 9 pm ET last night along with their own 15% off for the sale.

That would have been great, hopefully some people got the extra discount.

Now the next question needing answered is which downpipes work best? Apparently the Chevy G-body ones from Pypes work well and possibly better than the Olds A-Body ones. And, more importantly, if the column shift linkage presents any problems as opposed to the console linkage.

Would assume they are different, never owned a console shift car, and from pictures I've seen, the console linkage appears to be closer to the transmission allowing more clearance (?).

Could always fabricate downpipes, but the materials right now outweigh the cost of purchasing premade pipes.