'69 C10 307, with sbc 400 heads?

[w3lfd]

So, I bought my '69 last winter and finally got it road worthy, inspected, etc. after tons of metal work. Now I am finally digging into the motor itself.

When I bought it off of ebay, it had been listed as a 350. Upon picking it up and checking the block casting number, I determined that it was a '69 307 ( casting number 3956632).

I finally got to the tune up stage last week, and went to <insert copycutter chain store here> to get spark plugs. It currently had Autolite 25's in it. The auto store gave me the "wrong" spark plugs. This is when the box of worms opened up.

Pulled the valve cover and check the head casting number (3973493). I have checked a few places, and for the most part these are listed as early '70's 400SBC heads (76CC with 1.94 intake / 1.5 exhaust). One oddball site also indicated that they could have been used on 350's, but I digress.

Now the real question is this: How has this affected the performance of the motor? Is this a swap that was common, or am i dealing with a real oddball here?

It seems to run well, but I really have no frame of reference to determine. It is equipped with the original intake and 2 barrel Rochester carb. The truck does run quite rich, and fuel mileage is ridiculously low, I can only assume a side affect of oversized valves on a low compression motor.

I have ordered some 65k mile #416 Chevy 305 heads for the truck, as I had already planned on this swap. So it may be moot to discuss, but I am just curious what you all think.

[brn agn]

I am not an engine expert, but the 400 ci heads don't seem like the best match for the smaller displacement engine due to the larger combustion chamber.

I just did the 416 head swap on my 307 a couple of months ago (I also installed a camshaft with more lift at the same time). Unfortunately, I found issues with the lower end and switched to a different engine. The old girl still ran well with the new heads/cam. I think different exhaust and a better intake/carb would have really woke her up.

[raycow]

Originally Posted by w3lfd

Now the real question is this: How has this affected the performance of the motor? Is this a swap that was common, or am i dealing with a real oddball here?

It seems to run well, but I really have no frame of reference to determine. It is equipped with the original intake and 2 barrel Rochester carb. The truck does run quite rich, and fuel mileage is ridiculously low, I can only assume a side affect of oversized valves on a low compression motor.

The larger intake valves shouldn't cause a problem, but the combustion chamber volume will hurt your compression ratio. This will have a direct effect on the torque output. If the carb is stock for the 307 and has stock jetting, your rich running may be caused by high float level, an internal fuel leak, or the power valve hanging open.

Ray

[w3lfd]

Thanks for the info. I will have to check that out once I get these 305 heads on.

[67_C-30]

Originally Posted by w3lfd

Thanks for the info. I will have to check that out once I get these 305 heads on.

The 305 heads should make a significant difference. The factory 307 heads were around 65cc so the 400 heads drop the 307 well below 8 : 1 compression. The 416's have 58cc chambers are will put the 307 around 9 : 1 and it will be much more responsive down low but will still breathe adequately for the 307. I would recommend a carb and intake change when you do the swap to maximize the potential, unless you are trying to retain stock appearance.

[w3lfd]

Thanks for the info in the 307 displacement, I hadn't dug that up yet. So this thing is definitely running super low on the compression.

I am contemplating a carb and intake change when I do the swap. A small 4 barrel probably (looking into edelbrock carb / intake combos).

Also looking at a mild doing a mild cam from summit racing.

[bigunde]

Couldnt you get a 305 that had the 194 intake valves?

[w3lfd]

The #419 305 HO heads are a 1.84 intake valve.

I don't actually want or need 1.94's, that is just what is currently on the 400 heads that someone slapped on this thing.

I'd love to find a used quadrajet and intake to throw on this thing.