What can I expect?

[4th Gen C-10]

I have a 69 C-10 LWB that I am tearing into. Original plan was to take out the 307 and 3OTT and replace with a ZZ4 crate and 700R4 transmission. Thinking more about it I kind of want to keep the 307 for historical reasons. The engine has ~210,000 miles on it with one rebuild at ~160,000 miles. I am not sure exactly what was done to it.

My first question is what kind of HP and MPG can I expect out of it if I change out the cam, intake manifold, put on an efi, change out the exhaust manifold for headers, etc? So basically I am asking what would those of you that have alot of rebuild knowledge do with it?

Second issue: I do still want to swap out the 3OTT for an automatic tranny. What would you recommend to go with the rebuilt 307?

Third issue: This will be a nice daily driver with some long road trips (hopefully the Power Tour someday). What rear end gear ratio would you go with?

Thanks

[mechanicalman]

700R4 with TCC will get you low RPM's on the highway/better gas mileage but be warned engine efficiency can suffer if the cam is not in sync with the compression of the engine and the rear end gear ratio. Better to settle on rear end gear and trans to see what your cruise RPM is then select a cam to match those RPMs and your compression ratio.

I'd shoot for 9.5 compression but would recommend vortec heads/intake manifold and a GM cam like the HO350/330 HP hydraulic cam 212-222-112.

Headers and free flowing exhaust are the best thing you can do for it, however right now with the 3 OTT you likely have a bell housing cross-member that may limit your header selection in a negative way. Maybe start with "what is your rear axle ratio?" and install the appropriate automatic trans, 700R4 or T350 or T350C with the appropriate trans cross-member keeping in mind when you obtain a cross-member you might want to consider exhaust pipe clearance. With the long wheelbase you could probably add an "X" pipe or at least a cross-over pipe, also stick with 2.5" pipes all the way back.

Hard to predict what mileage to expect not knowing rear gear ratio, cruise RPM, intended transmission, etc.

Fuel injection is going to cost you. A well tuned Q-jet or AFB will do a great job but also critical is an appropriate ignition curve on the old distributor, and I'd advise you to get a new HEI as they are so cost-effective now on e-bay then a curve kit if necessary plus an adjustable vacuum advance.

[RManson]

I got around 14mpg from my 307 with a 4bbl Edelbrock setup. I think it was a 600cfm carb. Went to a Holley Avenger EFI setup and got around 18-20mpg. Those are highway numbers with a T5 trans, 3.83 rear gear, and a 29-inch tire. No cam, Speedway Motors stainless exhaust manifolds. I would estimate power around 200hp, give or take. Hope that helps!

[mechanicalman]

Originally Posted by RManson

I got around 14mpg from my 307 with a 4bbl Edelbrock setup. I think it was a 600cfm carb. Went to a Holley Avenger EFI setup and got around 18-20mpg. Those are highway numbers with a T5 trans, 3.83 rear gear, and a 29-inch tire. No cam, Speedway Motors stainless exhaust manifolds. I would estimate power around 200hp, give or take. Hope that helps!

Hi RManson-
Did the Holley Avenger control the distributor?

[hugger6933]

I would look into the Edelbrock in the 500 cfm range instead of the 600. Keep the cam small the smaller the motor the smaller it needs meaning a cam that works well in a 350 may be way to big in a 307 so theres that factor to deal with. I too would look into getting some Vortec heads off of a 305 maybe so the smaller chamber can work for you. A cam in the 250-256 duration[ advertised] probably would be really great. 2.5 inch exhaust is too large I'm afraid drop back to 2.25 and everything will be happier. The 700 will be a great choice with a 3:42 rear gear is hard to beat it is my favorite all around gear for the best of everything you could need. Jim

[tinlid]

You are saying keep the 307 for historic reasons but then you are changing it with headers, efi, transmission. Where is the historic value now with the changes? If it is not an accurate restoration why not go with your first thoughts of the ZZ4 crate motor?

[LSUMurse]

You may find this thread interesting:
http://67-72chevytrucks.com/vboard/s...d.php?t=429865

[4th Gen C-10]

Thanks for the input everyone! Definitely a lot to think about.

Anyone else have thoughts or opinions?

[Grumpy old man]

What can you expect ....... Long nights on the concrete with oil or trans fluid in your hair , a few busted knuckles , a sore back , and you'll find yourself wondering why when you drop a bolt/tool it always ends up directly in the middle under your truck ?

[KSWes]

What is the plan for the truck? Driver, hauler, cruiser, museum piece.

I agree with Cptn. You might also consider a late model LS truck motor swap if mileage and driveability is a primary goal.

[CaptRMW]

Putting heads, headers, cam, and intake on a 307 is kind of like putting a $100,000.00 saddle on a $10.00 horse. With a 3.47 rear gear and 50,000 miles on somebodys idea of an overhaul which may be a rering and bearings it may not pull a few bags of groceries to the house. The 307 will be good as a daily driver untill you get the 700R4 and funds to get that ZZ4 then you can do some real tire roasting. And for an additional $.02 worth of very important info 9.5-1 compression ratio will knock like the landlord after the rent unless you have some serious spark control. Good luck and keep us posted.