Engine compression questions

[Geronimo-6]

Need some advice and assistance from the collective wisdom of the forum. I am rebuilding my 1975 and have the block and crank at the machine shop. Will be bored .040 over from the current .030.

My plan is to add aftermarket heads, cam, intake and carb (probably the Edelbrock Power Package E-Street. I will also replace the stock exhaust manifold with headers (still debating long or short tube).

The Bronco will see lots of time on the street, no serious off road driving. Not a commuter, just Sunday afternoon drives. Gas mileage isn't a big concern, if it were I'd buy a Kia.

My questions have to do with increasing the compression ratio:

-From what I've read, the compression ratio was about 8-8.5:1. Correct?
-Is the low compression ratio a concern, considering the way I want to use the vehicle?
-What type of piston would increase my compression ratio, assuming it will benefit me?
-What else am I not considering? This is my first overhaul...not looking to do anything crazy. Crazy comes with the next engine I build. Probably try to put in a gas turbine or something.

Maintaining some originality is important to me. Building an engine, my first at age 49, is important to me. I am well aware that a 351w would have much more torque. I know this thing won't ever be a race car, but I do want to see some improved performance, even if it's just beating a mini-van from one stop light to the next.

Thanks,
Mark

 

[Rustytruck]

performance starts at the tires and axle gearing. as for the engine around town your only turning at most 4,000 rpm if your really stepping on it. higher speed and horse power would be used if your playing in the sand dunes. now think what you are looking at the Bronco to be. if your planning a pig heavy overlander then you might be looking harder at at 351 grunt motor. I would rather have a 351 than a large over bore 302 with high compression. how good has your cooling system been with your current motor? over boring and higher compression needs a very good cooling system.

 

[Timmy390]

All about how you want to spend your money.

Why not go with a modern 302 out of an explorer? SO many advantages over the 75 block. Roller block, one piece rear main, good iron heads (GT40 or GT40P) really good torque cam for a Bronco (I ran it for a while) and the best cooling front dress hands down. Throw in the alternator upgrade and the all the parts you need to go EFI if you ever decide to and it's win win win.....

Most everyone finds the Explorer 302's not needing a rebuild once they get into them. I've seen many pictures posted of the cylinder walls still having signs of cross hatching with 100K plus miles on them

I bought a van as my 351W donor. $850 Love it.....

Tim

 

[68ford]

I've been in your shoes. Sound like you want something that when you floor it, it gets a little exciting without being so rad the idle is insane?
While I'm a fan of roller cam, I would not buy an explorer engine just for that. None of the internals are any good really. They only made 210HP and 280tq. A mild 302 can easily make 350hp and over 300 TQ.
Just look at blueprints bronco 306 numbers and it's tame.
Depending on your budget, I would not spend any money turning, resizing and balancing you crank and rods. That's money spent, zero gained. Take that money and put it towards a stroker kit. Increasing the displacement with a extra 1/2 on stroke will be worth 70 ft lb of torque and make torque at a lower rpm.
If you don't mind always running 91 octane, you can run 11 to 1 compression with alloy heads. 10.5 to 1 of you really want to be super safe. A 11 to 1 347 ci stroker always makes over 400tq. Make it breath with good heads and cam and you'll make over 400hp. Cam doesn't need to be crazy. Little lopey idle. But super fun racing through 1st and 2nd getting on the freeway. Since you spending money on alloy heads, I would spend a little.more and get a little better than the ones you mentioned.
https://images.app.goo.gl/tJkxgQiFZtiy2ho1A
I would call CNC and see if they'll sell you a cam. 350ft lb by 2500rpm 400ft lb peak and 400HP at 5500rpm. Very usable

 

[jckkys]

A .040" over bore will only give you an additional 7.6 CIs. hardly worth mention. Since Ford already did the work of creating the 351W in '69. They're a cheap swap for anything that came with a 289/302. Really should have been an option in EBs. The added torque and HP come at all RPMs. A high HP/RPM 302 has to rev up to make power. In appearance the 351 is difficult to ID for anyone who doesn't see them a lot.
The compression ratio is basically established by the head's combustion chamber and the piston choice. The stock '75 302s were 8.0 to 1. They can work well with 9.5 to 1. It's just matter of choosing the right pistons for a given combustion chamber size. Your builder will just ask what you want. He'll know what's needed.

 

[73azbronco]

40 over to me means you have no other options. Trying to up the compression on a 40 over engine is really throwing red flags, I would not do that.

My explorer 5.0 with 80,000 miles was one of the ones that still had cylinder cross hatching, needed no boring except for pesky manufacturing defect one cylinder was misaligned from factory.

For the money you are throwing look at blueprint engines or other noted rebuilder of crate motors. Simple and in the long run, they provide pretty good product for a good price. Local machine shops can be very hit or miss on quality and price.