I am in the planning stages of building a 355. Theoretically if all my stock rods and crank were good would they be able to withstand compression ratios of around 10:1?

 

Higher compression has very little effect on the rods or crank assuming they are machined and the proper bearings are installed. Question to you is. Why bother with a higher compression engine? Its not like you will gain any real power difference between 9:1 compared to 10:1 and it will tolerate lower octaine fuels much better. I do have a 10.5:1 setup but it does me no good since the fuel that will allow the proper amount of advance is well over $4 a gallon.

 

Higher compression engine are more effecient engines. So a higher compression ratio will make more power and use less gas, all else being the same. But a higher compression ratio will require more expensive higher octance gas. That is unless you have the "new technology" heads with fast burn chambers, such as the Vortecs, AFR's, PRO, and other "good" heads. These heads produce faster flame propigation and travel, so less timing advance is required, reducing the chance of detonation.

As you can run a really high compression ratio engine on and gas even 85 octane if you never go to WOT. To make a really good gas mileage engine run 12:1 compression and limit yourself to half throttle with a throttle stop.

 

I guess closer to 9:1 then 10:1 would be better overall, because i could run a lower octane fuel and would make "about" the same amount of power that an engine closer to 10:1 with running higher octane fuel would. Thanks for the info.
-Bob

 

How do you plan on achieving this compression?

 

im running close to 11:1 with 36 degrees total 12 initial and 24 mechanical and all in 2,600.....it spark knocked a little before i backed off the vacuum advance...at cruise its 48 degress total with vacuum advance hooked up....i have no issues with spark knock anymore...now obviously with 110 octane i could increase timing substantially, but it runs like a champ on 94



to answer your question (again) you could easily run 10:1 with stock bottom end...since your running iron heads 10:1 would all id want to run, unless you polish the combustion chambers or run 110...

 

Stekman, i plan on acheiving this compression by having my machine shop decking my block and heads. Why do you ask?

 

Probably because some methods of boosting compression really don't help you much, and they can actually cause more problems than its worth.

Like, for example, putting huge domed pistons in a set of 882 heads to make 10.5 CR. Huge domed pistons won't unleash as much hp as a flat top piston with the correct heads, and the domes can cause problems of their own.

 

Adam, i am going to be running flat top hypers with the vortecs. Then i will have it decked. Why do you think that this combination will not help me much as you stated? I am an ameteur engine builder, this will be my 3rd (the other 2 being fords). If you have any input or advice id appreciate it. I was just going to go with a proven combo that others on this board have used.
-Bob

 

Nothing wrong with the way you are doing it. Your doing it the right way.
I was simply pointing out that some methods are better than others.

 

A higher compression ratio is actually easier on the engine, because the pressure is pushing the rod against the crank. The most stress occurs in the rod bolts, due to the inertia forces of the piston and rod when the piston changes direction at tdc. So a higher compression ratio will actually "cushion" the piston when it is going up and lower the stresses in the rod bolts.

 

AANNNNNNNDDDD itll thump harder =)

 

Yes but the bolts will break long before the rods will.

 

You'll be fine with the rods and crank... You'll also more than likely be fine with your compression and pump gas. Your camshaft plays a HUGE role in what compression you can run on pump gas... I know guys with over 11:1 with vortecs that run fine on 93 octane. I have somewhere between 10.3-10.5:1 (some claim vortecs are 64cc while others claim 62cc... who knows). My car runs fine on 93 octane. Like most Vortec headed motors, it runs best with only 32* total timing. I also have a pretty big cam which I'm sure bleeds off a lot of cylinder pressure.

 

Ok, thanks. The last thing that i want to happen is to build a motor and and the crank or rods to snap when i could have spent a little money more on forged internals. When my vortecs come in (next week) i will try to get my buddy to cc them to find a definate volume measurement for everyone.

 

I am running 12.5:1 on 93 octane (never had a detonation problem). All it takes is a good understanding of fluid dynamics and mechanical engineering. The best thing to do is learn everything you can and do the proper calculations.

 

You are correct but, unfortunately, the increased pressure on the assembly when it hits the downstroke negates any "cushion" during the upstroke.

 

Yep, that is exactly what i was getting at. There is more than 1 way to achieve higher compression. Some of course are more logical than others.

Sounds like youre on the right track.

 

Yes but on the down stroke the rod is pushing directly on the crank. Or the rod is being compressed. As i said earlier the rod bolts break when in tension. So what your saying does not have any effect on strength and durability of the components.