Stovebolt.com Forums The Shop Area The HiPo Shop "Baby Chicken" 261 stroker pistons- - - - -

Amazingly my high school in Woodstock, Georgia still offers an auto mechanics program.

When I bought my car in high school my dad told me I'd better learn how to fix it if it broke down because I'd go broke if I had to pay somebody to do it. I immediately signed up for the auto mechanics class. I took it as elective for every semester after that.

Most useful class I ever had in high school or college

I could fix my own cars (or, at least diagnose them) into the 1970s, but even my oldest* is almost completely beyond my comprehension. The sad part: also beyond the dealer's service department's comprehension.

* 1992 Lexus SC300 with 2JZ-GTE turbo engine

Sad for all young people. Watch and listen to HRL!

Ed

I admire your attempt to get the most out of one of these "tractor engines".
Who, among us, have not dreamt of doing the same?
I realize that the only true arbitror of horsepower and torque gains is the dyno, but with your vast experience, what is your guesstimate for what you will realize with your modifications.
What compression ratio will this engine have, static and dynamic?

A couple of old sayings come to mind- - - -"There's no replacement for displacement" and "Horsepower is to brag about- - - -torque wins races!" I'm not going to venture a guess about HP gains, because the cylinder head flow is so terrible. An extra 3/8" of stroke should help the low end and midrange grunt by a noticeable amount, reducing the need to downshift on hills or when trailer towing, etc. About 5 extra cubic inches per cylinder from the increased bore and stroke should raise the compression about one full ratio or slightly more, which will probably put the engine on the ragged edge of needing mid-grade gasoline- - - -close to 9:1 CR, I believe. Dynamic compression is a function of the camshaft profile, since pressure in the cylinder doesn't start to build until the intake valve closes. A long duration cam that uses up a lot of the compression stroke to take advantage of high RPM cylinder filling due to inertia of the incoming fuel/air charge needs a high static CR just to bring cylinder pressure up to what a stock cam with shorter duration used to make. The 261/Corvette cam or the very slightly more radical Melling cam that I'm planning to use doesn't have a radical lobe profile, so there should be a pretty good gain in actual cylinder pressure with more displacement.
Jerry

I did some reading about the Buick Nail Head engine, so named because of the relatively small valves for a large displacement engine.
It was indicated that since the head design was similar to a hemi (it was called a semi-hemi by Buick enthusiasts), the valves are not shrouded like what happens on big valve engine designs, thus breathed as well as infamously shrouded big valve designed heads.
There was also talk of how the Nail Head designers addressed CFM through these engines. When designing all of the components responsible for air flow through the engine, their focus was on the aspect of the speed of the airflow, and not just the final CFM number.
Is CFM static, or is it dynamic, taking place over time?
What I do not know is what did they do to accomplish the increased speed of flow aspect.

I heard a somewhat different story about the "nailhead" moniker- - - -The valve stems were almost vertical in the head, like someone had driven a bunch of nails in there. That made for a more direct flow path in and out of the head, although it made the pushrod and rocker arm geometry a bit more complex. Either way, the design produced an engine with plenty of low end torque. One of my competitors at the Merced California dirt track back in the late 1970's ran a nailhead engine, and trying to beat him off the turn with a tight-winding Chevy 327 was a real chore. He liked to run a little high in the turn and get a good slingshot down the straight. The only way to beat him was to hope he'd over torque it and spin wheels. He didn't do that very often!

Cubic feet per minute is simply a function of engine displacement and RPM. Port shape and valve size alters the actual volume, since it's impossible to get 100% volumetric efficiency most of the time. Here's a quick formula- - - -divide RPM by 2, and multiply it by the cubic inch displacement of the engine (cubic inches per minute)- - - -then divide that number by 1728- - - -(cubic inches in a cubic foot). For instance- - - -261/2= 130.5 X 4000 RPM=523,000 . Divide by 1728= 302 CFM. Since that's impossible, given the stovebolt port shape and gas flow, 90% of theoretical CFM is a more realistic number- - - -272 CFM.

Now- - - -how do some real world numbers compare to the "Use a 500 CFM carburetor" advice that some people give? The laws of physics are not "suggestions"!
Jerry

What are planning on using for a carburetor?

Most likely a Carter one barrel calibrated for a Ford 300 six. I might modify a stovebolt intake to accept a small bore Rochester 2 barrel for a 327 V8. Unlike the Rochester B, the 2G series carbs are very well designed and reliable. It's necessary to do some pretty extensive machining and welding to the manifold to extend the plenum under the 2 barrel throttle base all the way to the manifold runners. Using an adapter to mate the 2 BBL to the original manifold ends up getting less airflow than the original 1 barrel carb. I've also got an idea about using Keihin side draft diaphragm piston carbs for a Harley Davidson motorcycle on home-brewed individual manifolds, one for each intake port.
Jerry

The hemi comparison isn't accurate.
The Buick intake valve is vertical and inclined at 45 degrees to the bore axis (similar to some hemis), but so is its exhaust valve (parallel to the intake valve, which in a hemi has the reverse position).
The actual chamber shapes are completely different, the Buick is a very deep wedge with a closed chamber and quench.

W/r/t "261/2= 130.5 X 4000 RPM=523,000 . Divide by 1728= 302 CFM"
Easier, avoid 1 step:
261 X 4,000 RPM = 1,044,000. Divide by 3456= 302 CFM