To Stroke or not to Stroke

[Merritt5]

I have a fox body that I'm building that I would like to compete in the optima street car challenge with, but mostly it'll be a fun street car that i will autox.

Now, I have a stock short block lq9 and t56 sitting in my garage, along with a 4" crank and rods....just need pistons. I was originally planning on stuffing it into a drag car, but I've been heavily considering sticking in the fox.

Would a 408 stroker be too much for a fox to autox or should I just build the stock lq9?

[califconstruct]

...
A Stroker will get you more torque, which is essential in short power spurts. It definitely helps to stroke it.
You can do that now, knowing you can work on all the other options you have to add more power to it later.. heads, cam, springs, rods, intake, ect. go big.. you won't regret the horsepower.


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[WSSix]

x2! You have the parts now so you might as well. Learning to control the power and your cam selection will help a great deal towards making sure the car performs well. It's all about where you make the power and not just the overall number. Larger cubes will allow you to make broader power in general. I'd do it in a heart beat.

[Ron Sutton]

IMHO, the best power strategy is to moderate stroke & widen the cam duration split & LSA.

By Moderate stoke, I mean specifically in the 3.75" to 4.00 range as long as the bore is larger than the stroke. These strokes will create a manageable piston speed with today's quality components that are available. Don't scrimp on the quality of parts. When you have sizable cubic inches, with the 4.065"+ bore & 4.00" stroke, you can ... and should ... widen the duration split & LSA (Lobe Separation Angle) to build a broader, more usable & easier to drive powerband. Of course the optimum duration split is also influenced strongly by the cylinder head flow, you can increase the exhaust duration a few degrees and flatten the torque curve.

I would ask if you're running the cathedral or square port heads, as that will play a role in what duration split you target. If you're not familiar, the cathedral port heads have intake & exhaust ports flowing closer to what we consider optimum intake/exhaust balance ... then therefore do not need as much duration split. A lot of cams designed for cathedral ports have around an 8° duration split ... meaning they have 8° more duration on the exhaust side than the intake side. This is great for a 5.3 or 6.0 LS. But for your combo with a 4.00" stroke, that 8° duration will make the engine too peaky in the middle of the torque curve in my opinion. You'd have a hard time not spinning the tires off the corner on a road course & even worse on autocross tracks. Plus the engine would nose over at an lower RPM than optimum, so you end with a narrow powerband.

To solve both of these problems, I'd order the cam with a minimum of 2° more exhaust duration & as much as 5° more. A 10°-13° duration split will take away some of the mid range torque (that you won't be able to use without spinning the tires) and build power a few hundred RPM further up the range, making for a wider, more drivable powerband.

Cams made specifically for square port heads like the LS3 & LS7 heads come with a much larger duration split because the intake runners flow awesome, but the exhaust ports need some help. A lot of these cams are a 16° split (16° more exhaust duration) and that's pretty good. I only run more split than that when we're building power above 650HP & want the power band to pull hard all the way to 7000+ RPM. Then we'll run as much as 18°-19° more exhaust duration than intake duration.

The LSA ... Lobe Separation Angle ... plays a big role in the powerband as well. When we pick tight/narrow LSA's like 106°-108° ... the engine does build more power under the curve, but unfortunately concentrates the power in the 2800-3800 rpm range ... which is where we're coming off the corner. Torque feels awesome. It is fun & impressive to be pinned in the seat for part of the acceleration range, especially in straight lines. But you can have too much of anything. Too much torque right at the rpm we're coming off the corner makes it harder to be quick on track ... because the power comes on hard all of a sudden & spins the tires.

To make the power band wider, easier to drive & quicker on track, a wider LSA helps flatten the power band. Depending on cubic inches, stroke & power levle, we run anywhere from 112°-115° LSA. This strategy builds less power right there at the 2800-3800 range, but builds more power BOTH below that RPM & above that RPM. Simply put, this flattens the torque curve versus having a peaky torque curve. It is easier to drive hard on course & produces quicker lap times.

The strategy I've outlined above works very well if you have some cubic inches of at least 390" plus. On smaller engines with shorter strokes of 3.50" & less, we don't have enough low end power.

So IMHO, your 408" combo will be a great 550+/- HP autocross car utilizing a cam with:
* 11°-12° duration split
* 113°-114° LSA

Best wishes !

[Merritt5]

Thank you for the replies and thank you Ron for going deeper in your explanation.

I already have some ls6 heads that I planned to have ported. I will definitely consider the 11* to 12* split and larger LSA when I start to pick out a cam.



Merritt

[WSSix]

You'll be fine with the LS6 heads on top of the 408. Just make sure you get a quality port job done or purchased on exchange for your heads. Good luck!