destroking
 

Talking with a buddy of mine, he says what I should do to my 302 is destroke it. Knock compression down to 7:1 or so. Add a turbo, kick that dynamic compression up past 9.5:1 again. So you've recouped your power losses and gained some. The bonus there is that the crank isn't moving as much or as fast. So your redline goes up. Consider how fast that crank is moving when you stroke your engine and you've got a really, really high redline. Guesstimation suggests as high as 7,000rpm.

I realize this isn't a novel technique, but I can't find any information on it anywhere. Does anyone know whether this works / how well it does / etc?

Ok I just read some stuff on these forums that sort of answered my question.. but I'm still wondering about the forced induction thing.. it strikes me that a destroked 302 with the same compression ratio as a turbo/supercharged 302 will rev faster and thus accelerate quicker

if he takes the compression down to 7:1 its not going to run very well on the street. Unless its a full out race car I say the lowest a street car should go is 8.2.1 and mabye starting to get into the 7s but I wouldnt

7:1 was kind of just theoretical.. I was just interested in the idea of building a low compresson engine for turbo use.

When building a motor for forced induction you don't "destroke" the motor. You simply lower the compression with the proper piston dome/dish size to match your cyclinder head chamber.

For a street car you should run 9:1 or so, depending on boost to be used. For a race engine 8.5:1 or lower can be used, depending on boost again.

By adding a s/c with 10lbs of boost, you really make your motor go from 9:1 to ~19:1 compression, since your cramming the extra 10lbs of boost into the cyclinder head.

You crank doesn't technically turn slower or faster by changing compression, it stays the same.

The destroking concept lowers your total cubic inches of the motor and thus has less rotating mass which can then be revved higher. Whereas when you stoke a motor, you give it more displacement ( cu. in.) and more rotating mass, which forces the crank to turn with more force and at a lower RPM. Thats why the little Hondas can rev to 8000+ with no prob, since they only push 1.6L, as compared to your Mustang with 5.0L which is only around 6000.

no dont destroke it.. a destroked 302 is just a 289..

Just a 289 ? Are you saying those are bad motors or something ?

oh hell no.. i love 289's i've been trying to get one back from tireburner since i sold it to him.. lol


i'm saying just go buy a 289 in the first place.. their blocks are a little stronger.

I don't think it works like that! :eek:

It works exactly like that. That is why a boosted motor requires less compression. If your motor is 8:1, every psi of boost equals just under one compression point. So if you add 15 psi your are damn near at 20:1 for compression. So, now if you take a 15:1 motor and add 15 psi it will have high enough compression to be a deisel motor which means detonatoin very easily at that high of compression. Now all of this is just the basic idea. How well an engine flows will also effect how much compression boost is actually adding. Spark timing also will play a huge role in this too.

Adding boost doesn't have anything to do with compression ratio. It adds more air and fuel into the equation. The reason you would consider lower compression to run on a given octane of gas is because the now more dense (proportional to the boost) intake charge is still being compressed the same amount, so you will ultimately see a much higher cylinder pressure at TDC than you would without the boost. Basically, this means that the chances of preignition are greatly increased prior to TDC due to the increase in mass of air and fuel being compressed. This requires taking timing out of the equation, or running lower compression, etc.
You cannot state 1 psi = 1 point of compression, there is no real equation for how much manifold boost you see to how much boost you can actually run. Every engine and combination is different and that's where tuning plays a big role.
As for destroking, I think that is highly counterproductive. 7000 revs is nothing for a solid rotating assembly that a lot of quality quality stroker kits offer. I would be more inclined to stroke an engine to reduce it's operating rev range and still make the same amount of power, given that you are adding forced induction. RPM kills engines so ideally for a long lasting street car you would try to keep them as low as possible. If you were spinning to say 9000+ rpm all motor in a race car, then you would see the argument for destroking having real benefits.

It doesn't increase the actual compression ratio (since that's just an engine calculation of piston BDC cylinder volume compared to piston top dead center cylinder volume), but it does increase the compression pressure by increasing the amount of air forced in (psi of boost).

As for destroking, that works good for high revving race engines that have to run at higher rpms all the time, but for a street engine torque is a much bigger advantage plus the extra cylinder volume (Cubic inches) gives you more space to fill with a super or turbo charger. Not only that but a 302 can effectively run up to 11,000 rpm if race prepped (LOTS OF $$$). High RPM's will always require serious bottom end reinforcement since for (i believe this is the equation) every 1,000 rpm, force goes up by 4 times. horsepower is torque over rpm, so if you increase torque (cubic inches), the horsepower occurs at a lower rpm and with a broader torque curve.