Someone gave me the idea of building a destroked motor, and I have never heard of a destroked motor until the idea was addressed to me. I know what a stroked motor is, but what exactly is a destroked motor? Where can I learn more about a destroked motor?

 

a destroked motor is just that, a motor with a shorter stroke.

however, it would require some block work or taller pistons to keep the compression about the same.

the theory is that a shorter stroke, large bore motor can spin higher RPMs... then again, F1 cars do use that method... 40mm strokes and 90mm bores!!! (real close to that, but you get the idea)

the only way to shorten stroke is to change the crank... again, it would be lighter...

 

Chevrolet made a destroked engine back in '67. I'm talking about the 302 small block that the first gen Camaro had. Back then SCCA's Trans Am racing series had 2 classes: one for 2.0L and one for 5.0L engined cars. Chevy dropped a 283 crank in a 327 block, creating the 302. Early 327's had the small crank journals like the 283.

If you really want to make more top-end power, as in Trans Am road racing, you gotta rev the engine high. Therefore for a given engine displacement it's better to have a shorter stroke and a bigger bore because this reduces piston speed. There are limits to how fast a piston can move because the forces on it are so high. These forces multiply with engine speed. If the stroke is long then at any given engine speed the piston will move further than if it's stroke was shorter and the bore was increased.

You could drop a crank with a shorter stroke into your 350 block but you'd have to get longer connecting rods so that the pistons wouldn't be too far below deck at TDC. That would increase your rod ratio (rod length divided by stroke), also a good thing for revving high, but increases rotational weight. Chevy actually makes short deck racing blocks for this reason. Interestingly, when you look at that block's deck height and the resulting bore/stroke you'll notice that the dimensions are very similar to Ford's 302 smallblock which is almost never built to take advantage of it's high revving geometry.

Anyway a destroked engine would be great if you like to rev. I don't think it would make for a fun street car since you'd always have to rev to make power. The absence of bottom end torque would probably dissapoint. You could setup the intake system and cam to make low end torque but that would kill the high end power you worked so hard to get in the first place and all you will have done is decrease engine displacement.

 

That's interesting. I have a neighbor who's got a built 302 in his Mustang. He has only bored it out increasing the Piston size but kept the stock stroke.

I always wondered how it was possible for him to rev it up to 8000 without hurting the motor....now I know.

As for torque it has enough to put you back, but the top end is insane! I've never heard a V8 scream like this one does going through 7000 RPMs!!! He hasn't gotten the motor dyno'd by I know it's putting out around 500, we raced a vette one night and the thing might as well have been standing still.

 

I saw in Motortrend or one of the other car mags that Chevy made a 327 LS1 motor and put it in a 69 Z28 and it made like 450HP at 6800rpm or something crazy like that. I don't remember where I saw it though.

 

I think I remember the article. It was not an LS1 but an LT4 crate motor and destoked it to 302 cu in. It had a carb and optispark. Yes, it generated lots of HP at high RPM.

I believe a the most common destroke is going from a stock 400ci small block to 377ci using a 350 crank.

In the end, if you want max HP and max torque, go for more cubic inches. Unless there is a racing class rule you have to follow, it makes no sense to me to destroke a street motor.

 

Thanks for all the input on this, it is greatly appreciated. I have a little while to plan on what I am building. I am going to wait to build the motor till I am out of school, mainly because I am going to an auto tech school(UTI) and will know a lot more when I get out of there and be able to build it with more confidence.

 

strong 377's are just nasty little engines & definatly more suited to HP & rpm than a 383, i agree there's no reason to decrease cu on a street engine, but something sounds cool about a 8,000rpm 377 & a solid roller cam...actually a SB2 in a go cart isn't half bad idea either.

 

I did a small block 400ci and took it to a 377ci, it really turns some quick RPMs.

 

Just one thing to remember on destroking to get power at higher rpm's. The valve train needs to be addressed before the bettom end. Stock valve train in most engines is usually good to 5500. Look at the valve train work done on the LT-4 and the Z06 to get higher red line.

 

I ran a 377 (400 with 350 crank) in a 55 Chevy drag car that I used to have. It ran consistently in the 9's at over 150. But its not the best idea for a street car. You don't want your car to constantly have to be at 7K or above just to make good power. Not to mention the cost to do this reliably is astounding. You are better off with a larger displacement motor the revs lower but has gobs more TQ....383....396...

 

agreed lighten and stregthen the valvetrain.. Ti pieces are critical in lowering the valve train inertia and using a rev kit will substantially increase the efficiency of a higher revving engine!! much research and the experience of a builder should be consulted to obtain the best of both the valvetrain and bottom end rotating assembly geometry and material make up. best of luck.. a 377 is a nice motor!! one i have toyed with building before.. Solid Roller is a definite choice for that setup.. 8k is attainable at this juncture with ease.. however the only reason to do it is if thats what you want..you can make the same power at lower more parts friendly RPM with stroked cubes and use low end grunt to move you down the track, and its less finicky and more power on demand ready thena RPM motor.. lol But then again nothing is quite as hair tingling as the light vibration and shrill harmony of 8 pistons being pushed and pulled up and down 8000 times a minute and the resonance of that motion through a nice exhaust is nothing short of awe inspiring!!!

best regards,
Chris :cheers:

 

The TNN show Hot Rod TV has a show on this subject a few weeks ago. I really like their show. One of the points they made is that as bore increase, it has an effect of the speed of how the fuel burns, fills and empties from the cylynder. There is a point where increased bore hurts. So it is kind of a trade off stroke verse bore.