destroking?

[dodgepu360]

today I was reading hotrod from 1972 at the school library
there was a build up of a 70 duster 340 4speed that was runing 11.20's
the engine was a 340 destroked to 305 with a tunnel ram and a pair of 660cfm holleys it made 490 HP in the neighborhood of 8000 rpm
it seems backwards to go down in displacement so what are the benefits of destroking an engine

[Ausydad]

When "destroking" an engine, you do lose c.i.d.'s, true. In that process, you lower compression ratio and increase your horsepower rating, but lose critical torque. The motor will rev faster, but have much less strength. Depending on the weight of the vehicle, and whether or not you want to spend extra on custom height piston to try to keep your compression, as to whether or not this would be a good idea for you. In most cases, it's not! If you want to go faster, work with your heads, cam, trans, and the all important gearing. It may be cheaper, and definitely more dependable.

[6 packin]

ausydad is right. most destroked engines are made to spin above 8,000 rpms. and are mostly for race only cars. I would not reccomend building one because most likely a 360 or 340 woll make more power and torque at any givin rpm range below 6500 rpm's. mostly the torque will be fanomanal compared to the destroker.

[Locomotion]

Ausydad,

Destroking doesn't increase your horsepower rating, but this should clarify things:

If it was a "class" car for NHRA, IHRA or AHRA modified production or competition eliminator, that was a common practice because the cars had to weigh a certain amount per cubic inch. One can get more HP per cubic inch out of a smaller displacement engine (with the proper bore/stroke combo) so the cars could weigh less, but make more power per cubic inch - and go faster!

340 x 10.00 lbs per cubic inch= 3,400 lbs
305 x 10.00 lbs per cubic inch= 3,050 lbs

With the same amount of prep and mods for each engine, the 305 would be faster despite the smaller size at 10.00 lbs per cubic inch!

For the average person building a bracket car, the more cubic inches, you get more HP overall per dollar spent. (If you don't get carried away with stroker cranks, etc.)

Just to show how much progress there has been (with some help from rule changes), there are now Stock Eliminator 340's and 360's with OEM carbs, stock-lift hyd. cams, etc. running high 10's to mid-11's!

Myron

[Ausydad]

I agree that less c.i.d. doesn't automatically mean more h.p. But it does have less reciprocating mass that spins faster and higher. It is a higher RPM setup. But horse is how fast it will spin where torque is how strong it will spin. Either way, we agree that a destroked motor is for a lightweight non-streeted vehicle. Didn't mean to make my original post imply that one was hand in hand with the other. Good points!

[rumblefish360]

FYI;

The destroking of the 340 was due to the Trans Am racing scene. The rules were for 5 ltr. engines. I'm not sure if 305 cid was the limit though. But since GM had a 305 and Ford just needed a small overbore for there 302, destroking the 340 was the way to go. Not the 318 because the bore was smaller than the 340's.
Big bore, short stroke combos offer a high rpm and HP potental.
Long stroke small bore offers shorter rpm's with bigger torque down low. Cid being equal.

[451Mopar]

I'm not sure why the engine was de-stroked, but it really only makes sense at try to meet some "class" of racing.

The advantage of a short stroke is lower frictional losses due to slower piston speeds, which also helped ring seal at high RPMs back in the old days before crankcase vacuum systems, gas ported pistons and high tec piston rings.

The problem with shorter stroke Mopar engines, is that Mopars have a very high deck height, so a de-stroked engine would need a really tall compression height piston (to maintain compression ratio) or longer connecting rods which is going to weigh even more.

On the other hand the tall deck height works great for stroker engines. We can easely put a 4" stroke crank in a small block and still run are long connecting rods, the cam doesen't cause problems because it is located higher, and the pistons still have room for a good pin location (compression height of about 1.5".)
This is stuff GM guys would kill for, not to mention the 1/2" main and head bolts, and shaft mounted rocker system.

[Eternalfootman]

Rumblefish 360 is correct, if I remember correctly...

The reason for the destroking was for Trans Am racing. This is why the z28 Camaro was quietly introduced in 1967. (there are no badges on a z28 of that year. Once it made a name for itself, it became a more viable option in 68, and got badges for is own production) And that engine was the same as a regular 350 Camaro, just destroked to about 305 or so. (I forget if it was 305 or 302...?) Kinda like the AAR Cuda, which, again, if I remember correctly, competed in the Trans Am series with a destroked 340.

I could be wrong about some of this, as I am relatively new to this kind of history. But I think I am correct. Besides, I wasn't even alive then!

Dave

[sanborn]

FYI (in addition to Rumblefish)-
There was a circuit(don't remember if it was SCCA, USAC or Nascar) that ran for about 2 years in 1968-1970. The bodies were GM Camaros, Ford Mustangs and Dodge Barracudas. Full race rollcages, etc. were allowed. I do remember it was called the TransAM circuit.

The engine rules were pretty serious and the factories got into it pretty heavy.

305" maximum size-within the International FIA size!
iron block and heads -Anybody heard of TA blocks and heads?
full port/polish heads were allowed-Offset pushrods on TAs?
Multiple carbs were allowed-GM used 2x4 on offset intake, Ford and Mopar used 3x2 on aluminum intake-Six packs?

The Mopars used Crankshaft Company fully counterbalanced cranks, 6.25" rods( still got 7 somewhere around the shop) and a forged piston(about a 1/4" dome). The Mopar used (primarily) a Racer Brown 19X flat tappet cam.

Can't remember for sure but I think HP was somewhere in the mid 450s.

I'm sure it is all gone now but for many years Mopar still had the engine parts available for sale.

Several well known drivers competed in the series; Mark Donohue, Bobby Allison, Hershel McGriff, etc.

I think the series folded because of "lack of money".

Hope I didn't tell you more than you wanted to know!

[dodgepu360]

Z28 Camaros had 302's

Well today I went and read the article on the Duster when they took it to the strip. They were racing it MP/E class (modified production/eliminator?) and the driver complaned about no low end torque also through one season they had to replace three sets of rod/main brearings because the oiling system sucked.

[rumblefish360]

shaft mounted rocker system.

Jesel makes conversions for the brand "C" crowd to upgrade to the stock superior system in our MoPars.

Stike another one for the good guys!