gasxd

this is a bit of an obscure question that i've been thinking about for a long time.

For an all out race motor after maximum power out of a small block, what is the optimum stroke?

We all know that increasing the stroke increases capacity of the engine and the torque exerted on the crank by the piston (T = Fr). Both of these factors increase torque. Increasing stroke with a given deck height also increases the rod angle which creates increased sidewall pressure (friction) and increases stress. Increasing stroke also increases acceleration of piston at a given rpm which means more torque is neaded to drive it (F = ma).

What is the general opinion on the optimum stroke, i'm pretty sure you can't squeeze as much power out of a 289 (no matter how high you rev it) as a 351. Do you get to a point where you have too much stroke in a given block? For instance can you get as much all out power out of a 420 as a 351 or 393?

I'm talking about hypothetical race motors where the power band doesn't matter because transmission and diff ratios can be tailored to suit. I know it doesn't have much to do with real life but i'm just wondering...

brenx

It all depends on whether you want reliable power or high maintanence power. A machinist and I contemplated destroking a 302 I think it was. Worked out around 280ci I think. That engine would spin upto 11,000ish rpm. Problem is it'd go well but would require very high maintanence. Constant valvespring, bearings and ring changes.

The reason everyones going bigger cubes is more power with the same or less rpm.A 351 isn't a bad start point in regards to power stakes. They make good power if you have the right parts.

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XB Fairmont (street car) 11.07@123.02mph.

STROKEXD

Great question. I got told years ago that when it comes to Clevelands, a de-stroked capacity of 330 cubes gave the ultimate thearetical bore\stroke ratio. This was from a pro drag race outfit that was chasing the highest HP per cube record in a small block.

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LTD351V2

351 windsor would be similar to that 330 clevo you mention.

davis

Most of the destroked Cleveland drag engines of the 70s were based on weight rules that added big penalties for going over a certain cubic inch displacement. It wasn't the theoretical bore-to-stroke ratio that was involved, it was the weight "break" given to smaller engines. A lot of stroker BOSS 302 engines played in that game, because of the lighter, smaller block and rotating components.

A shorter stroke engine will rev more quickly and potentially higher in RPM than a longer stroke engine. Gaining RPM quickly under load is one of the keys to building a successful drag engine.


:davis:

davis

I wouldn't be so sure of how much power you can squeeze out of a smaller engine...

How does 1029 HP at 9500 RPM sound from a 330.8 CID BOSS 302 (based on a Dart block) with Yates heads? That's with a fairly big solid roller camshaft and 18:1 compression with a Hogan's "sheet metal" inlet and a pair of Ron's T2-Terminators running methanol. Peak torque of 622 at 7200 RPM...we're talking about 3.11 HP per cubic inch. That's fairly damn good. The bore is 4.155" and the stroke is 3.050". The bore to stroke ratio is: 1.3623:1.

Compared to a 393" stroker Cleveland (mine), which makes something like 630 HP at about 7500 RPM...1.60 HP per cubic inch or nearly half the power per cube of the smaller engine...with a bore to stroke ratio of: 1.0468:1.

More importantly is what you mention in your f=ma with regard to piston speed. For a given bore, we can't do much to reduce mass without grinding the skirts down to a point where they don't work very well. But we can easily change piston speed by shortening the stroke. Piston speed is represented by the following: fpm = (stroke*RPM)/6

For the shorter stroke at 9500 RPM we have the pistons moving at: 4829 fpm. The longer stroke engine at 7500 RPM we have pistons moving at: 4813 fpm. The two are nearly identical (16 fpm difference) and both well within the material limits. The BOSS piston is actually heavier due to the considerably larger bore, but still quite safely within limits at 4829...in fact, it can spin to 10,000 RPM if desired. It would be quite a bit of work getting the 393 to 8000.

Assuming that we were more closely comparing apples to apples with the two engines and both had the same compression ratio and fuel type (and induction system!), it is reasonable to suggest that the 393 would make somewhere around 870 HP at 7500 RPM. That makes for a better comparison, but it is still way behind the smaller engine in power.

To give you an idea of the cost differential for the two engines, the 393" stroker cost me about $15K US. The BOSS engine inlet alone cost $3800 USD. The heads bare were another $4800 for the pair...before porting. The Ron's Terminators were another $2900...and we're not talking about the cost of the Dart block, the ultra-light 4340 crank (the cost of the internal balancing alone was $800), titanium rods (another $2400), Ross custom pistons (you don't get an 18:1 Yates piston at 4.155" bore out of a catalog) another $950. The shaft-mounted rocker arms were $2300 and the valve springs were $900 for just the intakes! The titanium valves were $920 for intakes and another $810 for the exhausts.

All of this is to illustrate a point...it is a lot cheaper to go bigger and slower than smaller and faster...and, whether we're talking hypothetical engines or not, practicality will always play a role in every engine built. The "optimum stroke" for any engine is based on your budget!


Take that same 393" stroker, add a stouter block and a 400 HP dual stage nitrous kit and suddenly you're running 1000+ HP for not much more than about $20K US. That is some fairly serious grunt.



:davis:

gasxd

this is all true, the point which i was referring to in my 'F = ma' statement is exactly what 'davis' said, although i do appreciate the figures quoted.

It seems the consensus is somewhere between 302 and 351. It is obviously more efficient (in terms of wallet size) to build a stroker but the question concerned a hypothetical engine which was to squeeze the maximum power out of a given block (deck height and bore).

the reason i'm thinking about such an engine is because i was thinking about doing a thorough analysis on it for my thesis (and it is an interesting question).

my personal favoureite was Benny Gatts XA which i think used a 373 (i think) in the early 90's...

Is it a coinidence that Ford, chevy and chrysler decided on roughly the same capacity for all their high performance small blocks, or was it by chance?

davis

The old saying that the cylinder heads make the engine is fairly true. Today's smaller,multi-valve engines are producing much more performance than larger, pushrod V8s of years ago. For example, my 5.4L 3-valve engine in my F150 makes more power and torque than a 400 (6.6L) in a 1978 F150. To some people a 1.2L difference is an entire engine!

The "optimum" stroke for an engine is going to depend on by what metric its performance is measured. If HP per CI is the yardstick, the smallest possible engine with the biggest heads and a radical cam that doesn't even start making power until 7000 RPM is going to win the competition. That is basically a BOSS 302 in a nutshell. If you were to zoom that period into fast forward, you'd have something along the lines of a 4.8L modular engine in the DOHC variety...with a lot of work or even custom fabricated heads, one could see them pushing 10,000 RPM and making well into the 1300s HP.

If torque is the player, then the longer stroke engine that works well at about 5200 RPM is going to be the winner. However, it is not just stroke that makes tor que, it is bore, too. The reason a lot of engines start with a larger overbore is to increase capacity. Since the Cleveland blocks are notoriously not well-suited for large overbores, we're fairly stuck with stroking. Using a Clevor in a Dart block and alloy head combination makes the best sense in a SBF for maximum performance and longevity.

It isn't a coincident that all three major US manufacturers had the same sized engines. Various sanctioning bodies of various automotive racing associations dictated rules that specified displacement maximums or weight penalties for exceeding those maximums. What the racing bodies didn't do to decrease engine capacities, the EPA did...and/or OPEC when the fuel "shortages" of the mid-70s happened in the USA. The "fixation" of engine sizes by racing organizations was an attempt to level the playing field so that everybody had a chance and so that money didn't dictate the game. Rules are stupid when they try to control the game in such a manner, because it turned into a game where the guy with the most spare engines pushed to the maximum won. Six or eight spare Clevelands later and maybe you were taking home the Wally on Sunday.


:davis:

Ohio XB

This is an excellent topic with lots of excellent info.

A couple years ago a car magazine did an experiment on this exact topic. I'd have to do some weeding through my magazines to find it but I could.

They built two engines of almost exact displacement, within a couple cubes of each other. Same block but one was built with smaller bore and longer stroke and the other with bigger bore and smaller stroke and put to the dyno test.

The result? No appreciable difference in horsepower or torque.

Basically with both engines being equal with the bore and strokes being the only difference I would believe it came down to how much fuel was being burned. Since the displacement was the same and therefore the same amount of fuel being burned the power output was the same. This is for readings at all RPM. When one has more torque because of having a longer stroke the other one with the shorter stroke has more force applied to it with the larger piston (more surface area that force is applied to).

I have my own theory about longer rods. I disagree with the common notion that a longer rod with the same stroke will produce more torque. Think of it this away...

If you need to lift a weight you put your fulcrum next to it and then place a lever under the weight and over the fulcrum. Lets say the lever is 5 feet long. If you stand on the end of the lever you will exert your body weight times the length of the lever in force to lift the weight. If I weigh 150 pounds, times 5 feet, I am exerting 750 ft.lbs. of torque on the lever.

Now I lose 5 pounds and place a 5 pound box on top of the lever. Let's say this box it 2 feet by 2 feet. I stand on the box at the end of the lever and am now exerting how much force? Well, I am still at the same distance from the fulcrum but now 2 feet above the end of the lever. Since I now weigh 145 pounds and the box weighs 5 pounds I am still only going to exert 750 ft. lbs. of torque 5 feet from the fulcrum.

The box would be the connecting rod. I would be the piston, and the lever is the throw of the crankshaft (distance from crank bearing journal centerline to connecting rod journal centerline).

If 150 ft. lbs. of force is not enough torque to move the weight should I get a 5 pound 3 foot high by 1 foot wide box to stand on? No, I would need to get a longer lever (bearing journal to con rod journal).

The throw of the crank will need to be changed for a torque increase with the same amount of force applied to it if the force being applied is to remain the same.

As for the optimal bore/stroke ratio to maximize displacement and keep from having side load problems on the piston I don't know.


Steve

xrglen

Horse Power Sells Motors, Torque Wins Races!