Another Bore vs Stroke question

[slufkin]

I am looking for advice and I have obviously come to the right place..
The engine we are going to build is for a Pulling Garden Tractor. I have two roads I can go down, the Small Block or the Big Block Briggs & Stratton Vanguard V-Twin Engine.

Rules include; 38 cubic inch, 4500PRM Governor, single carb, stock position of valves, cam, crank, etc.

My question is;
Go with the Big Block and sleeve it down to get the cubic inches, or go with the small block and run pretty much stock bore/stroke. (the small block has no room for a long stroke crank)

The Big block could be configured one of two ways: 2.83" bore/3.07" stroke, or 2.66"bore/3.41" stroke.
Small block configuration would be 2.97" bore/2.76" stroke.

Given the RPM limit, would I be better off with the long stroke engine? Most conversations about this are in regards to engines running the highest RPM's possible, I just wonder if a 4500RPM limit would sway your answers toward the long stroke and if so, is the longer configuration too much?
Other considerations would be in regards to valve shrouding. I the increased flow potential of the larger bore more important then the old Bore/Stroke argument? At 4500rpm/19 cubic inches (/Cylinder) do I need to be that concerned with that valve shrouding?

 

[SMOKEY44211]

My vote is for small block, More favorable bore/stoke ratio. ----Phil

 

[patprimmer]

There is nowhere near enough information to begin to guess.

Missing data involves cylinder head airflow at different bores and rod to stroke.

There are relatively cheap engine simulation software packages that do a reasonable job of such calculations.

Regards
Pat
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[Tmoose]

so the engine will be governed to rev no higher than 4500 rpm?

 

[Jiujitsu10]

Bore to stroke ratio is a very delicate design parameter.

Engineers need a lot of information, and it is mainly based on the dimensions desired for the engine and its performance.

Usually Diesel engines will have larger strokes because of Surface/Volume ratio of the combustion chamber which is lower on a larger stroked engine, thus giving better efficiency due to less heat transfer. Also, the longer conecting rod associated with the longer stroke will enable the piston to stay put on top dead center longer, improving the combustion capability of the engine. Since the peak cylinder pressures are way higher in a Diesel, they have a constraint of inertial forces associated with the heavy swinging masses. Another thing is packaging of the engine, for the same displacement, an egine with larger Bore will have to be much longer, either having a slab head or individual ones because of the extra structural stuff you will have to put in the middle.

For a simple project, on a gasoline engine with limited RPM, i'd say go with the bigger bored engine because you will have more valve area and thus more air intake capability, being naturally aspirated it will make a huge difference, and since your not designing and not concerned with emissions or fuel consumption, i'd say its the best bet.

 

[slufkin]

Thanks for the responses.
Yes TMOOSE, it is governed at 4500rpm so it will under no circumstances see more that say 4600 (if the gov is overcorrecting) and never more than 4500 under load.
This is the main reason I am considering this long stroke. I wonder if it will make more torque at this lower RPM.
PAT, I wish I had flow numbers, but I have not built a flow bench yet. I plan on doing that this winter. I can tell you that the big block heads are designed to run with a 3.37" bore and I will need to sleeve them down to the 2.6-2.8" range. I assume this will make for considerable valve shrouding.
We can use any cam profile we want and I could use smaller valves, but that is all just talk without a flow bench to get on.

Thanks guys.

 

[BrianGar]

At those rpms, I would think fueling and intake manifolding is more important before you go pulling your hair out. Cylinder head flow capacity(at your preferred rpm) is also important as is combustion chamber shape/quench pads.
Id always prefer bigger bore but I deal a lot with 4 + 5 valve heads where bore shrouding is an issue, On a 2 valve, it can be less of an issue though. But, as Pat said, it depends on a lot of stuff.

Dont forget about traction, and at what point your max torque is being made. I did a lot of trials once in a Gti with various setups. No rolling roads, just a_road and two sign posts. I found that having bags of torque low down, results in nothing but smoke from tyres in the first 2.5gears...I know your case is different, but worth a mention.

Since we dont have those tractors here, I have no real Idea what you are using it for, be it racing, or just plain old pulling. If its pulling, Id fit any engine, gear it down lots, and add a half ton weight.

Brian,

 

[FoMoCoMoFo]

The second BB option has nearly the same bore & stroke as the SB..
But, I would guess it has bigger valves and more importantly, a longer con rod?

If So, I'd be leaning that way.

But, we ARE missing a ton of info.

 

[slufkin]

Unfortunately I cannt provide any flow info right now.
As far as Rod/Stroke ratio, I can buy pistons with a high pin and increase rod length up to the limits of deck height, but I do not know the deck height of the big blocks, as I do not own a block yet. I need to decide if that is the road I want to go down, then start buying parts.
As Jiujitsu said, I am not concerned with emmisions or fuel efficiency.
I am not designing a complete engine so size/packaging is not an issue.
Briangar, it is just pulling a weight transfer sled as far as you can, no speed component involved in the competition. You are on the right track, but there is a weight limit in the class so we can only add so much and everyone weighs the same so you cannot find an advantage in weight. There is, however some advantage to getting some speed up at the beginning of the run so momentum can carry you farther at the end, therefor super low gearing does not usually pay off unless the track has a LOT of bite in it.


Thanks for pondering it for me guys.

 

[Tmoose]

Are the intake and exhaust manifold designs "fixed"?

http://www.300f.com/ram01.jpg

A friend competes very successfully in full sized tractor pulling competition. Many ribbons in the garage.
He won't tell me a single detail about his theories on where the load should attach to the tractor, or why.

 

[slufkin]

Just looked at some more factory specs, and realised that I have one other Small Block option. I could use a 3.05" bore and 2.6" stroke.
I guess what I want to know is... all other things being equal, which configuration would be a better option? Bear in mind that my RPM is not limited by the engines ability to rev, but rather the rules set it at a solid 4500RPM limit.
Anoterh interesting point is I can add whatever flywheel weight (balanced with the assembly) I want to make "fake torque" at the end of the run when the engine starts to slow due to the increased loading.
My next question would be how much flywheel weight would be too much? Remember... acceleration is not an issue, it is brought to full RPM before sliding the clutch to start the run.

 

[slufkin]

Tmoose- The stock intake manifold must be used. We have added considerably longer runners in the form of an "Adapter" which is legal. Exhaust can be whatever you want. We run 1 1/4" tubes. 18" long, separated all the way. No $$ for dyno time to prove that these theorys work well together.

 

[FoMoCoMoFo]

Is this a new class?
Or do you know which configuration last seasons winner uses?

What's the rule on fuel?

Are these high pin pistons & long rods you mention readily available? affordable?

As weight is not a problem, and rotating weight may even be desirable.
I'm still going with bigger is Probably better.

Taller deck - Longer rod - Bigger valves/Ports - Stronger crank - Bigger flywheel etc.

 

[Tmoose]

I'd be investigating if the $$ to build combinations other than the biggest bore standard combo of max legal displacement could go toward development of intake and exhaust, and minor head work.

 

[patprimmer]

We can't even offer an educated guess without air flow data on the head. Improving power by normal means might move the peak power and even peak torque to well above your legal rev limit. We just can't even begin to guess.

One thing I can say, the exhaust seems to big and the pipes a LOT to short for power below 5000rpm.

A really big heavy flywheel sounds good to me. Huge flywheel. Very heavy drive wheels and tyres also I would think. Same deal as the F1 energy storage devices. Store it when you have more than you need so you have it for when you need it. Store as much as you posibly can.

Regards
Pat
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[slufkin]

Thanks Pat.
What would you recommend on the exhaust side? Should we tie them 2-1 and add some length, or should they be kept separate ? How much longer would you recommend?

 

[BigClive]

As with most contests involving engines - a good guide for a starting point is to see what other successful competitors do.

If high revs are not needed (and they are not in this case) I always tend to favour long-stroke engines.

 

[patprimmer]

Is there 360 deg between firing points for each cylinder? I doubt there is and if not, completely separate exhausts should work best.

ONCE AGAIN, there is far from enough data to do the sums, but I would take a real wild GUESS that it should be more like 40" or more and probably more like 1 3/8" dia.

Regards
Pat
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[Tmoose]

what kind of transmission ?

 

[slufkin]

Pat, No it is not even firing, the rods share a crankpin.
TMoose, It uses a Cub Cadet standard transmission, but it has been modified a bit. 7 1/4" Automotive differential and axles from the same automotive rearend, cut down and re-splined. Aftermarket gears to provide more useable gear ratios.
I am now in the proces of machining a new 15lb steel flywheel for his current engine to add some mass there, also considering adding a harmonic dampner to the other end of the crankshaft to add some inertia and also to help smooth things out a bit. Does that sound like a reasonable idea? I can modify one from a 350 chevy.

I attached a photo of my son at the last pull with his current engine.......I guess just because I am a proud Papa.