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?

 

[Tmoose]

There's been real, usable power in intake, carb, ignition and exhaust tuning that the 60 hours I spent at the Bridgeport making chips failed to find.

 

[LionelHutz]

Pat - with 19 cu-in per cylinder I was thinking the pipes should be more like 3/4" or 7/8" and around 3' to 4'.

I would agree with Tmoose. If you can't afford dyno time then you should consider researching a homemade solution. It really doesn't even need to accurately measure the absolute engine hp, just telling you if you gain or lose power would work well for tuning and experimenting. The old standard is a water brake but you can look at things like using a hydraulic pump or an eddy current clutch or brake for the load. Apparently, some road trucks had a "retarder" which is a eddy current brake.

 

[PackardV8]

No experience in lawnmower tractor pulling. However, in other areas, the most successful low speed engines in some other competitions have been square bore/stroke with a short rod/long piston combination. The short rod seems to develop more torque at the lower RPMs involved.

As to flywheel weight, the recommendation to look at what the winners are doing now is a good one.

jack vines

 

[Fmangas]

I always had the understanding you use as long stroke as possible at low rpm to maintain a longer piston dwell at TDC. This aids in complete compustion to increase combustion pressure and at 90degrees it increases the lever effect on the crank.

 

[SomptingGuy]

Wow! 23 replies before the old red herring (old chestnut?) of "lever effect" is brought up. That's got to be a new record in discussions here on bore/stroke ratios.

- Steve

 

[patprimmer]

Steve. Your not implying that at a longer stroke, for the same capacity, you are stuck with a smaller bore and therefore less force form the same cylinder pressure acting on that longer crank arm are you? ;-)

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[JackAction]

The 2.83/3.07 set-up looks good to me.

First, it has the largest displacement of all engines proposed (It is 1.6% larger than the 3.05/2.6 set-up). When RPM is limited, displacement is important.

The OEM RPM limit is 3600 rpm and we can safely assume that the manufacturer has sized its ports/valves to perform best at 3000 rpm. Increasing to 4500 rpm will require bigger ports/valves, which the big block should have since it is tuned for a bigger engine (but still at 3000 rpm).

If you can easily increase the ports/valves size, the OEM small block (2.97/2.76) would be my second choice. It could be also the best choice if valve shrouding is important. But, intuitively, I would tend to agree with BrianGar who says that it is probably less of an issue with 2-valve design.

Having a bore/stroke ratio less than 1 will tend to give a better shape for the combustion chamber as previously mentioned by Jiujitsu10 (Surface-to-Volume ratio and even better flame propagation). This could be important if you run a high compression ratio.

 

[140Airpower]

Generally, a smaller bore/stroke ratio makes a more thermally efficient engine, but a larger bore/stoke ratio gives increased volumetric efficiency. Do you want best fuel efficiency or best power?

 

[slufkin]

140 Airpower- Fuel Efficiency is not an issue, In Tractor Pulling you just make a short run at WOT. Like Drag Racing, you need not concern yourself with how much fuel it takes.