Buiding a short stroke 350

[leos]

I am looking into building a short stroke 350 with a 250 crankshaft and 71 mm bore cylinders to enable much bigger valves and higher revs. Has anyone an idea if something like this has been done in the past? Is there someone how has a 250 v crank for sale so I can try to fit it into my 350 crankcase? I am racing the 350 at the moment in the Dutch and Belgium classic championships.

[72degrees]

John Green of Elbymoto built a 325 (250 crank and 500 pistons) so I think, so it's not impossible. Unluckily for you I just sold a 250 engine minus rocker gear which was in good condition.

viewtopic.php?f=4&t=2531

[EVguru]

You used to be able to buy 500 pistons with 350 gudgeon pin bores, they're 20mm and 17mm as I recall.

[smotorboy]

I'd try to talk to someone that has done this....and here is why.

I tried shortening the stroke and increasing the bore in my Engine Simulation Software program and got no improvement to speak of...

I'll admit that the values I use for the supercharged 350 engine I'm running are probably some what different than normal Morini motors, but this should be taken into account...

I think the fact that Heron cylinder heads just don't flow well at high rpm is a major part of the problem of making power up high

In addition, high rpm will kill a street based engine's reliability,,, especially if larger heavier pistons are involved...

Good luck with this idea and I'd love to here what you learn,

Robert in California
World's Fastest Gasoline Pushrod 350

[norbert]

About 15 years ago I donated a complete 250 V motor for aproject like this. The guy was running quite succesful in the grab the flag series that days. One of this years he ended up to be the second in the SR500 Cup, which is not so easy because it´s much easier to find some more horse powwr in a tuned SR/XT motors than in a 500 or even 501 morini engine. As far as I know the project never had been finished but the motor is lost

norbert

[EVguru]

I tried shortening the stroke and increasing the bore in my Engine Simulation Software program and got no improvement to speak of...

Which simulation software? Most of the affordable packages apparently make a lot of assumptions and often don't give good results outside of a certain range. The Blair Associates and Lotus simulation packages do, but are very expensive (although there is supposed to be a free to use single cylinder version of the Lotus software).

A bigger bore means room for larger valves, resulting in potentially better cylinder filling. Very over-square engines needn't be very high revving and lacking in torque, the Benelli 650 Tornado and modern Morini are good examples.

[smotorboy]

If 250 cylinder barrels, pistons, and cylinder heads fit onto a 350 motor, I'd love to acquire a set for the Baron racebike...

Looks like a 250cc Baron would be able to run somewhere between 105 and 110 mph, and just crush all the existing records for pushrod 250 bikes...

Please let me know if these parts show up somewhere for sale...

Robert in California,,,,

PS: I'm considering taking off the rear cylinder and running as a 175cc single for the 2018 race season

[smotorboy]

Hi Paul,,,

I totally agree that a larger bore should help Morini's breathing just from "unshrouding" the valves from the cylinder walls if for no other reason...

My software ( Dyno Sim Advanced ) outputted within 1.2 peak hp all the bore/stroke combination I tried....

My conclusion is that the stock 350 exhaust valve is choking off the throughput of air the motor is processing, so changing things had almost no effect....

Supercharged motors are different enough that things sometimes don't relate well.

The real facts can only be determined with before and after dyno testing... Oh, if it were only available....

Robert in California

[EVguru]

If 250 cylinder barrels, pistons, and cylinder heads fit onto a 350 motor, I'd love to acquire a set for the Baron racebike...

You'd need the shorter stroke 250 crank too and by that stage you might as well acquire a complete engine. 250 heads are the same casting as 350, but with a smaller inlet seat and valve fitted.

PS: I'm considering taking off the rear cylinder and running as a 175cc single for the 2018 race season

You'd have to deal with the balance issue. The 250 single was comparatively low revving and had a rubber mounted engine. The 125 was slightly prone to stuff coming loose and I used to carry a tool box packed with spare bulbs.

[123morini]

Hi Leo ,

I gess that I have what you are looking for . I have build a complete engine with 250 cranck and 71 cylinder barrels and cylinder heads . In fact I tried this type of engine years ago . It is not so easy to fit a 250 cranck and a dry clutch because the 250 cranckshaft is 10mm shorter at the clutch side . The cylinders barels have to be shortened by 40mm , in fact I have done all modifications to fit everythings ok and the engine was runing well .
If you are interested please tell me at christiandelise@yahoo.fr

Christian .

[smotorboy]

Update on the horsepower vs short stroke 350 questions.....

I earlier posted that I was seeing no real HP improvement results with a short stroke 350 in my software simulation program and I now know why...

I just aquired a cracked 500 cylinder head for study, and I now know that the valves and ports in the 500 heads are just enormous compared to a 350's. I don't have exact numbers yet,
but the potential gains in airflow could be on the order of 35% or more under the right conditions,,,, turbo or NOS for example,,,,

I can tell you from my own experience that stock 350 heads with a "sports" cam grind have a maximum air processing ability of about 75 CFM at 7500 rpm under 10 psi of boost, and then they are DONE.... More boost or rpm does not result in more hp.....

As each single HP requires about 1.4 to 1.5 CFM real world flow,,,, that works out to 50 hp at the crank at 7500 rpm....

Does anyone know what the 500 turbo motor made HP wise ???? If my estimate is correct, I would say somewhere in the 65-70 HP range...

So, a short stroke 350 with 500 heads and pistons could really make power IF it can stay together at a high enough rpm to through-put big CFM numbers.....

Anyone doing this or that has done this, PLEASE let us know what you find or you found with a short stroke 350 .....

Robert in California
The Baron Race Team

[72degrees]

Does anyone know what the 500 turbo motor made HP wise ???? If my estimate is correct, I would say somewhere in the 65-70 HP range...

According to this article on the Dutch Club's site 70.5 at 8300.

http://www.motomoriniclub.nl/turbo.html

[morini_tom]

Robert,

I can't help with info on a short stroke 350 but can share some data I collected when building up my 501 motor.

I was able to flow test and take port moulds from the standard 501 head. Also, I measured lift curves for all of the camshafts available to me.

Bear in mind the cam lift curves are knifedge cam lift so you would need to factor in follower width, profile, pushrod and rocker kinematics and rocker ratio to get true valve lift.

The sudden change in the gradient of the mass flow curve after 6mm valve lift (plateaus at ~50g/s at STP) suggests that possibly the air flow is detaching from the port and generating some kind of charge motion.

The magnitude of swirl is quite low although I never got the complete story as I did not do a tumble test, so it's not clear whether the charge motion is intentional or indeed beneficial.

As the performance cams have significantly more than 6mm valve lift, it's probable that their airflow potential is limited by the ports.

I went ahead and ported my head, although time constraint to get the motor built meant I never got around to flow testing it afterwards. Certainly feels like an improvement by Arse dyno though. Certainly on a short stroke motor a 501 (or 500 if you can't get a 501) head would be a big improvement.

I'll post the pictures of the flow and lift curves later

[morini_tom]

Morini cam lift curves (NOT valve lift)

IMG_4073.GIF (123.5 KiB) Viewed 15892 times

501 port geometry

IMG_4072.GIF (78.32 KiB) Viewed 15892 times

501 cylinder head flow

IMG_4071.GIF (18.43 KiB) Viewed 15892 times

By the way- for those of you not in the Morini Riders Club, the above info I have previously shared with the club and indeed was in the excellent club magazine over a year ago