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Big bore/short stroke VS small bore/long stroke

  
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Big bore/short stroke VS small bore/long stroke

 
mines80 mines80
I watched NASCAR once | Posts: 6 | Joined: 07/10
Posted: 07/24/10
09:20 PM

Wanting to build a Ford racing engine for 3/8 to 1/2 paved tracks.  Although I am very concerned about weight, and of course Fords weight a good bit.  So, thinking  about building a 8.2" Mexican 302 block thats .060" over, hopefully some 5.5 maybe 5.6 rods, and a 3.290 stroke crank.  Gives me 341 CID.  Here's my ultimate question, will I have to turn the crap out of this thing, 8500 rpm + to make any horses and torque versus a longer stroke crank, say a 3.4 and shorter rods, 5.4 is the norm.  I firmly believe in running the longest dern rod you can fit in a block, y'all agree?  Please let me know what y'all think and ecspecially if you've tried something like this.  Successful or not.  Thanks.  lmstock80@yahoo.com  

mines80 mines80
I watched NASCAR once | Posts: 6 | Joined: 07/10
Posted: 07/28/10
07:22 PM

Come on guys, what do you think?  Mr McFarland, any input?  5.5" rod, 3.4" stroke (1,7 rod ratio) and 352 CID or 5.6" rods, 3.3 stroke (1.62 rod ratio) and 341 CID?  

tbirdtess tbirdtess
I’ve been to a few races | Posts: 89 | Joined: 02/09
Posted: 08/06/10
06:20 AM

Hello

What class?

Are there engine restrictions?

Gear or trany restrictions?

...

Smile  

ktownjay ktownjay
I watched NASCAR once | Posts: 4 | Joined: 11/09
Posted: 08/10/10
07:44 PM

According to Smokey Yunick (who knew much more about engines than I ever will) you want the longest connecting rod that you can stuff inside the block. Going in that direction is what I do. Torque is more than just bore and stroke. It has a bunch to do with camshaft selection and the heads on the engine. All the parts need to be dialed in with each other. In my experience, many people think they can just turn a bunch of rpm to go faster. Working on the chassis and getting the suspension to hook up coming off the corner will help more than turning 8,000 rpm.  

mines80 mines80
I watched NASCAR once | Posts: 6 | Joined: 07/10
Posted: 08/16/10
05:01 PM

Well its nice to see some folks reply finally.  Yeah, I'm a big believer in what Smokey always did, for the most part.  But believe I'm gonna settle on the 3.4" crank and 5.5" rods.  Was gonna have a real hard time fitting in those 5.6"s.  Class is asphault late model, straight rail car, 2800 pounds.  Engine rules are steel head with 14-1, roller cam, max 362 CID.  Or run a 9-1 alum head motor, big bucks there.  Run Q.C. rear, and 2 speed muncie.  

Fahlin_Racing Fahlin_Racing
I’ve been to a few races | Posts: 109 | Joined: 06/10
Posted: 08/22/10
05:13 AM

Well have you chosen a piston yet? You can dominate races with long or short connecting rods, its just a matter of piecing the engine together correctly and tuning. I would look at the heads and how they flow and connecting rod length to boot. When building an engine, you do not need to turn extreme rpms to make power and torque (just look at Engine masters), you late model guys get up to 7,500 - 8,500 at the highest revolution???? You want to adhere to your operation range closely when selecting camshafts/valvetrains, heads, ignition curves etc etc. You want the powerband to span the rpms you use the most.

If the heads lack some flow I would use a longer rod since the TDC speed is slower than a short rod at TDC. This allows a lower flow rate head to not chase the piston as much on the induction stroke. If the piston is too fast ATDC on the powerstroke as well, the flame front will not produce the maximum amount of pressure/power possible simply because it may just be filling a slight void right next to the piston crown as it moves down to TDC rather than actually forcing. We have to remember to look at the valve event timing as well. Look at burn patterns on the crown when the engine is torn down.

Getting the correct flow velocities, sufficient cfm, unshroud areas, add minor modifications (legal ones of course) along with optimum cam timing is a must.

I may not be a professional builder but I look up to Smokey's innovative, out-of-the-box thinking quite a bit. Every engine built should just tell you that there will always be something new or interesting out there to take note of. I hope this helps you out.  

tuffnuff tuffnuff
Moderator | Posts: 33 | Joined: 06/10
Posted: 01/10/11
02:52 AM

Icon QuoteFahlin_Racing:
Well have you chosen a piston yet? You can dominate races with long or short connecting rods, its just a matter of piecing the engine together correctly and tuning. I would look at the heads and how they flow and connecting rod length to boot. When building an engine, you do not need to turn extreme rpms to make power and torque (just look at Engine masters), you late model guys get up to 7,500 - 8,500 at the highest revolution???? You want to adhere to your operation range closely when selecting camshafts/valvetrains, heads, ignition curves etc etc. You want the powerband to span the rpms you use the most.

If the heads lack some flow I would use a longer rod since the TDC speed is slower than a short rod at TDC. This allows a lower flow rate head to not chase the piston as much on the induction stroke. If the piston is too fast ATDC on the powerstroke as well, the flame front will not produce the maximum amount of pressure/power possible simply because it may just be filling a slight void right next to the piston crown as it moves down to TDC rather than actually forcing. We have to remember to look at the valve event timing as well. Look at burn patterns on the crown when the engine is torn down.

Getting the correct flow velocities, sufficient cfm, unshroud areas, add minor modifications (legal ones of course) along with optimum cam timing is a must.

I may not be a professional builder but I look up to Smokey's innovative, out-of-the-box thinking quite a bit. Every engine built should just tell you that there will always be something new or interesting out there to take note of. I hope this helps you out.




+1 True,,,
There is only one glitch to long rod engines and what you gain in piston dwell time at and ATDC is also wasted time at and ABDC.
Your most effective cranking (combustion) pressure is from 15 to 55 degrees ATDC,,, beyond that, there is little effective combustion pressure left to turn the crank, besides, another cylinder is already going through the same cycle the previous one did.
Cylinder heads are where "ALL" your power is produced,,, pistons are nothing more than the combustion chamber floor that heads south, like an elevator going down.
Concentrating on VE (volumetric efficiency) is the key to power production,,,

This is the type of chamber to use,,, if rules allow it.

Vortecchamber

Grin  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

mines80 mines80
I watched NASCAR once | Posts: 6 | Joined: 07/10
Posted: 01/10/11
06:43 PM

TuffNuff, there aint a combustion there.  Just a couple holes to drop in valves!!  Well I got a beautiful 3.4" crank used from Roush Yates with Honda journals.  Got the block back machine shop, .060" over bore, = 352CID.  Got a nice set of Oliver 5.5" rods, JE pistons with no reliefs and 1" compression height.  Waiting on a set of rod bearings for this thing.  Either Calico or Clevite, got to be narrow.  Talking with Comp about a cam.  Eventually get working on heads.  Trying to figure out oil pan/pump setup.  Want to run a external pump and short wetsump pan(5"-6") to keep from dragging on the ground.  I'll keep the updates coming.  

tuffnuff tuffnuff
Moderator | Posts: 33 | Joined: 06/10
Posted: 01/10/11
07:24 PM

Icon Quotemines80:
TuffNuff, there aint a combustion there.  Just a couple holes to drop in valves!!  Well I got a beautiful 3.4" crank used from Roush Yates with Honda journals.  Got the block back machine shop, .060" over bore, = 352CID.  Got a nice set of Oliver 5.5" rods, JE pistons with no reliefs and 1" compression height.  Waiting on a set of rod bearings for this thing.  Either Calico or Clevite, got to be narrow.  Talking with Comp about a cam.  Eventually get working on heads.  Trying to figure out oil pan/pump setup.  Want to run a external pump and short wetsump pan(5"-6") to keep from dragging on the ground.  I'll keep the updates coming.


Chuckles,,, you're right, there is very little combustion chamber to speak of and compression is determined by the dish in the pistons,,, as it should be. This fast burn head, owes it's existence to Robert Yates, who in turn revived the design that Michael May worked on, many years ago.
The design came into existence at the end of World WarII, where it was used on RR Merlin engines running a crank driven centrifugal Super Charger with water/methanol injection.
Back to your engine,,, I'd get a pan with kick-outs on both sides and a swivel pick-up that follows the oil as it strays from G forces, during cornering, braking and accelerating.
Keep us posted on your build bud.

Grin  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

Fahlin_Racing Fahlin_Racing
I’ve been to a few races | Posts: 109 | Joined: 06/10
Posted: 01/23/11
07:31 AM

Tuffnuf wrote
...pistons are nothing more than the combustion chamber floor that heads south, like an elevator going down....

In a general sense, yes. The piston is also the surface our flame travel moves across. If we have a rod too long, we also block the flame propagation path in addition to employing too much dwell time ATDC. If we want optimum burn, we must pay attention to not only spark quality, but the combustion chamber design. Prostockers at the strip use domes for compression over complete chamber burn coverage. Us oval track guys need the complete burn among other things that induce power rather than straight compression as the most needed attribute.

This is where racers and engine builders NEED to conversate about engine combinations if there may be something special that is needed. I guess you could say like one of the CT articles in the recent past, the secret is in the burn, which really isn't a secret its just who pieces things together correctly and how the burn is persueded throughout the chamber creating a torque curve that will dominate if the chassis is setup better than other's.  

tuffnuff tuffnuff
Moderator | Posts: 33 | Joined: 06/10
Posted: 01/24/11
01:06 PM

Icon QuoteFahlin_Racing:
Tuffnuf wrote
...pistons are nothing more than the combustion chamber floor that heads south, like an elevator going down....

In a general sense, yes. The piston is also the surface our flame travel moves across. If we have a rod too long, we also block the flame propagation path in addition to employing too much dwell time ATDC. If we want optimum burn, we must pay attention to not only spark quality, but the combustion chamber design. Prostockers at the strip use domes for compression over complete chamber burn coverage. Us oval track guys need the complete burn among other things that induce power rather than straight compression as the most needed attribute.

This is where racers and engine builders NEED to conversate about engine combinations if there may be something special that is needed. I guess you could say like one of the CT articles in the recent past, the secret is in the burn, which really isn't a secret its just who pieces things together correctly and how the burn is persueded throughout the chamber creating a torque curve that will dominate if the chassis is setup better than other's.




That's true Fahlin,,, getting a flame kernel started and then to good flame propegation is best accomplished with a dished piston, as per diesels or boost engines,,, be it nitrous, turbo or blower.
I believe the key to converting thermal energy (combustion) to lineal energy (piston travel) and finally rotating energy (crank turns) is best accomplished with,,,
1) the right combustion chamber shape and polished, if rules permit.
2) undercut valve stems, less restriction
3) A ceramic, reflective coating on the piston crown, or polished
4) there is usually a .060" wrist pin offset on the piston, swinging the piston around takes advantage of a better rod angle

I'll stop there and dig up notes on the pros and cons of short vs long rod combos.

At lower RPM's the short rod out classes the long,,,
so pulling out of corners is a torque issue, keep in mind that extended dwell time at TDC is wasted time at BDC.

Grin  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

tuffnuff tuffnuff
Moderator | Posts: 33 | Joined: 06/10
Posted: 01/24/11
01:34 PM

This was my reply at CHP to a similar thread,,,

Icon Quotetucker44:
in that table did they use a 5.7" rod or 6.0". i know its not a real bir power adder but it does take less friction off the piston which in turn gives you power.



They, "FTLracing", did a comparison of a 5" (1.492) ratio rod vs 7" (2.0) ratio rod as an example.
This is a little, of what they had to say.

copy/paste

Effects of a longer Rod
* Less rod angularity reduces wear.
* Lower piston velocity and acceleration reduces tensile loading of the rods.
* Less ignition timing is required which resist detonation.
* Compression can be increased slightly before detonation is a problem.
* Less intake runner volume is required and high rpm breathing is improved.
* Reduces scavenging at low rpm (weaker low RPM power).
* Longer TDC dwell time. (high RPM efficency).


Effects of a shorter Rod
* Increased rod angularity increases wear.
* Increased piston velocity and acceleration increases tensile loading of the rods.
* Increases scavenging at low rpm (increased low RPM power).
* Reduced TDC dwell time. (Reduced high RPM efficiency).


What they forgot to mention about the long rod is that the positive gain at TDC, is partially offset by wasted dwell time at BDC. The other flaw IMHO, is the close proximity of the wrist pin hole and the bottom oil ring rail, on the piston.
They used a 3.5" stroke for both rods, which is very close to a 350's 3.48" stroke.
As the graph shows, even 2" longer rod does not perform miracles.

Crankangle

Rodratio

Crankrod


Grin  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

Fahlin_Racing Fahlin_Racing
I’ve been to a few races | Posts: 109 | Joined: 06/10
Posted: 02/02/11
05:58 PM

There doesn't seem to be much of a difference as far as the graphs tell. The largest area seems to be between the 165 to 225 crank degree window.

* Reduced TDC dwell time. (Reduced high RPM efficiency)

Would you please explain this more if thats no trouble?  

kabom1 kabom1
I watched NASCAR once | Posts: 10 | Joined: 08/11
Posted: 08/29/11
06:01 AM

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mines80 mines80
I watched NASCAR once | Posts: 6 | Joined: 07/10
Posted: 01/26/12
06:31 PM

Sorry its been so long since I've posted with this progress of my Ford engine build.  Been very slow, have yet to fire it up.  It is fully assembled and waiting for me to complete the exhaust system.  Went with a trick shallow aluminum wet sump oil pan with a nice external drive pump.  Have a great set of Schoenfeld three step "tri-Y" headers that I had ceramic coated.  I have alot of work to do to get my cowl functional.  The motor is set so far back, that the air filter is close to the windshield.  Gonna have to do some cutting.  This has custom engine build taken me a while, but wanted everything just perfect.  Hopefully it'll be a good investment and have alot of success with this engine.  

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