Ratio Questions

[BAKERSFIELD BOYS]

HERE IS AN IDEA TO CHEW .ON TAKE A ALUMN CHEVY BLOCK WELD ALUMN SQ TUBE IN THE SIDE OF THE BLOCK TO CREATE PORTS IN THE BOTTOM OF THE CYLINDERS THAT THE PISTON WOULD OPEN A LITTLE BEFORE BDC. USE A 8:71 CRANK DRIVIN WITH 25% OD TO FORCE FUEL AND AIR IN THE PORTS. USE BOTH VALVES IN THE HEADS FOR EXHAUST AND FIRE CYLINDER 1 AN 6 AT THE SAME TIME AND REST OF THE FIRING ORDER WHEN 2 PISTONS ARE AT TDC TOGETHER.CAM TO RUN SAME SPEED AS CRANK. WOULD IT MAKE A LOT OF POWER OR TORQUE 4.155 B 3.250 S WILL THIS WORK?

[JackD]

I did that with a Crosley and a blower in 1961.
 If you know how they were built you will realize how I made a barrel crankcase with another bottom end. We made a double lobe cam OHV 2 stroke.
 We silver soldered the plates to the rods to strengthen them.
 We put it in a Hydro and never ran it enough to get the correct pitch prop from those available.
 It is in the S D Automotive Museum collection.
 The first driver to go 200 mph at the lakes helped me with it.
 
  <small>[ February 10, 2005, 02:39 AM: Message edited by: JackD ]</small>

[ddahlgren]

Ok I need someone to show me the math that moves the piston 0.001 in the first degree of rotation. If we build an engine with a stroke of 3.5 and run a rod of 5.7 and compare to a 6.2 these are pretty common numbers.
 With a 6.2 rod RR is 1.77 peak piston speed is at 75 deg ATDC
 Travel down the bore is
 1 deg   0.00034
 5 deg   0.0085
 10 deg  0.03404
 20 deg  0.1345
 
 With a 5.7 rod The RR is 1.62 peak speed at 74 ATDC
 Travel down the bore is
 1 deg  0.00035
 5 deg  0.0087
 10 deg  0.03469
 20 deg 0.13705
 
 At 60 RPM or 1 per second the time to move 1 degree is 1/360 or 0.002777 seconds
 So at 6000 RPM 0.00002777 seconds per degree
 So over a 20 degree period the time available is 0.0005555 seconds and the difference in position is 0.003 inches..
 Tell me about the dwell story again.. Maybe should look at friction losses from side thrust and peak piston speed and position of peak piston speed these dramarically relate to port size and cam timing.. Not 'dwell'
 Math courtesey of 'Internal Combustion Engine Fundamentals', John Heywood
 Unless I have made some horrible mistake plugging Mr. Heywoods info into MathCad I fail to see the whole dwell issue.

[JackD]

I know why you fail to see the real world dwell issue.
 It's because you forgot bearing slack and oil preassure that will cause it to change.
 "Theoretical rod length produces theoretical horse power."
 ENJOY

[panic]

I give up. Of course, you're right.
 Hav a nice day.

[jacksoni]

The guys at Reher-Morrison, a pro stock engine builder co of some renown say for your engine size, pick your block, pick your crank, design your pistons (ring package, pin size, location, CH you want, (low so piston light) deck height etc, etc and then measure all and pick the rod that connects all together.  Forget the ratio. there are some suggested limits though.  A couple posts, one by Larry Meaux of Meaux racing heads and one by Darin Morgan, chief of cylinder head reasearch at Reher Morrison have some insight in addition to that already posted.
 -------------------------------------------------
 By Larry Meaux
 from experience Dyno testing various
 Engine sizes and Rod Ratios
 
 the trend so far shows
 increased BlowBy CFM (6 to 8 CFM) as you go less than 1.50 Rod Ratio
 if you can't run a Vac-Pump, i'd keep Rod Ratio above 1.50
 
 and with 1.43 Rod Ratio with same Hone finish
 8 to 10+ CFM BlowBy, but thats not much of a problem
 with a Vac-Pump ...the higher piston speeds and shorter rod ratios,
 are showing more HP/TQ per 1 CFM of Cylinder Head Flow up to a point.
 
 usually 1.60 to 1.65+ you'll have no major ring BlowBy problems
 and 1.6 to 2.0 can show 1.5 to 4 cfm BlowBy range
 
 if you have a very good Block , (Correct Hardness and Thick Walls),
 the BlowBy CFM numbers will be reduced from the above numbers
 _________________
 Meaux Racing Heads
 __________________________________________________
 Once again, what maxracesoftware said is right on the money. The piston rock and side loading become a big factor below 1.50 with piston speeds over 4500ft/min. If its a low rpm street machine that never sees the high side of 6000rpm or 4000ft/min, then the problems with blowby are much less but none the less still present. When I discuss Rod Ratio I always dismiss people who think there is an " ideal" rod ratio and are willing to compromise the overall engine design in order to achieve this " ideal" Rod ratio. I have never said that there are not both high and low limits one must avoid. A Rod ratio of 2.2:1 or a 1.4:1 is not going to help matters any. Anywhere between 1.65 and .1.85 is fine. any less and you run into high skirt loading, blow by and frictional HP losses. Anything above 2:1 and you run into pressure lag and have to run the cross sectional area of the ports very small and maintain a higher mean velocity in order to help make up for it.The little 265 and 302 Comp eliminator engines are usually a case study in pressure lag unless they run the Aurora block with a 8.5 deck and get there ratio into the 1.8 range.
 .
 _________________
 Darin Morgan
 R&D-Cylinder Head Dept.
 Reher-Morrison Racing Engines
 
 __________________________________________________
 My bigger concern with this engine is not the rod ratio, which unless you are using one of the specia low deck (8.5") blocks is in the 2.3:1 range and nothing really you can co about it since as Dave Dahlgren already said your CH is in the 1.75 range.  More ( a lot more) if you shorten the rod to give a more "ideal " ratio.  Makes a piston weigh a ton and pin too far down.
 To me the problen is the heads.  The ports may flow well for good power but be slow.  You need to think much smaller/faster flowing ports (but of course still with the cfm to make the power) and this may be tough to do.  Porperly designed, ported etc should do it.  The other thing is the chamber.  Putting a 18cc dome in a 66 cc head only gets you to a net of about 56 after accounting for gasket only, let alone quench, deck clearance, valve pockets etc.  You need something in the low 40's or better to start with to get rid of the dome as much as possible for not only more compression as noted above posts, but also for flame travel etc and get the piston out of the head as much as possible.  Look at something like the Chevy 15* head that starts at35-37 cc and can be milled more.  Other heads may but I think you need a smaller chamber.
 IMHO
 
 Jack
 
  <small>[ February 10, 2005, 03:57 PM: Message edited by: Jack Iliff ]</small>

[jacksoni]

You might consider a smaller bore block, 262 or 283 original with longer stroke( than your 2.57 )to fix some of these issues but, then shrouding valves and getting heads that are decent to fit becomes more issue.

[Bryan Long]

This looks like a fun discussion. I too have been wondering whether rod length is as significant as is sometimes claimed. I have run two analyses to try to improve my understanding.
 
 To assess the "longer dwell at/near TDC" I developed a graph of volume (basic combustion chamber volume plus the volume "swept" by the piston) for a 6.135" rod and a 6.7" rod version of a BBC. If I knew how to post the graph here I would; but trust me, you really can't see the difference at or near TDC. The biggest difference occurs around 55 deg ATDC and is still a small 1.2% difference. The slightly bigger volume swept by the long rod is a theoretical advantage, but a very small one I would say.
 On the matter of reduced friction due to reduced side thrust, I did a calculation that estmates the frictional loss due to that effect. Making what I consider to be unrealistically high values for friction factors and cylinder pressures, I get a 2.5 HP advantage for the longer rod. That is, 2.5 less frictional loss. I don't claim that this is exact, but to me it says we are talking about another small effect.
 
 Note that with a stroke of 3.76", both of these rods have rod ratios in the range recommended in previous postings.

[Sumner]

That is interesting information Bryan.
 
 Most of the numbers I've see from people like Vizard is 1/2% to 2% more hp.  Not much, but for a 600 hp motor that would be maybe as much as 12 hp and to some people that 12 hp is important.
 
 I never meant to imply that all the hp/torque of the motor in the Hot Rod article I have on my site is due to longer rods.  That motor has some great flowing heads (AFR) and a nice profile full roller cam.  They hit on a good combination, but some of it was by chance.  I talked to AFR about the heads that were used in the article and they said Hot Rod used them as they had them laying around from another project that never got done.  They (AFR) felt some of their other heads might have been even better for that motor.
 
 Personally I feel the best thing about having longer rods is the lower frictional loss you mentioned.  Friction uses up HP and in a street motor that will see a lot of miles the longer rod motor will probably not wear the cylinder walls as quickly.  The next 350 or 383 I build for my truck will probably use 6 inch rods.
 
 c ya, Sum

[John Burk]

One simple way to put together an E engine with a reasonable rod ratio is to take two pistons out of a 350 and destroke the crank .060" (258 ci)- Substituting a pair of of rods and pistons with the right clamp on weight adds no stress to the engine - Off set grind a 350 crank to 3.42" stroke to use 327 rods with 2" bearings - The engine will shake less than a Harley v twin - Any suggestions which would be the best pistons to remove ?

[JackD]

My 258 inch was a Buick V-6
 Crower built kits to remove jugs mainly for the gas crunch, but the hardware would be the same.

[RichFox]

Am I wrong again? Can't you buy such a V6 from Chevy? I remember working on a 300 inch one for Al Liest.

[BAKERSFIELD BOYS]

GUYS I NEED SOME HELP I WANT TO BUILD A FUEL 355 IN CHEVY S/B 6:71 BLOWER. BUGCATCHER.  WHAT PERCENT OD ON BLOWER AND WHAT COMPRESSION SHOULD I START WITH .IVE HEARD 10 OR 11 TO 1 COMP AND 15 TO 18% OD ON BLOWER  THANKS FOR ANY HELP. BAKERSFIELD BOYS RACING

[John Beckett]

After listening to all the comments about rod / stroke ratios and other factors in the design of an engine my conclusion is this: A V-6 is probably the best available engine for a salty ?E?. Buick and Chevy made some rather good ones a few years back when they ran V-6?s in the NASCAR Busch series.
 
 JB

[JackD]

Buick heads flow better and the blocks are stronger.
 They are available from the NASCAR market with a wide range of factory race parts.
 A 258" is an easy build with a motor size that small and the heads work well with out much preparation.
 Bigger is not always better.
 The best manifold was a circle track item that did not look correct for maximun HP on the dybo or drag racing.
 Correct handling of the air was important and not as traditional as you have seen.