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Old 10-23-2006, 06:49 PM
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SBC long rod, and clearance issues?

Just wondering if when you change rod length do you run into clearance issues? Curious because I remember hearing when people change the stroke (like the 383) there are some clearance issues with stock parts. I run a Late model race car, and our rules have changed. So now I have to build a new mill. I have most of the parts I need already, but I still need to decide on rods, pistons, and I need machine work done. Plans are to run 6.200" rods, instead of the stock 5.7 length, and 2vr Flat Tops.

I would talk to a engine builder, but... there are no local builders here. So I figured somebody here would know. I haven't built a lot of motors on my own, and none the caliber this one is going to be. I have researched what the real builders use for parts, but can't pay them 20k to do it for me.


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Old 10-23-2006, 07:01 PM
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You'll run into clearance issues with your cylinder head! you only have so much deck height on a small chevy (if thats what your building). You'll need pistons with a short compression height if you use a long rod. Heres a link to some engine calculators so you can play with some different setups.
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Old 10-23-2006, 07:07 PM
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Easy there killer rod ratio boy... Lol...

What cubic inch are you limited to?
What bore size are you going to use?
Stroke?

There's a advantage to rod ratio.. don't get me wrong.
Some of the mag's and articles say the rod ratio does not help but
I'm sold on the old attage that leaving the piston at TDC/BDC longer
and rod angularity helps with keeping piston skirt/bore scuff down.(friction)

Give us some of your class rules and we might be able to set you up with the right combo... where exactly do you live?
Smoky Yunick has a book with some good info in it. Visit Amazon.com for that book..
I got to speak with him several years before he died... Very smart guy when it comes to your classes and engine design.
~Scott
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Old 10-23-2006, 08:52 PM
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We build a lot of circle track engines and very few have 6.200 rods as we have to run fat tappet cams and with the longer rod you would not want to run the the same cam that is in your 5.700 rod engine as with the longer rod it breaths different.

Where are you located?
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Old 10-23-2006, 08:53 PM
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I did say I haven't built many of my own motors, but I did not once say I was completely stupid, just didn't think I had to say I wouldn't be using stock height pistons. I mean come on, has anybody EVER actually done that?

I have talked with a couple of guys who run these Late Model Tour engines, and have got the specs from them. Remember this is no street engine, just for the track. I just want to get a idea what problems I may run into. I will give you the basic rules we have. Max displacement is 358 for chevy's, I myself have a nice bowtie 350 block I will go .030 over on, and you MUST run stock stroke, so 3.48.

I have heard running longer rod engines performance gains are minimal, but why does every engine builder for these cars run 6.200" rods over stock rods? There has to be a plus side, and I already run on a smaller budget than most of the guys I run with so, I'm gonna atleast make a attempt at a mill that will make good power, and reliable HP. (which has been my problem in the past, with the old motor rules)

I like that there have been some people trying to help (and keep it coming_ but really the first deck height thing, that was just a attempt at being a smart A!

Also I'm not apposed to running stock length if somebody can tell me why they all seem to run the longer rods, I have a set of 6" rods I would far rather use (because I have them).

I'm in the Northwest, BC


bonuts
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Old 10-23-2006, 08:56 PM
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Are you in CANADA???????????
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Old 10-23-2006, 08:58 PM
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Yes BC Canada, but we run in Washington, Idaho, BC, and Alberta.


bonuts
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Old 10-23-2006, 09:05 PM
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What are the rest of your rules.
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Old 10-23-2006, 09:14 PM
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To be honest with you I can't give you much more of our rules because they are re-working them now. They may allow crate motors now, and then all this fuiguring would be a waste, because if they'll allow them, and give us some lower weight to run them I'll just run a GM crate motor, and save the hassle.

Steel head, 650 carb, and from there on in you can pretty much run what you want.


bonuts
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Old 10-23-2006, 09:42 PM
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Quote:
Originally Posted by bonuts
Just wondering if when you change rod length do you run into clearance issues? Curious because I remember hearing when people change the stroke (like the 383) there are some clearance issues with stock parts. I run a Late model race car, and our rules have changed. So now I have to build a new mill. I have most of the parts I need already, but I still need to decide on rods, pistons, and I need machine work done. Plans are to run 6.200" rods, instead of the stock 5.7 length, and 2vr Flat Tops.

I would talk to a engine builder, but... there are no local builders here. So I figured somebody here would know. I haven't built a lot of motors on my own, and none the caliber this one is going to be. I have researched what the real builders use for parts, but can't pay them 20k to do it for me.


bonuts
You can improve upon the tangential angle of the rod to crank ratio which helps an engine in a racing environment, but I have never seen any clearance issues in your application. You will be moving the piston up in the cylinder equivalent to the increase in rod length so you must check your deck height.

In my experience, I have only run into clearance issues with longer rods in the 400 sbc's, which required minor grinding of the sleeves at the lower end to rectify.

You would have a slight benefit to using longer rods in your engine, but you will need the proper pistons to compensate, get some with a higher pin to compensate for the rod length increase.
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Old 10-24-2006, 05:53 AM
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Quote:
You would have a slight benefit to using longer rods in your engine, but you will need the proper pistons to compensate, get some with a higher pin to compensate for the rod length increase.
Yep... good advice there.

If you've got a good set of 6" rods there would be little difference in performance than if you used a 6.2 rod. As stated above your putting that pin into the ring pack when a longer rod is used. The pistons for 6" rods are comon and on the shelf.
I remember getting into alot of grinding when we used a 4" stroke crank in a 400 block with 6" rods. (434cid) small block.

Since your stuck using a 3.48 crank... As mentioned before you should be ok for clearances. I'd mock everything up to make sure (rotate it in each cyl) with out rings. Depending on your cam profile and lift you may be required to have a small base circle cam with 6" rods. Call Lunati/comp/crane for advice there.... They have all the interferience info in their computer.
Investigate more on how you can get the heads to flow more that's where you'll see more power than rod ratio. I'd spend some more time and money on head work!
Good luck!
Scott~
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Old 10-24-2006, 08:42 AM
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Bonuts, I don't have the credentials to help you in your engine build other than so basic reminders. Competive power with superior reliability would be my goal. When it comes to racing week end and week out, you don't have to be 2-3 mph faster than the other guys. You know that 0.1 mph quicker over time can put you in front when the flag drops. This quicker time can come from things like handling, suspension, gearing, aerodynamics, driver skills, traction, and of course HP and torque. When it comes to getting more HP and torque from the motor, you may want to look where your competitors may be slacking. My heads would be smooth as baby butts every where I thought I could get an edge. I would spend a couple of days hand lapping the valves to get the most surface area possible for a super tight seal and excellent heat transfer. I would shy away from going to the maximum limits in any area that would jepordize reliability for a minute gain. And I think going to from a engineered design 5.7 rod length to a 6.2 rod length may be going to the edge. The 6.0 sounds like it might be a logical and more economical option for you. After a season of experience, you might consider exploring the outer limits (let your copetitors do the R&D for you!). I would be looking for every friction reducing option available under your rules and one of these would be a small journal crank.

Trees
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Old 10-24-2006, 10:17 AM
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Just so you know the competitors have been doing the R&D on these motors for years already. I realize that chassis set-up is huge, but the rest of the stuff you mentioned to gain on competitors is pretty well regulated, like aero dynamics, we all run the same bodies, same spoiler height, etc. Gearing won't help me, because the big dollar guys are running 8000RPM plus, I'd prefer not seeing that with mine.

And you are correct handling will make a lower HP car do well on the shorter tracks, BUT as soon as you see a bigger track, 1/2 mile HP makes you or breaks you. You stilll have to have a good set-up, but if your down on HP you won't have much of a chance.

And I absolutely agree with spending time with Cylinder heads to make power, I do have a good set of heads, but I will spend time porting, and other head related stuff, but I just wanted to know about the clearance issue for now, so I can figure out what direction we're going to go in.


bonuts
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Old 10-24-2006, 10:20 AM
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If the only thing I might say about it. DO NOT build your circle track engines like street engines. They are a whole nother animal all together.


With a 358" rule, I use a Dart block 3.48 and 4.125 bore and 1.5mm pistons with 6.125 rods. 38 pound crank, Dart Platinum 49cc head, 16:1 CR 650 carb by ? , Super Victor Jr or Wilson manifold and gear it about 5 points higher. Do they actually tech the carb? Or do the say something like "any 650 carb".

You need CR for any intake restriction rule.

You'll need to run it to 9600.
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Old 10-24-2006, 11:06 AM
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Quote:
Originally Posted by johnsongrass1
With a 358" rule, I use a Dart block 3.48 and 4.125 bore

3.48 x 4.125 is 372 CuIn.

I do agree with JGrass on using thin rings with horizontal gas ports. As far as clearance issues, going with the stock stroke but using the longer rod length won't cause interference issues in the bulkhead area. The angular difference at the rod journal is nill. The problem with too much rod length is the wrist pin occasionally has to be located into the oil ring groove which means a spacer is needed to support the ring package.
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