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effect of rod/ stroke ratio on detonation


datsphilly

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haha well.....if your going for a race application then why not have a custom crank built?? I mean if your going to take it all the way go have a company like SCAT build you a custom counterweighted crankshaft. I mean if we are talking race motors here we could go this far =P

 

I like Garrett76zt's build as well...overbore to 89mm, L24 rods and custom pistons with a stock L28 crank. I wonder if you could actually go further than that...f you can get close to 138mm rods you'll approach that "perfect" rod/stroke of 1.75. It just depends on how short you can get the custom pistons I guess (I wouldn't know the minimum you could go, I guess it would depend on whether you were NA or boosted as well).

 

 

well theres something i didnt know, i thought the higher the rod stroke ratio the better, whats all this 1.75 business?

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well theres something i didnt know, i thought the higher the rod stroke ratio the better, whats all this 1.75 business?

 

something I read somewhere, don't quote me on it XD Hence why I said "perfect." It's considered around ideal I believe for a street/track motor, again don't quote me on it. Building a higher rod/stroke should allow a higher revving motor period, so if you wanted to go real crazy, you could build it to rev more like a motorcycle engine (As far as I remember, most sports bikes and open wheel race cars have rod/strokes of over 2:1, but in either case they are very different than what you would drive on the street).

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Unless the L20A is fully counter weighted (none of the other L6's are so I don't see why It would be) it would still be missing the same 4 counterweights on 2 and 5, leaving the surrounding counterweights to make up for it. I was reading in my "how to modify you're 510 & 240Z" books that at a bit lower rpm than what that L20 was running the cranks still died fast due to harmonics above 8k.

 

Please understand that Frank Honsoweitz didn't actually do any of that engine work he just wrote about it, and in most cases he is relating stories that sometimes are of mixed origin/cross platform. The 'crank harmonics' thing is soooooo overblown it's not funny. In most instances people simply don't comprehend what they are reading---the issues with non-counterweighted cranks are limited to the first L24's to the country and nothing more. JDM L20A's had several variations, some of which were fully counterweighted (?!) the engine in the video is not one of them. Full counterweighting is not necessarily needed.

 

Trust me, that video is nothing.

 

The revving capabilities of the L Engine are shrouded more in myth and legend than actual hard data. I would suggest that anybody limiting their engine to 7500 due to 'crank harmonics issues' seriously consider unrestraining their motor.

 

This has nothing to do with Detonation and rod stroke ratio, but I can tell you the engine in the video has an L20A crank, L20A Block, and L20B Rods... do that calculation boys and girls. Maybe move this one to a new post now that I've mucked this one up totally throwing that tidbit out there...:mrgreen:

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:hijack:

 

Now I'm interested. Xnke has PM'd me with some very valid and solid information on the subject as there are harmonics in the crank at 7230 and 7490 Rpms due to the oil drilling in the crankshaft however there is no harmonic issue above this.

 

If anyone cares to unshroud this myth, I'd love to learn.

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This has nothing to do with Detonation and rod stroke ratio, but I can tell you the engine in the video has an L20A crank, L20A Block, and L20B Rods... do that calculation boys and girls. Maybe move this one to a new post now that I've mucked this one up totally throwing that tidbit out there...:mrgreen:

 

I kept shut because A: i didn't do any of this and B: I couldn't even come close to relating it all right anyhow, but I was hoping you would chime in after I threw your vid here.

 

Looooong rod, over 2:1 ratio, tight tight tight tight ring pack on the piston.

 

There are in fact, TWO replacements for displacement. One is boost... but the other is reciprocating frequency (RPMs) :burnout: 2 liters at 9300 = 6 liters at 3100, right??? :ass:

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7230, and 7490... now, please refer to the post from Chris at BHJ about what their product does...

 

And realize that the issue for ANY harmonic or resonant frequency is to pass through it as quickly as possible...

 

Centrifugal compressors all run through at least one critical speed on their way to running speed, usually before the second critical. Some older reciprocating engines had two criticals which had to be contended with during operation, especially during variable load/speed conditions. Experienced operators knew the speeds (on a 360rpm Cooper, 120 was one of the speeds, only a problem in compressor service, and then only early on when the engine was rated at 180rpms, later speed was increased, and then increased again making the engine pass through two criticals when reaching operating speed!) and could tell when they got close, and when you 'hit the sweet spot' things happened fast if you were heavily loaded. I wish I could get Andre to chime in here, but he's way too busy, he did developmental work on Sulzer Crankshafts and his knowledge in this area staggered me while we talked about it down in Australia last month. But alas, he's a Bike Guy and concentrates on his Ducatis and surfing now...

 

Continued operation above or below the critical speed is fine.

 

Do you think with simple gearing you can keep your car from loitering at 72-7400 rpms for extended periods? I think you can. This ceases to be an issue, especially with the current state of harmonic dampners (again Chris' Post elsewhere).

 

Remember the MYTH is that you can't run the cranks over 7500, so people limit their engines there. Putting them operating RIGHT on the critical speeds. They have a failure. They tell everybody: My engine came apart and broke crankshafts when I ran it at 7500, ergo you can't run them above 7500 rpms.

 

I worked with diseased hogs all day and ate an apple for lunch. I came down with Swine Flue. Apples cause swine flu...

 

Most racers in the 70's were running 8000+ for endurance events. What happened to the 7500 harmonics issue.

 

It's overblown. Yes, there may be a harmonic, but Chris does an excellent job (and linked a PDF from BHJ's Website as I recall) on the issue.

 

For the record, we haven't bought a BHJ Damper for the N/A engine, likely we will run either a BHJ or ATI for the Turbo car in both 2.0 and 2.8 configuration simply because of the crank impulses we plan to generate per hole.

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Grind you crank to match the toyota rod, centre the rod on the crank during this process. This will not weaken the crank, anyone who tells you this is not talking from experience.

You will then need to mill the pin end of the rod 2mm each side to fit the piston. Simple process, any competant machine shop should be able to do this. The pin diameter is 22mm for Z31 and 3sgte

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Grind you crank to match the toyota rod, centre the rod on the crank during this process. This will not weaken the crank, anyone who tells you this is not talking from experience.

You will then need to mill the pin end of the rod 2mm each side to fit the piston. Simple process, any competant machine shop should be able to do this. The pin diameter is 22mm for Z31 and 3sgte

 

Well, see, I was imagining a straight pull, not needing to modify the rod or your crank.

 

But this discussion motivated me to look a little deeper and i found this...

http://www.scribd.com/doc/7376862/Conrod-Application-July2408

 

It is a company's catalog in a digital, searchable document format.. I searched "53" and also "21" searching for rods that would only need to be altered at one end. Most (if not all) rods with 53mm big ends are too long to use, but there are some honda rods with 48mm big ends and 21mm pin journals that might work in the 136 and 138 range it looks like.. B16, B18, ans B20 motors.. Okay!! Listed: H23/B20A/F22 rod, 141.75mm length, BE is at 51mm, pin at 22mm, combined with a stock VG30 piston puts the piston .5mm above the block using that L20A crank, and the rod/stroke ratio is still above 2. Interesting document, I thought it might prove useful.

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Well, see, I was imagining a straight pull, not needing to modify the rod or your crank.

 

But this discussion motivated me to look a little deeper and i found this...

http://www.scribd.com/doc/7376862/Conrod-Application-July2408

 

It is a company's catalog in a digital, searchable document format.. I searched "53" and also "21" searching for rods that would only need to be altered at one end. Most (if not all) rods with 53mm big ends are too long to use, but there are some honda rods with 48mm big ends and 21mm pin journals that might work in the 136 and 138 range it looks like.. B16, B18, ans B20 motors.. Okay!! Listed: H23/B20A/F22 rod, 141.75mm length, BE is at 51mm, pin at 22mm, combined with a stock VG30 piston puts the piston .5mm above the block using that L20A crank, and the rod/stroke ratio is still above 2. Interesting document, I thought it might prove useful.

 

 

 

One type of L20A crank has a smaller big end journal (45mm's), like the L24E Maxima's. The larger L20A crank is 50mm's like the rest of the L series family.

I wonder how many rods you could find/use on the smaller L20A crank?

 

Hmmmm.....:mrgreen:

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Don't think that grinding down a 50mm journal to a small diameter doesn't weaken the crankshaft. This is done in VW engines all the time, and we all recognize that on some of the stroker cranks we are taking the life of the crankshaft in our own hands decreasing cross sectional width! Will it matter in THIS application? Maybe not, but there is no way a 43, or even 48mm Rod Journal diameter is as strong as a 50mm one...it's basic mechanics.

 

But one thing people miss is that by using that smaller diameter rod diameter, it's not a STRAIGHT cut from 50 to 48, but an OFFSET cut---this increases the stroke of the crankshaft without requiring welding.

 

Chevy guys do it all the time in little steps. Taking a VO7 and then offset grinding it, or even welding the journals and offset grinding it can make for a couple of MM increase of stroke.

 

But your rod ratio may go south...LOL

 

Ozconnection...I see you found my 'secret identity'! Sometimes I actually do, DO things! LOL

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Tony D: Thanks for the explanation, very logical.

 

As for the cranks, would you ever have to re-harden after you lop off .080? I know My M104 requires re-induction hardening after a shave more than .020.

 

And even though the 48mm does not have as much strength as a 50mm, it should be plenty. Nascar has been running the 283 main bearing journals with the small 2" rod journals at 9500rpm and 700+hp for a while now.

 

I do realize that a 2" rod is still 50mm, but it's a bit less than the standard SBC rod journal.

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One type of L20A crank has a smaller big end journal (45mm's), like the L24E Maxima's. The larger L20A crank is 50mm's like the rest of the L series family.

I wonder how many rods you could find/use on the smaller L20A crank?

 

Hmmmm.....:mrgreen:

 

I *did* know that, but couldn't find the specs anywhere (easily) so I just ignored it. Trust me, I was definitely aware of that one :) Its one of the reasons why, upon discovering a dearth of rods with 53mm BE bores, I still posted the info that I did up.

 

 

tony, your comment about offset grinding is appreciated; when Komdotkom mentioned grinding the pieces to match something seemed odd, but I couldn't put my finger on it. Another Rookie mistake from yours truly :bonk: The 1-3 mm potential increase in stroke can be taken care of using the VG piston now rather than the taller KA, and the honda (and 'yota) rods give us some real estate between 133 and 140 to play with.

 

 

But I gotta say, Honda engines are some of the best damn engines out there, and for anyone to hesitate before putting a honda rod into their engine because of a bunch of idiot post-tweens with more spoilers than brains, really doesn't fit this website. Ever since I started driving my CRX HF about six months ago I've started taking affront to anyone who assumed Honda parts were crap because Honda enthusiasts are largely wasting skin... so I;ve been meaning to get that one off my chest.

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Nascar have been running Honda big end rods for years now. The big advantage of this is that Honda supply bearings in minute increments which allows you to get things just right.

Grinding the big ends down gives you a number of advantages, you can increase the stroke without welding, but it also allows you to put a much nicer radius on the crank than standard. This needs to be done by a quality operator though, as too much radius can cause the rod bearing to pick up in the radius.

I always have my cranks nitrided and then ground, after the grinding it's off for cryo treatment. I consistantly spin my v07 crank to 8200rpm with 48mm big ends, it's never broken. If you are going to do it, do it once the right way.

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My take on it:

For most of us, it's not going to be very cost- or labor-effective to go to great lengths to optimize rod/stroke ratio, when the actual increase in CR you can run is going to be pretty minor. FWIW, I'm running the standard stroker r/s of 1.60:1. Also running the dreaded open-chamber N42 head (Sunbelt ported and cammed). I've been running this setup for years, at 11.4-11.5:1 CR (I had *thought* it was more like 11.1:1), on 93-octane pump, with full ignition advance (made same power from 34 - 38deg, so I run it at 35).

 

I will own up that it's pretty close to the edge. A few laps with the middle carb linkage stuck open recently cost me pistons #3 and 4 (doh). Some detonation damage to the head required some weld-up repair, and they took .006" off the head to clean it up, so now I'm at 11.63:1. I think I'm-a start running race gas at the track...

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