7age using Honda rods???

[RonRob]

Thought I would chime in as I am currently running a 7agze with Twinscrew supercharger. My boost is low, but I have no worries taking it to redline with stock modified rods. In fact, I have gotten guidence from former 7a racers that only use the stock rods modified to full floating and shot peened. They road raced their 7a's at 8500-9k on the stock modified rods and a fully balanced bottom end. Lets not forget that the 20v black top rods are about the same thickness as the 7a rods. Furthermore in all honesty, I have never heard of a stock rod failing on a 7a, yet the failure usually stems from bearings.

Ron

[wraith]

Shamelessly stolen from a Frod site.

The following maximum piston speeds are from the book, Performance Tuning in
Theory and Practice, by A.G.Bell.

Stock Motor - 3,500 fpm (cast crank, stock rods and cast piston)
Heavy Duty Motor - 4,000 fpm (forged crank, peened rods w/ good bolts, forged piston)
Drag Racing Motor - 5,000 fpm (forged crank, alum rods, lighweight pistons, etc.)

I really don't like raining on peoples parades, but the reality is that your heading into F1 territory with OEM hardware. As for "a couple of hundred more RPM"

with a motor, as you increase rpm, the loads experienced by all parts increase by the square or cube (can't remember now ) and THAT is the problem.

a motor that can last sayy.. 10,000km at 8000rpm may only last 10km at 9000rpm....

Think about it. Hast thou seen... or heard?

Some further reading...
http://www.toymods.net/forums/showthread.php?t=21404
http://www.toymods.net/forums/showthread.php?t=7018
http://www.toymods.net/forums/showthread.php?t=17716

Cheers.
Ash

[Mooro]

Shamelessly stolen from a Frod site.

The following maximum piston speeds are from the book, Performance Tuning in
Theory and Practice, by A.G.Bell.

Stock Motor - 3,500 fpm (cast crank, stock rods and cast piston)
Heavy Duty Motor - 4,000 fpm (forged crank, peened rods w/ good bolts, forged piston)
Drag Racing Motor - 5,000 fpm (forged crank, alum rods, lighweight pistons, etc.)

I really don't like raining on peoples parades, but the reality is that your heading into F1 territory with OEM hardware. As for "a couple of hundred more RPM"


Think about it. Hast thou seen... or heard?

Some further reading...
http://www.toymods.net/forums/showthread.php?t=21404
http://www.toymods.net/forums/showthread.php?t=7018
http://www.toymods.net/forums/showthread.php?t=17716

Cheers.
Ash

Yeh I understand the theory behind it and the problems with long stroke motors creating much higher piston speeds etc, however nothing beats real life and there are plenty of people all over the world who have built these engines and had them last with rev limits in the 8,000rpm to 8,500rpm range...and increasingly as I search I'm finding more people who claim even the stock rods are good for higher than 7500rpm if prepped properly.

Obviously things can be improved by using a longer rod, but this doesn't change the piston speed, only reduces the lateral loads on the piston and therefore reducing the chance of throwing a piston out the side of the block.

Even taking a conservative point of view most people will agree that that stock prepped rods are reliable for 7,500rpm. Now I haven't put a lot of thought into this so I may well miss something but taking a very simplified approach, the tensile load on the rod at TDC is,
Piston mass = M
Time for change in momentum = t (very small amount of time)
Stroke = 85.5mm
RPM = 7500
1 revolution, the piston travels = 2x85.5 = 171mm
7500 revolution = 7500x171 = 1282500mm = 1282.5m
Piston velocity = 1282.5/60 = 21.375m/s
Tensile load, F1 = m.a = m.V/t = mx21.4/t N

Now for 8500rpm
RPM = 8500
1 revolution, the piston travels = 2x85.5 = 171mm
8500 revolution = 8500x171 = 1453500mm = 1453.5m
Piston velocity = 1453.5/60 = 24.225m/s
Tensile load, F2 = m.a = m.V/t = mx24.225/t N

Now I realise the time for the change of momentum to occur will be less at 8500rpm and hence loads higher, I'm not really sure how to figure this out but I presume it is proportional to the rpm so therefore,
F2 = 85/75xF1/75/85 = 85²/75²xF1

So this agrees with Oldcorolla in saying that the loads increase by the square, ie it is not a linear relationship but we are only talking about a fairly small increase in RPM.

So the rod tensile load is 28.4% higher at 8500rpm compared to 7500rpm...maybe I'm wrong and I know this is an over simplified analogy but to me, given how weak a standard 7age rod is, it doesn't seam unreasonable that a chunkier rod like the MX5 unit would be that much stronger (ie bigger cross sectional area) at the weakest point. Also I would probably try keep the stock 20V cutout of 8200rpm so a 19.5% increase.

I know there are other factors like bearing wear rates etc at higher speeds but I'm looking at it purely from a catastrophic failure point of view.

Anyway just thinking out loud so feel free to argue .

[Mooro]

ive found they are a 132.5 rod length, any idea of the 7a v 20v pin height? in theory could you use a slightly shorter rod and a piston with a different pin height to acheive a decent (10-11.1)compression ratio?

Yeh I have wondered about this...I presume this is possible..except my understanding is you want a larger rod/stroke ratio, therefore you would want a longer rod (look at Honda rods...they are all much longer than toyota equivalent, ie B16A). As I mentioned though I don't think this affects piston speed and hence tensile loads on the rod, just the lateral loads on the piston...ie it is less like to throw a rod out the side of the block and I presume loads on rings and bearings are reduced.

Problem for me is that I'm coming at this more to find a budget alternative. If you have a decent amount of money to spend there a tried and proven, aftermarket rod/piston combos available which can be bought off the shelf.

[skiddz]

i now the 7a crank has 48mm mains and big ends yes? as does the CA16/18/20 they also have the 20mm small end, as for lenght the ca20 is about 148 and i am unsure at this stage of the ca18 and 16 lengths i shall head down to the engine place thingy after exams. im interested in this as it is VERY easy to get stock ca18det rods, last time i was quoted 200 for the set new from neeesan ca16 would be a peice of piss too, this is all assuming they are the right lenght to use a 20v piston with it

anyone know a 7a rod length and im talking definitave length not speculation same for the 20v pin height

[Mooro]

Hmm CA rod sounds promising...I'll try find out the lengths for CA18 and CA16. Perhaps for a 16V 7age you could look at some alternative pistons to overcome any changes in rod length. WIth a 20V you are a bit limited cos of the valve cutouts.

I'm fairly sure that 132.5 is the definitive answer when it comes to 7age rod lengths...I have come across it on numerous websites.

[Mooro]

Wow...might have hit the jackpot...

I just looked here,
http://www.importperformanceparts.ne...ssan-rods.html
and found the dimensions for aftermarket CA18 rods which are presumably the same as factory.

Rod length is 132.8mm, so the extra 0.3mm will give a slight increase in compression which is not all bad.

That site lists the BE diameter as 48mm and the pin diameter as 20mm. The big end width is 24.2mm.

Now I just have to find that thread with the technical drawing of the 7afe rods to see how they compare.

[skiddz]

excelent! im stuck on the 20v idea, cos im ghey and thats the way i roll

and yeah i thought it was odd that people were considering honduh rods and not looking at neesan may even pay to look to gm/opel i know my 18L engine in the pulsar has had the crap kicked outta it and still goes strong also a few people have turboed the 18LE's without rod or piston changes i am unsure of the results or longevity

also the b18a pistons have a 2 degree different fly cut to the 4a 16v head and they are deeper so there are no issues there also same bore.....

[Mooro]

OK, I managed to find the drawing of the 7afe rod but I have no more quota left

Anyway, as mentioned before the length is 132.5mm, and the small end diameter is 20mm, only problem is the big ends are 50mm bore...I have heard other people say it is 48mm so I'm not sure. So I wonder if machining the CA18 rod out to a 50mm bore would be a problem?

So can anyone, particularly those with engine building experience, see any flaws in my thinking or is it worth at least giving the CA18 rod a shot?

[jezza323]

the way i see it, you need to make friends with someone with a dead CA18, and friends with someone with a spare 7afe rod

[Mooro]

ALready onto it. I used to own a Silvia and I'm a member of SilviaWA forums, so I have already put a call for help out on that site, hoping to find someone with a busted CA who has a rod lying around they can donate in the name of research and development for the greater good.

Unfortunately I don't know anyone with a 7afe so if there is anyone in Perth who has one and is interested in exploring this further please let me know.

[Joshstix]

The big end of hte conrod is by far the most heavily stressed part, there is no way I would advise anyone to bore it out and reduce the strength further. If you have a 48mm big end bore in the conrod and a 50mm big end journal on the crank then it's time for an offset grind to get a bit more capacity and get the rods to fit the crank. It's also a good chance to get a nice big fillet cut at the edges of the journal to reduce stress risers.

[Mooro]

The following maximum piston speeds are from the book, Performance Tuning in
Theory and Practice, by A.G.Bell.

Stock Motor - 3,500 fpm (cast crank, stock rods and cast piston)
Heavy Duty Motor - 4,000 fpm (forged crank, peened rods w/ good bolts, forged piston)
Drag Racing Motor - 5,000 fpm (forged crank, alum rods, lighweight pistons, etc.)

I really don't like raining on peoples parades, but the reality is that your heading into F1 territory with OEM hardware. As for "a couple of hundred more RPM"

Just touching on this again, a CA18det has an 83.6mm stroke, the standard fuel cut is 7,800rpm, they have a reputation for being revvy buggers and are well known to be reliable at 8000rpm plus on stock components..the piston speed at 8000rpm is 4388fpm, and a 7age is only marginally higher (2%) at the same rpm. Not doubting the reference, but I don't think you can rely on a blanket rule like that. Sure the engine want last as long but it is not going to catastrophically fail even though it doesn't satisfy the requirements of that rule of thumb.

[Mooro]

Yeh thought that might be the case...if it was a little bit then I would consider it but 1mm radius is a fair chunk of metal, having said that though it would be interesting to compare big end sizes on the 7afe and CA rod given that the CA rods are from a high performance turbo motor...it might still be stronger in the big end even with the 1mm removed.

As far as I know the 7afe crank has 48mm journals so I guess the extra 2mm is to fit the bearing in.

Offset grind I guess is the way to go then, however I presume the machining costs are quite high for this sort of work?

I guess I would also have to look at knife edging the crank anyway if I want it to rev freely...Grrrr I can see crank machining costs being a killer!


Hmm so lets see, the extra 1mm stroke and rebore to 83mm (not sure if this is possible) give a capacity of 1872cc...hmmm sounds good!

[MRMOPARMAN]

grinding shouldnt be that expensive. i had my 4age crank ground a few weeks ago, cost me $180 with bearings.. you could do cheaper than that too.