Rod Ratio theory

[float_6969]

Alright, I'm bored and tired of reading the same ole' crap in here and there is nothing I love more than a good technical discussion.

If you don't know much about rod ratio's, do some research first, of if you'd like I can explain.

For whatever reason, Nissan likes to use really low rod ratio's in it's 4 cylinders. I've beeing thinking about the effects of a CA20DET conversion and how it will effect the engine's characteristics. Using a CA20 crank in the CA18 block gets the RR down to 1.51, which is relatively low.

What if instead, somebody took the CA18DET crank and put it in the CA20 block. That would drop you down to about a 1.9L and put your RR at 1.79. Also, if you swapped the 8 port head over, you would really make good use of that high RR. I seriously think that someone might be able to pull off 10-12K RPM off of that setup. Slap a big turbo on there and break the record CA HP level outside of Japan (currently around 650HP).

What I'm going to do for my daily/weekend warrior, is probably use the 4 port head w/a CPC manifold to help out the low RR on the CA20 conversion.

Both would be pretty darn good motors, and LOTS of fun.

Waddya think?

[ca18datsun510]

im not to familiar with ca20's, but dont they get there extra displacement fromt he stroke, and efectively have the same bore?

so putting a 18 crank in a 20 block would have the same ratio as a 18 crank in 18 block....

[ca18datsun510]

do the 18 and 20 have the same deck height?

[tyrannix]

im not to familiar with ca20's, but dont they get there extra displacement fromt he stroke, and efectively have the same bore?

so putting a 18 crank in a 20 block would have the same ratio as a 18 crank in 18 block....

do the 18 and 20 have the same deck height?

kinda and no

ca18det = 83/83.5

ca20 = 84.5/89

and a ca20 block is 20mm taller than a ca18 block

but I am interested. I've read that de-stroking is a neat trick used by pro racing developers to increase rev capacity and other fun goodness (check out racing 7Ms with shorter cranks)

and i just happen to have an extra crank or 2 sitting in my garage ... to go with the ca20 bottom end thats at the machine shop now? maybe

only problem is that swapping cranks with different strokes = custom rods with different lengths - and my tomei pistons are 84mm, not 84.5

But, if somone else doesnt do it first and document it, as i get time, i will do the expiramentation on different numbers to get the documentetion

I want to just get a ca20det working first, then i can work on other variants (in the ae86 if i dont have to sell it before i move.)

CJ

[iliketocrash]

this sounds like a big money project though. not for the feint of heart. with revs going into the 10-12k area you significantly increase stress on the internal moving parts and the motor then becomes strictly race and parts will have to be replaced regularly. i know that most people know this but a lot don't. i'm only throwing it out there cause every now and again we get someone new to the forums going "OMG!! i want a 12k redline!" and they don't realize the money and maintanance involved in such a beast.

[boost_boy]

I agree with you in the fact that the forum does have moments of boredom, but then again, part of our responsibilities is to try and lead the blind and lost and sometimes hopeless to the light. Now as for the rod thing, I'll be happy to share with you the specifications of the stroker kit I buy, especiall the rod ratio. I've gone through too much over the past 10years to be experimenting with mutt stroker components, so I decided to open-up my wallet and let flow go "You know" .

Dee

[davidricardo86]

Aftermarket Rod Choices

When it comes to aftermarket rods, you've got two decisions: length and weight. The brand question is an issue we'll side step in this section, choosing to concentrate on the technology instead. Rod length changes the piston speed curve, and the piston dwell times at TDC and BDC; these characteristics can be exploited by savvy engine builders to tune the engine's power curve and even its emissions. The shorter the rod, the more it dwells at the top and the bottom of the stroke, allowing the option of more bottom timing. Therefore, you can have a cam grind and timing to close the intake valve very late, allowing it more time to fill the cylinder. The disadvantage occurs on the power stroke, and, depending on rod ratio, after about 30 degree crank angle from TDC the piston begins to travel faster than the flame front, so you lose the force on the piston. The shorter the rod the quicker this occurs, so a longer rod increases the duration of the power stroke. The short rod on the other hand, speeds the piston up quicker so you'll have less concern about detonation and it's really moving the air column quicker so you should have better throttle response. However, you will still lose torque with a short rod because the force in the combustion chamber can act on the piston for less time and there is less crank angle.

As AEM's creater john Concialdi observed. "The very baddest, nastiest Honda engine is the 170-horsepower Del Sol Vtec B16A2 engine and specific power-wise it was the highest of any production motor, untill Ferrari came out with their new 355 engine. It makes the same power as the B18C1 Integra GSR motor but it's 200-cc smaller. And look at the rod ratios! The B18C1 has a 1.58:1 rod ratio and the B16A2 engine has a 1.74:1 rod ratio. They both have 81-mm bores, it's just the B16A2 has a 77-mm stroke." Just something to think about as you plan your engine build.

If you're interested in making horsepower, the rod length and rod-to-stroke ratio ussue is clear. The most powerful engine combinations use a long rod-to-crank stroke ratio. The weight issue is also an important tuning factor because light is not always right. In motorsports venues in which acceleration is important, such as drag racing or road racing on track with short straights, a lightweight rod works best. For longer races or fore tracks with long straights during which the engine revs higher for longer periods, a stronger, heavier rod is you choice. Your car accelerates more quickly with a lightweight rotating assembly. The connecting rods, as you should expect, are a large component of that weight.

If you really want to spend money, you can drastically reduce the weight of the spinning assembly by using titanium rods or aluminum rods. Titanium isnt any stronger than steel rods, says Rob Smith of RPS, but it is lighter. Titanium rods do cause problems though, because titanium's metallurgy is aggressive- it likes to grab on to other metals. Unless the connecting rod's big end bearing is designed to cover the sides of the rod, or the bearing area is specially treated, using titanium rods will gall the crankshaft. True, they are lighter and will free up some power during acceleration, but they are expensive and tricky to use.

The use of aluminum rods for drag racing was pioneered on big domestic V8 engines. Until recently, if you built aluminum rods strong enough to handle the power, they were so big they would'nt fit in the confines of a Honda engine. That is no longer true. PAC Motorsports, out of Colorado, offers an aluminum rod in their stroker kits. The rod is from GP racing, which developed an alloy that provided the strength-to-size required in a rod compatible with the Honda engine. Just keep in mind that aluminum rods aren't that much lighter than steel rods, and they have a far shorter service life. Still if you're looking to shave a few tenths of a second off you ET, an aluminum rod could be a solution. For most enthusiats though, just buy a good steel rod in a weight that makes sense for your application.

High Performance Honda Builder's Handbook Vol 1 - How to build and tune high performance Honda Cars and EnginesSA Design (www.cartechbooks.com)Joe Pettitt

Inside Stuff

A commonly known but often forgotten fact is that the internal geometry of an engine can affect the engine's power delivery. Bore size, stroke and rod length all have a profound effect on how an engine behaves in its power delivery characteristics.

Rod Length Ratio

An area of tuning that is just now being used by Honda engine builders is the altering of the stroke-to-rod lenght ratio. The bigger the stroke-to-rod length ratio the more dwell time the piston has around TDC. This accomplishes several things. Since the piston is near TDC longer, the combustion event has a longer time to impinge upon the piston, allowing better transfer of force to the piston, slightly improving the engine's thermal efficiency. The longer the dwell time also give more time to fill the cylinders during the intake stroke and more time to scavenge the cylinder during overlap, improving volumetric efficiency. With a short stroke and a long rod, the point where the crank pin to rod angle reaches 90 degrees, otherwise known as the point of highest piston acceleration, is further down the bore. Thus the piston accelerates more gently away from TDC. Since the piston is accelerated more gradually away from TDC, there is less mechanical stress on the crank, rods, pistons, and cylinder walls. Reduced rod angularity at the point of highest cylinder pressure (about 30 degrees after TDC on the power stroke for most engines) also reduces mechanical stress as the piston digs into the bore underside load less.

Higher rod ratios hace less velocity in the intake ports and there is a lower demand for the ports to flow as well because there is more time available to fill and sacavenge the cylinder. Conversely this can also mean stagnant flow at low-rpm power production. You cannot get everything for nothing!

To increase the stroke-to-rod length ratio, some Homda tuners are running a longer connnecting rod. Moving the piston up higher into the piston allows this. Some engines builders are even running deck plates to raise the engine's deck so a longer rod can be run. Some Honda motors have stroke-to-rod length ratios as low as the H22's 1.49:1, 1.7:1 or better. The high-revving B16A is considered good. The most highly developed 4-stroke engines in the world, such as Formula One and motorcycle engines, often have stroke-to-rod length ratios of over 2:1, and many tuners of production engines are attempting to emulate this.

Stroke-to-rod length ratio can also affect how the engine responds to changes in the cam profile. With a longer rod, since the piston travels away from TDC slower, there is more time to fill the cylinder with fresh fuel and air. Because of this, the camshaft's overlap period can be reduced and more cylinder pressure can be built at lower rpms due to the reduced blowdown and reversion resulting from less overlap.

Bore-to-Stroke Ratio

The bore-to-stroke ratio of an engine can also affect the engine's power characteristics. Oversquare engines, ones that have a bigger bore than stroke, have lower piston speeds and less internal stress at high rpm due to lower intertial loads. There is also more time to fill the cylinders because of the lower piston speed. Longer strokes with smaller bored engines, called undersquares, have more internal stress due to faster piston acceleration, higher piston speeds, which accelerates wear and can induce seal-killing ring flutter. Undersquare engines have higher, torque and low-end power producing intake port velocities to ensure more complete cylinder filling at low rpm. Honda automotive engines are undersquare.

Engine designers can get around things like engines being undersquare with short rods by designing around these things. Despite being undersquare with a low rod ratio, most Honda engines can still rev to the moon because their port configuration gives good airflow, even at high port air velocities, and the huge duration and lift of their high-speed VTEC cam lobes ensures good breathing at high rpm.

With a good bore-to-stroke ratio, stroke-to-rod length ratio and a modern, free flowing head, there is less need for a long overlap period as the piston has more dwell time in crankshaft degrees around TDC. Although the difference is literally milliseconds, this is a considerable amount difference. Engines with shallower valve-induced angles, like Hondas also tend to have more crossflow on overlap and also can breath well without long overlap periods. In general, the head's flow characteristics and the engine's bottom-end configuration have a lot to do with what kind of cam specs the engine will ultimately like.

Proof of this theory lies in practice. High performance motorcycles (with a few notable exceptions like the torquey Supertwin racing class bikes), and purpose-built racing engines, like the ones found in Indy and F-1, are oversquare for cylinder filling at hgih rpm and reducing piston speed reasons. These motors are usually very oversquare, have extremely free flowing heads and have big stroke-to-rod length ratios in the 2:1 or higher range. These motors run very little cam overlap, and sometimes, as in the case of Honda inline 4s found in the CBR series bikes, zero overlap! A less accessible, higly developed, 4-stroke are the ones found in Formula One cars whose cams have surprisingly little overlap considering the duration necessary to ensure breathing at 17,000rpm.

An engine may be limited in power production by a restrictive head design or, by rpm limits due to the weakness of its parts, or by the poor sealing ability of the rings due to flutter because of piston speed. To compensate, a tuner can destroke the motor with a shorter stroke crank, use longer rods and sleeve the block for a bigger bore to help with high rpm breathing and durability.

I feel that to further exploit their potential, Honda automotive engine tuners should look toward high tech ring packages that allow the piston pin to be placed into the oil ring groove area and even add deck height so longer rods can be run. Longer rods and a more oversquare bore and stroke combination work in motorcycle and unlimited racing engines. These ideas can probably be carred over to modifying production based Honda engines in the future.

Honda/Acura Engine Performance How to Modify D, B, and H Series Honda/Acura Engines for Street and drag racing PerformanceMike KojimaHP Books

[float_6969]

In stock form the CA18 is a 1.59 rod ratio, which is on the low side of the streetable range, at least from what I can gather. Most of what is discussed online is in regards to V8's and it seems to me that Japaneese 4's tend to run RR much lower than the V8 guys.

The accepted RR for V8's is 1.6-1.8 for a street motor. I have found that F1 cars run RR's in the 2-3 range. From what I can gather from various common Japanese 4's they run RR's in the 1.5-1.7 range.

My main concern regarding a stroked CA is the 1.51 rod ratio that occurs from increasing the stroke and not being able to adjust the rod length accordingly. It will help with the low end grunt and increase the width of the power band, but it will loose some of it's "revvyness" and peak RPM will drop.

On the opposite end of the spectrum would be a de-stroked CA20. It would have a VERY nice 1.79 RR. But to get it to make good power, you'll need to spin it pretty fast and the power band will be pretty peaky. My concern regarding this engine is the amount of time the piston will spend in the TDC area. Although it's volumetric efficiency would be greatly increased, the amount of time that it spends at TDC will also increase it's tendency to detonate. This along with the rev's needed would make practically unstreetable. At least on paper.

It would be tempting to build one for no other reason but to see how it behaved.

[r34 gtr]

i will volunteer to build it if someone else pays for it. give me enough cash and that baby will be running a RR of 10!

[float_6969]

LOL, you'd need an 836mm rod to pull that off. That would make for a VERY tall block. I hope you're not intending on fitting that under the hood of an S13.

[r34 gtr]

maybe if i cut the hood? give me a break, im drunk and its difficult enough to match your post-whoring skills as it is. seriously, i signed on and just saw your name the whole way down.

[good morning]

Two things:

1) Do not base everything off RR. It is a false sense.

2) Getting good cams to make good power over 9k is hard.

Only use RPMs as a way to increase power if you are limited by displacement.

[USsil80]

i love to learn.. and have a couple good books so far, but can someone reccommend a good engine build/design book that i could read... i love thread like this .. thanks

[r34 gtr]

and i would imagine that with the large amount of lift generated by the valves needed, you would need some massive valve reliefs in the pistons because of the stroke and whatnot. who knows, im pretty deep into this bottle....

[good morning]

and i would imagine that with the large amount of lift generated by the valves needed, you would need some massive valve reliefs in the pistons because of the stroke and whatnot. who knows, im pretty deep into this bottle....

I do not have the specs of the motor in question, but I will say that amount of lift and duration will be the least of your worries. The by-products they cause will be of importance. The valve spring will be a issue if it is not speced out and can not control the RPM.

[r34 gtr]

true, true. you would probably need some triple-spring action.

oh, and welcome to nico....and seeing as it is 1225, good morning!

[good morning]

true, true. you would probably need some triple-spring action.

Haha, pho sho, pho sho

]oh, and welcome to nico....and seeing as it is 1225, good morning!

Thank you, it is a bit later than that here

[float_6969]

Oh, I know RR isn't the only aspect of engine design that influences it's behavior.

As I previously stated, this "destroked CA20" wouldn't necessarily be a very good street motor. From what I can gather, a 1.8 RR on a street driven 4 cylinder is pretty high.

For comparison, the Nissan QR25 runs a 1.43 RR, which is VERY low and is one of the limiting factors of that motors ability to rev past 6000 rpm in stock form.

A stroked CA18DE(T) (CA20 crank in a CA18 block with new pistons) is at 1.51. From what I can gather, that isn't actually that low of a RR for a Japaneese I4.

My concern is how much am I going to loose in my top end from dropping from the CA18DE(T)'s 1.59 RR down to the 1.51 RR?

[good morning]

My concern is how much am I going to loose in my top end from dropping from the CA18DE(T)'s 1.59 RR down to the 1.51 RR?

Not much, if the engine is set-up right. Get a good flowing head and cams, and build your manifolds around that. That difference in RR is nothing.

A well pieced together set-up will make a lot of power.

[r34 gtr]

just use my method of engine building. put the damn thing together and give it hell. i figure if i built it, it should take all the abuse i can give it.

[float_6969]

Not much, if the engine is set-up right. Get a good flowing head and cams, and build your manifolds around that. That difference in RR is nothing.

A well pieced together set-up will make a lot of power.

From what I've gathered a lower RR motor makes good power with large port and plenum volume, and an emphasis on cam lift as opposed to duration. This is because of the small amount of time that it spends at TDC.

My thinking is to use the UKDM 4 port head and large volume plenum with short runners. Probably the one from Custom Plenum Creations.

My ultimate goal is to still have a streetable motor, but I want a good powerband and I am hoping to gain some of the bottom end back that the CA lacks, but at the same time, I don't want to give up too much in the top end as this is still a small motor and will be making power in the 7K RPM range.

[r34 gtr]

doubt you need to go as far as de-stroking a ca20 for those goals. make sure its a monster though, will ya? maybe a little gt40 powa, with some c16 pwnage. some stupid lift/duration tomei procams wouldnt hurt either. 10.35mm lift, 292 duration ftw!

[float_6969]

No, the destroked CA20 would be a totally different motor. It would have lots of duration, little overlap, and long intake ports. It would also have to be built to take some revs to make the best use of the long RR.

[dash]

I normally stay away from these sorta deep technical discussions and stick only to proven recipees, as without solid feedback from those experienced, you're really 'flying blind'. A hi-rev ca18 may wind up an expensive proposition. Plus I wouldn't label a ca18 particularly 'durable' even with our 8k redline, let alone 10-11k.When would one need a "powerband" more than ~4000rpm wide ? I can't imagine any circumstanc where 8500rpm redline won't suffice.I'd think, simply sizing a turbo to your needs + learning quality tuning would help surpass any goals.

After seeing a few 10-11000rpm stock bore/crank toyota 4AG buildups, I'd try one just for 5hits & giggles, if I ran out of projects.As a comparison; Hi-rev(10k+) fwd 20v 4AG managed a 10.5sec ETA heavier 'street' mr2 ET'd 10.6 with a 'traditional' 16v 4AG build.I can only guess what a rush 10k+ shifts would be tho.For street/strip, I really don't see any advantage of a hi-rev motor.My buick GN with its lazy 5500rpm redline, taught me you don't need revs to go ballistic... mind-boggling acceleration.

[good morning]

From what I've gathered a lower RR motor makes good power with large port and plenum volume, and an emphasis on cam lift as opposed to duration. This is because of the small amount of time that it spends at TDC.

My thinking is to use the UKDM 4 port head and large volume plenum with short runners. Probably the one from Custom Plenum Creations.

My ultimate goal is to still have a streetable motor, but I want a good powerband and I am hoping to gain some of the bottom end back that the CA lacks, but at the same time, I don't want to give up too much in the top end as this is still a small motor and will be making power in the 7K RPM range.

It spends less time at TDC, depending on the build.

I have never flowed a 4 part head, but I am guessing it is not the best flowing head in the world. I think you are going to hit a road block with it.

I think you should do a Ca20 bottom end first then build it.

[float_6969]

I normally stay away from these sorta deep technical discussions and stick only to proven recipees, as without solid feedback from those experienced, you're really 'flying blind'. A hi-rev ca18 may wind up an expensive proposition. Plus I wouldn't label a ca18 particularly 'durable' even with our 8k redline, let alone 10-11k.When would one need a "powerband" more than ~4000rpm wide ? I can't imagine any circumstanc where 8500rpm redline won't suffice.I'd think, simply sizing a turbo to your needs + learning quality tuning would help surpass any goals.

After seeing a few 10-11000rpm stock bore/crank toyota 4AG buildups, I'd try one just for 5hits & giggles, if I ran out of projects.As a comparison; Hi-rev(10k+) fwd 20v 4AG managed a 10.5sec ETA heavier 'street' mr2 ET'd 10.6 with a 'traditional' 16v 4AG build.I can only guess what a rush 10k+ shifts would be tho.For street/strip, I really don't see any advantage of a hi-rev motor.My buick GN with its lazy 5500rpm redline, taught me you don't need revs to go ballistic... mind-boggling acceleration.

Well that's part of it. I don't really KNOW that it's not going to make power up until that high. I just think it would be fun to build and see how it performed. I SHOULD have a much higher volumetric efficiency, but who knows? Regardless, I wouldn't build a motor like that unless it was just for fun.

It spends less time at TDC, depending on the build.

I have never flowed a 4 part head, but I am guessing it is not the best flowing head in the world. I think you are going to hit a road block with it.

I think you should do a Ca20 bottom end first then build it.

I just got my 4 port head a couple of days ago. I'm still trying to get a beat up 8 port head so that I can flow both of them and see how they compare. From what I can tell, the UK guys seem to think they flow fine on the intake side, but are restrictive on the exhaust side.

I plan on putting the CA20 crank in the CA18 block first....

[r34 gtr]

if it really does rev to 10, i call shotgun first ride. 8k is scary as hell when you are still making 18lbs of boost, feels like everything wants to let go! i cant imagine what 5 digits feels like - boosted that is. my firend's prelude revs to 11, and its just wrong in so many ways.

[Nunook]

I think it would nice to see the results. And you do make a point about the rod lengths.

Id say 1st-> get a ca20det running.2nd -> Find someone that makes the length rods that u will need, and make sure the sides dont touch anywhere on the block.3rd -> tune the fuel/spark with the stock cams4th -> get custom cams for that set up.

I wish I was rich.

[r34 gtr]

i wish i was rich too. then i wouldnt have to whore myself out to every store manager in north alabama because i need a job so damn bad. plus i could do things to my car.