The engine builders "black art" (AKA Rod ratio)

[Rusty67]

Project.65's thread about his 351w build got TOTALLY hi-jacked by this discussion (sorry dude) but there was a lot of good info and debate in there. I'm going to post quotes from that thread in here and hopefully we can get some great engine tech info into this thread.

Be prepaired for a long initial read, we have a debate raging already. (I'm not trying to start a fight, I think this is important info for all of us. Lets keep this tech related and civil please.)

 

[Rusty67]

Related stuff from the first page:

Rod ratio is sort of a 'black art' in our hobby...

What I know about it is:
Longer rods put less pressure on the cylinder walls, because they travel at less of an angle the longer the rod gets.

Longer rods spend more time at the top of the cylinder, which is said to allow it to absorb more of the combustion explosion to convert into downward force, when it starts going back down.

Longer rods allow less crank stroke.
More crank stroke makes FAR more power than more rod length, so stroke wins the arguement over building a long rod engine!

actually the rod ratio does have an effect on the dynamic compression ratio. remember that with a shorter rod, the piston accelerates and decelerates much quicker than an engine with a longer rod. the more consistent piston acceleration means lower pressure spikes in the combustion chamber, which in turn lowers the dcr. that said, you can also affect the dcr with cam timing as well. with the right cam you can run fairly high compression ratio's and still avoid detonation at low speeds and high loads(which is where detonation is most prevelant), even with 10-12 degrees initial timing.

remember that with any engine build you have to take a systems approach to make power, fuel economy, and drivability.

exactly, with a higher rod ratio the piston stays parked at TDC longer effectively boosting compression and promoting a longer burn and with the smaller ratio the piston doesn't stay at TDC very long at all effectively decreasing compression. personally, i'll be staying with a stock stroke 351w but a long rod version using 351m/400 rods and KB pistons, with an aluminum head you can run as much 11:1 compression on pump gas and get better mileage to boot, not to mention that the higher rod ratio will increase the engines longevity. there is a reason why most straight 6 engines run forever and it has everything to do with rod ratio

 

[Rusty67]

Related stuff from the second page:

I am confused about how extra time at tdc helps reduce detonation. It seems like since it moves slower at tdc, you will get less turbulence from the quench pads and have the mixture compressed for a longer period of time. Both of those should cause an engine to be more prone to detonate. There was more than a couple Engine Masters contestants noted these things when talking about thier engine combonations a few years ago.

with the piston parked at TDC longer it's not pulling air in as quickly so it promotes a more complete burn because the incoming air is not putting out the fire so to speak. it's all about flame front propagation.


also, if you have a piston dish that mirrors the combustion chambers you get even more benefit since the fuel/air combo is not getting pushed away from the flame front as much.

the best combo would be small chambers with dished piston that mirrors the combustion chambers, long rods, with the right lift/duration and LSA to match the static compression ratio and provide a good dynamic ratio and a head that with good flow numbers but small enough ports to keep the mixture velocity up. i like the valves to have a 30 degree back cut as well. also, i prefer smaller valves as well but that's just me.


we could go on and on about all this and start talking about things that nobody understands but as far as the rod ratio thing goes there are more advantages than there are disadvantages to running a longer rod from everything i can tell. longer engine life due to less side loading, better combustion, more torque and hp lower in the rpm range and still able to rev as well. they can also help in lowering emissions and increasing fuel economy as well.

that's what i've found from researching the long rod combo's for the 351w anyway.

Now I am really confused. At tdc the intake valve is on the seat. So yes it is not pulling in air as quickly. It is not pulling in air at all.

Dwell time has NO effect on dcr. A longer rod does lower the piston in the bore at a given intake closing. If the piston is in the same place at intake closing it will have the same dcr whether it is a long rod or short rod.

Long rods require more octane for a given dcr because there is more time for detonation to occur. When dwell time is decreased there is less time for a wild fire to start somewhere in the chamber other than at the sparkplug. A slow moving piston will decrease the airspeed with in the chamber as well.

There are advantages to longer rods, but most engine builders will tell you it is overhyped. Reducing detonation is not an advantage, it is a disadvantage.

sorry i meant to say that the piston stays at TDC longer but that's not necessarily saying that the cam and valve train are, i also didn't say that a long rod had any affect at all on DCR i was talking about the cam and the rest of the combo. also long rods do not require a higher octane fuel, in fact they require a lower octane fuel that is one of the advantages, you can run a higher compression ratio and use a lower octane fuel than the same engine with a shorter rod. i don't where you heard that a long rod makes an engine more prone to detonation but it's wrong, totally incorrect, an engine that is burning the fuel more efficiently is less likely to detonate. a long rod does not create a hotter combustion chamber it creates a more efficient one. also the long rod doesn't create a slower moving piston, it actually moves just as fast as a shorter rod because you're not changing the stroke, it just stays at the top and bottom of the stroke longer piston speed in the cylinder is still the same. it may be moving a tiny bit slower from full speed to a dead stop and from a dead stop to full speed but in reality that's where the main advantage comes from.

i'm not sure where you got your info that a long rod motor is prone to detonate from but it's flat out wrong. also, with piston staying parked at TDC longer the exhaust valve has more time to get the spent gases out actually make it even less prone to detonation than the same engine with a shorter stroke which is why they can run on less octane.


look at this way. in a regular 351w for instance, the piston goes down in it's bore and the intake valve opens and the piston starts drawing the mixture, on the way back up right before the piston reaches TDC the ignition lights off the mixture and the piston finishes compressing it and the exhaust valve opens and the piston starts back on it's way down. in the same 351w with a long rod when the piston reaches the top of the stroke it slows down a little more than the stock engine right at the top of the stroke and it stays at the top just a little bit longer and burns the mixture better and and then exhaust valve opens and can pull more of the spent mixture out before the piston starts going back down, this cools the mixture some, then the intake valve opens as the piston starts going back down the bore since it stayed at TDC a little longer this allows the cylinder to fill just a little more adding to the cooling effect started when the exhaust valve was able to pull a little more of the spent gases before the piston started back down.


now do you see why the long rod combo requires less octane, makes more power, produces less emissions and does NOT make the engine any more prone to detonation that it was before? if not, then i don't know a better way to explain it.

Do you read much on the tear down articles from the Engine Masters competition. They like shorter rods when limited on octane so that the the piston does not stay at the top of the bore. That is when detonation occurs. This is mentioned in some articles I have read on pump gas drags builds as well. People that have forgot more than both of us know.

Your valve timing events are off in the third paragraph. The intake valve opens before tdc and stays open after bdc. The exhaust valve does not open until the piston is over half way down in the bore and stays open until after tdc. If the valve events are correct for a stock rod, then there is no real advantage as far as cylinder filling goes. Todays heads are good enough to fill a cylinder with a short rod.

I am not too sure about long rods lowering emissions and having more efficient combustion either. These things are determined by the combustion space and spark timing.

 

[302 coupe]

some of it looks appealing on paper, but the fact is, it doesn't add up in real life, especially on a street engine. Rod ratio has negligible effect on wear, or torque, or fuel economy. OEM rod ratios are all over the place, and just about any OEM engine will last 150k, 200k, maybe more miles without any danger of popping a piston through the block due to some mythical side loading issue that exists on paper. Anybody can regurgitate stuff from a magazine article, an article which is paid for by someone selling "long rod" parts/kits....so take it for what it's worth. Work out the comression you need, stoke and bore, then let rod ratio fall where it may.

 

[bnickel]

again, i state that i was talking about a long rod stock stroke 351w not a long rod, long stroke stroker motor and in that application the rod is more than .500 longer than the stock rod, this combo using a 400 ford rod, so there are big advantages to this combo. in a longer stroke engine ther is not signifacant gain. most long rod 351 stroker motors use a 6.2 rod vs the stock 5.956 rod so, yeah, not much of a gain.

 

[Hack]

some of it looks appealing on paper, but the fact is, it doesn't add up in real life, especially on a street engine. Rod ratio has negligible effect on wear, or torque, or fuel economy. OEM rod ratios are all over the place, and just about any OEM engine will last 150k, 200k, maybe more miles without any danger of popping a piston through the block due to some mythical side loading issue that exists on paper. Anybody can regurgitate stuff from a magazine article, an article which is paid for by someone selling "long rod" parts/kits....so take it for what it's worth. Work out the comression you need, stoke and bore, then let rod ratio fall where it may.

Well said.

 

[ratio411]

I was all gung ho about rod ratio for a while...
However, after going through it in my head over and over for months, I came to the conclusion that no matter how 'optimum' your rod ratio is for power, it can't come close to designing in more displacement.

So I agree with the previous post about building your displacement for what works and rod ratio be dammed.

I get in on the discussion from time to time because I know there are benefits, but they are in opposition to stroking an engine... therefore stroking becomes the lesser of two evils when seeking streetable power.
If you could stroke an engine a great deal, AND plan in a generous rod ratio, that would be awesome. But we all know what that means: A taller block, more weight, fitment issues... so it is a losing battle.

 

[rbohm]

you cant pick one area to concentrate on when building an engine. you have to use a systems approach, and you have to match the parts to each other, and to your own theory on engine building. this is one of the things offenhauser found out in the 60's when long rod engines first came into vogue on the race track. they went to the long rods, but they lost power. why? because they did not rework the rest of the combination to suit the longer rods. head ports, cam timing, combustion chamber shape, header tube size, valve size, intake runner size, shape and length, piston dome shape, all have an effect on engine performance. sometimes the effect is small on the dyno, but huge on the track. sometimes it is the other way around. why is this? you now need to get into the transmisison gear ratios, and the rear gear ratios you are running, the type of racing you are doing, even the tires you are running.

 

[bnickel]

you cant pick one area to concentrate on when building an engine. you have to use a systems approach, and you have to match the parts to each other, and to your own theory on engine building. this is one of the things offenhauser found out in the 60's when long rod engines first came into vogue on the race track. they went to the long rods, but they lost power. why? because they did not rework the rest of the combination to suit the longer rods. head ports, cam timing, combustion chamber shape, header tube size, valve size, intake runner size, shape and length, piston dome shape, all have an effect on engine performance. sometimes the effect is small on the dyno, but huge on the track. sometimes it is the other way around. why is this? you now need to get into the transmisison gear ratios, and the rear gear ratios you are running, the type of racing you are doing, even the tires you are running.

exactly. that's what i was trying to say in the other thread. i'm all for someone doing a stroker motor but i'm not building a high hp engine and don't need a stroker. i want to stay with the stock cube motor just make it more efficient, i'm doing it more for mileage, emissions and the added benefit of a little more hp and torque under the curve. it will not have a radical cam more a mild hyd. roller, but it will have some nice aluminum heads, probably either E'brock, ford racing x303 or AFR 165's still undecided. it'll have cobra roller rockers, stock exhaust manifolds or shorty headers and my old c90x aluminum intake and a holley commander 950 pro-jection system and if the budget allows i'll do some coatings throughout the engine, probably the home applied variety but i'll send 'em out if i can afford it.

i'll use stock 400 rods(ill have them modded locally) with KB pistons and again if the budget allows i'll either use a Scat cast stock stroke crank or an eagle forged one. i wish i could find an aftermarket 400 rod but there appears to be no such critter. my plans have changed a little since i'd originally planned to use F4TE roller block because KB didn't have a piston for the shorter deck 69/70 block but they do now so i'll be able to use the original block from the 69 cougar that this motor will be going in. i'll just have to use a retro-fit cam or the crane link bar lifters, probably the link bar lifters so i can use any roller grind i want. the heads and lifters will be the most expensive part of the build. hopefully i'll be able to keep the standard bore on the 69 block too since it's a low mileage original, KB supposedly has these pistons for standard bore motors now too. that'll save me a bit more money too.

 

[ratio411]

If I was to build a long rod 351w, I would use the 400 rod.
Nice and long. Like 6.58" IIRC...
Built that engine in my head a few times.
I imagined all sorts of exotic parts, like titanium pins (floating of course!).

There is a few years where the engine block was a few thousandths taller.
Would help slightly with piston selection.

 

[ratio411]

i'll use stock 400 rods(ill have them modded locally)

What needs modded on 400 rods?
They should be bolt in...

Or are you refering to general modding, like bore reconditioning, beam polishing, ARP bolts, and the like?

Btw: You stole my thunder by posting the 400 rods while I was typing!

GMTA!

 

[bnickel]

What needs modded on 400 rods?
They should be bolt in...

Or are you refering to general modding, like bore reconditioning, beam polishing, ARP bolts, and the like?

Btw: You stole my thunder by posting the 400 rods while I was typing!

GMTA!

you have to narrow the big end by like .050" (.025 on each side) and remove the protrusion on the top. nothing major. KB is now making the long rod pistons for the shorter deck too, which i like. BTW, what does GMTA mean?

 

[ratio411]

what does GMTA mean?

Nothing really.
I just made it up at that moment.
Great minds think alike...

 

[ratio411]

you have to narrow the big end by like .050" (.025 on each side) and remove the protrusion on the top. nothing major.

This surprises me.
I am not saying it isn't so, but back in the day, the 400 crank was machined slightly and used as a stroker crank in the 351w block, often with W rods.
Seems like the rods would not fit the crank if the stock rods were that much wider.

I knew about the big chunk of metal on top of the rod, but had forgotten about that.

 

[mikethebike]

some of it looks appealing on paper, but the fact is, it doesn't add up in real life, especially on a street engine. Rod ratio has negligible effect on wear, or torque, or fuel economy. OEM rod ratios are all over the place, and just about any OEM engine will last 150k, 200k, maybe more miles without any danger of popping a piston through the block due to some mythical side loading issue that exists on paper. Anybody can regurgitate stuff from a magazine article, an article which is paid for by someone selling "long rod" parts/kits....so take it for what it's worth. Work out the comression you need, stoke and bore, then let rod ratio fall where it may.

Must disagree with you here..from my personal experience with a BOSS-302 that used a beefed-up hi-po 289 rod at 5.15 and it broke 7 pistons at 7500 rpm but my 271/289 with a 1/8" shorter stroke never broke the first one and my 302 Z/28 with a 1.9-1 ratio would rev 8500 every day of its life and never broke a piston or scored a cylinder wall says otherwise.
And those engines you talk about are rev limited either by electronics or should they be old enough to not have those the hydraulic cams stopped them around 6000 rpm.
All the engine builders I spoke with back then agreed the short rod in the Ford limited the max rpm of the engine. To quote Dan Perrin:

"10,000 rpm heads, 10,000 rpm crank, 6500 rpm rods, 6500 rpm cam.....sounds like a good combination to me....if you want to come in 2nd place."

 

[bnickel]

Nothing really.
I just made it up at that moment.
Great minds think alike...

ok, yeah we do don't we.

well if the 400 crank in the windsor used 351w rods then it would probably work just fine since the 400 crank is slightly wider than the 351 crank, you could run a larger fillet on the rod journals or even just using it as is they would probably work just fine just with a little larger rod side clearance.

speedomotive modifies the rods this way and i found the exact amount on a post somewhere on the mustangs and more site. when i get ready to do this i'll mock up the rods unmodified and check the clearances to see if it's really that much or not. better safe than sorry, i'd rather check them unmodified than realize after they've been modded that they shouldn't have been modified in the first place.

i did find a place called wheeler racing (i think) that sells a stage II 400 rod that is shot peened with polished beams and ARP already installed.

 

[Helmantel]

To all of you who make all kinds of claims about how the rod lenght affects the combustion process, have you ever calculated the differences in piston position and combustion chamber volume, like blkfrd (and me) did? I think you will be surprised.

 

[jasonn]

If you are building an all out, no expense, high rpm, race motor?? Then I would definatley look into it. But otherwise??

some of it looks appealing on paper, but the fact is, it doesn't add up in real life, especially on a street engine. Rod ratio has negligible effect on wear, or torque, or fuel economy. OEM rod ratios are all over the place, and just about any OEM engine will last 150k, 200k, maybe more miles without any danger of popping a piston through the block due to some mythical side loading issue that exists on paper. Anybody can regurgitate stuff from a magazine article, an article which is paid for by someone selling "long rod" parts/kits....so take it for what it's worth. Work out the comression you need, stoke and bore, then let rod ratio fall where it may.

 

[bnickel]

so some of you guys aren't believers in the long rod theory, i am. i'm going to build my stock stroke long rod 351w and do it my way, if i'm wrong it's easy enough to switch back to parts.

i am however going to take the best systems approach i can and try to match every part to the next as closely as possible. i want to end up with around 10-10.25:1 compression, smallish hydraulic roller cam with 93 cobra 1.7 rockers, 165-170cc almuminum heads with 64cc chambers to meet my compression goals with my piston choice, or 60cc heads and have a small dish milled into the heads to match the chamber shape of the head. if budget allows i'll do some coatings on the piston skirts and tops, combustion chamber faces, crankshaft journals and intake and exhaust coatings. if the budget allows i'll get a stock stroke forged crank as well. the idea for me is to make the engine as efficient as possible on my limited budget,with no really exotic or hard to come by parts to keep it simple.

that's my plan and i've been planning it and thinking about it for a few years now, hopefully before next summer i'll be able to start on it. i figure if i shop around and by as many good used parts as possible i can build this motor for under $2500.

 

[brianj5600]

If you stick an off the shelf cam in with a long rod i don't think you will see any gain at all, maybe loose some.