View Full Version : relationship between rod ratio and port volume


ILLINTENT
06-25-2007, 02:53 PM
Hello all, curious to hear some input from the elite engine theorists that frequent this forum-

I'm building a 360" turbocharged engine. The stock 6.0L cranshaft even with a stock bore has too much stroke and would exceed the 360 cube limit. This means either a custom crank or offset grinding to keep the displacement below 360 inches.

In adition, I am limited to a 220cc intake runner and .550" maximum valve lift. (duration is unrestricted) My thinking is that 3.5" stroke, 6.2" rod and 4.040 bore would be the best combination.

In this type limited scenario- would the positive manifold pressure take advantage of the longer piston TDC dwell time of the shorter stroke/longer rod and small ports? Or would the limited valve lift change this theory? I have been desperately searching for someone who really knows from expierience, regretably I don't have the budget to build multiple shortblocks for testing.

Old SStroker
06-25-2007, 07:57 PM
Hello all, curious to hear some input from the elite engine theorists that frequent this forum-

I'm building a 360" turbocharged engine. The stock 6.0L cranshaft even with a stock bore has too much stroke and would exceed the 360 cube limit. This means either a custom crank or offset grinding to keep the displacement below 360 inches.

In adition, I am limited to a 220cc intake runner and .550" maximum valve lift. (duration is unrestricted) My thinking is that 3.5" stroke, 6.2" rod and 4.040 bore would be the best combination.

In this type limited scenario- would the positive manifold pressure take advantage of the longer piston TDC dwell time of the shorter stroke/longer rod and small ports? Or would the limited valve lift change this theory? I have been desperately searching for someone who really knows from expierience, regretably I don't have the budget to build multiple shortblocks for testing.

"elite engine theorists"? I don't qualify but here are some thoughts:

If you are talking about a 6.0L aluminum block LS2 engine, you'll need to resleeve, so I think you mean an iron block engine.

I'm confused. Why would you want more piston dwell in an FI engine just as the intake is opening (TDC)? There wouldn't be much volume for the air to occupy. As long as you are forcing in a denser mixture why do you want more dwell time@ TDC during the burn? Won't that make any detonation problem worse?

With today's available cam profiles, a "lift rule" isn't much of a problem. You can use aggressive profiles and dwell the valve full open to get the required area under the lift curve to satisfy the engine's desires. Valvetrain parts are available to allow that and quite high revs if you need to go there.

With a 220 cc port limit, a very good head porter can make LS heads do what you need. There are a few around.

With a cleanup bore (iron block) to 4.010, offset grinding to a 3.55 stroke should allow use of the stock crank, unless you are planning on more hp than it can stand, which is considerable.

A good engine cycle analyzer (simulator) should be able to evaluate all the possible shortblock combinations and their effects on power, detonation, etc. With the correct valve events for each bore/stroke combination you may be surprised how close power will be from one shortblock to the next within the small range you are discussing.

BTW, what is the purpose for this 360?

Formulated
06-26-2007, 12:15 AM
Don't get worked up about port volume, it's all relative. Rather, consider the minimum cross sectional area of the port. A ratio of bore area to port minimum area multiplied by mean piston speed will give you a good simple metric of port speed. Based on this, you can select a port for the engine speed you intend to target for peak power. As for what that psudo port speed should be... you will have to figure it out. In the context of naturally aspirated engines I've always heard that 100m/s is a good target.

ILLINTENT
06-26-2007, 11:25 AM
Don't get worked up about port volume, it's all relative. Rather, consider the minimum cross sectional area of the port. A ratio of bore area to port minimum area multiplied by mean piston speed will give you a good simple metric of port speed. Based on this, you can select a port for the engine speed you intend to target for peak power. As for what that psudo port speed should be... you will have to figure it out. In the context of naturally aspirated engines I've always heard that 100m/s is a good target.

I totally see what you are saying, and the first responder as well. I think the logic behind the port area/piston speed issue is more relative to charge velocity, which seems like it would be almost negated by positive manifold pressure, but on an NA combo, yes it would be critical.

Regarding the piston dwell time, I am still a little unclear as to what you mean. I understand that a "rectangular" lobe, so to speak would be Ideal, leaving the valve open at max valve lift as long as possible without getting rediculous with nominal duration- but I am limited there as well, the stock hydraulic roller lifters have to be used, and it is my understanding that that steep of a ramp will colapse the piston of a hydraulic lifter.

Thanks everybody, hopefully we can keep this discussion going and I can get a free education :)

P.S. this is for the 360"/Turbo combination to run in NMCA's S/R class. I am using an Iron 6.0 block, an aluminum block is not legal for my car.

TATER
06-26-2007, 11:40 AM
Sounds like NMCA Street Race class rules. As said above, you'll have to resleeve. That or pick up a 5.3 block, bore it to LS1 size, and keep your 3.622 crank. With a turbo, all the dwell, port volume, rod ratio, etc. pretty much goes out the window. By the way, an aluminum block IS legal for NMCA S/R as long as it's a factory casting and going in a car that HAD an aluminum block. (1998-2002 F Body or 1997 - Current Corvette)

1.15 BLOCK

BLOCK: Engine block is restricted to an accepted type, cast iron small block or big block with OEM design & factory bore spacing. OEM aluminum blocks are accepted for use only if exact vehicle (year and model) came OEM-equipped with aluminum block.

On an engine with small runner volume, just turn the boost up. :)

Note: I just re-read where you said an aluminum block is not legal FOR YOUR CAR. Sorry about that. Will a 4.8 crank fit the internal design of a 6.0? I've never tried it, but it might work?

Old SStroker
06-26-2007, 12:58 PM
I totally see what you are saying, and the first responder as well. I think the logic behind the port area/piston speed issue is more relative to charge velocity, which seems like it would be almost negated by positive manifold pressure, but on an NA combo, yes it would be critical.

Regarding the piston dwell time, I am still a little unclear as to what you mean. I understand that a "rectangular" lobe, so to speak would be Ideal, leaving the valve open at max valve lift as long as possible without getting rediculous with nominal duration- but I am limited there as well, the stock hydraulic roller lifters have to be used, and it is my understanding that that steep of a ramp will colapse the piston of a hydraulic lifter.

Thanks everybody, hopefully we can keep this discussion going and I can get a free education :)

P.S. this is for the 360"/Turbo combination to run in NMCA's S/R class. I am using an Iron 6.0 block, an aluminum block is not legal for my car.

Yes, with stock hydraulic lifters getting valve dwell will be tricky. The cam doesn't have to be all that aggressive. "Rectangular" lobes aren't necessarily desirable anyway for a number of reasons. Picture the lift curve the engine would like with unlimited lift, draw a line @ .550 and imagine a lobe that smoothly transitions from the opening flank to the dwell and back to the closing flank. Spring choice will be important.

What kind of rpm are you planning?

ILLINTENT
06-26-2007, 01:19 PM
Sounds like NMCA Street Race class rules. As said above, you'll have to resleeve. That or pick up a 5.3 block, bore it to LS1 size, and keep your 3.622 crank. With a turbo, all the dwell, port volume, rod ratio, etc. pretty much goes out the window. By the way, an aluminum block IS legal for NMCA S/R as long as it's a factory casting and going in a car that HAD an aluminum block. (1998-2002 F Body or 1997 - Current Corvette)

1.15 BLOCK

BLOCK: Engine block is restricted to an accepted type, cast iron small block or big block with OEM design & factory bore spacing. OEM aluminum blocks are accepted for use only if exact vehicle (year and model) came OEM-equipped with aluminum block.

On an engine with small runner volume, just turn the boost up. :)

Note: I just re-read where you said an aluminum block is not legal FOR YOUR CAR. Sorry about that. Will a 4.8 crank fit the internal design of a 6.0? I've never tried it, but it might work?

...Must have posted just before my last post, good eye. You are assuming it is going into a car that came with an aluminum block, it is not. What about going with a larger bore and running a 4.8 crank? It looks as though the main journals are different size though...

I am on board with the positive manifold pressure negating the piston speed issues, but keep in mind everyone is only alowed a 76mm turbo, and everyone will be running the cheater exhaust housing. (if they have any sense) That being the case, the guy with the most efficient combo is going to make the most power, considering everyone will theoreticaly have the same compressor; and same boost capability.

in other words, the boost will already be turned all the way up.

I am thinking in the neighborhood of 7K RPM, to be honest, I am unsure what this kind of combination will run to efficiently, I would think that the exhaust housing and heads and cam could all lower power peak RPM.

Old SStroker
06-26-2007, 04:06 PM
...Must have posted just before my last post, good eye. You are assuming it is going into a car that came with an aluminum block, it is not. What about going with a larger bore and running a 4.8 crank? It looks as though the main journals are different size though...

I am on board with the positive manifold pressure negating the piston speed issues, but keep in mind everyone is only alowed a 76mm turbo, and everyone will be running the cheater exhaust housing. (if they have any sense) That being the case, the guy with the most efficient combo is going to make the most power, considering everyone will theoreticaly have the same compressor; and same boost capability.

in other words, the boost will already be turned all the way up.

I am thinking in the neighborhood of 7K RPM, to be honest, I am unsure what this kind of combination will run to efficiently, I would think that the exhaust housing and heads and cam could all lower power peak RPM.

Where is your budget?

Can you use the LSX GM block? That would give you some bore options. You'd need to get to about 4.180 with the 83 mm 4.8 crank. Because you are using LS1 or LS6 heads, you don't need that much bore, IMO. Also, 7000 isn't a problem for the 3.55 in. stroke.

Here's a thought. Sleeve an iron 6.0 block and rebore it to 3.97 (effectively a .030 de-bore) with the stock 92 mm (3.622) crank. That's enough bore for modified LS6 heads to work well. Sleeving the iron block is MUCH less expensive that Darton sleeving the aluminum block.

Keep thinking like you have been. You are correct about treating the airflow in the engine efficiently. Think NA. Hiring the right person to do the heads, and getting the right pistons and valvetrain should make or break this type of engine. A ton of development work has been done on the LS6 heads with limited port volume. You just don't see it on the net, the street or the strip.

Good luck.

ILLINTENT
06-26-2007, 04:34 PM
Thanks so much for the reply. I love the idea of the 4.8 crank/4.180 bore, Combine that with an L92 head, and you would have a nice little 8,000 RPM screamer. :devil: ... to bad the L92 have way too big of an intake runner to be legal :cry:

Regretably, I do not have the budget for LSX, and I would certainly use an aftermarket head casting if I thought it the gains would be representative of the difference in cost. (when dealing with a small port situation)

The sleeve Idea may work, considering the block I have may not clean up with a reasonable overbore. If it does work out, for testing purposes I will probably just run the stock crank and 4.030 bore and take the weight penalty for too much displacement. It's not like the car will be competitive this season anyway, I just need to get it together and on the track to do some testing, then really iron it out for next year.

Asuming I take the cheap way out, is it the general consensus the the stock length rod would be the way to go? Would going with a longer Rod represent any other benifits in side load friction, wrist pin location, etc...


...or is there evidence that better rod ratio would be better from a combustion-efficiency standpoint that we just havn't heard yet?

Wnts2Go10O
06-26-2007, 05:55 PM
there was a big thread here about rod ratio. the consensus seemed to be that a longer rod gives you better reliability and thats about it.

oldstroker, how different are the sbc cranks from the ls cranks?

TATER
06-26-2007, 07:04 PM
Firing order is a big difference. :)

Wnts2Go10O
06-26-2007, 08:37 PM
Firing order is a big difference. :)
im talking physical pieces. like main to main distance, snout length, etc. if it were possible my idea wouldve prolly been done by now.

Old SStroker
06-26-2007, 09:46 PM
LS cranks are specific and different from SBC. The stock crank is good for a lot of horsepressure and inertia loading, but if you are looking for 800 or 900+ go aftermarket forged. There is some choice there. Use the rods you need for the loads. There are plenty to choose from.

When Cadillac went racing in the CTSV-R they convinced SCCA to let them run a 5.7 L LS using the 4.8 stroke and a 4.125 bore with LS7 (style) heads about 2 years before the LS7 appeared. The got something like 7900 rpm for the first race with the new car (Sebring). One car was on the pole, and the second car started from the back for some reason, but finished second by driving thru the entire field. Of course by the next race they got weight penalties and a reduction in rpm to 7100 or so. The GM race hydraulic lifters we buy now were used maybe for the first time in these cars. Now they are back to LS6 style heads and still win. I think Katech had a bunch to do with the CTSV-R engines. You might look at their site.

I can't stress how important it is to get the LS1/6 heads working efficiently with high-quality airflow if you want to be competitive. You made a good point that even turbo'd, everyone will have about the same turbo flow, so building the best NA engine will make the most power. If you treat boost like really dense air instead of a crutch for a poor airflow system internally in the engine, you will have the best chance of coming out on top.

To address a few other comments: firing order has nothing to do with the crank. A 2-plane (90 firing between cylinders) crank for a V8 can be setup with a number of cylinder firing orders. A single-plane (180 or "flat") crank is another story. Forget that.

Long rods give less side loading, but within the confines of your deck height, strokes and the need for a decent compression height (CH) on the piston, there is basically zilch one way or the other. IOW, determine the deck height, the stroke and the CH recommended by the piston manufacturer for the amount of boost you are going to run. Do the subtraction to get rod length. Buy the closest catalog rod length and adjust the CH fractionally as required before you buy pistons.

Find a good engine builder, listen to him and don't go for an internet "consensus" on how to design your engine. If you are serious about being competitive and serious about not wasting you money, the design is not a DIY project nor is the headwork. I don't do either, BTW, but I did stay at a Holiday Inn Express recently. http://ls1tech.com/forums/images/smilies/LS1Tech/gr_stretch.gif.

ILLINTENT, you are on the right track. Keep your eye on the ball, and forget the fru-fru. Spend your money where it counts: air in/air out efficiently.

ILLINTENT
06-27-2007, 09:01 AM
Thanks so much, very logical argument. I would love to have the best in the buisiness put my motor together, but regretably the budget just isn't there, especially going with the new LS platform this year. I will definitely outsource the head work and cam design, but the shortblock is going to have to be homegrown- this year anyway. Does anyone know of a reputable machine shop in the southeast that could prep this block? I have been burned by machine shops more times than I care to remember. There is no way to be competitive this season, especially coming in this late. I like to think of it more like real early testing for next year.

racer7088
07-01-2007, 09:54 AM
You can bore an iron 5.3 block (ultra cheap to free besides the boring cost) to OEM 5.7 size + .020 or so and have the stock crank offset and stroked to 3.720 and run a small journal chevy rod. You'd be at around 359 then. How big is the turbo you are limited to?

racer7088
07-01-2007, 10:00 AM
Rod length will have no impact on almost anything but piston design and weight. I would design the piston around the popular on-center LS1 rod lengths of 6.125 and 6.200 and run one of those. Even with the 6.200 long rod you could run a killer Turbo piston that would still be pretty light and have at least a top ring land of .300 with the better power adder rings. Just remember the ring selection is kind of sparse in that bore size.