Connecting rods?

newbtrying

Member
I have a 2007 LSJ that I am turbo-swapping, converting to E85, and dropping ZZP Stage 3 cams into. Also waiting on my kit to ship (#457). I had to re-sleeve the engine (waiting on that to be done currently) so I'm totally rebuilding. I'm installing 10.5:1 compression RaceTec pistons, headstuds, and main studs. While I'm here, I want to upgrade my connecting rods, but everything is currently sold out. I've been looking at K1 and Molnar rods with no luck in finding them. Does anyone have other suggestions that I could find in stock? Or a secret stash they'd be willing to sell?
 

newbtrying

Member
You considered these? Why 10.5:1 compression on a turbo engine? Lower compression and more turbo boost would give you more hp.
Yes, that is the same knowledge that I have... lower compression is better for boosted engines, but some guys that have build 100% more cobalt engines than myself, and race them, have convinced me that I should go with the 10.5:1 pistons. I guess I can always change them if it's not what I wanted ;)

The ZZP rods are definitely on the list of rods i'd use, but like all the others, they are sold out and won't be made for half a year. :( I was told by a guy at Molnar that their next run is 6 months out, and K1 was even further out. I'd prefer not to wait that long to even reassemble my engine considering my goblin kit is supposed to come in soon.
 

Desert Sasqwatch

Goblin Guru
I hear that NASA has a prototype set they are working on, made of unobtainium - element 121 - that are infinitely strong and weigh next to nothing! LOL!! :p:cool:
 

newbtrying

Member
Yes.... but they don't. It seems popular for websites to have "in stock" show up until you go to their website and it says they are on backorder, or are shipped by the manufacturer. AKA, not in stock. :(
 

Robinjo

Goblin Guru
Yes, that is the same knowledge that I have... lower compression is better for boosted engines, but some guys that have build 100% more cobalt engines than myself, and race them, have convinced me that I should go with the 10.5:1 pistons. I guess I can always change them if it's not what I wanted ;)

The ZZP rods are definitely on the list of rods i'd use, but like all the others, they are sold out and won't be made for half a year. :( I was told by a guy at Molnar that their next run is 6 months out, and K1 was even further out. I'd prefer not to wait that long to even reassemble my engine considering my goblin kit is supposed to come in soon.
Have you looked on the Cobalt/Ecotect Groups on the Books of Face? I see rods pop up in there a little bit. Of course, people want a premium for parts that are out of stock.....
 

Rttoys

Goblin Guru
Yes.... but they don't. It seems popular for websites to have "in stock" show up until you go to their website and it says they are on backorder, or are shipped by the manufacturer. AKA, not in stock. :(
Ugh. Ya, that is the norm now. “In stock”, buy, get email it’s backordered.
 

newbtrying

Member
Have you looked on the Cobalt/Ecotect Groups on the Books of Face? I see rods pop up in there a little bit. Of course, people want a premium for parts that are out of stock.....
Great suggestion. I'll go looking there. I'll pay a little extra for them if needed.
 

Tinkles

Well-Known Member
You considered these? Why 10.5:1 compression on a turbo engine? Lower compression and more turbo boost would give you more hp.
Yes, that is the same knowledge that I have... lower compression is better for boosted engines, but some guys that have build 100% more cobalt engines than myself, and race them, have convinced me that I should go with the 10.5:1 pistons. I guess I can always change them if it's not what I wanted ;)
Lower compression in engines allow for lower grade fuels to be used. Boosted or not. That's it. The power difference comes from the different amount of ignition timing and/or boost. Whichever one you decided to sacrifice to prevent detonation. With the proper high grade fuel that you will be able to run your desired boost level and the ignition timing that the engine wants. Depending on CR and the amount of boost, pump E85 could work or you might want to look at Ignite Red(E90) or something similar. It all comes down to fuel.

But even on low grade fuel the higher CR will give your engine better power when it is not in boost. Which IMO makes the car better in the "around the town" situations.

As for rods. CNC Motorsports lists K1 rods in stock, but I suggest calling 1st. I have never dealt with them. But I am in the same boat waiting for valvesprings to be restocked for a 4B11T I am rebuilding.
 
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Ross

Goblin Guru
Shorter connecting rods or short-stroked crankshaft will reduce the engine compression ratio, and allow for more combustion room in each cylinder. With that extra room, you can cram more air and fuel, by raising the total compression (turbo boost * engine compression ratio) to what ever your fuel will allow. Forced induction is essentially variable compression. The power does come from ignition timing & boost, but also the amount of fuel you can burn.
So more air & fuel in each cylinder = more hp.
Higher compression engines can't cram as much air in, so lower hp potential, for a given fuel.
 
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Tinkles

Well-Known Member
Close. I can see your line of thinking. I am having trouble putting this into the right words, so I guess read the links for the technical stuff. But short version is higher CR = more stored energy from compressing the air/fuel and the longer the burning air/fuel is expanding/pushing downward on the piston. Like playing with a spring. It increases engine efficiency which in turn will increase power output.

Motortrend

Even more detail



So more air & fuel in each cylinder = more hp
Higher compression engines can't cram as much air in, so lower hp potential, for a given fuel.
This sounds like you are mixing the compression ratio up with cylinder displacement. Since the air/fuel mixture is compressible, a 10 cubic foot area and a 50 cubic foot area can both hold 100 cubic feet of air/fuel.

Higher compression does create higher cylinder temps and pressures. Higher cylinder temps are not exactly a good thing and higher cylinder pressures will increase stress on the internals. But there are ways to combat cylinder temps and stronger parts that will handle the pressures.

I hope I didn't make that sound insulting, I didn't intend to be. Which is honestly odd for an internet forum.



Back to the connecting rod issue. Any thoughts of doing a 2.1L LSJ stroker? Parts might easier to get.
 

Ross

Goblin Guru
Love our odd internet forum... I'm learning, like most builders here, so no insults taken, just trying to understand your perspective and opinions, and trying to get it to align with my understanding of engine physics.

The Motortrend article is talking about a naturally aspirated engine. I agree that higher compression on a NA engine will make more HP.
It mentions: "So, looking at our rifle analogy, we've got the same barrel length and diameter, the same bullet (piston), the same charge (air and fuel), only we're starting the bullet farther down the barrel. The farther down the barrel the bullet starts, the less expanding force the gas can exert on the bullet. " Which is true for a NA engine... but my point is that a boosted engine can have more air and more fuel in the chamber.

The State of Speed article is mostly about NA engines, but at the end, finally mentions: "Careful tradeoffs have to be made with forced induction engines to balance out the effect of boost with reduced static compression in order to keep detonation at bay – the efficiency lost by running lower compression ratios is more than offset by the increased power offered by intake pressurization. " It is interesting that it mentions tradeoffs have to be made.... but then conceeds that it increased power.

That 2.1L stroker engine is what Justin Reed is now running in his Goblin.

This one guy (hoogin89) says you are right for a track car, but I am right for a drag car.
"Low comp high boost yields turbo lag and a weak low end but room for huge gains in the top. So, in short, track car would be more optimal in the high comp low boost range, more area under the curve power wise. Drag benefits from low comp high boost because you want the huge top end. This is crazy over simplified but it's the basic gist."

This guy (ItsAllSomething) expresses my opinion best: "Compression ratio helps with efficiency but won't make up for the sheer amount of power more air brings"

Ecotec LSJ: 1998cc (122 ci) Compression ratio: 9.5:1. So using math, it has a head volume of 52.6cc.
If I lower the compression ratio (say 7:1), we could have a head volume of 71.4cc.
That head volume is 35% bigger, and can hold 35% more air, and burn 35% more fuel, theoretically making 35% more power if you don't consider the extra exhaust resistance the turbo will have.
 
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Tinkles

Well-Known Member
In drag racing you see better ET improvements in the 1st half of the track. A good 60' can make or break a run. I won't say all because there are some bonkers people out there, but the majority of fast guys are running high CR and high boost. 11 or 12:1 CR, 40-50 psi from huge turbos, and methanol making 3000-4000hp on 500+ cubic inch engines running 5-6 second ETs.

I couldn't find the quoted post in that thread, but the quote sounds like they are limited by fuel octane are talking about making a specific amount of power. Low CR/high boost and high CR/low boost will both make 500hp and on the same octane fuel, but will deliver it differently. But that is a different point that I am trying to make.

With everything else the same, aside from CR. Same engine, same displacement, same turbo, same boost level, same amount of ignition timing, and the only difference is CR. The higher CR engine will make more power do to it's increased efficiency(ignoring fuel octane).

I'm picking a 2000cc 4-cylinder engine for the ease of the math. 2000cc divided by 4 cylinders equals 500cc per cylinder.
That 500cc number is how much air the cylinder can move in a single 360* rotation and where the potential power is.
That 500cc of air will both fit in a 50cc and 70cc area because it is compressible.
The 500cc/50cc setup will be under higher pressure and higher temperature than the 500cc/70cc.
The 500cc/70cc setup will be under lower pressure and lower temperature than the 500cc/50cc.

Going back to my spring example. The more you squeeze it, the more force(power) it springs back with. Say the spring is 500cm long and you compress it down to 50cm it will return to its 500cm length with more force than when you compress it to 70cm. Same thing with air. Higher pressures produce more power when pressure is released.

The increase in pressure and temperature is what forces compromises. Do you spend the money on more expensive higher octane fuels? Do you make less power by running less ignition timing or do you lower the boost(decrease the CFM of the engine)?
 

Ross

Goblin Guru
"Low CR/high boost and high CR/low boost will both make 500hp and on the same octane fuel, but will deliver it differently. But that is a different point that I am trying to make."

If your engine parts are limiting you to 500hp, and you don't mind the expense of running high octane fuels, then yes, high CR is better.
The power delivery is better for high CR/low boost. That is why hoonin89 says it is better for track. I didn't think about this when we started this thread. Heck, if you can get to 500hp naturally aspirated, then that would be even better power delivery. Ditch the turbo, run high CR and high octane.

If getting more air thru your engine is the limiting factor (not the engine parts), then low CR/high boost will get you more power, at the expense of delivering it at later RPM, as turbos will do.

Your example of squeezing 500cc into 50cc is only using stage 2 of the engine. The turbo being stage 1, and the cylinder's CR being stage 2. So in your example, yes you do get more power as long as you don't hit the limit of the fuel's octane. Why would you run an engine at a lower than optimum compression level for a given fuel?

My proposal to run low CR/high boost will bring the compression level back to optimum for the fuel. The fuel's compression limit can be reached with both engines, but the engine with 70cc head volume can have 40% more fuel and oxygen than the engine with 50cc head volume. You just have to run stage 1 with 40% more boost, and be able to intercool that extra air. That extra fuel/air will spring back on the piston 40% harder. Both engines running the same octane fuel, same ignition timing... one engine makes 500hp, the other makes ~700hp. Yes the power delivery is more top end RPM, which makes it more challenging to drive. Turbos will do that, especially as you get into higher boost levels.

High CR/low turbo boost engine is just a middle ground between a NA engine, and a high boost turbo engine. Maybe that is why top fuel runs high octane and a super charger. Nice even power delivery and high power. Ditch the turbo if you want better power delivery.
 
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Tinkles

Well-Known Member
Your example of squeezing 500cc into 50cc is only using stage 2 of the engine. The turbo being stage 1, and the cylinder's CR being stage 2.
Maybe it is the fact that it is close to 4am and I need to go to bed, but that is making 0 sense to me.

I have been going back and forth trying to find another way to explain it and all I had determined is that I am too tired. At this moment my best assumption is you are getting hung up on the cylinder volume and total volume of air that needs to be moved to make X amount of power. Which are different.

Turbo technology has come along way, so it is possible to have a very wide power band on a turbo engine.

I most will likely disappear again after this post. I got some remodeling on my house to do this week after work and am not exactly sure how long it will take.
 

Ross

Goblin Guru
Running an engine at 7:1 compression ratio on E85 leaves a lot of power potential unused. Which is what happens on a big turbo engine before the turbo spools up. A big turbo engine needs that room, because once it gets up to full pressure, it should be operating near the octane limit of the fuel. A small turbo engine doesn't need as much room, so you don't have to sacrifice as much at the low RPM, and can run a 10:1 CR engine. Leaving the CR at 7:1 does leave a lot of room for big power.

Disabling the bypass in your Harrop supercharger will give you smoother power delivery and make the most of the E85 fuel. Running the engine CR at 7:1 wouldn't be effective as the supercharger can't build enough pressure to utilize the extra headroom. A turbo can, but with the tradeoff of turbo lag. Top fuel superchargers (around 6.5:1 CR) also work, but frequent seal replacement is not practical for most people.

I am focused on the head volume in the combustion chamber, as it determines the amount of compressed air and fuel available for ignition by the spark plug. Whether it's stage 1 or stage 2 that brings the 70cc to optimal pressure doesn't matter, as long as the engine can achieve 70cc of pressurized air and fuel. This can deliver more power than 50cc. If the engine can't achieve 70cc of pressurized air, then a smaller combustion chamber would be more effective.

I hope my comments didn't discourage you. I enjoy learning from others and your build has inspired me. Although mine is currently stock, I am gathering parts for a performance build and hope to gain more knowledge on engine building soon.
 
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Desert Sasqwatch

Goblin Guru
Wow, this discussion has been enlightening for those of us who are not well versed with turbocharged engines. Both sides of it are definitely something that seem to have a more specific application than the other - track manners versus street manners. The GM philosophy for a stock LNF is directed towards street manners - low compression/high boost - for obvious reasons. On the other hand, a little higher compression ratio and a little lower boost would be more friendly to the breathing capability of a K04 (if not jumping up to a bigger turbo). Looks like finding the sweet spot between the compression and boost is where we all want to be, if not building a Goblin just for track time. Great info!
 

Tinkles

Well-Known Member
I feel like this conversation has derailed quite a bit.

Disabling the bypass valve on my Harrop would kill the drivablity and destroy my engine. It's job is to provide an outlet for the extra airflow when it isn't needed. If it is inoperable it will either not build any boost(which is the most common when the valve fails) or make all the boost it possibly can all the time.

Running the engine CR at 7:1 wouldn't be effective as the supercharger can't build enough pressure to utilize the extra headroom. A turbo can, but with the tradeoff of turbo lag. Top fuel superchargers (around 6.5:1 CR) also work, but frequent seal replacement is not practical for most people.
This makes me feel like you need to dive deeper into supercharging theory. It sounds like you do not believe that superchargers are not capable of producing the same psi of boost that turbos can. PSI is just a measurement of resistance. The CC or CFM is the important number. Efficiency differs too much between engine and turbos/superchargers to make psi a useful number.

I am getting a bit discouraged. Your understanding is 95% there, but I cannot think of another way to make it click that lower compression does not make more power. With boost and even with some wicked N/A setups when you are supercharging the cylinders(i.e. 1000cc of air into a 500cc cylinder) that extra 20cc means nothing in terms of volume. The 1000cc will fit in both. That 1000cc is were the potential power is. The only difference is that the smaller head volume will compress the 1000cc of air into a higher pressure that will push the piston back down with more force once ignited. It uses the 1000cc of air more efficiently and the increased efficiency is what creates more power.

The GM philosophy for a stock LNF is directed towards street manners - low compression/high boost - for obvious reasons. On the other hand, a little higher compression ratio and a little lower boost would be more friendly to the breathing capability of a K04 (if not jumping up to a bigger turbo). Looks like finding the sweet spot between the compression and boost is where we all want to be, if not building a Goblin just for track time. Great info!
Eh, it is more along the lines of making a reliable engine that can run safely on a range of different grade fuels. Some regions of the US do not have 93 octane and others have 94. Up north we have different fuel blends for winter and summer. You really need a source of fuel where the grade and quality is consistent in order to use an engine to it's full potential all the time.

My cousin and I got into the high/low compression debate awhile ago. I am a fan of efficiency. I want to make as much power as I can with the lowest boost pressure I can while still running my 2.6" pulley on my Harrop. He just wants to make 700hp and is perfectly fine having to run his turbo at a higher pressure.
 
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