B20 Vtec power.

**aaronng** · 18-10-2009 11:44 AM

Originally Posted by string

The 10% more powerful engine has a 10% larger load. The loads absorb energy such that both engines are at constant angular velocity. Read the example again.

Ahhh, I missed that 10% load increase that you posted earlier. My bad. Thanks for pointing that out to me again.

Back to the stress on the rod bolt, my understanding is that it is based on the acceleration of the piston/rod being pulled down by the crankshaft. F = m x a, so the additional acceleration is from the increased force which can be said to come from the engine's additional torque output. So if someone frequently enjoys full throttle short 1st gear acceleration, the rod bolts are subject to fatigue quicker than someone else with an ultra torquey engine but rarely uses very quick engine RPM increases. I guess you could say that a commo V8 doesn't snap rod bolts eventhough it has 3 times the torque of a b20vtec because the acceleration of the engine components is kept low by using higher gearbox ratios.

**string** · 18-10-2009 11:53 AM

Originally Posted by aaronng

Back to the stress on the rod bolt, my understanding is that it is based on the acceleration of the piston/rod being pulled down by the crankshaft. F = m x a, so the additional acceleration is from the increased force which can be said to come from the engine's additional torque output. So if someone frequently enjoys full throttle short 1st gear acceleration, the rod bolts are subject to fatigue quicker than someone else with an ultra torquey engine but rarely uses very quick engine RPM increases. I guess you could say that a commo V8 doesn't snap rod bolts eventhough it has 3 times the torque of a b20vtec because the acceleration of the engine components is kept low by using higher gearbox ratios.

If you were to blast quickly through 1st gear (full throttle, or you have a 10% more torquey engine for example), the rod bolt stresses and piston accelerations rise to the same levels, but more quickly. Their values remain identical with respect to the engine speed; engine speed however, now grows faster with respect to time.

The rotating assembly (including the load) is one connected unit. If your load offers 1000 times the resistance of pulling the piston down, it receives 1000 times the force. A V8 with 3 times the torque offers it to the load - if this triple torque happens to accelerate the load quickly, then the rod bolts motion is "accelerated" - but the rod bolts/rods/pistons are always accelerating up and down in their bores. Engine angular acceleration results in a growth in the piston acceleration - the derivative of acceleration, jerk. Newton gave us force = mass x acceleration with no mention of jerk.

The only way to keep the engine component (rod/pistons) acceleration low is to keep the engine speed low. Engine angular acceleration results in engine component jerk.

Engine angular acceleration is non-issue - unless you happen to know something I don't about high tensile steel's behavior in jerk.

**Terrano08** · 19-10-2009 11:55 PM

so listen, lets say i dont want to change my rod bolts coz im a lazy **** !! when should i be shifting gears ??? and what little things (not internals) can i do to enhance power??

**string** · 20-10-2009 12:00 AM

Don't exceed the stock rev-limit of the B20: 7000-7200. Stories of LSVTECs with spun rod bearings 3 months down the track are plentiful. Rev high on the stock bolts and you will regret it. The difference in forces between 7000rpm and 8000rpm is much bigger than the numbers alone might suggest.

The usual advice is as follows: Breathing mods (intake/header/exhaust), then tuning to make the most of what you have. A stock ECU is doing you no favours.

IIRC there is a stock block (high comp) B20VTEC in the Build forum with bolt-ons and tuning - ~125kW. Sounds good to me...

**Lerlo** · 21-10-2009 03:55 PM

I heard of issues of Con-rod stretch as well. Is the stock B20 conrods able to handle, say 8500-9000 rpm, without the piston hitting the head?

**Terrano08** · 22-10-2009 11:11 AM

Just wondering what type of bolts they are, are they a dingle bolt with aa thread on the other side of the rod or are theynut and bolt. And what is the tightneing torque required? thanks

**lithium** · 22-10-2009 12:00 PM

this is an interesting discussion guys, i learnt a lot from it

can i try to summarize, is this right?

increasing the peak torque output from an engine will result in a greater maximum compressive force on the rod of the piston on the power stroke. (this is why boosted evos et al. use forged rods to prevent rod failure due to excess compression). however increasing peak torque has no effect on the max force acting on the rod bolt.

on the other hand increasing the maximum engine speed increases the max force applied to the rod bolts. increasing max engine speed has no affect on the maximum compressive force on the rod.

so if you want to raise your rev limiter without increasing peak torque you only need to change the rod bolts. however if you increase peak torque you need to consider changing the rods themselves.

(in practice i don't think anyone changes rods without also uprating the rod bolts while they have the engine apart!)

**GSi_PSi** · 22-10-2009 01:00 PM

Originally Posted by Terrano08

Just wondering what type of bolts they are, are they a dingle bolt with aa thread on the other side of the rod or are theynut and bolt. And what is the tightneing torque required? thanks

ARP ROD BOLTS - 208-6001,. Tightening instructions will come with it.
As with molylube etc

**string** · 22-10-2009 01:10 PM

The torque requirement is more than the stock rod bolts (more clamping force). This means that when you install them on the stock rods, the big end (the actual rod material) is compressed ever so slightly more. The internet seems to think this is a problem which requires remachining of the big end bore - YMMV.

Originally Posted by lithium

on the other hand increasing the maximum engine speed increases the max force applied to the rod bolts. increasing max engine speed has no affect on the maximum compressive force on the rod.

Increasing engine speed affects the compressive stress of the rods. The pistons are accelerated both downwards (rod tension) and upwards (rod compression).

Is the increase enough to warrant a forged rod? I don't know. Seems to be plenty of builds out there using stock LS rods with uprated bolts whilst revving sky high.