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RPM max increase

 
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Old 06-24-2016, 10:26 PM
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RPM max increase

Hello all, so my question of the time being is that with my motor being built (B21) with h23 forged rods, Eagle forged crankshaft will this let me be able to rev or drive past the stock 6500 RPM limit without causing major problems? If not what would be needed to let me do so? Thanks!
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Old 06-25-2016, 11:54 PM
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Re: RPM max increase

Well, to put it simply, no. You can't really safely increase the RPM limit, at least not much.

It's not a matter of how strong the bottom end is, the reason a number of Honda motors don't love to rev even when built is due to a low rod to stroke ratio. This is the ratio of the rod length to the diameter the crank rotates around (the stroke). In a standard B21 this is around 1.492:1 which is pretty darn low. For comparison a stock B16A (which love to rev) is 1.74:1 and a K20 is 1.64:1. Even a H22 is not that great at revving due to its 1.577:1 ratio, hence why most people who build a H22 aim to make the most from the RPM they have rather than try to increase the limit. The problem a low rod ratio produces is the piston will spend relatively longer at TDC and BDC (dwell time), and have to accelerate quicker and travel faster in between. Due to the relatively large size of the crank, the rods have to travel further laterally during a cycle too. The rapid acceleration and large lateral forces is what destroys engine components at higher RPM. Unfortunately for engine builders this is always a trade-off as they are trying to get decent compression and displacement in the smallest package possible, and for a road car 6500RPM is more than enough. Again, it's not a matter of strength but basic engine geometry.

By putting the H23 rods in, you have only increased the rod ratio to 1.494:1, a meaningless increase really but at the expense of dropping 0.17mm off your rod length which will lower your compression ever so slightly. You're on the right track, it appears the H23 rod swap is a common one but due to the FRM liners in the B21 you can't run forged pistons.

Finally, there is practically no benefit to increasing the RPM limit without some major headwork. If you look at the stock power curve of stock B21 and you'll see peak power is made around 5500RPM and it's making the same power but much less torque at 6500 than at 4500RPM. There is just no reason to rev harder than this, change up a gear. Sure, with some serious (custom) cams and a few thousand dollars spent on a port job you might move the peak power up a few hundred RPM but will likely lose power in the low to mid range as a result.

Is there a good reason why you want to rev higher?
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Old 06-26-2016, 12:59 AM
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Re: RPM max increase

I thankyou for the fast detailed response for my top end I was prehaps thinking of shaving the head and/ or putting ground cams and building the head to what I believe was .420 grinded cams(I might be wrong if so my apologies) but I suppose I will keep it how it is mainly like you said not too bad for a small street car, thankyou again!
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Old 06-27-2016, 10:00 AM
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Re: RPM max increase

Quote:
Originally Posted by tassievteclude View Post
Well, to put it simply, no. You can't really safely increase the RPM limit, at least not much.

It's not a matter of how strong the bottom end is, the reason a number of Honda motors don't love to rev even when built is due to a low rod to stroke ratio. This is the ratio of the rod length to the diameter the crank rotates around (the stroke). In a standard B21 this is around 1.492:1 which is pretty darn low. For comparison a stock B16A (which love to rev) is 1.74:1 and a K20 is 1.64:1. Even a H22 is not that great at revving due to its 1.577:1 ratio, hence why most people who build a H22 aim to make the most from the RPM they have rather than try to increase the limit. The problem a low rod ratio produces is the piston will spend relatively longer at TDC and BDC (dwell time), and have to accelerate quicker and travel faster in between. Due to the relatively large size of the crank, the rods have to travel further laterally during a cycle too. The rapid acceleration and large lateral forces is what destroys engine components at higher RPM. Unfortunately for engine builders this is always a trade-off as they are trying to get decent compression and displacement in the smallest package possible, and for a road car 6500RPM is more than enough. Again, it's not a matter of strength but basic engine geometry.

By putting the H23 rods in, you have only increased the rod ratio to 1.494:1, a meaningless increase really but at the expense of dropping 0.17mm off your rod length which will lower your compression ever so slightly. You're on the right track, it appears the H23 rod swap is a common one but due to the FRM liners in the B21 you can't run forged pistons.

Finally, there is practically no benefit to increasing the RPM limit without some major headwork. If you look at the stock power curve of stock B21 and you'll see peak power is made around 5500RPM and it's making the same power but much less torque at 6500 than at 4500RPM. There is just no reason to rev harder than this, change up a gear. Sure, with some serious (custom) cams and a few thousand dollars spent on a port job you might move the peak power up a few hundred RPM but will likely lose power in the low to mid range as a result.

Is there a good reason why you want to rev higher?
^^Great post!
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