CBX Racing

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Re: CBX Racing

Postby Rick Pope » Thu Jul 11, 2019 7:57 am

What your measuring is cranking pressure, not compression. Changing cam timing affects cranking pressure. Check a sport or racing engine, then check something built for lugging power, say GoldWing or KLR. The softer performance engine likely has more cranking pressure.

Long winded way of saying, you might not need be concerned about your engine's health.
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Re: CBX Racing

Postby Warwick Biggs » Fri Jul 12, 2019 8:14 pm

Can you explain that a bit more please Rick? I can understand that more overlap could produce less pressure but if the valves are both closed (and the clearances suggest they are) and the pistons and rings are basically stock (or first overbore in my case), why wouldn't the compression be closer to the 170 psi that Honda specifies for the CBX, rather that the 110- 140 I measured? And what does the variation suggest?
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Re: CBX Racing

Postby Rick Pope » Sat Jul 13, 2019 9:26 am

You mentioned you had non-stock cams. They likely hold the valves open longer than stockers, thus wasting part of the compression stroke. Thus you aren't getting the benefit of the full stroke of the cylinder being squeezed into the final space.

EMS can explain this better than I as he's a "for real" engineer. Or, he might just tell us I'm wrong....... :oops:
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Re: CBX Racing

Postby Gearheadgregg » Sat Jul 13, 2019 12:49 pm

Correct , Racing cams in my experience in my Big Block Chevy race boats, The overlap was for higher rpm meaning the valves were hanging open longer to permit more time to to charge cylinder but low end would suffer from loss of cylinder pressure, Most racing cams will recommend higher compression . Now ? to Supercharge or Turbo Stock profile is more like a compressor so boost does not blow by the lobe separation overlap , More like a water pump Greg
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Re: CBX Racing

Postby EMS » Sat Jul 13, 2019 12:59 pm

Not sure that I should get involved in that....
My view of the difference between "Compression" and "Cranking Pressure" is that:
"Compression" is measured on a non-firing engine. The pressure you measure is the result of the piston compressing the volume of the cylinder into the combustion chamber.
"Cranking Pressure" should be the pressure measured above the piston when fuel/air mix is ignited ( a tad more difficult to do)

I tried to re-read the posts and could not find a detailed description of what Warwick measured and how and what brought up Rick's comment. (But I suffer from ADHD)
As far as the comments about the difference in racing engines and "lugging" engines are concerned, racing engines are usually tuned for horsepower, which is an indication for sustaining speed, while 'lugging engines" are tuned for torque, which is acceleration out of lower RPMs.
The combustion pressure directly indicates torque, because it can be calculated as a force pushing the piston down.
As far as the valve timing and overlap and opening of the valves are concerned, the RPM range is an important consideration. At higher RPM, the inertia of the mass of the gas helps the "load exchange" i.e.: the intake of fresh air/fuel mix and exhaust of the burned gas. On a race engine, the valves are open longer because the moving gas will fill and empty the cylinders in addition to the movement of the piston.
That is the simple reason, racing engines have poor idle, in most cases.
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Re: CBX Racing

Postby Rick Pope » Sat Jul 13, 2019 6:08 pm

Thanks Mike and Gregg. But to the original question, would a "racing cam" give a lesser reading on a "Compression guage"?
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Re: CBX Racing

Postby EMS » Sat Jul 13, 2019 8:52 pm

I am not sure it would. The compression cycle occurs when both the inlet valve and exhaust valve are closed. The exhaust valve should be almost close when the piston on the exhaust stroke reaches TDC. This is when the inlet valve should have started to open. Here you have overlap. The theory is to flush all burned gas by having fresh mixture streaming in through inertia. When the piston reaches BDC, the inlet valve should have started to close and on the upstroke, it should be closed completely. I don't see how you lose any compressible volume.
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Re: CBX Racing

Postby Warwick Biggs » Sun Jul 14, 2019 1:48 am

That was my thinking too EMS. There may be some small loss of pressure from the fractionally wider bore expanding the combustion chamber by about 10% altho' I don't know whether the compression I am measuring would be affected in direct proportion to the increased capacity.

In any event there is a big difference between the stock 170 psi and my lowest reading of 110 psi. I used the conventional method altho' I had the CR"s fitted when I did it, so I held the throttle open. I also had the exhaust pipes and muffler fitted and all plugs removed while I cranked it on the starter for 5-10 seconds.

Roly opined that the compression would read lower because of the race cams, presumably along the lines that Rick and Greg were thinking but I wasn't so sure. The motor starts well and idles fairly well for a 'cammy' engine at around 1,000 -1200 rpm, so a bit more than stock, for sure. It only bogs down under load and that was why I was originally so convinced it was a fuelling issue. That and the fact that the 2 cylinders showing noticeably lower temperatures and fouled plugs were on separate coils.

My best guess is that the valves are not sealing properly because of the carbon build up due to incomplete ignition caused by the misfiring and I was going to do a leak test to confirm that until Roly suggested it was probably unnecessary and I might as well wait and see how the new ignition goes. He wasn't bothered by the low compression readings which he put down to the race cams.

As a matter of interest he reckons he was quicker on the Beast when he went back to stock cams altho' the faster riders like Trevor and Michael preferred the race cams. However, the Beast has 10.5:1 pistons, Carillo rods and a lightened and balanced crank and makes its peak power 1,000 rpm higher than the Lump so its not really comparable.

When Hugh originally ported my motor he was getting around 115hp on his dyno. That head was over ported and blew out through a valve guide. The second head was less extreme but I could not get much info out of Hugh on what he actually did. I reckon by seat of the pants that I lost about 5-10 hp in the process but it was still a strong engine.

To be honest I am losing interest in the CBX. It is a rather silly race bike, especially when it refuses to run properly. That is my fault because I'm a lousy mechanic and no longer have the assistance of any of the skilled engineers and mechanics that I had at the beginning because I have moved to a very remote part of the South coast. It is fun to ride fast and is capable of hustling TZ750's which is a surprise to many but its just such a pain in the ass to work on. Its ironic because reliability was one of my original reasons for choosing a Honda. But I chose the mother goose of all Hondas.

Maybe I should forget about old bikes and just get a cheap late model 675 Daytona like everybody else Then only have to worry about tyres for a change?
R.
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Re: CBX Racing

Postby Gearheadgregg » Sun Jul 14, 2019 10:49 am

Good info for this question> So back to those two identical engines with different cams; if you did a compression test on either of these two 9:1 static compression engines, the engine with no overlap (or even negative overlap) would probably have about 140 -150psi or so in the cylinders. The engine with more overlap may only have 110 - 120 psi or so, depending on how much overlap the cam has, how narrow the lobe separation angle is, and most importantly... what the intake valve timing is.http://www.badasscars.com/index.cfm/pag ... /prd68.htm
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Re: CBX Racing

Postby Rick Pope » Sun Jul 14, 2019 3:15 pm

A long time ago, Kevin Cameron did an article on cam timing and cranking pressures. I seem to remember he compared a Gold Wing to a sport bike, each with comparable compression ratios, but different cam specs. I bet Dave McMunn has that article somewhere in is attic......... 8)
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Re: CBX Racing

Postby Warwick Biggs » Tue Jul 16, 2019 3:14 am

After thinking about all of this it now occurs to me that my port job would have been more effective had I installed higher compression (10.5:1) pistons at the same time. However, this modification would have necessitated stronger rods and then if I am going to spend thousands of $ on rods and pistons I may as well bore out the motor to the capacity limit of 1300 cc. Then have the crank lightened and balanced and pretty quickly I have spent $20-30K and completely obliterated my original thinking in choosing the CBX as a cheaper alternative to the CB1100R replicas.

I could always reverse direction and install the more user friendly stock cams which is what Roly has been suggesting all along. Maybe I should try a stock set and see how they work with the ported motor and 2mm oversize valves? But first I have to get it running cleanly. At least, thanks to this forum, I am less concerned about the lower compression than stock, so I appreciate the feed back guys.
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