Cams, compression, revs and power...


Kozy

Kozy

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I've been havin' a little thinky about this subject, and something puzzles me about the way we tune our Hondas with regards to cams, and the crazy compressions we run. This is only a little theory, to promote a bit of discussion on the matter, so feel free to shoot holes in it/me.

The basic way a cam is selected for an engine should depend on it's capacity, it's head flow characteristics and the target power band. The stock cams are a blend of midrage and top end power, with a bias towards midrange on the B18C and one towards top end on the B16B.

When we stick an uprated cam in, what happens? Well assuming we increase both lift and duration by an equal amount, we lower the dynamic compression ratio (DCR), and raise the point at which the flow requirements of the engine outstrip the supply from the cams. The lower DCR reduces midrange power until the point where inlet port velocity overcomes the port reversion allowing the cylinder pressures to gradually increase with engine speed, from this point the increased valve open time increases power and holds it further up the revs. So you lose some midrange, gain some top end and extend the power band a little, sounds logical right?

So now lets look at some 'm4d tizzle' cams, proper gnarly race spec stuff. They can feed your stock B18C to 11000rpm, but have dropped your DCR to the low 9s. Your midrange will SUCK with this setup. So then, the usual reccomended solution is to go and drop in some 13+ CR pistons to return to the stock engines mid 10 DCR. In theory this is great, we have enough flow for 11k, and a stock DCR, so it'll drive around town like a factory car and tear chunks out of ex BTCC cars on track.

Except as far as I can see, it won't...

Can anyone tell what flaw I am looking at here? :blinx:
 
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Very interesting.

I was very supprised that my Toda C's only made power up to 8k then started to die off.... I don't know what would be need to rev the up to 10k and make power...
 
IMO looking at B16A and B16B,
B16A makes 170ps at 7800rpm
B16B makes 185ps at 8200rpm
Increasing CR from 10.4 to 10.8 doesn't look like helping with the low end torque (DCR)
with the increased cam profile which 15ps after 400 rpm.
Just my 2 cents - if the goal is the look for a motor for daily use and the chunks, increase capacity! CR and the duration can never be balance from the road to the track.
Although VTEC is a nice system to increase "usable" power band, 3K to 11K is too much to ask for... Might have better luck with K20 I-VTEC witch have VTC to play around.
 
Kozy said:
Can anyone tell what flaw I am looking at here? :blinx:
Click to expand...

I'm thinking that a huge static CR could actually be detrimental to the performance when using huge cams. Seems a bit controversial to conventional wisdom, but I've seen tell tale signs of this in numerous builds on HT.

ijwhiteman said:
Very interesting.

I was very supprised that my Toda C's only made power up to 8k then started to die off.... I don't know what would be need to rev the up to 10k and make power...
Click to expand...

I have an idea... What cams/compression are you running?
 
ian runs toda c's with 13.2:1 static CR

if you want to make power high up the revs on a B18, you've got to be able to maintain the torque, any thoughts on that?

ALOT comes down to the cylinder head and inlet/exhaust. if you think about it, the cams will hold the valves open for a certain lift and duration, the pistons will provide whatever CR, but the shape of the combustion chamber and way the air is fed in and out of the combustion chamber is vital............
 
Ha ha

Toda C with static compression of 13.2:1 I think dynamic is just under 13....
 
I'd love to know where these are tuned and what dyno's are producing these numbers
 
stevenEK9 said:
if you want to make power high up the revs on a B18, you've got to be able to maintain the torque, any thoughts on that?
Click to expand...

Do you have to though?

Ian your DCR is nowhere near 13, try mid 11s with those cams! Going on my admittedly crude calculations, those cams should be good to flow up to around 9k. The problem I think you may have, which is exactly the problem I am alluding to, is that you've tried too hard to patch up a hole in the midrange left by having those big cams, which actually works against them at the top of the revs.

That compression is helping you loads in the midrange by keeping the cylinder pressures in the optimal range for ignition timing. What is happening to those cylinder pressures as the engine speeds, and subsequently the port velocities rise though?
 
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Kozy said:
The basic way a cam is selected for an engine should depend on it's capacity, it's head flow characteristics and the target power band.
Click to expand...
Hmmm i agree to an extent. Your application wether it be street, track only or drag only muct be a major factor too.


This is a fascinating subject and one which i did some research on before deciding on what parts to build my engine with.
There are a lot of factors to consider.
1. Cylinder Capacity
2. Compression ratio (and what sort of pistons you will use)
3. Your cams
4. Point 3 above should determine what intake you choose
5. Exhuast gas flow-header type-reversion. Point 3 and 4 should influence your choice and if you decide to modify.
6. Money LOL!

Kozy said:
When we stick an uprated cam in, what happens? Well assuming we increase both lift and duration by an equal amount, we lower the dynamic compression ratio (DCR), and raise the point at which the flow requirements of the engine outstrip the supply from the cams. The lower DCR reduces midrange power until the point where inlet port velocity overcomes the port reversion allowing the cylinder pressures to gradually increase with engine speed, from this point the increased valve open time increases power and holds it further up the revs. So you lose some midrange, gain some top end and extend the power band a little, sounds logical right?
Click to expand...
I think you have the basics of this. Also remember that the overlap on an aggressive aftermarket cam like the Toda C is quite long. That means that the intake valve will be open along with the exhaust valve for a longer period of time than normal. With a stock header this will drain power significantly as 'reversion' will take place and exhaust gasses will travel back into the cylinder, and as most of us know that is bad for power. With an aggressive cam like the 'C' there are a few ways to combat this.
1. Perform anti-reversion on your header. It's designed to keep the exhaust gasses from flowing backwards into the cylinder before the exhaust valve closes. This will help ensure that the cylinder gets filled with the freshest intake air that isn't contaminated with leftover exhaust gasses, which will make more power.
2. Get a performance intake like ITB's. 'C's generally are a good cam to mate with ITB's because the ITB's can flow large amounts of air at mid-range revs. Along with anti-reversion your DCR should be at least mid 11's IMO (POSSIBLY HIGHER). At top end rev's the 'C's cam profile should provide pretty high top end power and if you have tapered ITB's you can net serious gains up high too and have a seriously linear output all through the rev range.


If anyone disagree's feel free to correct me.

stevenEK9 said:
ALOT comes down to the cylinder head and inlet/exhaust. if you think about it, the cams will hold the valves open for a certain lift and duration, the pistons will provide whatever CR, but the shape of the combustion chamber and way the air is fed in and out of the combustion chamber is vital............
Click to expand...

Agree 100%
 
Anti-reversion and valve overlap do not affect DCR, DCR is affected only by the bottom end geometry and intake valve close event. It is calculable and constant for a given cam profile and timing. Although named 'dynamic', it is in fact just another static ratio, except one taking into account the EVC event.

Cylinder pressures on the other hand are affected by anti-reversion and overlap. These are non-calculable and for the most part, completely unpredictable, as they vary throughout the rev band and depend on throttle position and all the factors you mentioned above. They are a crucial point that needs controlling when tuning an engine, as when considered with the ingition timing, dictate a large proportion of the engines thermal effeciency.

Whilst not calculable, it would be reasonable to assume that with an increase in engine speed, the intake port velocity increases and the cylinder pressures increase. If you've boosted the low speed cylinder pressures into the optimal range with a massive static CR in order to patch up the low DCR, what is going to happen as the cylinder pressures increase with revs?

Another thing to consider, what is that massive dome on the pistons doing to the airflow at TDC, on the overlap? How is that going to affect the volumetric efficiency?
 
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Intake runner length, volume, plenum volume will all play a big part too, so will the fuel used against c.r, when you talk of overlap this becomes less of a problem at high rpm's due to time. You can dial most of it out any how
 
rs250nut said:
Intake runner length, volume, plenum volume will all play a big part too, so will the fuel used against c.r, when you talk of overlap this becomes less of a problem at high rpm's due to time. You can dial most of it out any how
Click to expand...

I don't follow? Dial out the overlap? Or the 'problem'?

ijwhiteman said:
I'm mega confused... :run:
Click to expand...

Me too, I think people are missing my point so far. No saying it's a correct point at all, but that's the point of discussion!

Rich has stayed quiet in the corner munching his popcorn so far, I reckon he might know what I'm getting at. Either that, or he's chuckling at me barking up the wrong tree...
 
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rs250nut said:
Intake runner length, volume, plenum volume will all play a big part too, so will the fuel used against c.r, when you talk of overlap this becomes less of a problem at high rpm's due to time. You can dial most of it out any how
Click to expand...

Yep, unless you hit exhaust back pressure from a restricted exhaust/manifold. This is one of the reason's i always say that a cam(s) is only as good as the exhaust setup.

Another thing to note is that if you have excesive backpressure it won't matter what "anti reversion techniques" you have tried. Your cylinder's will still choke on exhaust gas causing a increase in compression pressure/heat resuling in pumping losses, lower inlet charge density, lower combustion pressure (less downforce on the crank),ect.
 
Sorry Kozy, my brain has been out of order thanks to me birthday. I need to sit there and properly read through it all few times... Yummy popcorn ! ! :p
 
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