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how does the duration and lift really work?
What i mean is, from my mini days, a 266 cam was like the mg metro cam, droves loverly.a 286 in my mates mini 1340 used to idle very lumpy, and the power was all at the top.
Now i have a volvo b230, it has opening duration 300 degrees, cam lift 12.9mm, Lobe center line angle 108 degrees, this is not that lumpy and the power is low down. But i have been told i need to get the static compression right up and itll work better. My engine does 160 at the flywheel.
However, my mate had a 2500 penta motor, that made 156 at the wheels and 170 lbs and that was 12:1 compression, and the volvo K cam..
K 11.95 270 257 110 Standard B23E (81-83), B19 Sport, B230 Penta marine engine
So , the less aggressive cam made a lot better power, so what cam do i go for? I don't understand how it all works now!
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have you still not sorted the compression ratio yet?
its all pretty straightforward what the different figures mean, however its bordering on a dark art/science actually specifying something to work in any application.
my engine has newman ph4's, 286 degree hydraulic cam, 108 peak lift, 11.something peak lift, 2186cc and 240 bhp.
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well, its been a nightmare, i bougth a 531 head, i had it skimmed but i honestly cant rememeber by how much, and never measured the chambers. I put it on with a 0.5mm cometic head gasket, and took it to the rolling road. It did 160 bhp, it pulls ok, but its still not the big power i wanted.
My mates boat 2500cc had 531 head skimmed 2mm, k cam, cast manifold, and split 50;s. Now ok he has better induction than me, but he had 156 bhp and 170 lbs and 169 lbs AT THE WHEELS!
So that would be fantastic power in my car. My cam is 300 degs, its a kl racing t5, so not sure if its the cam aswell. I need to take the head off again and measure everything.
Just don't know what to do!
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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camshafts.slater
@slater
Club Retro Rides Member 78
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Re the compression ratio you need to look up 'dynamic compression ratio'
In a nutshell if the cam has more duration the inlet valves close later on the compression stroke meaning that the piston has already travelled part way up to bore before its started actually compressing if you like. This means cams with more duration need a higher static compression ratio to work properly.
To be honest what cam you choose is your choice. I'm sure you understand the more duration will generally mean torque shifts higher up the rev range and if you do the math having the torque higher up the rev range is going to give more power (power=torque×revs basically). This doesnt nessicary mean the car with more power will be faster. It depends if the engine can be kept at higher revs all the time. Thinking of it in terms of 'xxx cam gives more power' is a red herring. It's all about matching the torque curve of the engine to how you will use it. At a bare minimum you need to see if you can get some information on power band each cam will give and if you can get actual dyno comparisons of the different cams you have to choose from. There are other factors involved like capacity and bore stroke ratio and all sorts really. There no substitute for trying them out or finding information from somone who has.
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right ok, is there a prefaired dynamic compression ratio then? As in, if i havea 300 deg cam or a 256 cam, id need to skim different amounts off of the head?
I need a long spread of power, at the moment its no where near what rogers car had, he said he changed gear at 5,500, thats fine for me. His overall power at the flywheel is like 190 bhp and over 200 lbs i think.
my injection system can cope with 180 or 190 bhp depending on who you talk to in standard form, so I'm well with in there, and rogers car had the same cast manifold, and i have the same bore collector.
So my 300 deg camshaft is too much then?
still struggling with this.
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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camshafts.slater
@slater
Club Retro Rides Member 78
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Not as such. The problem is you cant really measure the dynamic ratio to any level of accuracy. It is by it's very nature dynamic! It changes with engine speed and all sorts of things. Basically all you need to do is take into account that longer duration cams want a higher static ratio than short duration ones.
Exactly how much and what cam will do what is somthing I cant tell you. Like I say you need to ask the cam manufacturer for information and see if you can get some idea of what other people who built simlar engines have done. The only other way is trial and error or maybe some kind of multi million pound combustion simulation software.. Unless your an oem engine manufacturer your pretty much stuck with an educated guess.
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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camshafts.slater
@slater
Club Retro Rides Member 78
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My 'educated guess' on your situation based purely on what you've said here is you have a high duration cam in an engine that's not really got the right modifications to make use of a high duration cam. 12:1 sounds like it's in the right ball park to aim for but your lower than expected power is probably down to a load of different things like not having a good induction and exhaust setup as well as the wrong compression ratio. Higher duration cams will be more sensitive to that kind of thing.
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stealthstylz
Club Retro Rides Member
Posts: 14,812
Club RR Member Number: 174
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camshafts.stealthstylz
@stealthstylz
Club Retro Rides Member 174
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Nov 10, 2020 10:08:33 GMT
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Has it got a adjustable cam pulley and is the cam timed in properly? With a big head skim and stock pulleys it can throw the timing out a fair whack.
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Nov 10, 2020 12:15:35 GMT
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My understanding is that the longer duration enables the outgoing exhaust to draw more fresh charge in behind it, more air means you can burn more fuel and therefore more power. This effect tends to be work better at high flow rates and therefore higher engine speeds which is why you generally end up with power higher up the rev range. It is also dependant on the intake and exhaust, if you are choking it somewhere else you won't be able to benefit from the cam, conversely if part of the system is too large the flow speed will drop and there won't be enough momentum to draw air in. As mentioned above the compression ratio will vary, where more air is drawn in you will have a higher compression ratio than at low flow points, so on a well set up engine you might have a static 9:1 but a dynamic 11:1 at 5000rpm, this can only really be measured with very exepensive equipment.
The bottom line is manufacturers spend millions of £ and use test cells which cost several million £ each to get the most out of an engine, the best thing we can do is research what others have done and use that, collectively we have probably spent even more than the oem's!
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lebowski
Part of things
Hillman Avenger, Clan Clover
Posts: 476
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Nov 10, 2020 14:11:29 GMT
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I think that some of the posters above are conflating dynamic compression ratio with volumetric efficiency. The static and dynamic compression ratios of an engine are mechanically fixed (unless you have variable cam timing), and do not change with rpm.
Static compression ratio is the ratio of the volume above the piston at BDC, compared to at TDC (i.e. the maximum possible comp ratio).
Dynamic compression ratio is the ratio of the volume above the piston at the point the inlet valve closes, compared to at TDC. This is because you can't start compressing your inlet charge while the inlet valve is still open. It'll just head back out of the open valve. It also means dynamic comp can only ever be lower than the static comp ratio. If you know the point at which your inlet valve closes then you can calculate this.
If you've put a long duration cam in which closes the inlet valve a long time after BDC, then as mentioned above, you should raise your static comp ratio otherwise your dynamic comp ratio will be very low and performance will be poor. It's the reason why cars with wild cams usually have a very high compression ratio.
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Nov 10, 2020 20:05:22 GMT
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right ok, i am understanding it a bit more now..
first the lh2.4 is good for 180 bhp some say 190, and timos motor say they can get 210 with the right head and exhaust. The ports match the head, and the only restriction with it i can see is the injectors are right by the head and sit up a bit.
My exhaust is cast manifold, that is the same size as the exhaust ports, the down pipes are 50mm, and then it goes into a small box under the car and a straight threw back box, the pipe is 61mm.
i think i need to take the head off again, and have a measure of the chambers and see what size they are.
ive learnt quite a bit reading above, thankyou
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Nov 11, 2020 15:55:33 GMT
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Proton Jumbuck-deceased :-( 2005 Kia Sorento the parts hauling heap V8 Humber Hawk 1948 Standard12 pickup 1953 Pop build (wifey's BIVA build).
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madmog
Club Retro Rides Member
Posts: 1,152
Club RR Member Number: 46
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camshafts.madmog
@madmog
Club Retro Rides Member 46
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Nov 12, 2020 21:58:38 GMT
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I've found this explanation on youtube really good
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