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Old 09-15-2010, 05:14 PM
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Cam and compression ratio...will it work or is it to small?

I have a comp cams extreme 4x4 cam, specs are

comp cams X4270H
1800-6000 operating range
lobe-center angle 111
intake centerline 107
duration 270 intake 278 exhaust (advertised)
226 int 234 exh @.050"
intake lift .48
exhaust lift .498

Will this work with 10.5:1 compression or is it to small and will it cause to much cylinder pressure? thanks

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Old 09-15-2010, 05:21 PM
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You will need to post more info on the total package, no one can answer this question yet.
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Old 09-15-2010, 05:31 PM
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383
Scat crank
AFR street 195cc runner 64cc chamber
10.5:1 compression (proposed)
Weiand dual plane stealth intake
770cfm holley carb
comp cams X4270H
1800-6000 operating range
lobe-center angle 111
intake centerline 107
duration 270 intake 278 exhaust (advertised)
226 int 234 exh @.050"
intake lift .48
exhaust lift .498
1.6 roller rockers
hooker headers


its in a 75 K20 pickup with a 4 speed manual 4.56 gears, 35" tires
desktop dyno estimates 51 degrees valve overlap based on the comp cams spec sheet.

Last edited by 383_Gladiator; 09-15-2010 at 05:37 PM.
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Old 09-15-2010, 06:18 PM
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Quote:
Originally Posted by 383_Gladiator
383
Scat crank
AFR street 195cc runner 64cc chamber
10.5:1 compression (proposed)
Weiand dual plane stealth intake
770cfm holley carb
comp cams X4270H
1800-6000 operating range
lobe-center angle 111
intake centerline 107
duration 270 intake 278 exhaust (advertised)
226 int 234 exh @.050"
intake lift .48
exhaust lift .498
1.6 roller rockers
hooker headers


its in a 75 K20 pickup with a 4 speed manual 4.56 gears, 35" tires
desktop dyno estimates 51 degrees valve overlap based on the comp cams spec sheet.
Go here http://www.kb-silvolite.com/calc.php with this data and plug it into the DCR calculator. 8 is street-able with 91 octane. 8.5 is getting iffy, but a manual gear box and stiff gears will let you stretch it on the high side.

Bogie
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Old 09-15-2010, 07:02 PM
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Let me help you out a little here, because first you have to find the intake closing point at 0.050" tappet lift. I'm able to do this pretty quickly. The figure is 40 degrees for that cam, so adding 15 degrees to that means you have to enter 55 into the calculator and are rewarded with a DCR of 9.009:1 at 10.5 SCR with 3.750" stroke and a 5.703" rod.

My first impression is that there is no pump gas on the planet that will allow operation at 9.009 DCR without the motor detonating like shaking a can full of rocks. Needs less SCR or more cam. Even if the DCR were in line, I would err on the side of caution (like maybe 8.0:1 DCR with a very tight squish) because of the excellent cylinder-filling characteristics of the AFR heads and Stealth intake.

I might be full of beans, but that's my take on it.
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Old 09-15-2010, 10:37 PM
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What would you say the highest compression I could go with? Will be running 93 octane
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Old 09-15-2010, 11:10 PM
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Can anyone reccomend a good comp cam for use with 10.5:1 or 10:1? Truck will be used off-road and a mug drags so a good torque down low off the line would be nice with decent power up top to keep up wheel speed once the motor revs out
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Old 09-16-2010, 01:45 AM
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Here's the power, but I don't know if it will operate on 93 octane....
10.5:1 SCR
Crane 110742 flat tappet hydraulic cam and kit
http://www.summitracing.com/parts/CRN-110742/
1 3/4" open headers
RPM HP TQ
2000 143 376
2500 190 399
3000 243 426
3500 307 460
4000 372 488
4500 431 503
5000 485 510
5500 516 493
6000 530 464
6500 527 426
7000 512 384
Peak volumetric efficiency 99.7% @5000
Peak BMEP 200.9 @5000
Call up the tech guys at Crane and discuss this with them. They may have a completely different direction for you and some way to lighten up the cylinder pressure and make hp at the same time.
http://cranecams.com/?show=technicalhelp
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Old 09-16-2010, 07:16 AM
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I would say that cam would work with 9.5:1 smogger iron heads, 9.75:1 with iron vortec heads, and 10:1 with aluminum heads.

I would just use an old school 280H compcam with your set up. 280/280 advertised with 230/230 at 0.050" with 110 lsa. This cam will work with your heads and 10.5:1 cr.

Plus your heads don't need any extra exhaust duration due to a really good intake to exhaust flow ratio. Furthermore, the 280H will have a better chance for a long engine life due to softer lobe ramps.
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Old 09-16-2010, 10:47 AM
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The info you are being fed here is flawed based on inaccurate data input.

"garbage in = garbage out"

The calc assumes that there is 30deg difference between the .050" duration and the actual running duration which is the real valve open close event duration of the engine when it is running. The point of intake valve closing of the running engine must be established to determine the start of compression to make this calc work. A hyd flat tappet cam with 226 @.050 duration will typically have between 44 and 56deg difference between the .050 duraion and the seat duration. Half of that would be 22 to 28 deg added to the .050" closing point to get the seat timing closing point.

Seat timing does not = advertised duration. ever. Cam companies do not publish the actual running seat duration of their cams.

You have to stick the cam in a motor and actually measure that to have data to stick into a formula to even come close to prediction the cranking pressure.
Or dynamic compression of a running motor.

The formula only works for cams with 30deg difference. Some solid rollers and very fast ramp solid flat tappets are like this subject to the running valve lash etc.

The whole effective dynamic cr stuff is way over simplified and will not predict the cylinder pressure of the actual running engine |"BMEP" at the running rpm.

It only attempts to predict the cranking compression pressure.

10.5:1 would be near the upper end of practical compression for 93 octane.
You do not want to run at the limit.

Here is the thing. You engine will not be operating under ideal conditions all the time. Off roading involves hard running at low speed. The motor is going to get hot at times. You need to take that into consideration when designing the engine. You do not want to build for the most possible compression ratio.
You want to build so that the motor will not detonation under the worst conditions, like when running hard thru the mud at low speed but heavy throttle etc. Things get hot fast.
You do not want to build an engine that runs right on the edge of reliability.
Especially a hard running off road motor.

Never select a cam to correct the compression. Select the cam to give you the engine torque curve you need. Then select the cr based on real world operating conditions and the fuel you will use everyday.
Do not over cam, fix the compression ratio.

I suggest 9.8:1-10:1 measured compression. The cam you are looking at will work fine.


Select a appropriate dished piston with a 64cc head or a flat top piston for 70-72cc heads on the 383.

Its nice to have a piston to head clearance of .040 to .055" with the gasket included. You need to determine the piston deck clearance at TDC to calc the compression ratio.

Excessive compression ratio will not make you go faster or make you more popular with the girls.
Over cammiing a motor will not fix it. It just builds a crappy motor with the wrong cam in it.

Last edited by F-BIRD'88; 09-16-2010 at 10:55 AM.
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Old 09-16-2010, 10:57 AM
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Thanks for the input F-BIRD. I'm always interested in learning from some of you other guys.
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Old 09-16-2010, 11:28 AM
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My heads are 64cc so if I run a dished piston of the appropriate dish to achieve 10:1 and run the 4x4 270 cam and 93 octane I should be fine? The 270 according to desktop dyno shows a strong torque curve I like.
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Old 09-16-2010, 03:19 PM
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Quote:
Originally Posted by techinspector1
Here's the power, but I don't know if it will operate on 93 octane....
10.5:1 SCR
Crane 110742 flat tappet hydraulic cam and kit
http://www.summitracing.com/parts/CRN-110742/
1 3/4" open headers
RPM HP TQ
2000 143 376
2500 190 399
3000 243 426
3500 307 460
4000 372 488
4500 431 503
5000 485 510
5500 516 493
6000 530 464
6500 527 426
7000 512 384
Peak volumetric efficiency 99.7% @5000
Peak BMEP 200.9 @5000
Call up the tech guys at Crane and discuss this with them. They may have a completely different direction for you and some way to lighten up the cylinder pressure and make hp at the same time.
http://cranecams.com/?show=technicalhelp
Have I missed somthing?Dident Crane get out of bussnis?
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Old 09-16-2010, 03:41 PM
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Quote:
Originally Posted by 383_Gladiator
I have a comp cams extreme 4x4 cam, specs are

comp cams X4270H
1800-6000 operating range
lobe-center angle 111
intake centerline 107
duration 270 intake 278 exhaust (advertised)
226 int 234 exh @.050"
intake lift .48
exhaust lift .498

Will this work with 10.5:1 compression or is it to small and will it cause to much cylinder pressure? thanks
Run it, I ran a similar cam in a 350 with 10:1 compression. Had no problems with pinging.
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