Cranking Cylinder Pressure... Tuning Aid or Myth?

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 8:58 am

I had always thought 225 to 235 was the optimum cranking compression for drag racing engine combos.

Well my current combo 3.875 x 4.165 (422) SBC with SB2.2 heads and 15.25 compression isn't even in the balpark. (160 psi)
Based on the time slip I feel I'm about .03 slow in sixty foot.

Is cylinder pressure a myth or is my thinking correct?

Current cam is 279/291 @ .050 on a 112 LSA installed @ 108

I think it needs to be 110/108 or 108/108

Fuel is C16

Thanks.

Procision-Auto · Post by **Procision-Auto** » Tue Jul 03, 2007 9:21 am

It's quite a myth. There is a guideline you can use to determine how much
cranking pressure you can expect with cam timing and static compression .

As for tuning: the spark timing, pulse tuning, valve train responce,
breathing characteristics of the motor, etc. can't be reproduced or
analyzed at cranking RPM.

Having read your combo, 160 PSI seems a touch low but nothing to be
concerned about. The intake closing will make a huge deal.

Have you tried advancing the cam a couple of degrees?

P.S. Just out of interest, I had a customer car that jumped ~ 60 PSI
on cranking from removing 10 cc of chamber volume and resetting
the valves. This was a 383 cube engine.

Rick360 · Post by **Rick360** » Tue Jul 03, 2007 9:21 am

Why would cylinder pressure at cranking RPM have ANY correlation to how an engine will run at 8000rpm? or whatever rpm makes peak HP.

If you want more cranking cylinder pressure put a 16V battery in your car, but it won't run any quicker.

Rick

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 9:43 am

If you want more cranking cylinder pressure put a 16V battery in your car, but it won't run any quicker.

As a matter of fact a friend of mine has a combo that's pumping 150 psi with a 16volt battery spinning the piss out of it.

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 9:45 am

Have you tried advancing the cam a couple of degrees?

It's already advanced 4, should I go even further?
With it being on a 112 wouldn't that throw the exhaust off even more?

Procision-Auto · Post by **Procision-Auto** » Tue Jul 03, 2007 9:54 am

Scott Smith wrote:
Have you tried advancing the cam a couple of degrees?
It's already advanced 4, should I go even further?
With it being on a 112 wouldn't that throw the exhaust off even more?

Sorry, I misread part of your post. I see that it's a 112 @ 108 now.

What type of cam is this? Roller, hyd/solid?

Greezer · Post by **Greezer** » Tue Jul 03, 2007 9:57 am

Have a nice day

Ron C. · Post by **Ron C.** » Tue Jul 03, 2007 10:13 am

Scott, it's not related to the cranking cylinder pressure but I can tell you the C-16 is not a good fuel for your N/A engine. C-16 is designed for high mechanical heat load (blower) and you will see a power loss for your combination. The VP C-14 or 14Plus would increase performance for you.
Don't fall into the thinking that more octain is always better also. Over octaining an engine will show up as the engine being sorta doggy or unresponsive to tunning.

Blessings...........Ron Clevenger.

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 10:35 am

Greezer wrote:
Based on the time slip I feel I'm about .03 slow in sixty foot
So what might that be?

E.T. and vehicle weight, too.

Total combo???

Greezer

Well I guess that opens up a big can of worms but here goes..
Peak head flow is 418 @ .700
Combustion chamber is 46 cc with a single valve relief flat top piston.
15.22 compression (Yes I CCed every detail)

2600lb Tube chassis door car. 16x33 tires
5.13 gear
1.80 first glide
converter stalls to 7000
Peak head flow is 418 @ .700 (399 @ .800)
Currently shifting @ 7600 and falls to 6950 on the 1-2 shift.
When I shifted @ 7800 I lost a tenth of G force at the top of first over 7600.
ET range has been a best of 5.43 @ 127 with 1200' DA and a 1.22 60'
Worst has been 5.53 @ 125 with a 1.23 60' with 3400' DA
CHipping at 5000, and tried 6000 but only picked up a little react.
Ignition timing has been from 27 to 30. 27 seems to be the best.
Simular combos with smaller cams and alcohol are 60 footing in the mid 1.teen range.
I know this is a sleeping giant when the air gets good but I still feel there is a lot left on the table.

Ron,
I had a long conversation with Jay at VP.

He said 14 plus was just 14 with 2 more grams of lead and was a dirty fuel.

Out of time I'll be back.

Strange Magic · Post by **Strange Magic** » Tue Jul 03, 2007 11:46 am

Fuel is C16

Scott take that nitrous fuel out and put some C14 in as mentioned in the above posts.

Use someone elses cranking compression gauge. I am having a hard time accepting that it is 160pds of cylinder pressure with 422 cubic inches, 15.2 and 279 duration.

If you happen to come out with the same findings than I would have that camshaft cam doctored to determin exactly what it is. You also might want to find out what that core is intended for. There are two offered blocks and the camshafts definately differ.

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 11:52 am

Strange Magic wrote:
Fuel is C16
Scott take that nitrous fuel out and put some C14 in as mentioned in the above posts.

And pick up what .02?

I have to run N20 to run the 5.30 index as it stands.

Procision-Auto · Post by **Procision-Auto** » Tue Jul 03, 2007 12:05 pm

Use someone elses cranking compression gauge. I am having a hard time accepting that it is 160pds of cylinder pressure with 422 cubic inches, 15.2 and 279 duration.

I wouldn't worry much about the cranking pressure. YOu can alter those
results with valve lash alone..and in the end, you might lose power to satisfy
the reading you see on the gauge.

It's not worth it. If the car traps higher and runs a quicker E.T., don't let
the cranking pressure spin you in circles.

Old School · Post by **Old School** » Tue Jul 03, 2007 12:13 pm

I agree with strange magic. Something is wrong that you are only pumping 160. Should be 240 give or take a little.

Do you have an aluminum block?
Have you tried another gauge?
Any chance valves are too tight?
Are all the cylinders at 160?

Every .010 change in the valve lash should be about 10 lbs pumping compression. If you tighten .010 it should be 150-if you loosen .010 it should be 170. You might try this and see what happens.

I still think you have a faulty gauge. Best of luck - let us know what you find.

k-star · Post by **k-star** » Tue Jul 03, 2007 12:46 pm

I think there is something wrong with 160 PSI also.

Somebody else can check my numbers but with 15.22:1 and a intake closing event of 71.5 degrees you should have 235/240 psi of cranking compression...

Keith

Scott Smith · Post by **Scott Smith** » Tue Jul 03, 2007 1:21 pm

Old School wrote:I agree with strange magic. Something is wrong that you are only pumping 160. Should be 240 give or take a little.

Do you have an aluminum block?
Have you tried another gauge?
Any chance valves are too tight?
Are all the cylinders at 160?

Every .010 change in the valve lash should be about 10 lbs pumping compression. If you tighten .010 it should be 150-if you loosen .010 it should be 170. You might try this and see what happens.

I still think you have a faulty gauge. Best of luck - let us know what you find.

Dart Little M block.
I checked 5 cylinders. All were exactly 160 on 5 strokes.

Intake lash is .025 Ex is .027 cold with 1.88/1.80 rockers.

I have another gauge I can try but I think it will read the same.

If it had 235 I would smile like Paris Hilton when she got out of jail.