Dynamic Compression Ratio

[Autoholic]

I've been doing a bunch of calculations, for fun, and after a particular DCR I figured it is worth talking about with this awesome lot. I've been looking at the DCR for the original SOHC crate engines and I'm not sure if I believe the rather high DCR I've found. It would explain the decent performance of the crate engines but I'm not sure if they are accurate as such a high DCR could lead to trouble. Below are all the factors I used to compute the dynamic compression ratio.

Bore: 4.232"
Stroke: 3.784'
Rod Length: 6.489"
Static Compression Ratio: 12.01:1
Intake Closing Angle ABDC: 57

   The ICA value comes from the following cam specs and calculation.

   Factory Cams Profile
   Lift:  Intake - 0.433"  Exhaust - 0.433"
   Duration (seat to seat):  Intake - 258  Exhaust - 258
   Lobe Separation Angle - 108

   ICA = (Intake Duration / 2) + LSA + total advance (ground in advance + timing advance) - 180
   ICA = 129 + 108 + 0+0 -180 = 57

Altitude: 0 feet, sea level
Boost Pressure: 0 PSI

Plugging these values into this calculator: http://www.wallaceracing.com/dynamic-cr.php  Gave me a DCR of 10.08:1 with a dynamic cylinder pressure of 213.82 PSI. I know this is largely effected by the static CR and the ICA. If you're running high compression, you almost have to run an aggressive cam to bleed off some pressure. But the cam used is rather mild, resulting in a low ICA. So I wanted to check with all of you if this made sense. Below are my two sources for the specs.

http://www.racedyne.com/Racedyne_Inc./427_SOHC_Cams.html
http://www.carmemories.com/ford-engine-specifications/ford-427-428-429-specs/427-motor.html

I would also really like my sources to be confirmed or be corrected on the specs. I care a lot about historical accuracy.

[jayb]

Using your numbers I get the same DCR, but I don't think your cam specs are correct.  Pretty sure that seat to seat duration on the SOHC cams was over 300 degrees.  Those might be @.050" numbers, the SOHC cams had a really slow ramp at first...

[Autoholic]

Using your numbers I get the same DCR, but I don't think your cam specs are correct.  Pretty sure that seat to seat duration on the SOHC cams was over 300 degrees.  Those might be @.050" numbers, the SOHC cams had a really slow ramp at first...

Well Racedyne has them listed as seat to seat, so that is why I used 258. If they actually were @ 0.05", then the numbers wouldn't be usable because compression can only happen once the valves are closed. I'll dig into some files I have, someone ran a test that mapped valve lift for 2 or 3 different cams and one of the cams used was the factory spec. If the cams actually had a number close to 300, then the DCR would be more practical.

[Autoholic]

Alright, I found the info I was looking for...

http://www.network54.com/Forum/74182/thread/1239079728/SOHC+Valvetrain+Testing+-+Old+School+Cam+vs.+New+Tech

This data shows just how long the duration is. The problem is reading it accurately. I looked at the Comp 8440 at first to try and read it correctly. On the first image, I see the peak being around 460 degrees. Looking at the zoomed in image, I see the duration starting at 0.005" at 305 degrees. This would match the rated duration of 310 for this cam, 2(460-305) = 310. Using this as a starting point, I see the factory as starting at 270 degrees at 0.005" of lift. So 2(460-270) = 380. Does that seem realistic for the factory cams, 380 degrees of duration? If that is correct and let's say the LSA was 108, the cam profile would be...

Lift: Intake - 0.433" Exhaust - 0.433"  (this does match the data)
Duration: Intake - 380 Exhaust - 380
Lobe Separation Angle: 108

This would result in a ICA of 118 with no advance. That can't be right. So the duration has to be more than the comp 8440 but not by a drastic amount. If we can figure out the cam data that Bill did, we could get a pretty accurate idea of the factory cam specs. Unless someone has them, then that would be a lot easier.

[jayb]

I think where you are getting tripped up is that the data in the chart is measured using the base circle as zero.  The valve of course has lash, 0.016" for the factory SOHC cams on the intake side, and that is where the seat to seat timing should be measured.  Bills post says that the factory SOHC cam lobe duration is 328 degrees, which rings a bell with me, so I'd use that for the DCR calculation.

By the way, the data shown in Bill's post is data I took, with my cam checker 

[Autoholic]

LOL, completely missed the advertised duration in the post. I remembered the data and went straight to it once I found it. So, using that information, the factory cam specs would be...

Lift: Intake - 0.477" Exhaust 0.477"
Duration: Intake - 328  Exhaust - 328
LSA: 108?

ICA would be 92 with no advance. This would give a DCR of 7.14:1 with a DCP of 137.03 PSI, which makes sense. Thanks for your help Jay. What CR are you running on your engines? Have you ever seen a SOHC piston like this before? I imagine it would have a rather low CR but with a mild cam, a decent DCR. Or if you were to run the Robert Pond heads, probably still end up with a good CR.

[jayb]

That sounds about right for the specs, although the data I took didn't show the same lift that Bill listed.  I will try to get a chance to measure one of my stock cams tonight and see if I can measure the actual lobe lift (before rocker ratio).

I've run anywhere from 11.5:1 to 13.4:1 on my SOHCs.  Those engines really seem to like compression.

That piston in your post is probably a blower piston, maybe 8:1 or something like that.  All the naturally aspirated SOHC pistons I've seen have much bigger domes than that one.

[Autoholic]

How well does this engine run on pump gas with high compression? And what DCR are you seeing with high compression? I think I read somewhere you were running the 8440 cam, or something similar.

[jayb]

The 700-800 HP engines I've done have had DCRs in the 8.3-8.4 range, and they have run well on the street with 92 octane pump premium.  But on the dyno and the track, I always use race gas, because I don't want to risk the engine with pump gas that may be suspect.  The engine I'm working on now (over 900 HP) also will run fine on pump gas, but also I keep my foot out of it; I just get from point A to point B with the pump gas.   The DCR is something like 9.3:1.  I have a pump gas tune I put into the EFI system to pull out some timing and richen up the mixture, and then I drain the pump fuel at the track and replace it with race gas, and upgrade the EFI tune, before I run.

[Autoholic]

Will you be doing another build write up for this new engine? I learn something new each time you share your build process.

[machoneman]

Not to hijack the thread Jay and I do know you have spoken of this before: any ideas or plans to allow increased lift on a SOHC head?

This is a stumbling block for even more hp yet I don't know if you have written off any plans to address same. 

[Autoholic]

I know there was a roller rocker designed by Jay that was improved on by a few others that would come in at a 1.5:1 ratio. Any updates on that Jay? This does play into the DCR, better rocker ratio allows for more aggressive cam profiles.

[jayb]

The high lift rocker is still on my project list but I haven't made any progress on it for a couple of years.  Too much other stuff to do, I guess.  My current engine is already together, but most of it has been detailed here at one point or another; the engine uses an offset bore Shelby block to get to 4.700" bore spacing, to allow a 4.500" bore.  After the connecting rod broke last year, I had the block repaired and resleeved, and basically put it back together the way it was.  I'm still working on optimizing the intake manifold and header configuration on the engine, but I should have it in the car in time for Drag Week.  Last year the engine made 970 HP at 7100 RPM, and this year its doing a little better, but I'm keeping the numbers confidential until Drag Week.  This is with a .739" lift cam, which I find pretty impressive; I don't know of any other naturally aspirated engine that makes that kind of power with that little lift.  I think more lift may show a big power improvement...

[Autoholic]

You're gonna have a 1000+ hp at drag week, won't you? You're pretty much at the limit of valve lift for these rockers if I remember correctly, something like 0.015" of clearance. Are the heads on this beast Pond or Coon's special high flow oval heads that are even harder to get than his regular ones?

[jayb]

Actually, I'm keeping the specs on this engine confidential until after Drag Week.  Also, I don't know anything about an oval port Coon head.  I try not to think about Bill Coon 

PS - I just measured one of my stock cams and got 0.415" lift at the lobe.  With the SOHC rocker ratio this is about 0.550" lift at the valve, which is close to the same as what is shown in the lobe comparison chart.