BB rebuild - too much compression?

[Ted S.]

Re: BB rebuild - too much compression?

Here's a link on Dynamic Compression Ratios. It also has some links to DCR calculators. It looks like the cam is the same as the L78 396 cam and it closes the intake at 80* ABDC @.006". The piston is quite aways up the cylinder so your DCR will be quite a bit less than your static compression ratio. With iron heads you want to target below 8:1 DCR which I believe with that cam and 11:1 static compression ratio (or 10.5 as it may have been originally) you'll be fine. For a cam with that late of intake closing you'll want to have a static compression ratio above 10:1.

Search the archives as Duke and other have posted on this topic several times.

http://www.empirenet.com/pkelley2/DynamicCR.html

[Duke W.]

Re: BB rebuild - too much compression?

It looks like the cam is the same as the L78 396 cam and it closes the intake at 80* ABDC @.006".

All SHP big blocks use the same cam lobes and lobe phasing. The difference between the '65-'66 and later cams is the groove in the rear journal of the early cams. I don't know how you came up with this closing point, but it's not even close.

DCR is just a guideline and can be very misleading. For a relatively short duration cam like the 327/300 cam 8:1 DCR is okay, but to achieve this with SHP cams takes in the range of 12:1 SCR.

Most DCR calculators don't define what inlet valve "closing point" you need, and even if they do, most available camshaft specifications aren't sufficiently granular to determine that point.

The maximum SCRs I recommend for OE configurations are based on anecdotal evidence from known original, unrestored engines, actual tests of restored engines that had their SCRs precisely measured, and a bit of engineering intuition.

Duke

[Clem Z.]

Re: BB rebuild - too much compression?

the specs i have for cam #143. tappet lift @ .050, intake open 13 BTDC,close 49 ABDC, exhaust open 61 BBDC, closes 1 ATDC. ,max lift intake 108 degrees, max lift exhaust 120 degrees

[Duke W.]

Re: BB rebuild - too much compression?

...and .012" of that .050" tappet lift is clearance ramp...

Duke

[Ted S.]

Re: BB rebuild - too much compression?

He indicated that he was using a comp cams equivalent. Here's the spec card for what I understand he is putting in the engine. As you'll note on the card it is the grind comp uses as a replacement for the 3863143. On the spec sheet it shows the cam intake closing event "Valve Timing @ 0.006" Lift: 80* ABDC"

http://www.compcams.com/Cam_Specs/Ca...px?csid=4&sb=0

Having researched DCR for running engines on pump gas. The general consensus is 8:1 for iron heads and 8.5:1 for aluminum heads is pretty much the maximum DCR you'd want to run. If you approach these numbers you need to take special care (i.e. smoothing the pistons, chambers, etc) to eliminate hot spots in the chamber that may cause pre-detonation. Given that I generally try to match static compression ratio and cam timing to go at least .1 - .3 under these guidelines. I've built multiple big blocks and small blocks with no detonation problems on pump gas.

The problem is you go into a generic machine shop and everyone instantly says you need to run 9 or 9.5:1 static compression to run on pump gas. That simply isn't true. If you really want a dog of an engine run that type of a comprssion ratio with the cam indicated. To run the factory muscle cams you need compression and yes they will run on the current 92-93 octane pump gas.

[Ted S.]

Re: BB rebuild - too much compression?

Joe, Just my .02, If it was mine I'd run:

1. Factory replacement pistons (i.e. factory target compression ratio as others stated due to variances in machining/component parts (ie head gaskets) will probably end up somewhat lower)
2. Run a factory replacement cam (i.e. comp 11-106-3)
3. Make sure the distributor is to factory specs with a factory vacuum advance unit (i.e. no generics)
4. Factory rated thermostat

With that you should be fine with running on pump gas. I ran the metrics with the information I had and roughly the engine would calculate to a 7.4 - 7.6 DCR depending upon the actual static compression ratio.

FYI - In a pinch if you were experiencing problems you can add xylene or tolulene to pump gas to raise the octane. If you look at the formulation of race gas it primarily has a higher concentration of aromatics (usually toluene -this is not to say that is the only difference). These are generally available at most paint or hardware stores. Toluene is 114 octane. Here's one of many links available discussing this topic.

http://www.best-chemical.com/racingfuel.htm

[Duke W.]

Re: BB rebuild - too much compression?

He indicated that he was using a comp cams equivalent. Here's the spec card for what I understand he is putting in the engine. As you'll note on the card it is the grind comp uses as a replacement for the 3863143. On the spec sheet it shows the cam intake closing event "Valve Timing @ 0.006" Lift: 80* ABDC"

http://www.compcams.com/Cam_Specs/Ca...px?csid=4&sb=0

What about the clearance ramps that are .012" above the base circle? Is this ".006"" above the base circle or above the top of the clearance ramp? Where is lift measured -at the valve or lifter?

That's the problem with all these "specs" that everyone throws around. Rarely is the full contex of the data stated.

Like IT guys say - garbage in, garbage out!

Duke

Duke

[Ted S.]

Re: BB rebuild - too much compression?

Do you have the specs for a GM 3863143 cam? You indicated that the 80* ABDC spec from Comp "wasn't even close" so I assume you have the specs for the GM part number, correct? What is the spec for the intake valve closing event?

One would believe if the major manufacturers (i.e. Crane, Comp, etc) are making "blue print" cams that the cams would have the same specs as the GM original.

Joe is trying to determine if his 427/425 will run on pump gas with "factory replacement" pistons. Given the variances in machining of the heads/block/etc, parts used in the rebuild, etc. most parties replying view that the compression ratio will be somewhat less than the advertised 11:1 static the factory represented. With using the 80* ABDC and an 11:1 static the DCR calculators show a 7.6 DCR which should run fine on 92-93 octane pump gas. At 11.5:1 static it goes up to 7.95 which is pushing the limits on 92-93 octane pump gas. At 10.25:1 static the DCR drops to 7.13 which should be able to run on 87-89 octane pump gas.

Likewise if the intake closing event is at 75* instead of the 80* it again pushes the envelope on pump gas with 8.02 DCR

Just trying to help Joe out.

[Duke W.]

Re: BB rebuild - too much compression?

I understand you are trying to help - so am I - but you are placing total reliance on this DCR calculation using a vague "inlet closing point" and using 8:1 as a line in the sand. Relying on such simplistic logic and vague calculations will get you in trouble. It's just not that simple!

As I said earlier, short duration cams will tolerate 8:1 DCR, but SHP cams with tolerable SCRs will come in with DCRs in the low to mid sevens.

I have a considerable body of evidence for the maximum true SCR that various OE SB configurations can tolerate on today's pump premium. The evidence for BBs is not as great, but for L-72 I recommend a maximum true, measured SCR of 10.25 (if your available pump preimum is 93 PON) with a maximum variation of 0.1.

Duke

[Ted S.]

Re: BB rebuild - too much compression?

So you don't have the specs. Hmmmmmmm! I have experience too and mine doesn't show the same results as yours. Hmmmmmmmmm! Easy to be a critic where's your evidence? I have virtually the same engine only its a '69 396/375 .060 over currently running in a '69 camaro, factory replacement pistons, same cam. 840 heads....... NO DETONATION PROBLEMS and you?

[Michael H.]

Re: BB rebuild - too much compression?

I understand you are trying to help - so am I - but you are placing total reliance on this DCR calculation using a vague "inlet closing point" and using 8:1 as a line in the sand. Relying on such simplistic logic and vague calculations will get you in trouble. It's just not that simple!

Yup, that theory completely ignores "after BDC cyl filling" and air flow. There's a LOT going on between BDC and the point where the intake valve closes that can/does increase cyl pressure, which automatically increases C/R, the same as increasing the mechanical compression ratio. Calculators won't give you the answer.

Just put the heads on with steel gaskets (coated with aluminum paint) and go for a ride.

[Ted S.]

Re: BB rebuild - too much compression?

Yup, that theory completely ignores "after BDC cyl filling" and air flow. There's a LOT going on between BDC and the point where the intake valve closes that can/does increase cyl pressure, which automatically increases C/R, the same as increasing the mechanical compression ratio. Calculators won't give you the answer.

Just put the heads on with steel gaskets (coated with aluminum paint) and go for a ride.

Michael, I couldn't agree more with your last statement. If the block hasn't been decked the steel shim gaskets are great. Someone mentioned using thicker gaskets to lower the compression ratio. Anyone that has done their homework on max performance on pump gas knows that quench is one of the critical components if you're pushing the envelope. Increasing quench = increasing detonation problems.

[Duke W.]

Re: BB rebuild - too much compression?

The importance of quench clearance is way overblown. Taylor says once quench is greater than .005 time bores the effects on detonation are minimal. That's .020" inches on a 4" bore.

GM double gasketed SHP/FI engines beginning as a running change in mid-'62 at least through the end of '63. The second .018" shim gasket drops actual CR about half a point, and mitigated the detonation despite the increase in quench clearance.

To the OP - John McRae started a discussion on a detonating L-71 that had been decked. He removed the heads and took the measurements to compute SCR. It was defiinitely too high. Some discussion and suggestions ensued and you might find the tread useful.

Duke

[Ted S.]

Re: BB rebuild - too much compression?

Duke, I give up, despite your ancedotal "evidence". I've spent the last several years researching and experimenting with "running engines on pump gas", have built both small and big blocks based upon that research and experience. I have had good success following the research and the experiences of others and myself. I never said that DCR was the bible nor that Joe should target 8.0:1 DCR. 8.0:1 DCR is a guideline for iron heads. There are other factors that do come into play. Spend some time and research the topic, you may surprise youself on the breadth of knowledge that is out there. Everyone out there is not an expert but several have done considerable experimentation in this area to see what works and what doesn't.

Joe, My experience indicates you'll be fine. Check the other aspects to be sure they are within GM specs (i.e. quench, cam timing, distributor specs, etc.). Some steps you can take are eliminating potential hot spots in the chamber (i.e. smooth piston edges, smooth the combustion chamber, etc - and yes Duke I do realize these actions will lower the SCR which also lowers the DCR.)

Good day sir.