Opel GT Forum banner
1 - 10 of 10 Posts

·
Opel Rallier since 1977
Joined
·
2,312 Posts
The cranking pressures reported above indicate a static CR of around 12 to 12.5, with a typical cam, not 14 or 15. So it is not quite as bad as you think. Dividing standard psi atmospheric into the cranking readings is not how it is computed... you have to convert you gauge reading to absolute psi then account for the intake closing angle on the compression stroke...

But that would be with a normal cam.... you don't have a normal cam any more.... just a hunk o' junk LOL. This damaged lobes with shortened durations, and thus an early intake closing angle, will raise the cranking pressures considerably, probably at least 20 psi, and it will throw off normal comparisons. So I would agree that these are more like 11 to 11.5: SCR pistons.

That piston to bore clearance is pretty common for the older materials in forged pistons. It will have plenty of blowby and the pistons may rattle a bit at cold start-up. The rings will not live as long either. So those are not a great recipe for a long lived street engine. I used some old forged units that I had for years in an engine about 1 year back, and it has those characteristics.

If you keep the pistons, then 10:1 true SCR is a good place to shoot for with a mild street cam. But realize that your end cranking numbers are a combination of cam and SCR. It'll take some modest attention to timing and tuning to make it run without detonation but that is my standard street SCR target, so that I end up with around a 155-160 psi cranking pressure with a modest duration cam that helps the low RPM torque (as much as you can on a 1.9L).

Here is a pix of how that last pistons looked in the block with the domes massaged a bit to get to the desired SCR; this was with larger valves and a Felpro head gasket. These pistons stick about 0.012" above the block deck in the quench side (the large flat area) so this 'dome' looks lower.
Engineering Machine Space Metal Auto part

Actual cranking pressures on this engine are 170-175 psig at 2400' elevation with the Isky OR-4H cam. The slow opening and closing ramps on this cam profile make the cam durations a bit on the short side so that raised the cranking compression numbers above what I had planned. But it runs without detonation.

BTW, when you get the crank set up and do an initial re-assembly, but before you have milled the domes, put together one piston-rod assembly and assemble it in, and measure your flat quench area relative to the block at TDC. This is important to push up CR with while avoiding detonation. Take that clearance and the head gasket thickness and work out the quench/squish gap and see if you have something like .035"-.045". Setting that gap in that range (between the flat quench area of the piston top and the flat area on the head) is a known good method to fight detonation. Closer is better, but too close and you risk a piston kissing the head. You can massage the block's deck height and the gasket thickness and the quench are on the piston head to dial this in. This may be getting in deeper than you want, or more than the budget allows, but I wanted you to be aware of this gap and its beneficial effects.

BTW, your local barometer readings are not the actual absolute air pressures in your local atmosphere. Barometric readings are normalized to sea level pressure.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
And you can always open up the combustion chambers in the head to lower compression ratio.... I have purposely dropped 1 full point in CR that way.

And FWIW.... 150 closed and 300 open presures are doable with flat tappets. It's done all the time. Try to find what oil was used and determine the ZDDP content. That is the most likely cause of the tappet/cam failures.... too many people are not up to speed on the need for adequate ZDDP when spring pressures are increased. Soft, stock pressure setups seem to survive with the newer lower ZDDP oils, but higher spring pressures will cause an almost 100% guaranteed fail of the lifters/cam without the older, higher ZDDP levels in tyhe oil.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
Kwil... are you familiar with computing DCR.... dynamic compression ratio? That is what you really want to know and relates directly to cranking compression. You set the SCR to work with the cam duration and timing to achieve the desired DCR number. For street running on premium gas, you ought to shoot for around 8:1 DCR. That invokes some reasonable degree of tuning, timing, and carb mixture control, but not an inordate amount. Most gearheads can work with that DCR on premium and avoid detonation.

If you want to use a cam with an advertised duration of around 268, then for a DCR of 8.0 then the SCR needs to be right at 10.0:1. If the deck height is 0.000" (i.e., the top of the piston's flat is 0.000" relative to the deck at TDC, then the net volume of (dome volume - valve reliefs volume) needs to be 2 cc's. This assumes a head gasket thickness of .040" and a chamber volume of 50.0 cc. Once these things are known (deck height, head gasket thickness, and chamber volume), then the net dome volume can be dialed in.

Please let me know if you want further help in this. The DCR calculator that I used is the Pat Kelley calculator that can be downloaded here: Dynamic CR

Some knowledge of how to use this is helpful; let me know if you need assistance.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
Keith, just so it’s clear, dynamic CR never looks at the cylinder pressures compared to atmospheric pressure. That’s not how it’s calculated. Wallace Racing has a good DCR calculator and it will give a rough idea on dynamic cylinder pressures.
While atmospheric pressure does not enter into SCR or DCR computations, DCR is a step to reach cranking compression estimates, which can follow on with average local atmospheric pressure to compute actual cranking pressures... which reflects the loss in cylinder pressures in general as altitude increase in a normally aspirated engine.

I start with The Pat Kelley DCR calculator as it is so easy to enter measured parameters from engine builds directly, and things like dome volumes and eyebrow volumes (or the net of the 2) can be entered directly from piston catalog data to get SCR and DCR, plus it gets you a good view on cam timing events. If I want to know altitude effects, then I jump to the Wallace calculator with the Pat Kelley outputs.

Good feedback on the ZDDP.... hope it is all true..... seems like he would know but that sure is suspicious. Look forward to hearing the spring pressures... and an examination of the head and the oil pump to see if just a plain ole head oiling failure could have been the culprit. 7 of 8 is a big failure.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
I hadn't contemplated the effect of the damaged cam on Dynamic Compression Ratio (DCR), which, to some extent, a compression test exhibits. It makes perfect sense that the early intake valve closure and late exhaust opening would increase the DCR, and hence compression test readings. It is surprising to me how much it affects it.
Which is why smaller cams are used for street engines or in any applicatoin where lower RPM 'umph' is desried.... it creates more cylinder pressures at low RPM's (when the exhaust scavenging has not yet kicked in) and thus more low RPM torque. You just have too much of a good thing now LOL
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
Bob, is Cometic Gasket's part listings for the head gaskets the uncompressed height or compressed height?
Everything I have ever read, and every number I have used for Cometics, is that their specified thickness IS the compressed thickness. The uncompressed thickness all over the map due the multi-layer make up of these gaskets, plus the embossed sealing ridges. One example of an actual measurement done in the field:

Be aware that Cometic spec's a finer head and block milling finish for their gaskets. They have softened their insistence on this in the last few years, but it is something to be aware of. From what I have learned in using them, the older flat grinder methods of finishing blocks and heads (like Blanchard grinders) typically produced a rougher finish that might give the Cometics some sealing issues. Newer milling methods tend to produce a finer surface finish that does not give any problems.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
Good deal on the springs.... it's unlikely that those pressures would damage the cam IF the ZDDP was maintained with good break-in practice. Make sure you check out the whole head oiling system thoroughly to make sure it is all good. My last spring install was:
  • At an installed height of 1.60 to 1.55, the closed pressure is 120 lbs + or -. (Stock numbers that I have are 85 lb IN & 74 lb EX.) For my .407 lift cam, open pressures are going to be around 210-220 lbs, vs the stock 150-160 lbs.
  • This works out to 235 lb per in spring rate.
It's all surviving well so far.... the lifter were re-conditioned units from OGTS... I trust the old metalurgy more than new. (And I think Gill feels the same.) Break in was with Gibbs oil and I have switched to Mobil1 15W-50 to get a better cold start viscosity to keep down some of the leakdown and start-up lifter rattle. (The higher spring pressures cause more leakdown issues.)

The piston milling oughta work well. Is the deck height above or below the deck? Just be sure of the sign convention used in the calculator.... ditto for the sign convention used for the valve reliefs and domes. This is one spot where folks go astray. Sounds like the piston to head clearance on the piston flat will be 32 to 36 cc (depends on the deck height being above or below block deck ). Either would do well for quench/squish effect to help combat detonation so that is good. I just get a bit nervous down around 0.030"; my last was .028", but so far, no evidence of pistons kissing the head!

BTW, the the cam duration of 268 that you used is the right number for DCR computations if you got it out of the OGTS ads, and would be called 'advertised duration' by US cam companies. Just be aware that the slow ramps of the old Isky hydraulic cam designs will make the duration effectively shorter than 268 in its effect on DCR (and raising the cranking compression numbers). I'd like to hear your cranking compression numbers when it is broken in. A real DCR of 8 at 3000' computes to a cranking compression of around 150 psi.
 

·
Opel Rallier since 1977
Joined
·
2,312 Posts
FWIW, I noticed that the fire rings were sealing well on the pix of the head. As long as they seal, there is no need to go bigger. It is only to make sure that the rings register well around each cylinder.

When I have selected Cometic's in the past for Mopar V-8's I have typically gone to holes .080" (2 mm) larger than the actual bore, but that extra is just to insure that any bore mis-registration is not going to slip under the edge of the fire rings. Sometimes bores DO get registered off-center a bit when cylinders get bored. (And on occasion, that is intentional...)
 
1 - 10 of 10 Posts
Top