Duke,B28 can

[Jim Cear]

Duke, cam in my car is the "097" and on my vac. gauge(Sears) does read approx. 15"(new rebuild and have only reset valves once to 8/18)I've ordered a VC1765/B20 for the safety factor, but have not detected any detonation in the brief period I've driven it with the B28, although it seemed a bit peppier (could have been my imagination).As I said the idle had stabilized and I'm convinced your solution was spot on. Thanks

[Duke W.]

Re: Duke,B28 can

What is the normal idle speed you use? Idle vacuum should always be listed as vacuum@idle speed because higher/lower idle speed yields higher/lower manifold vacuum.

If you aren't getting any detonation you can advance the initial timing. The proper range is 10-14 (and set it below 700 to keep the centrifugal at zero), but I ran as high as 16-18 back in the sixties with Chevron Custom Supreme when I went out cruising looking for trouble. Advancing the timing up to just short of the detonation limit will give crisper throttle response, better fuel economy, better low end torque, and a little more top end power.

Does is still have the OE type forged, domed pistons? As OE built the engine was "double gasketed" - two steel shim gaskets. This practice began on SHP/FI engines during the '62 model run.

As built with typical deck height and two shim gaskets the actual CR is no more than about 10.5, and the same if the engine is rebuilt with a typical composition gasket.

With the relatively late closing inlet valve on Duntov and other SHP cams, dynamic CR is well under 8:1, and with no more than some ignition map tuning these engines will run detonation free on available unleaded premium.

Installing "low compression pistons" is a mistake because low compression and high overlap is the worst possible combination and will create a "soggy dog".

The OE advertised CRs were about as realistic as the OE gross HP ratings. Even the 300 HP engine with nominal deck height (and most are 5-10 thou high) and a shim gasket is only 9.75:1 versus the advertised 10.5 (later is was dropped to 10.25, but that was still way optimistic). If a typical 300 HP engine is rebuilt with a composition gasket the actual CR is probably in the range of 9-9.25, and they might even run on 89 PON midgrade.

Duke

[Jim Cear]

Re: Duke,B28 can

To follow up Duke, I was probably idling @ about 650 RPM when obtained vac. reading of about 15". I'm currently running the initial at 10 degrees to be on the safe side with a fresh motor(1000 mi.), it was rebuilt with TRW forged flat top pistons, Crower rods(your suggestion), the original forged crank,(40 over block), and was assembled with a composition Fel Pro head gasket. I tried some 89 oct. and it didn't like that at all. I've since had the ignition curve returned to stock so possibly the 89 might work now. What the actual CR ended up at I really don't know other than my machine shop trying for a 9.5CR. It certainly doesn't seem like a "soggy dog" at all, has good throttle response, seems to pull strong and sounds great. Havn't had it above 4500-5000 rpm yet, but I know I'm gonna give in to that one soon.

[Duke W.]

Re: Duke,B28 can

Unless they measured deck clearance and ran all the data through a CR calculator the CR is unknown, but is probably in the range of 9-9.5.

That's why I always harp about measuring deck clearance on teardown. With deck clearance dimesions, the proposed piston specs, and proposed head gasket thickness, you can compute the actual CR of the new engine, and select alternate head gasket thicknesses (or at worse machine a little off the pistons crowns or domes if you have to) to acheive in the range of the target max/min. If you buy "10:1 pistons" the actual CR may be as little as 9:1 depending on deck height and head gasket choice. This is the MOST OVERLOOKED aspect of engine overhaul. Not even "professional engine builders" pay any attention to it. Especially with a big cam (including the OE SHP cams) you don't want to leave any compression on the table. High overlap and late closing inlet valves REQUIRE high compression to perform to potentitial including reasonable low end torque. The blanket advise by "professional engine builders" to drop the compression ratio of vintage high compression engines is just brain dead. The system engineering should be accomplished for EVERY individual engine. It is neither difficult, nor time consuming - just attention to very important details.

Your replacement cam appears to have much less overlap than the Duntov based on your manifold vacuum readings, which will make up for the lost low end torque. Low compression and mild cams make decent low end torque. A big cam and low comression is the forumula for a "soggy dog". In fact, your engine might have more low end torque than OE because the replacement cam is probably milder than the Duntov, but the top end will not be as strong.

Essentially what you have done is "detuned" your 340 HP engine to something closer to a 300 HP engine. That's okay if that was your objective, but it's not the way I would have done it.

Pocket port the heads, LT-1 cam, OE replacement forged, domed pistons set up at about 10.25-10.5 (about what you get with typical deck height and a compositon gasket) and the Crower rods - 350-360 honest gross HP (dyno test proven!) at 6500 (about 320 net and 270 at the rear wheels with SAE air density correction), and it will rev safely without worry to 7000. All this with about the same low end torque as OE built and for reference an OE built 340 HP will only make about an honest 300 gross @ about 5500, but most did rev strongly to 6500. So you can achieve 20 percent more on the top end without a loss of low end torque and no visual clues that your engine is "modified" - all without serious detonation issues.

Also, for reference, a decent OE built 300 HP engine has about 225 net and will measure about 190 RWHP. SHP/FI engines are in the range of 260-275 net, 220-230 RWHP. FI won't make much more power because the OE machined heads are choked. Open up the heads and FI starts to pull away from their carbureted cousins, but FI has less low end torque because of the single plane manifold configuration.

The advertised OE power ratings were GROSSLY overstated, but are achieveable with some head massaging and a reasonably high CR that will live with today's premium gasoline.

That's why I recommend pocket porting the heads. It's the single best modification you can make - more top end power and revs without loss of low end torque, and it's undetectable. The LT-1 cam's very late closing inlet valve reduces DCR to about 7.4 with 10.5 static, so detonation is not a problem with a reasonable (not too agressive) centrifugal curve. (With the Duntov cam try to limit static CR to about 10.25 as it has an earlier closing inlet, which increases DCR.)

Duke

[Clem Z.]

to get a true CR reading

you must calculate the area of the open space between the cylinder wall and the top of the piston down to the top of the top compression ring.most performance head gaskets give the CC volume of the gasket opening in their catalogs or even on the head gasket package .

[Duke W.]

Re: to get a true CR reading

On a racing engine where you are looking for the maximum possible CR that space you refer to should be calculated and added to chamber volume.

For street engines I just use the measured deck clearance (adjusted for any compression height change for the new pistons), head cc, piston net volume, and gasket thickness and OD to calculate the CR using:

http://www.csgnetwork.com/compcalc.html

That added volume means that it's a tenth or so lower than calculated. On a street engine the "spec" should be "not to exceed...", and it doesn't hurt to have a little margin so any error is on the low side.

Duke