300hp Motor rebuild....

[Jim V.]

Would appreciate any and all engine rebuild advice for an original 65 300hp SB. Criteria includes:

1. Use original 870 block, top end (small valve heads, carb, intake, dist), and stock exhaust.
2. Must run on Pump gas
3. Realistic limited budget... (3k)
4. Civilized but spirited driver motor with

[Duke W.]

Re: 300hp Motor rebuild....

With a little "under the covers" work you can raise the RWHP from about 190 to about 215, which is close to an OE L-79 without loosing the much stronger bottom end torque and smooth idle, and it will pull strong to 5500+. The key is the heads. If you want the extra power in the upper third of the rev range, pocket port/port match them and do a three angle valve job. Do not install larger valves. You can do the porting/matching work yourself using the guidelines in "How to Hotrod your SB Chevy" and David Vizard's book.

Use all OE or OE quality replacement parts from Tier 1 vendors like Federal Mogul. No "hotrod" parts! Your engine has the "early" 300 HP cam, which is obsolete and no longer available. The replacement cam is 3896929, which went into production in 1967. It has refined dynamics, and the "929" is now serviced by 14088839, which has further refined dynamics.

Assuming your original block stamping is intact, BE SURE the machine shop understands that the block is NOT TO BE DECKED!!!

Your engine has the weak early 327 rods. As a minimum, they should be Magnaflux inspected (along with the crankshaft). If okay, they can be reused on a 5500 RPM engine, but I recommend replacement with stronger aftermarket rods on (pre-'66) L-79s and mechanical lifter engines.

Prior to disassembly of the short block carefully measure piston/deck clearance on all eight cylinders. You must have this dimension to compute the CR of the assembled engine, and you should also compute the actual CR of the engine before rebuild. I recommend that actual CR be kept at no greater than about 9.5:1, which will likely occur with OEM machined decks, the OEM or equivalent replacement pistons, and a typical .038" compostion head gasket. You will probably find that the original CR even with a thin shim OE type gasket was considerably less than OE spec.

I CANNOT OVEREMPHASIZE the importance of taking the required measurements to compute ACTUAL CR.

You can use the following online CR calculator.

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

The OE replacement pistons are fine, but some prefer aftermarket hypereutectic pistons, which are okay, but you MUST pay attention to compression height and notch volume if you want to achieve the target CR, which will allow the engine to operate detonation free on current unleaded premium. The OE pistons compression height is 1.671" and "dome volume" is -5.4 cc. The entire new/reworked rotating/reciprocating assembly should be balanced.

If the pistons you select have different compression height and volume, you can still calculate the true CR because the deck clearance measurements you took PRIOR to disassembling the block will allow you to compute the new deck clearance with pistons of any compression height and volume, and then run them through the calculator.

Taking the measurements to compute actual CR is the most overlooked engine rebuilding/blueprinting procedure. Many just install "low compression pistons" and end up with an engine that will probably run on regular unleaded, which is okay if that's your objective, but you will lower the torque curve and fuel economy by 5-10 percent.

Spend some time searching the archives for more information.

Duke

[Jim V.]

Re: 300hp Motor rebuild....

Thanks Duke...

On the cam issue, what is the difference between GM 12364051 and 14088839? I know the first is the"929" replacement kit which, I think, includes lifters and lists for $116 while the number you gave shows a price of $178.

On the "carefully measure piston/deck clearance on all eight cylinders" thing. I simple terms what am I missing if I don't do this? Are you saying without this number (average of eight cylinders..) I can not select the proper pistons or CC of head chamber in order to achieve the maximum compression ratio. Sorry to be a bit thick on this one!

On the rod thing I am trying to source the later design beefier rods but am having little luck. Since the motor will live under 5000rpm...suppose this maybe wasted time and $$.

Thanks as always. Your kind help is greatly appreciated.

JimV

[Christopher R.]

Re: 300hp Motor rebuild....

How do you determine and plan the compression ratio if you are unable to measure before the pistons are removed from the block?

I'm putting an engine together from a bare block that was disassembled before I got it.

[Duke W.]

Re: 300hp Motor rebuild....

The 839 has a very slight change in dynamics relative to the 929. If you want to e-mail me I can send you the several thousand numbers that describe the lobes and dynamics if you're analytically inclined.

If you don't take the measurements and used specified component data to determine CR, you don't know what it is. You want to keep it as high as possible, but not too high. It's about precision engine building and getting the most out of your configuration. You can just through in new bearings and hope the clearances are okay, or you can measure and insure that the clearances are in the proper range. Same thing!

For example the difference between the OE thin shim gasket and a compostion gasket it about half a point. A few thousanths here or there makes a real difference in CR.

Duke

[Duke W.]

Re: 300hp Motor rebuild....

It's tough to "plan", but you can take the measurements after assembly and compute the CR. If it's in the proper range, fine, If not, you'll have to make some "adjustements" like finding head gaskets of the proper thickness to achieve the target range.

Doing the measurements and planning UP FRONT allows one to select components that should achieve the target range, and it can be verified on assembly.

Duke

[Brian Monticello]

Re: 300hp Motor rebuild....

Jim,
I can help you out here. I took the readings Duke is recommending AFTER assembling the new shortblock. I ended up getting lucky because I had already selected my pistons and heads. With some gasket adjustments I got my compression ratio right under 10.0:1. I wanted 10.25 though. It was lower because the piston/deck clearance was much larger than the factory specs.

I also have a buerette which we can use to CC your heads. To take the piston/deck measurements, all you need is a digital caliper and some clean pistons. I have some later design rods that you have have for free. You'll just have to have the old piston pins pressed out and have them reconditioned. It may or may not be economical to do so.

Brian

[Greg S.]

Re: 300hp Motor rebuild....

Is there any way to determine compression ratio with an assembled engine? When I bought my 62 340 horse ten years ago the previous owner said the engine, casting 870, was rebuilt with flat top pistons and the compression ratio was 9.5 to 1. It runs happily on regular unleaded gas with stock 091 cam and double hump heads so I never questioned his statement. Rarely sees RPM in excess of 5500 but does start to pull harder over 4000 RPM. Your statement about weak rods on early 327 engines makes me ask if you would risk running this engine to redline at 6000+ RPM?

[Duke W.]

Re: 300hp Motor rebuild....

I don't know of any way to determine CR on an assembled engine. Cranking compression pressure means nothing because of variations cranking speed and, in particular, inlet valve closing point. (Note: It has NOTHING to do with overlap!) For example the 327/340 specified compression pressure is 150 psi versus 160 for the 327/300. Despite the higher static ratio, the later closing inlet valve of the Duntov cam reduces cranking compression pressure relative to the 300 HP engine.

If your 340 HP engine runs detonation free on regular unleaded then the actual CR is probably no more than 9:1, and the combination of low CR and high valve overlap is a recipe to kill the torque curve across the range and reduce peak power revs, which is probably okay if the rods are original. The '71-'72 LT-1s were specified at 9:1 and unleaded regular, but like most GM engines of the era, "as built" with production machined decks they were probably closer to 8.5:1.

It's a matter of how much time they spend at high revs. If rarely taken to the redline, the rods probably won't break, but engines that are run hard a lot are in jeopardy. I've previously discussed the badly cracked rod on my 327/340 that was probably only a few jaunts from the redline away from breaking and holing the block, and I've known of several early mechanical lifter 327s over the years that suffered broken rods.

Rebuilding any vintage engine with "low compression pistons" is a myth. I don't think most guys understand and appreciate that double shim gaskets or a typical composition gasket will drop the CR half a point relative to a single shim gasket. Also, the advertised CR is based on nominal deck height, but most were machined high by 10-15 thou average, which is nearly another half point, so your "11.25:1" was really much lower, and it can be lowered more with head gasket selection and a little chamber grinding down to the range of 10.25-10.5:1 using the OE pistons.

Knowing the highest ACTUAL CR you can run and then planning your project accordingly using OE pistons (or equivalent) is the best choice. Usually just running a typical composition head gasket and maybe a little chamber gringding will lower the actual CR enough that combined with the typical high decks will drop the CR to a good range for current unleaded premium - no more than 10.5:1 for SHP small blocks and 9.75 for medium performance. For big blocks, maybe subtract another quarter point for some extra margin since the bigger bore is more prone to detonation.

Duke

[Tom D.]

Re: 300hp Motor rebuild. Very timely 4 me! *NM*

[Duke W.]

Re: 300hp Motor rebuild....

I find the best way to measure piston/deck clearance is with a machinist bar and feeler gages. Due to piston rock using a caliper or depth mic can be tricky, but if you can slide two feeler gages, one on either side of the piston with equal drag, you probably have a true measurement.

Duke

[Mark #28455]

Duke, what's your experience

In a properly assembled BB with iron and aluminum heads, how high of a dynamic compression ratio (taking into account valve timing and rod length as in the KB compression ratio calculator) have you been able to run with 93 octane pump gas?

Thanks in advance,
Mark

[Duke W.]

Re: Duke, what's your experience

...don't have any direct experience with BBs - just anecdotal evidence.

In terms of DCR, on SBs about 8:1 is the limit, and my guess is that the limit should be about a quarter point lower for a BB with iron heads. Aluminum heads can probably take about the same as an iron head SB.

DCR isn't the only parameter. High overlap can promote detonation even if the DCR is relatively low, particularly at low revs because the high exhaust gas residual heats up the mixture.

Duke

[Greg S.]

Re: 300hp Motor rebuild....

In 62 the 300 horse engine varied from the 340 horse by camshaft, distributor and pistons. Block and heads were the same. Yet, the advertised compression ratio of the 300 horse with the flat top pistons was 10.25 (I think). Was this optimistic? Why would a 340 with the same flat top pistons produce a C.R. of 9.0-1? I don't doubt you I just want to understand.

[James W.]

Re: 300hp Motor rebuild....

Duke,
Duke,

Do you know if there are any OEM type steel shim head gaskets availble today or are they all the composistion type? The Fel-Pro Perma Torque/Blue head gaskets have an advertised thickness of .041" with a 4.166" bore and others like the SCE copper gaskets are .031" thick with an advertised 4.060" bore. The SCE gaskets have an optional thickness gasket of .042" and .062", the same sizes are offered for both bore sizes of 4.060" and 4.155". Milodon offers a .030" thick copper gasket for both the 4.060" and 4.155" bore small block chevrolet engines.

When I rebuild my 327/300hp engine in my '64, I'd like to keep the CR as close to the advertised stock CR as possible. I plan to use a Keith Black, Silv-O-Lite piston set as replacements, there are tow different CR's; the first CR with the flat top piston is 8.4 to 10.3:1 the other is 9.1 to 11.3:1 with a .100" piston dome, both with a .030" overbore. They advertise that these pistons can be bought in standard bore size as well as .020", .030", .040" and .060" oversizes. I would like to keep the overbore to .020" to .030". I also like the Sealed Power piston sets also, but the ones I've looked at advertise the CR's at 10.35 with a 64cc chamber and a .125" piston dome and the other specs at 7.65 CR with a -21.1cc "D" shaped cup.

Any comments on the head gaskets ans pistons I'm looking at above?

Thanks,

James West