65 Fuelie - 11:1 compression - Octane Booster?

[Joe L.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

11 to 1 .30 over forged pistons in 66 L-79 and I have had no problems with detonation on premium (92/93 octane). Just a side note on the rebuild, my builder like many builds a lot of race engines and is quite experienced advice me not to use hardened valve seats as my heads didn't need them. I thought he was wrong and insisted but after reading posts on this board by some of the more experienced engine guys looks like my builder knew what he was talking about on this subject too.

Peter------


In addition to the fact they're not needed, there's a risk to installing hardened exhaust valve seats 2.02/1.60 heads. Doing so can cause one to "strike water" and ruin a head. It can be done successfully, but the risk is there. It's just not worth taking it to install something that you don't really need in the first place.

There's little risk to installing the hardened exhaust seats in 1.94/1.50 heads. However, they're not needed for those engines, either.

[Joe L.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

if you are not going to deck the block and the heads have not been cut you will not be at 11:1 CR. make sure the engine builder removes all sharp edges from the pistons and combustion chambers in the heads.

clem------

Yes, and if a valve job is done on the heads that will create a little more combustion chamber volume due to the usual slight recession of the valves and consequently lower the compression ratio. Also, if good composition head gaskets are used in place of the original steel shim type, that will also have the effect of lowering compression even if there is slight decking of the heads or (God forbid) the block.

[Duke W.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

There is also anecdotal evidence of hardened seats dislodging, which creates significant havoc in the cylinder. Hardened seats are OE on aluminum heads and they rarely dislodge, but rebuild work has a much larger range of quality variation.

The bottom line is that hardened seats are unnecessary and a total waste of money on a vintage SB rebuild.

Unless you go through all the measurements to compute compression ratio, nobody knows what it really is.

In fact, it would be somewhat difficult to get an OE engine to actually achieve the OE specified compression ratio without decking the block and milling the heads and using a THIN head gasket!

Test data I have from those I have consulted in SHP engine rebuilds indicates that the maximum actual target TRUE, MEASURED compression ratio should not exceed 10.5:1, and it is usually easy to achieve 10.25-10.50 on typically machined OE blocks/OE replacement forged domed pistons with proper head gasket thickness selection once measured final combustion chamber cc and final deck clearance data is available.

That's why I referred the OP to the CR article in The Corvette Restorer.

Duke

[Brooks R.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Hi Duke,

Again I want to thank you and everyone else for all the guidance and wisdom. There is nothing like experience and you folks certainly have a bunch.

I think I have settled on the 30-30 cam for nostalgia's sake (I just remember how much I liked the sound). I will have the heads only pocket ported, port matched, and the valves/seats ground as you described. It looks like I will settle for 10:1 compression, realizing that this may cost me a little on the power side of things but I plan to drive at least 3k miles a year and don't want to worry about boosting the octane.

Looks like H beam rods and light weight forged racing pistons are going inside.

The engine will of course be balanced and I have asked that the flywheel and harmonica balancer be done separately, in case one need replacement I won't have to worry about the system being out of balance.

The folks performing the engine work have geat reputations and Jim Lockwood will be handling my fuel injection.

I also realize that timing is critical and will discuss this with the builders.

[Jerry G.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Don't forget to relieve (to unshoud) around the valves on the combustion chamber back wall. Special cutter needed. Very important

[Joe L.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Don't forget to relieve (to unshoud) around the valves on the combustion chamber back wall. Special cutter needed. Very important

Jerry-----


...but resulting in further increase in combustion chamber volume and consequent reduction in compression ratio.

[Duke W.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

The '65 2.02/1.6" valve 461 heads had the intake relieving operation done by GM, but part of "head massaging" is beveling back any overhang on the sides of the chambers. There will probably be a bit on the inlet side, and a little more on the exhaust side, which is the most critical to correct.

This hand beveling operation removes very little material. When done, big valve 461 head chambers should be about 65 cc and 492 and 186 about a cc larger because the small quench area on the spark plug side of the 461s was eliminated by a bevel in the mold tooling.

Duke

[Jerry G.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

True true but in a vintage race engine compression is easier to get than air flow out of these old heads

[Joe L.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

True true but in a vintage race engine compression is easier to get than air flow out of these old heads

Jerry------


Yes, in a vintage racing engine that is true.

[Duke W.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Most of my engine system engineering work involves restoration of OE engines that must maintain their normal driving characteristics, including idle behavior, so they will easily pass a PV test without the judge suspecting that the engine has been modfied.

When one gets into serious racing, it's a whole different ball game. Many aftermarket heads for Gen I small blocks when given a little massaging will outflow vintage massaged heads, but the latter, when properly massaged will provide substantial performance improvements in the upper half of the rev range (including the additional 500-1000 useable revs over OE) without impacting visual appearance, or idle behavior.

If the racing rules require vintage heads, then you are giving away power, but so are your competitors if they, too, are following the rules.

As I have always said, road engines and racing engines are totally different animals, which require a different system engineering approach to attain maximum performance for their intended use.

Duke

[Domenic T.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I believe the engine should remain the way it was designed by the factory engineer,(to remain original).

We strive to keep the body and all other parts original then modify the engine that brought all to life.

I use the same compression and cam it was designed with and want it to run and sound the way it did when it was born.

The compression gets a little higher if the heads are milled or the block is decked, but that's life and also in the repair/ rebuild manual.

They rumble and run great with good HI octane also.

Build the gas for the engine not the engine for the gas.

DOM

[Timothy B.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I agree with Dom and Duke, stick with the stock engineered parts, (piston dome etc.) and tweak with some head work and head gasket thickness. Stick with stock camshaft and valve train but pay attention to the rods, lots of good info here at our site.

[Joe C.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

I believe the engine should remain the way it was designed by the factory engineer,(to remain original).

We strive to keep the body and all other parts original then modify the engine that brought all to life.

I use the same compression and cam it was designed with and want it to run and sound the way it did when it was born.

The compression gets a little higher if the heads are milled or the block is decked, but that's life and also in the repair/ rebuild manual.

They rumble and run great with good HI octane also.

Build the gas for the engine not the engine for the gas.

DOM

That is all well and good until you realize that much factory "engineering" is done for financial expediency and not for performance optimization. The "bean counters" rarely let engineering have their way with things. Most if not all designs can be improved upon if a systems view is taken with optimization being paramount.SBC Cam Intake Valve Closing - SCR_DCR Yields.jpg

[Timothy B.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Joe,


Correct me if I am wrong but the stock piston domes are the same size for these four camshafts you listed. Dynamic compression would change (get higher) as piston speed increases faster than the incoming air can escape. The 346 camshaft would be a screamer with 12.3 pistons if you could find fuel to keep it out of detonation.

I have a set of TRW 2211's around here and they have a big dome, perfect for that #346 cam but they are .060 so they would make even more compression pressure.

[Joe C.]

Re: 65 Fuelie - 11:1 compression - Octane Booster?

Joe,


Correct me if I am wrong but the stock piston domes are the same size for these four camshafts you listed. Dynamic compression would change (get higher) as piston speed increases faster than the incoming air can escape. The 346 camshaft would be a screamer with 12.3 pistons if you could find fuel to keep it out of detonation.

I have a set of TRW 2211's around here and they have a big dome, perfect for that #346 cam but they are .060 so they would make even more compression pressure.

Timothy,

The pistons used for SHP SBC engines between 1962 and 1968 (all for 327 inch displacement, 3.25" stroke) had 5.3cc domes. STATIC as well as DYNAMIC compression is directly proportional to stroke. All other things being equal, two identical engines, except for stroke will have different DCR's, with the engine with the longer stroke having a higher DCR than the engine with the shorter stroke. The Duntov cam was originally designed for the 283, which had a 3.00 inch stroke. When it was installed in the 1962-63 327, with a 3.25" stroke, these engines experienced detonation because the DCR (all other things being equal) increased. The 097 equipped 283 engines were already at the "ragged edge" of detonation based on a well thought out design which optimized SCR/DCR. The pistons used for the 3.48" stroke LT1 may have had different volume domes than the 1962-1968 ones. They were NOT the same pistons and are visibly quite different. I can't say whether or not the volumes were the same at 5.3cc. The pistons used for the 283 are also visibly different and MAY have had different dome volumes as well.

As you know, the Duntov equipped engines are very susceptible to detonation (especially the SHP 327's) because the 097 is much milder than the LT1 and the 30-30 based on its much earlier intake valve closing. Referring to the above chart makes this clear.
"Dynamic" compression ratio has nothing to do with piston speed. It is a metric which is based on intake valve closing @ J604d. SCR is found by multiplying the SCR by the amount of piston travel left after the intake valve is on it's seat (J604d duration). The piston speed becomes more important as valve overlap increases and impulse tuning becomes more effective at higher engine speeds. You are correct in stating this, although the term and calculation of "dynamic compression ratio" does NOT take this into account.

One of the more useful things about that chart which I made up, is to COMPARE the intake valve closing events for all 4 of those camshafts @ J604d: 63*, 76*, 82*, and 85*........................the 097 is the mildest, followed by the 151, and then the LT1, finally the 346 being the most radical.

GM was clearly "mixing and matching" available parts to minimize costs during this era, with minimal attention being paid to thorough analysis. Thus, the quick fix for the early 327 SHP engines with 097 cam by installing double steel shim head gaskets to knock back the (stated) SCR from 11.25:1 to 11.0:1. The chart makes it clear that thorough analysis and planning is necessary in order to optimize static compression ratios based upon intake valve closing (@ J604d).