Your thoughts on compression ratio etc...
02-29-2008, 12:31 PM
#1
Melting Slicks
Thread Starter
Your thoughts on compression ratio etc...
Ok, what compression ratio would you shoot for with iron heads and today's gas??
10:1 less or more??
what are your thoughts on copper head gaskets?? I may have to use them to get the correct compressed thickness.
10:1 less or more??
what are your thoughts on copper head gaskets?? I may have to use them to get the correct compressed thickness.
02-29-2008, 12:52 PM
#2
Drifting
I am sure you will get a ton of opinons so here is mine.
I recently rebuilt an L-79 and lowered the CR to about 9.7:1 from a claimed 11:1 (actually I think these were 10.5:1). I did this for two reasons. First, I plan on keeping the car a long time and second, the quality and octane of gas is not going up anytime soon. I am fortunate to live in a state with 93 octane fuel (and 110 leaded racing gas at the station down the steet). This is getting to be the exception and not the rule. Many parts of the country have 91 octane max and even that may be reduced in the future due to requlations, supply and demand and a host of other rules, regulations or market forces. I strongly believe you will not see fuels increasing in quality or octane in the future given the way things are going.
To help mitigate the reduction in compression ratio I put in a slightly more aggressive cam and changed to a full roller setup. The engine dynoed at 345 HP and 380 ft-lb of torque, and will run all day on 91 octane fuel with 14 deg initial advance without a wisp of detonation.
I had a 70-LT1 with 11:1 stock compression. The car and engine were in excellent condition. I had a constant headache trying to find of mix of fuels to stop it from pinging. I constantly shopped different fuels, mixed in AVGAS or racing fuel, always left the distribution loose enough to retard the timing when I had to......etc. Got real tired of this after a while for 10 to 20 HP.
In conclusion, if I were to plan on keeping the car for a long time I would reduce its requirement for octane given the future of gasoline. With some planning and a bit of engine work one can compensate for slightly lower CR without sacrificing real world performance. In my case the engine looks 100% stock on the outside but is built for today and tomorrow's fuels (and lubricants) on the inside.
I recently rebuilt an L-79 and lowered the CR to about 9.7:1 from a claimed 11:1 (actually I think these were 10.5:1). I did this for two reasons. First, I plan on keeping the car a long time and second, the quality and octane of gas is not going up anytime soon. I am fortunate to live in a state with 93 octane fuel (and 110 leaded racing gas at the station down the steet). This is getting to be the exception and not the rule. Many parts of the country have 91 octane max and even that may be reduced in the future due to requlations, supply and demand and a host of other rules, regulations or market forces. I strongly believe you will not see fuels increasing in quality or octane in the future given the way things are going.
To help mitigate the reduction in compression ratio I put in a slightly more aggressive cam and changed to a full roller setup. The engine dynoed at 345 HP and 380 ft-lb of torque, and will run all day on 91 octane fuel with 14 deg initial advance without a wisp of detonation.
I had a 70-LT1 with 11:1 stock compression. The car and engine were in excellent condition. I had a constant headache trying to find of mix of fuels to stop it from pinging. I constantly shopped different fuels, mixed in AVGAS or racing fuel, always left the distribution loose enough to retard the timing when I had to......etc. Got real tired of this after a while for 10 to 20 HP.
In conclusion, if I were to plan on keeping the car for a long time I would reduce its requirement for octane given the future of gasoline. With some planning and a bit of engine work one can compensate for slightly lower CR without sacrificing real world performance. In my case the engine looks 100% stock on the outside but is built for today and tomorrow's fuels (and lubricants) on the inside.
02-29-2008, 12:59 PM
#4
Le Mans Master
Donny, you are the Man!
What do you mean by having to use copper head gaskets to get the correct thickness. Is this because of the deck height? What is the deck height of this block now? These engines run great at 10-10.35:1 with a GM cam, ("151", L-82, 30-30 or LT-1), correct timing and carb settings.
A Speed Pro L-2166NF piston is the OEM replacement for a starting point. If you are near zero deck, then the 2165 forged piston w 4 reliefs might be required or a 2 valve relief H660CP to get you close to 10:1.
A set of custom Ross, JE etc would also be an option to get the compression ratio you want, however you should be able to rebuild this with off the shelf parts. A 1/4 point shy of 10:1 won't be a big deal.
Even if your block has a deck height as low a .010", with a .027" head gasket and a L-2165 piston you're sitting at 9.7:1. With a H660CP, you're at 9.9:1. I wouldn't worry too much about having to use some very thick copper gaskets.
What do you mean by having to use copper head gaskets to get the correct thickness. Is this because of the deck height? What is the deck height of this block now? These engines run great at 10-10.35:1 with a GM cam, ("151", L-82, 30-30 or LT-1), correct timing and carb settings.
A Speed Pro L-2166NF piston is the OEM replacement for a starting point. If you are near zero deck, then the 2165 forged piston w 4 reliefs might be required or a 2 valve relief H660CP to get you close to 10:1.
A set of custom Ross, JE etc would also be an option to get the compression ratio you want, however you should be able to rebuild this with off the shelf parts. A 1/4 point shy of 10:1 won't be a big deal.
Even if your block has a deck height as low a .010", with a .027" head gasket and a L-2165 piston you're sitting at 9.7:1. With a H660CP, you're at 9.9:1. I wouldn't worry too much about having to use some very thick copper gaskets.
Last edited by Scott Marzahl; 02-29-2008 at 01:12 PM.
02-29-2008, 01:20 PM
#5
12.14 w/ the original 327
We drive our car a lot. The engine was built somewhat milder, so my wife and her girlfriends could putt around in L.A. traffic reliably. My take on it is, build an engine that can run on 87 if you want to drive it a lot. If you don't mind paying for premium, or hesitating to take long drives because of premium gas prices, build it with 10:1 + compression and run premium. Under no circumstances would I build a street car that needed to blend race gas. Too expensive to maintain, and a PITA to fill-up. It will definitely limit your driving.
Our 62's 327 was bored .040" over and stroked with a 3.85 crank that was offset ground to fit the SJ block. We went with a dished piston, that would give us a calculated 9.5:1 compression. Our cam is a cheapie re-grind from Camonics in North Hollywood.
.520" I/E (w/ a 1.6 rocker), 236* I/E @ .050", 284* I/E adv, ground with a 112* LS. Lash is .012" I/.014" E.
The heads are '62 #461 castings with a little clean-up work and 2.02/1.60 valves.
The good news is we can run 87 octane, even in 105* heat. The cam has enough lope to sound OK. The car beat some heavily modded cars at LACR, including a BB Camaro and a modded C5.
My complaints are, the engine doesn't have the crisp sound and throttle response of a higher compression engine. We are also prevented from running more cam due to the lower compression ratio.
Our 62's 327 was bored .040" over and stroked with a 3.85 crank that was offset ground to fit the SJ block. We went with a dished piston, that would give us a calculated 9.5:1 compression. Our cam is a cheapie re-grind from Camonics in North Hollywood.
.520" I/E (w/ a 1.6 rocker), 236* I/E @ .050", 284* I/E adv, ground with a 112* LS. Lash is .012" I/.014" E.
The heads are '62 #461 castings with a little clean-up work and 2.02/1.60 valves.
The good news is we can run 87 octane, even in 105* heat. The cam has enough lope to sound OK. The car beat some heavily modded cars at LACR, including a BB Camaro and a modded C5.
My complaints are, the engine doesn't have the crisp sound and throttle response of a higher compression engine. We are also prevented from running more cam due to the lower compression ratio.
02-29-2008, 01:28 PM
#6
Race Director
I have a 70 LT-1 short block in my '62. I got tired of paying crazy prices for leaded high-test and bought a set of Dart Iron Eagle Platimum heads with 72cc. Effectly lowed the CR to approx 10 to 1. Car runs great on any premium unleaded gas. I used the Fel-Pro head gaskets which seal perfectly. No need for copper head gaskets.
02-29-2008, 02:30 PM
#7
Le Mans Master
IMO 9.5 min and 10.0 max will give peace of mind and performance.......... now if you want to switch to some aluminum heads I would add 1.0 to the above numbers along with a nice power increase, annnnnnnd that is not even considering what you would gain from better flowing heads 
02-29-2008, 02:39 PM
#9
Melting Slicks
The maximum compression ratio on pump gas without detontation depends heavily on CAM CHOICE and head choice. The CR numbers thrown around are almost always static CR numbers while dynamic CR is really what you need to consider.
My point:
a 10.5:1 static CR 327/300 motor may ping on 92 octane while a 10.5:1 static CR 327/365 will not.
I run 10.0:1 (static measured) 327 with an LT1 camshaft on 92 octane on a super agressive curve and it has nevr had a problem. I suspect I could go to 89 if needed. Quench also matters. 10.5:1 with good quench will be less likely to ping than a 10.5:1 with a super bug quench.
You need to design your engine as a system - not as a collection of individual parts.
brian
My point:
a 10.5:1 static CR 327/300 motor may ping on 92 octane while a 10.5:1 static CR 327/365 will not.
I run 10.0:1 (static measured) 327 with an LT1 camshaft on 92 octane on a super agressive curve and it has nevr had a problem. I suspect I could go to 89 if needed. Quench also matters. 10.5:1 with good quench will be less likely to ping than a 10.5:1 with a super bug quench.
You need to design your engine as a system - not as a collection of individual parts.
brian
02-29-2008, 02:53 PM
#10
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I have no issues with ping... I run 2 parts 94 with 1 part 110......
Have run just 94 with the timing backed down to 36 total.... just regular driving, nothing spirited
The 151 cam has a lot of duration, and the dynamic compression is not really that high......... and I have a real tight quench.
Have run just 94 with the timing backed down to 36 total.... just regular driving, nothing spirited
The 151 cam has a lot of duration, and the dynamic compression is not really that high......... and I have a real tight quench.
Last edited by Donny Brass; 02-29-2008 at 03:02 PM.
02-29-2008, 02:55 PM
#11
Melting Slicks
I am surprised this did not get mentioned sooner. Among other things, high overlap cams need a lot of static compression to have any power.
03-01-2008, 02:41 PM
#13
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03-01-2008, 02:48 PM
#14
Drifting
Exactly - a 30-30 or LT-1 cam with 9:1 compression makes a turd.
So in order to avoid a big brown smelly mess, what compression minimum is recommended for the 30-30? In order to run pump gas.....
Thanks,
RK
03-01-2008, 02:54 PM
#15
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03-01-2008, 03:04 PM
#16
Drifting
with the correct timing map and manifold vacuum-connected vacuum advance (neither of which are as the factory shipped it), octane isn't an issue.
Appreciate it,
RK
03-01-2008, 03:38 PM
#17
Melting Slicks
IMO 9.5 min and 10.0 max will give peace of mind and performance.......... now if you want to switch to some aluminum heads I would add 1.0 to the above numbers along with a nice power increase, annnnnnnd that is not even considering what you would gain from better flowing heads
03-01-2008, 03:58 PM
#18
Drifting
The maximum compression ratio on pump gas without detontation depends heavily on CAM CHOICE and head choice. The CR numbers thrown around are almost always static CR numbers while dynamic CR is really what you need to consider.
My point:
a 10.5:1 static CR 327/300 motor may ping on 92 octane while a 10.5:1 static CR 327/365 will not.
I run 10.0:1 (static measured) 327 with an LT1 camshaft on 92 octane on a super agressive curve and it has nevr had a problem. I suspect I could go to 89 if needed. Quench also matters. 10.5:1 with good quench will be less likely to ping than a 10.5:1 with a super bug quench.
You need to design your engine as a system - not as a collection of individual parts.
brian
My point:
a 10.5:1 static CR 327/300 motor may ping on 92 octane while a 10.5:1 static CR 327/365 will not.
I run 10.0:1 (static measured) 327 with an LT1 camshaft on 92 octane on a super agressive curve and it has nevr had a problem. I suspect I could go to 89 if needed. Quench also matters. 10.5:1 with good quench will be less likely to ping than a 10.5:1 with a super bug quench.
You need to design your engine as a system - not as a collection of individual parts.
brian
Very true. My old 427/435 still runs fine on today's premium (valve seats for unleaded gas have been installed). That engine still has the original 11:1 compression and the original long duration cam (306 degrees duration, if I remember right). This greatly cuts the effective compression at mid rpms's and at high rpms you get "ahead" of the pre-ignition/detonation.
03-01-2008, 05:45 PM
#19
Race Director
Now that you have all your {LT1} parts ready to assemble, copper head gaskets are about as good as it gets and maybe over kill. And if the engine is still square, the OEM steel shim gaskets are just as good for max available compression. It`s what I prefer. However Felpro makes several quality head gaskets too.