Pistons for 1970 350/350 ???
09-03-2015, 11:14 AM
#21
Instructor
Thread Starter
For others that may have an interest in the contents of this thread. more research has uncovered the following advice:
A good approach when building an engine is to determine the duration and LSA needed for the desired RPM range. Once this is know, manipulate the chamber size and piston valve reliefs (and sometimes the cam advance) to provide a DCR around 8.2:1. Now that the correct piston volume and chamber size is know, enter the actual crankshaft stroke in your CR calculator to see what static CR to build to. Often the needed SCR is higher that you would expect. Note: The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance.
Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.
A good approach when building an engine is to determine the duration and LSA needed for the desired RPM range. Once this is know, manipulate the chamber size and piston valve reliefs (and sometimes the cam advance) to provide a DCR around 8.2:1. Now that the correct piston volume and chamber size is know, enter the actual crankshaft stroke in your CR calculator to see what static CR to build to. Often the needed SCR is higher that you would expect. Note: The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance.
Alternatively, with the SCR known, manipulate the cam specs until a desirable DCR is found. When the best intake closing time is derived, look for a cam with that intake closing timing, that provides the other attributes desired (LSA and duration). Often times the best cam is smaller than one might expect. Sometimes a CR change is needed to run a cam with the desired attributes.
09-03-2015, 11:51 AM
#22
Instructor
Thread Starter
And a little more....
High Compression: Cam companies have long preached the gospel of conservative cam specs for a true street car, meaning that you're always better off to go one step smaller on the cam instead of one step bigger. Most of the time that's true, but there are instances where a street car should run an aggressive cam, and sometimes bigger is indeed better.
Overlap (lots of duration and tight lobe-separation angles) decreases cylinder pressure, especially at low rpm, which allows an engine to run a higher compression ratio and still work on pump gas. High cylinder pressure, which is caused partly by a high compression ratio, is what makes an engine detonate on pump gas. Decreasing the cylinder pressure by adding duration is just like taking compression out of the engine, but mostly only at low rpm. A compression ratio of 11.5:1 running on pump gas is not unheard of when the cam has enough overlap and duration to bleed off the low-speed cylinder pressure.
High Compression: Cam companies have long preached the gospel of conservative cam specs for a true street car, meaning that you're always better off to go one step smaller on the cam instead of one step bigger. Most of the time that's true, but there are instances where a street car should run an aggressive cam, and sometimes bigger is indeed better.
Overlap (lots of duration and tight lobe-separation angles) decreases cylinder pressure, especially at low rpm, which allows an engine to run a higher compression ratio and still work on pump gas. High cylinder pressure, which is caused partly by a high compression ratio, is what makes an engine detonate on pump gas. Decreasing the cylinder pressure by adding duration is just like taking compression out of the engine, but mostly only at low rpm. A compression ratio of 11.5:1 running on pump gas is not unheard of when the cam has enough overlap and duration to bleed off the low-speed cylinder pressure.
09-03-2015, 01:37 PM
#23
Pro
For what it's worth, I'm running..........
350 .060 over with TRW L2304 (no F) pistons with a measured dome volume of 2cc.
Factory cast iron 3927186 cylinder heads with a measured combustion chamber volume of 61cc.
.012 piston to deck height with Mr Gasket 1134-G head gaskets. This gives me a SCR of 11.92:1 at sea level and a quench of .040.
Solid flat tappet Comp Cam 12-673-4 camshaft with a intake closing point of 70* ABDC.
Factory cast iron intake and exhaust manifolds, Quadrajet carburetor with a Muncie wide ratio and 3.55:1 rear gear.
But here's the kicker. I live at 6300' above sea level which really weakens my compression ratio. At this elevation I effectively have 10.6:1 SCR and 7.92:1 DCR. I'm running 91 octane pump gas (best we can get out here) and have zero detonation problems. Occasionally it will run on after turning the key off but not always.
I know it could run a lot better with some aftermarket performance parts but I want to stay with the factory look. Even with the factory stuff it pulls really hard above 2800 RPM but starts running out of breath about 5500. I honetly think it would give my 70 Chevelle 454/4 speed a run for its money.
I hope my ramblings will give you some useful info you were looking for.
350 .060 over with TRW L2304 (no F) pistons with a measured dome volume of 2cc.
Factory cast iron 3927186 cylinder heads with a measured combustion chamber volume of 61cc.
.012 piston to deck height with Mr Gasket 1134-G head gaskets. This gives me a SCR of 11.92:1 at sea level and a quench of .040.
Solid flat tappet Comp Cam 12-673-4 camshaft with a intake closing point of 70* ABDC.
Factory cast iron intake and exhaust manifolds, Quadrajet carburetor with a Muncie wide ratio and 3.55:1 rear gear.
But here's the kicker. I live at 6300' above sea level which really weakens my compression ratio. At this elevation I effectively have 10.6:1 SCR and 7.92:1 DCR. I'm running 91 octane pump gas (best we can get out here) and have zero detonation problems. Occasionally it will run on after turning the key off but not always.
I know it could run a lot better with some aftermarket performance parts but I want to stay with the factory look. Even with the factory stuff it pulls really hard above 2800 RPM but starts running out of breath about 5500. I honetly think it would give my 70 Chevelle 454/4 speed a run for its money.
I hope my ramblings will give you some useful info you were looking for.
09-03-2015, 03:14 PM
#24
Instructor
Thread Starter
Thanks GTLow. Exactly the info I am looking for. The more I have been reading the more I am convinced I can run the stock pistons with 11:1 CR and not have any problems as long as the cam and quench are correct. I am going to order the L2304F pistons and get on with putting the bottom end together.
How is your idle and vacuum with that cam? Did you have your head surface ground or that .012 deck height original?
How is your idle and vacuum with that cam? Did you have your head surface ground or that .012 deck height original?
09-03-2015, 03:58 PM
#25
Pro
Idle vacuum is fairly low at about 8" but I don't have any problems with the vacuum accessories. I also don't have power brakes. Idle is pretty choppy but I kinda like it.
Click on the video to hear the idle quality.
Oh, I forgot to add, I'm also running an intake gasket set with blocked off heat crossover passages and a 160* thermostat to keep the intake temperature as cool as possible.
Last edited by qtlow; 09-03-2015 at 04:09 PM.
09-03-2015, 06:36 PM
#26
Le Mans Master
Since you are a .060 over then it looks like with a 64cc head you'll need about 1 cc pistons to get 11:1. So may be a small dome with valve cutouts?
I used this calculator when figuring my CR.
http://www.jeepstrokers.com/calculator/
It allows you to see how altitude affects your DCR.
I have 9.9:1 with 12cc pistons 8.2 DCR and run at 5000 ft DA on average. I can run 85 octane with no detonation, 380ish HP. 270/270 advertised 219/219@.050 with a 108 LSA. Love the effect of the 108.
However I took many steps to insure that intake temps were kept down. CAI, oil splash shield on the intake, blocked crossover, 180* thermostat, aluminum radiator with electric fans. Also used aluminum heads. Highly recommend them for detonation reduction. I also have a TH350 with a 3.08 out back, so worst case scenario for high load at low RPM, so I added a 2600 stall torque converter as well.
With your 4.11 rear you can get away with more CR and higher duration cam.
I used this calculator when figuring my CR.
http://www.jeepstrokers.com/calculator/
It allows you to see how altitude affects your DCR.
I have 9.9:1 with 12cc pistons 8.2 DCR and run at 5000 ft DA on average. I can run 85 octane with no detonation, 380ish HP. 270/270 advertised 219/219@.050 with a 108 LSA. Love the effect of the 108.
However I took many steps to insure that intake temps were kept down. CAI, oil splash shield on the intake, blocked crossover, 180* thermostat, aluminum radiator with electric fans. Also used aluminum heads. Highly recommend them for detonation reduction. I also have a TH350 with a 3.08 out back, so worst case scenario for high load at low RPM, so I added a 2600 stall torque converter as well.
With your 4.11 rear you can get away with more CR and higher duration cam.
09-03-2015, 11:44 PM
#28
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Just wanted to clear up a couple of things.
First..overlap has nothing to do with compression..though it is often quoted as such. Think about it...both valves are open during overlap...no compression is occuring. The confusion comes from the fact that larger duration cams "usually" have more overlap unless the LSA is widened. The longer duration and more accurately the intake closing point as described above determines when compression will begin. It's possible to have a lot of overlap with a short duration cam especially of you have some aggressive asymetrical lobes. You "could" say that overlap might help cylinder filling and create higher cylinder pressures at high RPM even.
Next, I always caution folks with getting too hung up on the DCR calculators. Yes...DCR is a real thing and it's what we're all working to achieve...but variables such as head flow, intake, carb, exhaust can all dramatically change the dynamics of a running engine. For example...the calculators will tell you that the cam I ran in my 555" when it was naturally aspirated could handle near 13.0 compression on pump gas. Yep...I could cruise around on it no issues...but as soon as I would have put my foot in it and those killer heads started really flowing some air and the cylinders got really full...I would have hammered the bearings out of it in no time. It DID run, race and dyno just fine on 93 octane with 11.56 compression. Probably could have pushed a whisker more. But it had extremely tight deck clearance (.028") and I kept an eye on the tune.
One of the best examples were the old Busch NASCAR engines. Had to have 9.0 compression. They didn't use really huge cams..but made stupid power at high RPM. They had incredible heads/intakes that would really flow some air. No way they would operate on 93 octane once they started pulling even with 9.0 compression.
So if the heads really don't flow as well and/or the RPM isn't that high..you can sneak up on DCR a little more. Just be careful when you swap something to make it really breathe.
JIM
First..overlap has nothing to do with compression..though it is often quoted as such. Think about it...both valves are open during overlap...no compression is occuring. The confusion comes from the fact that larger duration cams "usually" have more overlap unless the LSA is widened. The longer duration and more accurately the intake closing point as described above determines when compression will begin. It's possible to have a lot of overlap with a short duration cam especially of you have some aggressive asymetrical lobes. You "could" say that overlap might help cylinder filling and create higher cylinder pressures at high RPM even.
Next, I always caution folks with getting too hung up on the DCR calculators. Yes...DCR is a real thing and it's what we're all working to achieve...but variables such as head flow, intake, carb, exhaust can all dramatically change the dynamics of a running engine. For example...the calculators will tell you that the cam I ran in my 555" when it was naturally aspirated could handle near 13.0 compression on pump gas. Yep...I could cruise around on it no issues...but as soon as I would have put my foot in it and those killer heads started really flowing some air and the cylinders got really full...I would have hammered the bearings out of it in no time. It DID run, race and dyno just fine on 93 octane with 11.56 compression. Probably could have pushed a whisker more. But it had extremely tight deck clearance (.028") and I kept an eye on the tune.
One of the best examples were the old Busch NASCAR engines. Had to have 9.0 compression. They didn't use really huge cams..but made stupid power at high RPM. They had incredible heads/intakes that would really flow some air. No way they would operate on 93 octane once they started pulling even with 9.0 compression.
So if the heads really don't flow as well and/or the RPM isn't that high..you can sneak up on DCR a little more. Just be careful when you swap something to make it really breathe.
JIM