Compression ratio's
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The advice I got from an engine builder was not to run more than 10:1 for normal 95 octane fuel. But you can run up to 11.5:1 on 97/100 octane.
Don't know if its true but I took his advice.
Don't know if its true but I took his advice.
Doc Watson
Lifetime Supporter
I was told up to 10.5:1 compression for unleaded BUT you need stainless valve seats to prevent wear.... also the higher the comp ratio generally the higher the tickover.... a min of 3000rpm tickover is not good in traffic.... im going for between 10:1 and 10.5:1.
With the power you want I assume you will be stroking a 302 block??? more inches more power but be careful of the increased piston wall side forces as the rod ratio drops....also look out for pistons that have the oil ring partially machined away due to the large stroke.
Hope this helps
Andy
With the power you want I assume you will be stroking a 302 block??? more inches more power but be careful of the increased piston wall side forces as the rod ratio drops....also look out for pistons that have the oil ring partially machined away due to the large stroke.
Hope this helps
Andy
Doc - I believe that's "Stellite" valve seats rather than Stainless.
91-92 Octane Unleaded or leaded
10.5:1 is about the limit for most engines while using Alloy cylinder heads. drop that to 9.5:1 with iron heads.
If you have electronic timing controls you may be able to eek out 11:1 on Alloy and 10:1 on iron - but it's not going to run that well (retarted timing).
You can also decrease the compression ratio dynamically by increasing the valve overlap (Webers HATE this). By closing up the lobe separation angles - you decrease the cranking compression and low RPM dynamic compression. This will help the engine survive without detonation at low RPMs and low loading - However - when under heavy loads at mid-high RPM, your engine will detonate if timing is not controlled well and set within the parameters and limits of the Cylinder Head / Static compression ratio limits.
Throw in any sort of Turbo/Supercharging and all bets are off...
91-92 Octane Unleaded or leaded
10.5:1 is about the limit for most engines while using Alloy cylinder heads. drop that to 9.5:1 with iron heads.
If you have electronic timing controls you may be able to eek out 11:1 on Alloy and 10:1 on iron - but it's not going to run that well (retarted timing).
You can also decrease the compression ratio dynamically by increasing the valve overlap (Webers HATE this). By closing up the lobe separation angles - you decrease the cranking compression and low RPM dynamic compression. This will help the engine survive without detonation at low RPMs and low loading - However - when under heavy loads at mid-high RPM, your engine will detonate if timing is not controlled well and set within the parameters and limits of the Cylinder Head / Static compression ratio limits.
Throw in any sort of Turbo/Supercharging and all bets are off...
One thing I might suggest is that you consider aluminum heads (if you're thinking about 400ish HP, you probably have already invested in aluminum heads). The increased heat dissipation of the aluminum will allow for about half a point higher compression ratio.
Fuel here in the U.S. seems to run around 87 octane for regular unleaded, 89 for "unleaded-plus" (which can sometimes be less expensive than regular unleaded due to the inclusion of ethanol in the fuel, which raises the octane rating). Very seldom do you find "premium" fuel above a 91 octane rating where I shop....93 seems to be a dream and you probably need to mix in some av-gas to get up to a 97 octane rating.
If it were me (and it will be soon), I would also target somewhere between 10:1 and 10.5:1 with aluminum heads and 91 octane. As Doc says, good quality valve seats are a must to avoid valve seat recession with the combination of high compression and poor quality fuels.
Doug
Fuel here in the U.S. seems to run around 87 octane for regular unleaded, 89 for "unleaded-plus" (which can sometimes be less expensive than regular unleaded due to the inclusion of ethanol in the fuel, which raises the octane rating). Very seldom do you find "premium" fuel above a 91 octane rating where I shop....93 seems to be a dream and you probably need to mix in some av-gas to get up to a 97 octane rating.
If it were me (and it will be soon), I would also target somewhere between 10:1 and 10.5:1 with aluminum heads and 91 octane. As Doc says, good quality valve seats are a must to avoid valve seat recession with the combination of high compression and poor quality fuels.
Doug
Ron Earp
Admin
Be careful when discussing octane ratings across the pond.
I wrote a thread about the octane ratings between the US and UK some years back. The two methods of calculation are different and US 93 is equivalent to 97-98 in the UK format.
93 US RON has always been commonly available in all the SE states. That is weird that Texas, where a good bit of gas and oil come in the country, wouldn't have the 93 around but go figure.
It is the lack of lead that dictated the need for Stellite seats, not the Octane rating.
I have been using a 289 with a measured 10:1 compression with Iron block & heads with 91 Octane fuel for years without issue. Swapping to alloy heads requires an increase in compression (usually about 1/2 a point) to regain the heat that is (too) efficiently dissapated by the alloy heads causing a loss in power.
I had a friend years ago with a Lotus Super 7, 1600 Twin cam. It was an absolute dog until it got heat into the head.
I have been using a 289 with a measured 10:1 compression with Iron block & heads with 91 Octane fuel for years without issue. Swapping to alloy heads requires an increase in compression (usually about 1/2 a point) to regain the heat that is (too) efficiently dissapated by the alloy heads causing a loss in power.
I had a friend years ago with a Lotus Super 7, 1600 Twin cam. It was an absolute dog until it got heat into the head.
In case it wasn't clear, the ceiling on compression is usually dictated by pre-detonation. In other words, the heat created by compressing the atomized fuel/air mixture, along with the heat (or hot spots) in the cc, can light off the mixture too early, or at the wrong time - that "pinging" sound you hear.
There are some tricks for adding a little more compression while avoiding predetonation. Alloy heads is one - this takes the head temps down to a level which lessens pre-detonation, as the alloy dissipates the heat faster. Good combustion chamber design is another....no localized hot spots that act to ignite the mixture. Corners and sharp edges tend to creat localized hot spots.
There are other tricks such as using twin plugs. This will allow another .5 compression. Has anybody here built a SBF with twin plugs?? And, as Randy said above, decreasing the dynamic compression by increasing the overlap (roughly) can also help. Obviously, very precise control of mixture and timing goes a long way as well (EFI).
There are some tricks for adding a little more compression while avoiding predetonation. Alloy heads is one - this takes the head temps down to a level which lessens pre-detonation, as the alloy dissipates the heat faster. Good combustion chamber design is another....no localized hot spots that act to ignite the mixture. Corners and sharp edges tend to creat localized hot spots.
There are other tricks such as using twin plugs. This will allow another .5 compression. Has anybody here built a SBF with twin plugs?? And, as Randy said above, decreasing the dynamic compression by increasing the overlap (roughly) can also help. Obviously, very precise control of mixture and timing goes a long way as well (EFI).
Believe it or not, for street use I have gotten by on local pump 93 octane in a 640+ HP 434" SBC running well over 12:1 CR. It has ported Dart AL heads and a BIG solid roller cam. During the build, combustion chamber squish was carefully optimized. It is carbureted, and has an MSD ignition with knock sensor timing control.
One real key(besides the big overlap cam) is that the engine is in a full out 911 race car weighing about 2150 lbs. (930 trans, of course). It has a very good cooling system, and I run a 160 degree thermostat(important).
The reality is, whenever you you have this much power in such a light vehicle, you just can't use full throttle very long on the street. This helps keep you out of detonation trouble that a heavier car would experience.
On the track, I run race gas. Or pump premium with some AV gas in a pinch.
This vehicle is an extreme example(and I don't run it on the street often), but you can run a high cr IF everything is optimized. Maybe not 12:1 CR in an everyday car, but I'll bet 11:1(maybe more) with AL heads in a light GT40 should be doable.
Jack
One real key(besides the big overlap cam) is that the engine is in a full out 911 race car weighing about 2150 lbs. (930 trans, of course). It has a very good cooling system, and I run a 160 degree thermostat(important).
The reality is, whenever you you have this much power in such a light vehicle, you just can't use full throttle very long on the street. This helps keep you out of detonation trouble that a heavier car would experience.
On the track, I run race gas. Or pump premium with some AV gas in a pinch.
This vehicle is an extreme example(and I don't run it on the street often), but you can run a high cr IF everything is optimized. Maybe not 12:1 CR in an everyday car, but I'll bet 11:1(maybe more) with AL heads in a light GT40 should be doable.
Jack
For a healthy street/track performance SBF you're going to be running enough cam to make 10.5 to 1 c.r. the practical MINimum, and it will be fine on premium pump gas. To give you an idea of the margin of safety, I run a 10.6 to 1 383SBC in my road car with a mild performance hyd roller cam and recently added a wee bit of forced induction and it handles 6psi of intercooled boost using 26 degrees of total timing with no det.
Cheers, Andrew
Cheers, Andrew
Andrew - What kind/octane fuel and what sort of timing (lead) are you running in your 383? Cam profile specs?
26 degrees is not a lot for a small block chevy. Many I've run were 40-42 total - all in around 3200 RPM. But that's not boosted either...
26 degrees is not a lot for a small block chevy. Many I've run were 40-42 total - all in around 3200 RPM. But that's not boosted either...
Gidday Randy,
when naturally aspirated ran 20 initial, 34 total all in by 2500, cam is comp hyd roller 218/224 dur @ 50, 274/286 adv duration on 110 lobe centre, with 1.6 rockers. Obviously that's the WOT timing - I run up to 50 degrees total at light cruise 2k and above. Got street ally 170cc runner heads on the car with a reasonbly modern design of combustion chamber.
When I boosted it I started by just adding 3 more rows to my timing table for 2.5, 5, and 7.5 psi, taking off 1 degree per psi from the WOT curve above. That initially gave me 28 at 6psi and I was getting det in peak tq region - I could smother the det by dumping fuel and running 11.5 AFR but ended up running harder with 26 timing and AFR of 12.0
Cheers, Andrew
when naturally aspirated ran 20 initial, 34 total all in by 2500, cam is comp hyd roller 218/224 dur @ 50, 274/286 adv duration on 110 lobe centre, with 1.6 rockers. Obviously that's the WOT timing - I run up to 50 degrees total at light cruise 2k and above. Got street ally 170cc runner heads on the car with a reasonbly modern design of combustion chamber.
When I boosted it I started by just adding 3 more rows to my timing table for 2.5, 5, and 7.5 psi, taking off 1 degree per psi from the WOT curve above. That initially gave me 28 at 6psi and I was getting det in peak tq region - I could smother the det by dumping fuel and running 11.5 AFR but ended up running harder with 26 timing and AFR of 12.0
Cheers, Andrew
Interesting Andrew.... Never been north of 45 degrees on a SBC.
What type of induction (Carb / EFI / Port / TBI etc?)
You can do a lot to reduce detonation by mixture cooling but at a cost of power..
SBF's have a more efficient cylinder head / port layout than SBCs in my opinion. That's why Ford's require so much less timing...
What type of induction (Carb / EFI / Port / TBI etc?)
You can do a lot to reduce detonation by mixture cooling but at a cost of power..
SBF's have a more efficient cylinder head / port layout than SBCs in my opinion. That's why Ford's require so much less timing...
Howard Jones
Supporter
91 octane, 10 to 1 CR, 280 duration, 550 lift Hyd roller, current aftermarket Alum heads with 2.02/1.60 valves, Holley 750 DP, 36 degrees total advance all in @ 2500, Mechanical secondaries and advance, 6300 rev limit, free breathing exhaust.
This combo should get you 1.25 - 1.3 HP per inch and a long life, trouble free, high value to $ ratio ($10-14/HP) , street motor that can hold it's own on the track in a 2500 lb GT40 with good suspension setup, modern tires, and very good brakes.
Use highest quality rod bolts and don't go cheap on the balance work.
I have a similar one in my car and it work's good.
This combo should get you 1.25 - 1.3 HP per inch and a long life, trouble free, high value to $ ratio ($10-14/HP) , street motor that can hold it's own on the track in a 2500 lb GT40 with good suspension setup, modern tires, and very good brakes.
Use highest quality rod bolts and don't go cheap on the balance work.
I have a similar one in my car and it work's good.
Howard Jones
Supporter
91 octane, 10 to 1 CR, 302, 280 duration, 550 lift Hyd roller, current aftermarket Alum heads with 2.02/1.60 valves, Holley 750 DP, 36 degrees total advance all in @ 2500, Mechanical secondaries and advance, 6300 rev limit, free breathing exhaust. Use highest quality rod bolts and don't go cheap on the balance work.
This combo should get you 1.25 - 1.3 HP per inch and a long life, trouble free, high value to $ ratio ($10-13/HP) , street motor that can hold it's own on the track in a 2500 lb GT40 with good suspension setup, modern tires, and very good brakes.
More than that and the question becomes the rest of the drivetrain
I have a similar one in my car and it work's good.
This combo should get you 1.25 - 1.3 HP per inch and a long life, trouble free, high value to $ ratio ($10-13/HP) , street motor that can hold it's own on the track in a 2500 lb GT40 with good suspension setup, modern tires, and very good brakes.
More than that and the question becomes the rest of the drivetrain
I have a similar one in my car and it work's good.
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