Lowering Comperssion?????
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Lowering Comperssion?????
Yeah, I sad lowering compression. Here's the problem: I have a 72 F-100 on a 76 highboy chassis. It's got about 4" of lift on it and 38" Ground Hawgs. I love my truck and I love driving it but the 8mpg is killing me and the roughly 10.7:1 compression means 92octane min. OUCH
The engine came out of a 66 T-Bird and they had 10.5:1 stock. I figure w/ the overbore and new pistons it has been bumped up a couple of 10th's. Did the later model FE's, such as the "emissions years" have larger chambers in the heads or did they just use dished pistons to lower the compression? I really don't want to pull the pistons out as this engine has >10k mi on it and runs beautifully. I could get a set of mid 70's heads pretty cheap and if that would drop the compression enough to let me run 87 octane that would be great. Any advise for a poor college boy tired of paying $2/gal to feed a truck getting 8mpg round town????
The engine came out of a 66 T-Bird and they had 10.5:1 stock. I figure w/ the overbore and new pistons it has been bumped up a couple of 10th's. Did the later model FE's, such as the "emissions years" have larger chambers in the heads or did they just use dished pistons to lower the compression? I really don't want to pull the pistons out as this engine has >10k mi on it and runs beautifully. I could get a set of mid 70's heads pretty cheap and if that would drop the compression enough to let me run 87 octane that would be great. Any advise for a poor college boy tired of paying $2/gal to feed a truck getting 8mpg round town????
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From: Jackassville
well, id hate to decrease the power of that high boy, my recommendation is get a cheap Japanese car or something to drive around around town and just drive your truck on the weekend. its a crime to intentionally decrease the power of your vehicle.
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From: Long Island USA
I'm not sure about the good old stock 10.5:1 4bbl 390, but the cam is matched to the compression. To lower the compression enough to run 87, the cam is not going to make near it's normal power, and you just might blow the mileage even further, making the move to 87 worthless.
Have you even tried a lower octane? Back off on the timing, you'll lose idle vacuum, but you might be able to get away with 87 with the right timing and a well-cooled motor - and then, you could go back to 92 at will when you wanted the power.
Run-on might be a problem though
Have you even tried a lower octane? Back off on the timing, you'll lose idle vacuum, but you might be able to get away with 87 with the right timing and a well-cooled motor - and then, you could go back to 92 at will when you wanted the power.
Run-on might be a problem though
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w/ the timing set at 6deg BTDC I tried mixing about 5-7gal of premium in the tank w/ about 5gal of 89 and it spark knocked so bad I swore I'd never try that again. I don't think it's possible to back the timing off enough to run 87.
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From: Brea CA
Your heads (if they're C6 something) already have bigger chambers than any of the later heads from the "emission years." Thicker head gaskets will lower the compression but also will negate the quench zone in your heads. What you gain in one hand you will lose in the other. Hey, since you already got some compression, how about running a bigger cam with more overlap? It's the overlap that will save you, increasing it will bleed off some of the effective cylinder pressure to something you can live with pump gas.
Let me explain this a little better, it's not just the overlap by itself but the relationship of the Lobe Seperation Angle (LSA) for a given overlap. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure but the decrease/bleed-off of compression is not due to the overlap only, it is due to the intake closing and exhaust opening events.
Having said that, maybe Ratsmoker and the guys here with dyno programs can help you pick a better cam.
Let me explain this a little better, it's not just the overlap by itself but the relationship of the Lobe Seperation Angle (LSA) for a given overlap. By increasing duration for a given LSA, the overlap will increase, the intake closing will be delayed, and the exhaust opening will occur earlier. This will decrease cylinder pressure but the decrease/bleed-off of compression is not due to the overlap only, it is due to the intake closing and exhaust opening events.
Having said that, maybe Ratsmoker and the guys here with dyno programs can help you pick a better cam.
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From: Long Island USA
Originally posted by BB
Hey, since you already got some compression, how about running a bigger cam with more overlap? It's the overlap that will save you, increasing it will bleed off some of the effective cylinder pressure to something you can live with pump gas.
Hey, since you already got some compression, how about running a bigger cam with more overlap? It's the overlap that will save you, increasing it will bleed off some of the effective cylinder pressure to something you can live with pump gas.
Wish I had thought of it
Quick edit: I wonder how long it will take to make up the cost of the cam with the cheaper gas?
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Sounds like you guys have covered most of the good solutions.
Here are a few more "low cost" solutions to think about.
Did the engine knock like that from day one, or did it get worse
over time? It may be excess carbon buildup in the chamber. You could clean it with the water trickle method or use a bottle (or two) of Techron Concentrate.
Another thought is water injection. It kicks in a water mist only at WOT. I've not had a need to do it myself, but in theory, it should be a good solution -- just don't forget to fill up the water tank.
Another thought that may be to run a cooler thermostat, slightly retarded timing (are you sure the marks on the balancer haven't slipped?), and then get away with the RUL/PUL or mid-grade mix.
But you know, that is a lot of compression, so maybe the best bet is still to check into that cam swap.
Good luck and let us know how it goes.
Here are a few more "low cost" solutions to think about.
Did the engine knock like that from day one, or did it get worse
over time? It may be excess carbon buildup in the chamber. You could clean it with the water trickle method or use a bottle (or two) of Techron Concentrate.
Another thought is water injection. It kicks in a water mist only at WOT. I've not had a need to do it myself, but in theory, it should be a good solution -- just don't forget to fill up the water tank.
Another thought that may be to run a cooler thermostat, slightly retarded timing (are you sure the marks on the balancer haven't slipped?), and then get away with the RUL/PUL or mid-grade mix.
But you know, that is a lot of compression, so maybe the best bet is still to check into that cam swap.
Good luck and let us know how it goes.
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From: LAT 46.55501° & LON -122
I have been interested in this topic for the same reasons as the original poster.
Fel-Pro makes some .020 head gasket shims. I don't know much about these like if they can be stacked, or how much they reduce compression, but I would guess the effect would be the same as thicker gaskets.
Other than the power loss from lower compression, how badly does increasing the quench area affect performance?
I was thinking of trying colder plugs, but I haven't been able to find any Autolites colder than 45's.
I was thinking of a different cam also. The one I have now is a Comp Cams 268. How does this compare to Crane's 901 in relation to LSA and duration like "BB" was talking about?
Fel-Pro makes some .020 head gasket shims. I don't know much about these like if they can be stacked, or how much they reduce compression, but I would guess the effect would be the same as thicker gaskets.
Other than the power loss from lower compression, how badly does increasing the quench area affect performance?
I was thinking of trying colder plugs, but I haven't been able to find any Autolites colder than 45's.
I was thinking of a different cam also. The one I have now is a Comp Cams 268. How does this compare to Crane's 901 in relation to LSA and duration like "BB" was talking about?
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From: Evanston, WY
Just a thought. Make sure you are jetted correctly. A lean condition will magnify a detonation problem. Running a little fat will sometimes make enough margin to alleviate the problem.
-Scouder
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Originally posted by Like M. Fords
...Other than the power loss from lower compression, how badly does increasing the quench area affect performance?
...Other than the power loss from lower compression, how badly does increasing the quench area affect performance?
I think DeskTop Dyno 2000 showed a bigger effect, but, even though I love that program, I trust a real dyno more.
Smokey Yunik (sp?) used to recommend using a double head gasket for knock problems, but Ive never tried it myself.
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72F100
I am an A&P instructor, I teach people to work on airplanes. One of areas that is tought is fuels. The octane rating is actually the ratio of iso-octane to normal Heptain, with the Iso-octane being the active portion and the anti-knock ingrediant. Aviation fuels have ranged from 80-88 octain to 115-130. To get above 100 many additives are used. One of the most common addatives in fuel is lead. If you are not running a C-Converter, adding lead is one of the more cost affective ways of reducing knock.
Use a good brand of fuel, you may have had some cheap stuff. Shell or amaco (BP), make sure that there is no MTBS or gasahol and run a little lead, then try adjusting your timing. If knocking still persists, try enriching the mixture. A hotter running engine will knock before a cool running engine so you might try running a cooler thermostat.
Enjoy being a poor college kid, pretty soon you will be a poor married guy with two kids like me. Your truck will sit in your back yard or worse, your wife will comandeer it. In order to be able to afford to keep the truck, I drive an 89 toy camery with 246000 miles on it.
I am an A&P instructor, I teach people to work on airplanes. One of areas that is tought is fuels. The octane rating is actually the ratio of iso-octane to normal Heptain, with the Iso-octane being the active portion and the anti-knock ingrediant. Aviation fuels have ranged from 80-88 octain to 115-130. To get above 100 many additives are used. One of the most common addatives in fuel is lead. If you are not running a C-Converter, adding lead is one of the more cost affective ways of reducing knock.
Use a good brand of fuel, you may have had some cheap stuff. Shell or amaco (BP), make sure that there is no MTBS or gasahol and run a little lead, then try adjusting your timing. If knocking still persists, try enriching the mixture. A hotter running engine will knock before a cool running engine so you might try running a cooler thermostat.
Enjoy being a poor college kid, pretty soon you will be a poor married guy with two kids like me. Your truck will sit in your back yard or worse, your wife will comandeer it. In order to be able to afford to keep the truck, I drive an 89 toy camery with 246000 miles on it.
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Here's a nice reference:
http://www.chevron.com/prodserv/fuel...rgas/ch5.shtml
Look at Figure 5-9 near the bottom of the page. It shows things that affect octane requirements for engines.
For example, each compression ratio number increase needs a 3 to 5 octane number increase to prevent knock.
http://www.chevron.com/prodserv/fuel...rgas/ch5.shtml
Look at Figure 5-9 near the bottom of the page. It shows things that affect octane requirements for engines.
For example, each compression ratio number increase needs a 3 to 5 octane number increase to prevent knock.
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let me restate my title, I am a poor, married, college boy who's paying his own way.
I really don't have any trouble w/ detonation unless I run something other than premium. I've considered adding lead to the fuel but w/ the crazy compression this engine has I doubt it will be enough to drop down a fuel grade. The engine runs pretty cool w/ the 180 T-stat thats in it so a cooler stat will probably not do much either. I hate to think that I'm stuck w/ a premium loving gas hog but it may be an unavoidable problem. In the end I may have to end up selling my baby to get something a little easier on the wallet
If I didn't already have 3 cars I would probably just buy some cheap little gas sipper so I could keep the beast.
I really don't have any trouble w/ detonation unless I run something other than premium. I've considered adding lead to the fuel but w/ the crazy compression this engine has I doubt it will be enough to drop down a fuel grade. The engine runs pretty cool w/ the 180 T-stat thats in it so a cooler stat will probably not do much either. I hate to think that I'm stuck w/ a premium loving gas hog but it may be an unavoidable problem. In the end I may have to end up selling my baby to get something a little easier on the wallet
If I didn't already have 3 cars I would probably just buy some cheap little gas sipper so I could keep the beast.
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