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#1
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Increasing Compression
As I understand it there are a number of ways to increase the compression,
Shave the heads to reduce the area around the valves, Use different pistons that have a higher or larger top, My question is can I also use longer rods to acheive the same thing, If so then by how much would they need to be longer by? Tony
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1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#2
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Re: Increasing Compression
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Higher comp pistons would be the best. Harvey.
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One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#3
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Re: Increasing Compression
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It still requires new pistons, but hey, boring is relatively cheap! Its the pistons that are expencive.
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www.svxfiles.com The first SuperCharged SVX, the first 4.44 gears, the first equal length headers, the first phenolic spacers, the first Class Glass fiberglass hood, the first with 4, 4.44s in his driveway Fiberglass Hood thread My locker 4.44 Swap link |
#4
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Re: Increasing Compression
Quote:
Tony
__________________
1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#5
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Re: Increasing Compression
Harvey's right Tony...I shaved my heads and am having difficulty getting the timing right. Didn't realise that the poofteenth I took off would impact the timing as much as it has.
If you're interested in pistons, and you need some extras to get going, then I'll be interested in a set. Mike offered me a set a few years back, but I just wasn't in a position to do anything then. He may have the specs and contacts that could help? What ratio are you thinking? And how did you work it out? Are you thinking E85 as well? So 13:1? M |
#6
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Re: Increasing Compression
I was think of trying 14 to 14.5,
pretty sure the pistons I have in the motor from Mike were 13, I want to go higher.
__________________
1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#7
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Re: Increasing Compression
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If your HP was 350, (for example) at 13 to 1 compression, and you bumped it up to 14.5 to 1, the increase is only to 358 HP. IMHO not worth the expence! Bowling's Compression Ratio -> HP Calculator Computation Results: Engine Horsepower (peak) is 350 Old Compression Ratio is 13.0 New Compression Ratio is 14.5 Computation Results: Computed New Engine HP is 358, a 2 percent change
__________________
www.svxfiles.com The first SuperCharged SVX, the first 4.44 gears, the first equal length headers, the first phenolic spacers, the first Class Glass fiberglass hood, the first with 4, 4.44s in his driveway Fiberglass Hood thread My locker 4.44 Swap link |
#8
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Re: Increasing Compression
So Harvey what does your calculator say about bumping up the compression.
I don't get the 2% gain only, If thats all that is in it why have a number of race rules forced the compression ratio down. You would think for 2 % it would make stuff all difference. Tony
__________________
1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#9
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Re: Increasing Compression
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The same NA engine going from, say 7 to 1 to 9.5 to 1 has a MUCH different outcome. Bowling's Compression Ratio -> HP Calculator Computation Results: Engine Horsepower (peak) is 350 Old Compression Ratio is 7.0 New Compression Ratio is 9.5 Computation Results: Computed New Engine HP is 384, a 10 percent change
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www.svxfiles.com The first SuperCharged SVX, the first 4.44 gears, the first equal length headers, the first phenolic spacers, the first Class Glass fiberglass hood, the first with 4, 4.44s in his driveway Fiberglass Hood thread My locker 4.44 Swap link |
#10
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Re: Increasing Compression
No sorry, I ment 0.06"
Quote:
Whats that say Tom? Harvey.
__________________
One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#11
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Re: Increasing Compression
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Bowling's Compression Ratio -> HP Calculator Computation Results: Engine Horsepower (peak) is 350 Old Compression Ratio is 11.0 New Compression Ratio is 14.0 Computation Results: Computed New Engine HP is 370, a 6 percent change
__________________
www.svxfiles.com The first SuperCharged SVX, the first 4.44 gears, the first equal length headers, the first phenolic spacers, the first Class Glass fiberglass hood, the first with 4, 4.44s in his driveway Fiberglass Hood thread My locker 4.44 Swap link |
#12
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Re: Increasing Compression
svxfiles, your right they were 11 to 1.
Matt what did you bring your compression up to by the head mod? Tony
__________________
1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#13
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Re: Increasing Compression
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The overall factor that we work against is the actual combustion pressure that is developed, to expand the air, to push the piston down, and this needs the maximum pressure to happen at around 15* After Top Dead Center. If you work back to why increase the ratio? The simple answer is the speed of the gas/air burn in the combustion chamber. If we are to run the engine at a faster speed, the burn has to be faster, we get this by running the highest comp pressure that the fuel can stand till detonation limits it. So we tend to go to higher-octane fuels to achieve this. In the quest for more power we have to go to alcohol fuels, these have a much slower burn rate than petrol, so to speed the burn up, up goes the compression ratio, to increase the combustion pressure, to get the burn done in the necessary time. Of course it has other benefits that help us up the next step to increase the power. Harvey.
__________________
One Arm Bloke. Tell it like it is! 95 Lsi. Bordeaux Pearl, Aust. RHD.149,000Kls Subaru BBS wheels. 97 Liberty GX Auto sedan. 320,000Kls. 04 Liberty 30R Auto Premium. 92.000kls. |
#14
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Re: Increasing Compression
bazza seems to agree with Matt that we can use E85 and get around a heap of problems.
My question is can I weld to the inside of the head and increase the compression ratio. I understand that it is band by some rules but could it be done. If we weld the head then CNC the metal away to get the right ratio. Tony
__________________
1995 - SVX 700,000 K Mine, DMS Struts to lift car 2in. Tyres Wrangler Silent Armor 235/70R16, PBR Radiator. 6 speed with DCCD and R180 rer diff, Heavy duty top strut mounts front and rear. Speedo correction box fitted. New stero (gave up on the old one). Back seat removed and 2 spare tyres fitted for desert driving. ECUTune SC sitting in the box for the next SVX. 1992 - SVX 255 K Wife (Want to stay Married so not allowed to fit SC) 1992 - SVX Pearl with black roof race car roll cauge etc ready to race. Ex Tasman Targa car. 1995 - SVX Green low k mint condiation. 1995 - SVX Rally car, ex Matts car. Now to be used on track. 1992 - SVX red & Black being converted to Mid Engine. 1995 - SVX Red 143,000 bit rough. Owned 5 others Subaru back to a 1974 1400 GSR. |
#15
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Re: Increasing Compression
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Also the theory of E85 making more power is good, high compression + E85 will make more power. 98RON is such a nasty fuel that its very hard to get near the engines true potential due to detonation. Stock Subaru tunes are unbelievably aggressive. A mate of mine couldn't run more than 14 psi on street 98RON for example with the stock ecu running timing - major detonation. We put in E85 and a microtech to control fuel only and it was clearing 30 psi without a hint of detonation - car was absolutely ballistic. Timing was around 25 degrees at peak power. I think we could push it even harder and get close to 30 degrees and make a bit more. It actually snapped a rod it had so much torque. Engine now rebuilt to 2.2L with forged rods and running 25 psi with 25 degrees timing... madness!! So I think trying to bump up the compression is a great idea. My own engine has had the block and heads shaved, a cometic head gasket and stock pistons. I'd assume the compression is a touch higher than stock. One thing in turbo applications is to actually use E85 and reap proper benefits you need high compression - something I've noted is the low compression turbo engines don't get the gains as the high compression ones. Also did a 2 second image search for E85 vs 98RON in NA applications - this is a random V8 I found: Last edited by bazza; 05-29-2012 at 06:46 PM. |
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