Compression Ratio: 8.5:1 vs. 9.5:1

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Compression Ratio: 8.5:1 vs. 9.5:1

Post  96Mustang460cid on September 22nd 2010, 10:12 am

For a large bore, large cubic street driven turbo application, would you guys recommend 8.5:1 or 9.5:1. This is going in a 4,800 lb truck.

A few of my thoughts:
1) The large cid will help with low end torque
2) The higher compression will increase cylinder pressure and may encourage detonation. A well designed camshaft should fix that, though, right?
3) Higher efficiency
4) Bigger engine will utilize as-cast A460 cylinder heads at a lower RPM better.
5) ???
6) ???

Combo:
-- 4.5" bore
-- 4.3" (547 cid) or 4.4" stroke (560 cid)
-- A460 cylinder heads (as cast)
-- < 7,000 rpm
-- Custom camshaft designed around application
-- mid-mount 98mm turbo (< 15 psig)
-- C6 trans w/ ~3,000 stall
-- Carb'd
-- Belt driven A/C, P/S, alternator, and fuel pump
-- ~5,000 miles/year (hopefully)
-- 1,000+ hp
-- Pump gas (91+ octane)


I'm interested in hearing your thoughts on the matter Smile.

Have a good day!
Michael

Edit: One more downside: bigger cid will bring the hp potential of the turbo down a little further. I believe this turbo is getting close to too small already...


Last edited by 96Mustang460cid on September 22nd 2010, 10:38 am; edited 1 time in total (Reason for editing : Adding information)
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  Tennessee Bullitt on September 23rd 2010, 8:51 pm

why not do a roots or twin screw with that much weight? I have seen 400 cid small blocks in cars have to much fun when the big turbo hits. It almost seems that much weight would like more hard hitting TQ. Or twins might be better for that idea. P51 heads might be more user friendly for the street or runnig A460 heads would be great for a n/a 600+ cubes.
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  whatbumper on September 23rd 2010, 9:14 pm

8.5:1 CR and the smaller cubic inch

You're going to run out of turbo a long time before the 7000rpm number.

I still have the 101 for sale if you need it. just put your hot side on it and sell the 98.

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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  96Mustang460cid on September 24th 2010, 12:13 pm

Tennessee Bullitt wrote:why not do a roots or twin screw with that much weight? I have seen 400 cid small blocks in cars have to much fun when the big turbo hits. It almost seems that much weight would like more hard hitting TQ. Or twins might be better for that idea. P51 heads might be more user friendly for the street or runnig A460 heads would be great for a n/a 600+ cubes.

I'm more familiar with turbo stuff and I think a roots might damage the sleeper persona I'm looking for Smile. I agree with you...my original build plan was 600+ cid N/A. That's why I bought the A460 heads and IDT block. Now that I've got it, though, I would lose money by trying to switch to P51 heads. I've already bought pistons, intake, fabricated headers, ect. I like the added security of 18 head bolts, though. I know I'm using the A460 stuff outside of its intended range, but I think it'll work okay still. One of the good things about non-N/A stuff is I can add boost if I want more power. If I ever want to really step this engine combo up, I'll swap camshafts and turbo(s) and be ready to roll some big'er' hp #'s.

whatbumper wrote:8.5:1 CR and the smaller cubic inch

You're going to run out of turbo a long time before the 7000rpm number.

I still have the 101 for sale if you need it. just put your hot side on it and sell the 98.

You're probably right..the 98 is too small. If/when I step up to a larger turbo, it'll likely be a 106mm drunken.

Thanks for you input, guys!

Have a good day!
Michael
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  whatbumper on September 24th 2010, 12:41 pm

I have one still! Precision 106mm g trim $1400 plus shipping. or I will be at Tulsa on October 16th for the OTT5 race. you just have to swap your bigger exhaust side on it.

Let me know. It would be a bad ass ride then for sure!

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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  Tennessee Bullitt on September 24th 2010, 5:19 pm

Cool, if you already have parts then turn it loose. The big turbo like the 106 you are talking will scream. I thought you didn't have any parts yet. Hell if the big turbo is a little lazy then spray a little HeRo juice on it to wake it up on the street Twisted Evil

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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  whatbumper on September 24th 2010, 5:40 pm

Tennessee Bullitt wrote:Cool, if you already have parts then turn it loose. The big turbo like the 106 you are talking will scream. I thought you didn't have any parts yet. Hell if the big turbo is a little lazy then spray a little HeRo juice on it to wake it up on the street Twisted Evil


it won't be lazy with that many cubes.

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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  83-88T-Bird Guy on September 24th 2010, 9:10 pm

With all those cubes and head flow , I would go with 8.5 to 1.
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  Filucky on December 12th 2010, 12:32 pm

96Mustang460cid wrote: I know I'm using the A460 stuff outside of its intended range, but I think it'll work okay still.

I think the A460 18 bolt heads would be a great turbo head! 18 head bolts, great flow numbers and one of the biggest CC chambers for a BBF head, wheres the down side? Very Happy
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  dfree383 on December 14th 2010, 10:56 am

Filucky wrote:
96Mustang460cid wrote: I know I'm using the A460 stuff outside of its intended range, but I think it'll work okay still.

I think the A460 18 bolt heads would be a great turbo head! 18 head bolts, great flow numbers and one of the biggest CC chambers for a BBF head, wheres the down side? Very Happy

the 18 bolt stuff has smaller chambers than the 10 bolt heads.
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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  Outlaw5.0 on January 19th 2011, 1:24 pm

Stay at 8.5:1 for 91 octane.

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Re: Compression Ratio: 8.5:1 vs. 9.5:1

Post  EVE on January 20th 2011, 2:47 am

I ran 10.8 on E85 in the 502". The new motor is a little bit less but is getting more boost.

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