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lamune

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PostSubject: Compression ratio   Tue May 09, 2017 12:17 am

What's the compression ratio I should not exceed when building a motor for 87 octane or 91 octane? Stock is 10.5:1, so is 11:1 problematic?

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Rev Bob



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PostSubject: Re: Compression ratio   Tue May 09, 2017 5:53 am

Depends.

The compression ratio on a computer controlled engine is not the only indicator of the tendency to pre-ignite(knock). Modern PCMs have provisions to adjust ignition timing, fuel mixture, and even EGR flow to enable compression ratios that would destroy carbureted engines. As a last resort, knock sensors (microphones) that detect pre-ignition bias the maps in the PCM to compensate for a particularly bum batch of fuel, extreme temperatures, altitude, ETC.

The choice of camshaft profile, connecting rod length, piston top shape, head material, normal operating speed, ECT. will also affect what compression ratio is doable for a particular combination.
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lakeffect
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PostSubject: Re: Compression ratio   Tue May 09, 2017 7:16 am

So given that information with no mention of an engine swap, can someone tell us what they have run
(successfully) in their LT1?
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Fred Kiehl

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PostSubject: Re: Compression ratio   Tue May 09, 2017 7:18 am

The material of the heads will also help determine the max compression ratio. You can go a little higher with aluminum heads. You would probably need 93 octane with 10.5. Reference the early 90s LT1 Corvettes for compression ratio and required octane.
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jayoldschool

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PostSubject: Re: Compression ratio   Tue May 09, 2017 9:12 am

No. STOCK compression ratio is 10.5 to 1. With iron heads. The 94-96 B/D cars are tuned for regular 87. You can advance timing to pick up a little power and run 89, 91, 93. Many of us have done that for years. You can also run a thinner head gasket to increase compression slightly, and combine with timing, and run 91/93.
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jasonlachapelle

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PostSubject: Re: Compression ratio   Tue May 09, 2017 10:36 am

don't get bogged down with static compression numbers. Aim for 8.5-9.0 Dynamic, your static compression ratio should sort itself out. You might be able to run regular below 8.5. If I were rebuilding an engine, I'd aim for 9.0 DCR and tune for 91 since we can get it without ethanol up here.

The successful B-body cams (crane 227, gm 845) put the DCR pretty much right on the money.

I've run up to 9.2X on 91 octane
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lamune

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PostSubject: Re: Compression ratio   Tue May 09, 2017 1:21 pm

Aha, of course this is probably a more complicated question than I thought. Let me add some qualifying info here that may make a more specific recommendation possible.

I ask this question because I need to pick pistons. We're pretty much decided on getting a set of Edelbrock heads (sorry Jason Smile ) which have 54cc chambers. With a 6.2l bottom end that's 775cc per cylinder so with flat-top pistons I get over 11:1, with dish tops I get closer to 10.5:1 (if I'm doing the math right)

What I'm hearing is that going over 11:1 may be too much.

I'll have to look and see what the process would be to compute DCR, Jason. Smile
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phantom 309

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PostSubject: Re: Compression ratio   Tue May 09, 2017 9:03 pm

google dynamic compression ratio vs static ,..  it all depends on what cam you're going to run,.and which rods,.

pick a piston that works with your final deck height to give you maximum quench,..

also be advised that 6.2 litres are thirsty,. Wink

but do what you want,. you're gonna anyway,. being an engineer n all.
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lamune

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PostSubject: Re: Compression ratio   Tue May 09, 2017 11:33 pm

I have some more homework to do.

To be fair, I am not an automotive engineer Very Happy
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jasonlachapelle

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PostSubject: Re: Compression ratio   Wed May 10, 2017 7:49 am

consider getting your stock cam reground at delta cams in Tacoma. Since you're going with a stroker you can afford to go with a bit more duration than you would in a 350 (+5-ish degrees).

just so it's clear from Nick's comment, research quench too. Interpret maximum quench as optimal quench.
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lamune

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PostSubject: Re: Compression ratio   Thu May 11, 2017 12:34 am

Thanks, I still have some schooling on a few of these details. Smile

I don't think the stock cam is usable, it was underwater for a while and it's pretty nasty! Besides, the kit comes with a new cam anyway. I'll see what those guys suggest as well.

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phantom 309

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PostSubject: Re: Compression ratio   Thu May 11, 2017 9:08 pm

lamune wrote:
Thanks, I still have some schooling on a few of these details. Smile

I don't think the stock cam is usable, it was underwater for a while and it's pretty nasty! Besides, the kit comes with a new cam anyway. I'll see what those guys suggest as well.


Cam comes in the kit?

Interesting,.
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lamune

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PostSubject: Re: Compression ratio   Fri May 12, 2017 4:08 pm

Yep, I'm getting close to getting the parts, I hope. I understand what the DCR is now to a much better degree and why that's really important.

Looks like quench is better with flat-tops than dished, but I might be way too high (statically anyway) with the flats. We'll see what they say.

What's the dynamic compression ratio on the stock LT1 anyway?
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phantom 309

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PostSubject: Re: Compression ratio   Fri May 12, 2017 10:41 pm

lamune wrote:


What's the dynamic compression ratio on the stock LT1 anyway?

nobody cares really,.

but it you feel motivated you need to get the stock cam specs ,. then do the after bottom dead centre colsing point stuff,. rod length,. stroke etc,..blah blah, all formula's easily found on the internet,.just plug all the math in,.

let us know too,.it'd be interesting.
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lamune

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PostSubject: Re: Compression ratio   Sat May 13, 2017 12:51 pm

Finding some of those specs isn't so easy as it turns out. I'm still looking for the closing angle and intake lobe centerline
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