highest C/R for 93

[427Bullitt]

Hey guys, n00b here and I'm building a 427 and was wonder what the highest compression ration you can have and still run on 93 octane. I'm trying to get all the horsepower I can out of the motor, but don't want to set the compression too high and mess with the timing so much that i defeats the purpose of it being a high comp. motor. Any input would be appreciated, about the fuel and what parts have worked best for hp gains.

Foti

[olddog]

I don't have the expertise to give you an answer, but I do know enough to tell you that you need to give more information, if you want reasonably accurate answer.

Aluminum heads or cast iron?

The combustion chamber and piston combination can give more or less swirling and quench, which can have an impact.

Cam timing can give a much lower dynamic compression than the static compression.

All these factors combine to make big differences that is over my head.

My rough rule of thumb is stay around 9.5 with iron head and 10.5 with aluminum heads for a street engines. I could be dead wrong depending on your combination.

[427Bullitt]

Originally Posted by olddog

I don't have the expertise to give you an answer, but I do know enough to tell you that you need to give more information, if you want reasonably accurate answer.

Aluminum heads or cast iron?

The combustion chamber and piston combination can give more or less swirling and quench, which can have an impact.

Cam timing can give a much lower dynamic compression than the static compression.

All these factors combine to make big differences that is over my head.

My rough rule of thumb is stay around 9.5 with iron head and 10.5 with aluminum heads for a street engines. I could be dead wrong depending on your combination.

Well I was thinking of using flat top's with -58cc 255cc AFR aluminum heads but that would give me about 13:1 compression, with a custom ground cam from Comp Cams. And then I remembered that if it's too high you'll need race fuel.

[Cashburn]

You can make your own race fuel with a trip to Home Depot.

[427Bullitt]

Originally Posted by Cashburn

You can make your own race fuel with a trip to Home Depot.

how? is it cheaper?

[blykins]

If you wanna stay safe, shoot for about 10.5:1. If you keep an eye out for chamber shape, sharp edges, camshaft, quench, etc, you can run higher.

[Gaz64]

Originally Posted by olddog

I don't have the expertise to give you an answer, but I do know enough to tell you that you need to give more information, if you want reasonably accurate answer.

Aluminum heads or cast iron?

The combustion chamber and piston combination can give more or less swirling and quench, which can have an impact.

Cam timing can give a much lower dynamic compression than the static compression.

All these factors combine to make big differences that is over my head.

My rough rule of thumb is stay around 9.5 with iron head and 10.5 with aluminum heads for a street engines. I could be dead wrong depending on your combination.

I'd say the above is fairly wise words from someone who doesn't believe he has the "expertise".

It's all about effective compression ratio (static CR versus cam duration, specifically inlet valve closure), also known as dynamic compression ratio.

You can have an engine of 9:1 with an economy cam (short duration) rattle its head off from detonation, and yet still have a 10.5:1 motor with a long duration cam not ping at all.

So if you're limited to 93 octane, I'd suggest building more cubes at milder engine spec settings. You'll make more reliable figures that way.

[Cashburn]

http://www.turbobuicks.com/forums/bu...dvertised.html

[bobcowan]

It's all about cylinder pressure. Pressure makes power.

There are a number of facters that influence pressure: static compression, altitude, cam design, etc. There are some other facters invoilved, like quench and chamber design. But they're very difficult to calculate. What you want is to get your cylinder pressure up around 190'ish psi. That's about the max pressure you can use with pump gas.

For example, my static compression is 13.2. But I can run all day on 91 octane pump gas.

http://www.wallaceracing.com/dynamic-cr.php

[zrayr]

on my 289 w/ stock pistons & stock iron heads I have a CR of 10:1 and have no problems with 93 octane gas. Advance is 40 degrees, or 33 degrees with vintage Paxton. Its been several years since I've ran an FE on 93 octane so I can't comment directly as gas formulations have changed.

Z.

[427Bullitt]

Ok cool thanks a lot guys, especially that dynamic calculator!!!! Is it true that two carbs give you more hp, i was planning on just one but if tuning two is worth it I my might just go for it.

[blykins]

No...

On anything less than a tunnel ram setup, you probably won't see a gain over a good single carb intake. It can be a headache to get the carbs tuned as well.

[DanEC]

On my stroked 428 with ported iron heads, I couldn't talk Keith Craft into going more than 9.0. Still made plenty of HP for what I was wanting - a reproduction appearance motor. The dyno operator said it loved 40 deg timing as a result. I suspect it could have tolerated another half point but that's how he wanted to build it.

Dan

[Dwight]

Coach Mike had Keith Craft to build him a stroked 428. 601 hp with 9.0 c.r.

Lainhart had Keith to stroke his 427 to 487. It made 617 hp with 9.0 -9.5 c.r.


If you have the correct head, cam, intake set up you can make big HP with out high compression.

427Bullitt, what is your hp goal?


Dwight


Coach and Lainhart drive their Cobras on the street, so 600 HP is just right

[427Bullitt]

Originally Posted by Dwight

Coach Mike had Keith Craft to build him a stroked 428. 601 hp with 9.0 c.r.

Lainhart had Keith to stroke his 427 to 487. It made 617 hp with 9.0 -9.5 c.r.


If you have the correct head, cam, intake set up you can make big HP with out high compression.

427Bullitt, what is your hp goal?


Dwight


Coach and Lainhart drive their Cobras on the street, so 600 HP is just right

Around the 600 area, more if that's possible, but honestly as much as possible with pump gas. Are those numbers whp, or to the crank? Probably wheel horse power, but i'm just double checking.

[ERA Chas]

Originally Posted by 427Bullitt

Well I was thinking of using flat top's with -58cc 255cc AFR aluminum heads but that would give me about 13:1 compression, with a custom ground cam from Comp Cams. And then I remembered that if it's too high you'll need race fuel.

FE's have chambers north of 77cc's and AFR does not make FE heads. I think you're talking about Windsor heads. Please clarify what we're talking about.

[blykins]

Originally Posted by 427Bullitt

Around the 600 area, more if that's possible, but honestly as much as possible with pump gas. Are those numbers whp, or to the crank? Probably wheel horse power, but i'm just double checking.

Those would be flywheel horsepower numbers that were quoted above.

600 hp to the wheels would be up over 700hp at the flywheel. Not that it isn't possible, but money and maintenance goes up while the reliability and longevity go down.

[ERA Chas]

Originally Posted by ERA Chas

FE's have chambers north of 77cc's and AFR does not make FE heads. I think you're talking about Windsor heads. Please clarify what we're talking about.

HELLO!
Are you building a 427 Windsor??? You're calling it an FE, posted it the FE Forum, and they're giving to FE information.
Everybody sleeping through this or am I wrong?

[427Bullitt]

Originally Posted by ERA Chas

FE's have chambers north of 77cc's and AFR does not make FE heads. I think you're talking about Windsor heads. Please clarify what we're talking about.

Yea sorry, its a windsor block but I'm stroking it to a 427.

Originally Posted by ERA Chas

HELLO!
Are you building a 427 Windsor??? You're calling it an FE, posted it the FE Forum, and they're giving to FE information.
Everybody sleeping through this or am I wrong?

Sorry about that, I wasn't thinking straight. Mindset was i'm building a motor that will end up being a 427ci motor, a 427's an FE motor, but a still a windsor based block. Sorry guys

[ERA Chas]

OK, now we're getting somewhere.
Tip: In future post in 'Small Block Talk' forum. You'll get more help 'cause those guys almost never look at the FE forum.

You don't say if track car or street only-just that you want max HP without big compression. At 11:1 on 93, you can easily and comfortably get 550 to 575. I would use 255's or something in the 235cc range, with 2.08's and 1.60's, an hydraulic roller around 245 deg and .660" lift and a 900+ cfm Holley or even a 1050 Dominator-if not in a Cobra.

Brent Lykins on here specializes in good Windsor builds.

The more info you give the more effective advice you'll get.