compression ratio versus driveability

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compression ratio versus driveability

Post  fordguy69 on June 11th 2018, 9:06 pm

I know everyone loves hypotheticals, so here goes.


I am putting together a 521 for an pro street style fox body. C6 with 3500 stall 10" convertor.  31x18 Mickeys, rear gear TBD.

I am going to use AFR heads.  With the 75 cc heads, I will have 10.4 CR, or with the 85 cc heads 9.4 CR.

I will have a custom cam ground.

I am thinking that the lower CR will increase the streetability on pump gas (93 octane), easier starting, etc. (I am in Florida, so have to deal with a lot of heat and low elevation.)

However I don't want to lose the throttle response and power and sound that the higher compression will afford.

So what's the thought process of you guys with way more knowledge and experience than me on this?  What's the trade offs going with the lower compression ratio?  

Or am I seriously over thinking this?

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Re: compression ratio versus driveability

Post  BBFTorino on June 11th 2018, 11:05 pm

With aluminum heads, you can get by with a little more compression because it dissipates quicker, which in turn leads to the cooling system. It must be more than adequate to shed the heat and keep the motor running 'cool'.
Several other things can be done to compliment this as well, cooler plugs, a bottle of octane booster with every fill up!! etc.

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Re: compression ratio versus driveability

Post  dfree383 on June 12th 2018, 7:19 am

10.4:1 will run fine on 93 in the combo you have described.
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Re: compression ratio versus driveability

Post  Wayne Pearce on June 12th 2018, 11:08 am

I'm running a 545 cu. in. 11.3 to 1 compression ratio, with A429 cylinder heads, likes 93 octane, starts easily, and driveability has never been an issue. You'll be just fine.

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Re: compression ratio versus driveability

Post  DaveMcLain on June 12th 2018, 7:42 pm

It all depends on what you want to do with the engine and the availability of fuels where you're going to run the car. Pushing compression ratio too high is never a good idea if there is any chance that you might have to use fuel that's marginal and the gains don't offset the hassle. A customer of mine runs a Mopar on the Hot Rod Power Tour and all of the engines that he's had me do for him in recent years are built to run fine on 87 octane and that means about 9:1 compression. In the past he's had a rough time finding real 91 or higher octane out on the road and in that situation having 10.5:1 compression even with an aluminum head is a bad situation.

With that said if you're using 93 octane and quite a bit of duration 10.5:1-11:1 even shouldn't be a problem with an aluminum head and a coolant temp of about 185 degrees.


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