• Jimmy
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Topic: Compression Ratio Question

So we are planning on building a spare engine later this year ( decade?) . I have had decent survival success with low compression stock domestic V8s ( Mopar 400 big block  )  engines from the 70s. The compression ratio for those is about 8.2 :1

We have also run a 383 big block Mopar and it is living . I believe the compression ratio on that is about 9.2:1. ( it was used and not exactly sure what's in it )

My question is how high a compression ratio can you go ( domestic V8s)  in Lemons and still maintain longevity?

Cordoba
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Re: Compression Ratio Question

My opinion on V8 is that its not compression as much as RPM. You can run higher compression but then you need to use higher octane fuel. I'd set compression to be safe on regular pump case. The $$$$ cost to run 91 is not worth it.

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  • From: Camarillo, CA
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Re: Compression Ratio Question

Compression ratio isn't really much of a factor with engine longevity unless your on the extreme end with a poor tune and low octane fuel. I wouldn't consider 9.2:1 high at all, as that's pretty standard for 90's era cars running regular 87 pump gas. You should be able to run regular 87 fuel on 9.2:1 without issues of knock.

As mentioned above, RPM is more of a killer, especially on old V8's. Give the engine a once-over, shift early, make sure it doesn't have oil starvation issues, and send it.

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  • Jimmy
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Re: Compression Ratio Question

Thanks for the responses

I guess my question is if you were building a motor from scratch and were going to get new pistons....How high a compression would you be comfortable with. is 10:5 to 1 to high? 11.5:1? Say I am willing to bite the bullet and run 93 Octane .

Our dream is to put a stroker crank in a 400 and get 450 - 475 cubic inches out of it . ( still have to do home work on that. )  Prob going to use the 452 stock cast heads to keep the budget down. I thinking that all that extra displacement will increase torque significantly. Getting these girls up to speed after a hairpin is quite the challenge. I think those extra cubes would come in handy.  These cars are automatic so I can keep the revs down .

All this would require new pistons/rods...so what compression pistons do I order?


Thanks

Cordoba
  • Mr.Yuck
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Re: Compression Ratio Question

Jimmy wrote:

So we are planning on building a spare engine later this year ( decade?) . I have had decent survival success with low compression stock domestic V8s ( Mopar 400 big block  )  engines from the 70s. The compression ratio for those is about 8.2 :1

We have also run a 383 big block Mopar and it is living . I believe the compression ratio on that is about 9.2:1. ( it was used and not exactly sure what's in it )

My question is how high a compression ratio can you go ( domestic V8s)  in Lemons and still maintain longevity?

Our 84 T/A had 9.5:1 (L69) no issues. I'd run the 383, I'd even deck the block, mill  the heads for 9.5:1. I'd call summit and order there t-chain set, 465/488 cam, along with new springs and rod bearings and roll. That cam has a great power band. The 400 is going to be hard to make power without buying new pistons, and they are heavy. Plus the 383 ha a steel crank. I guess you could use an older 440 crank in the 400 and make it a 451.

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  • Mr.Yuck
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Re: Compression Ratio Question

Jimmy wrote:

Thanks for the responses

I guess my question is if you were building a motor from scratch and were going to get new pistons....How high a compression would you be comfortable with. is 10:5 to 1 to high? 11.5:1? Say I am willing to bite the bullet and run 93 Octane .

Our dream is to put a stroker crank in a 400 and get 450 - 475 cubic inches out of it . ( still have to do home work on that. )  Prob going to use the 452 stock cast heads to keep the budget down. I thinking that all that extra displacement will increase torque significantly. Getting these girls up to speed after a hairpin is quite the challenge. I think those extra cubes would come in handy.  These cars are automatic so I can keep the revs down .

All this would require new pistons/rods...so what compression pistons do I order?


Thanks

The main problem will be pistons. If you want 10:1 or  higher you are going to need aftermarket or custom pistons. The factory LY rods will be fine with 10:1. The best thing you can do to improve your track numbers is go to a 4 speed. I'm a Mopar guy and my friend has buckets of parts. email me jhunter163@msn.com and let me know what you are looking for.

"get up and get your grandma outta here"
  • Jimmy
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Re: Compression Ratio Question

Thanks Mr Yuck! Most appreciated.

A 4 speed would be amazingly wonderful but talk about a project... . Man its has taken me three weeks to fix a sway bar. Its impossible to find time. Like everybody -  Life gets in the way. But I am always thinking......

We are super happy with the cars reliability and I don't want to mess that up too bad.  We have had a total of 13 different drivers this last year.  I bet 1/4 of our drivers don't drive a stick and the others well lets say...we have had to have a team meeting about putting the car in Drive and not 2nd before going on the track. ( I am not kidding - and I have also done it ) . I love my team but we are not Race Car drivers.

If you make it to Pitt or NJ stop by our paddock and we can totally nerd out rapping about old Mopar stuff.

Cordoba
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Re: Compression Ratio Question

Jimmy wrote:

Thanks Mr Yuck! Most appreciated.

A 4 speed would be amazingly wonderful but talk about a project... . Man its has taken me three weeks to fix a sway bar. Its impossible to find time. Like everybody -  Life gets in the way. But I am always thinking......

We are super happy with the cars reliability and I don't want to mess that up too bad.  We have had a total of 13 different drivers this last year.  I bet 1/4 of our drivers don't drive a stick and the others well lets say...we have had to have a team meeting about putting the car in Drive and not 2nd before going on the track. ( I am not kidding - and I have also done it ) . I love my team but we are not Race Car drivers.

If you make it to Pitt or NJ stop by our paddock and we can totally nerd out rapping about old Mopar stuff.

We are hoping to make it to NJ. this will be the 1st race with the new car...which is basically the old car in black, as we transferred everything over. It might be a tad lighter.

"get up and get your grandma outta here"
  • Registered: 2020-01-30
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Re: Compression Ratio Question

On non-computer controlled engines, the highest compression to run 87 octane is about 9.5:1. Concur with previous posts about RPM.

For added longevity, I'd add an engine oil cooler and three-row radiator. Maintain air flow through the engine compartment to decrease the likelihood of vapor locking the carb and fuel lines.

  • From: Queens, NY
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Re: Compression Ratio Question

static compression ratio and cam timing go together. their baby is dynamic compression ratio (dcr). dcr of about 8.5:1 is max for 87, 10.25:1 max for 93, thats my rule of thumb. I usually aim for 91 octane which is about 10:1. Cranking pressure would be about 215psi You might run into too much mean pressure for head gasket. or heat soaked head if its iron that can cause pre ignition.

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

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  • Jimmy
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Re: Compression Ratio Question

kakarot1232001 wrote:

static compression ratio and cam timing go together. their baby is dynamic compression ratio (dcr). dcr of about 8.5:1 is max for 87, 10.25:1 max for 93, thats my rule of thumb. I usually aim for 91 octane which is about 10:1. Cranking pressure would be about 215psi You might run into too much mean pressure for head gasket. or heat soaked head if its iron that can cause pre ignition.

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




I think I would try to stay around 9.5 - 9.75 CR on my dream stroker .  That's prob a good spot for a low buck Lemons rebuild

The nice thing about the low compression 400 motors is that  they do crank and start relatively easy even after coming off the track hot. I noticed the the 9.2:1 ( ish ) 383 that we run can have a little attitude when try to restart it start on a hot pit fuel up. Its always a fingers crossed scenario. I imagine more compression only makes things worse.

Cordoba
  • Mr.Yuck
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Re: Compression Ratio Question

Jimmy wrote:
kakarot1232001 wrote:

static compression ratio and cam timing go together. their baby is dynamic compression ratio (dcr). dcr of about 8.5:1 is max for 87, 10.25:1 max for 93, thats my rule of thumb. I usually aim for 91 octane which is about 10:1. Cranking pressure would be about 215psi You might run into too much mean pressure for head gasket. or heat soaked head if its iron that can cause pre ignition.

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




I think I would try to stay around 9.5 - 9.75 CR on my dream stroker .  That's prob a good spot for a low buck Lemons rebuild

The nice thing about the low compression 400 motors is that  they do crank and start relatively easy even after coming off the track hot. I noticed the the 9.2:1 ( ish ) 383 that we run can have a little attitude when try to restart it start on a hot pit fuel up. Its always a fingers crossed scenario. I imagine more compression only makes things worse.

When I drag raced I had a spray can, like you'd use for roundup, full of ice cold water. I'd spray the radiator down between hot runs. It only takes 30-45 seconds. It cools and engine down quick. If you are worried about heat sink, give it a try. You don't even need to lift the hood.

"get up and get your grandma outta here"