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383 engine build, how to lower the compression?

Tiny
ETEDESCHI72
  • MEMBER
  • 1985 CHEVROLET TRUCK
  • 7.4L
  • V8
  • 4WD
  • MANUAL
  • 150,000 MILES
Hi,
I've been watching Ryan's videos on building the 383 engine. I've got an '85 Chevy K20 that currently has a tired 454 in it. I want to build a 383 for it but I've never built an engine before. I asked Ryan a question about building a 383 truck engine on his latest 383 video and he said that this build would be perfect for a truck application, but he said that I'd want to lower the compression to about 9:1 and put in a different cam. So, my questions are, exactly what do I do to lower the compression? Is it a matter of just putting in different pistons and if so, exactly what pistons do I need and exactly what cam do I need? Thank you.
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Saturday, December 7th, 2024 AT 8:01 AM

6 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,867 POSTS
Hi,

Thanks for stopping by the site. I have forwarded your post to Ryan specifically to help answer your questions.

Take care and good luck with the build.

Joe
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Saturday, December 7th, 2024 AT 9:23 PM
Tiny
RYAN L
  • ADMIN
  • 261 POSTS
Hey there!

Thank you so much for watching the 383 series! So for lowering the compression ratio you either need a bigger combustion chamber in the heads or bigger dished pistons.

To decide on which to do check out this awesome compression calculator from Summit Racing https://www.summitracing.com/popup/calcsandtools/compression-calculator?srsltid=AfmBOor9OVeILCrtBhOANTvZvfXQiMxU7D7B85aI_YPnNPpKLJ2HGuih

If you use the same cylinder heads I am using, you can put these pistons in and get 9:1 which could run on 87 pretty easily and have good power still. https://www.summitracing.com/parts/uem-kb101-030/make/chevrolet

Just double check the wrist pin size and the cylinder bore!

For a camshaft something like this would be a good bet for a truck 4x4 application https://www.summitracing.com/parts/sum-1102/make/chevrolet

I have uploaded the calculator to show what it would look like with this combo.

Also here are the heads I plan on using, we have not gotten to this part on the build. https://www.summitracing.com/parts/SUM-162112

Good luck on your build, let me know how it goes!

If you have any other questions, please let me know and thank you for using 2CarPros!

Best,
Ryan
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Tuesday, December 10th, 2024 AT 10:58 AM
Tiny
ETEDESCHI72
  • MEMBER
  • 3 POSTS
Hey Ryan,
Thank you for directing me in the right direction. This is probably a silly question but, the stroke on the pistons you're installing in the series is 3.750 in. And the stroke on the pistons you suggested to me is 3.480 in. That's not a bad thing, right? Thanks
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Tuesday, December 10th, 2024 AT 3:33 PM
Tiny
RYAN L
  • ADMIN
  • 261 POSTS
It would change the wrist pin height and affect the total volume of the cylinder a bit. My apologies. These should be everything right https://www.summitracing.com/parts/uem-kb121ktm-030/make/chevrolet but again check the wrist pin diameter. With these your compression ratio should be high 8's which is still solid for a street truck.

Best,
Ryan
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Thursday, December 12th, 2024 AT 12:27 PM
Tiny
ETEDESCHI72
  • MEMBER
  • 3 POSTS
Great Ryan, wrist pin looks to be the same size. 2 more questions lol. So, these would be good for a 4x4 that I tow with once and a while and, will the piston rings you used in the video work with these? Sorry for all the questions but I've never built an engine before and just want to get it right the first time, for my application in a 4x4 that I tow with sometimes and daily drive. Thanks again
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Thursday, December 12th, 2024 AT 2:38 PM
Tiny
RYAN L
  • ADMIN
  • 261 POSTS
Very cool! No problem!

Yeah, the camshaft I linked should be pretty good for your application. As far as the rings go the ones, I'm using are gas ported which are pretty high performance and pretty pricey. So, you could get away with this https://www.summitracing.com/parts/hsn-2m139030/make/chevrolet and be perfect. I'm also assuming your bore is 4.030 as I did with the pistons.

I get that! But if you make a mistake, remember, it is just an engine and it can be fixed.

Best,
Ryan
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Tuesday, December 17th, 2024 AT 9:56 AM

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