75cc heads lowering the compression

[Spanks]

Hey guys I'm Spanks from Iowa, I have a 69 roadrunner and a dog named Mr mojo risin.
This forum has helped me a lot in the past by just lurking.
I just purchased the edelbrock rpm performer 75cc heads 60829. I want to install them on a 68 440 non hp. Engine is stock. My concern is the compression will be to high for pump gas 91 octane. Is there any tricks to lower the compression to 9.5-10.1. By my rough calculations with the rpm heads and stock flat top pistons the compression would be around 11.6 (I'm really not sure).
If I would have done more investigating I would have went with the 84cc heads. The engine is currently torn apart and I am open to any suggestions.
Manual brakes, 4 speed, 3.23 sure grip
Go easy on me and use small words for I have no idea what I'm talking about.

Thanks in advance

 

[RemCharger]

Were your pistons around 50 60 thou down?

 

[Spanks]

Were your pistons around 50 60 thou down?

I believe 50

 

[kramer]

Assuming the deck is uncut and the pistons should be around .050" in the hole with 75 cc heads the compression should be around 10.3 to 1 with a .040 head gasket and you can go to a thicker and bigger bore gasket to easily get the compression around 10 to 1 or slightly lower, although you would have no quench.

 

[RemCharger]

So the trick you're looking for is, fatter headgaskets?

 

[Spanks]

Assuming the deck is uncut and the pistons should be around .050" in the hole with 75 cc heads the compression should be around 10.3 to 1 with a .040 head gasket and you can go to a thicker and bigger bore gasket to easily get the compression around 10 to 1 or slightly lower, although you would have no quench.

The head gaskets I have on hand are fel-pro .039 compressed.

 

[1 Wild R/T]

Assuming the deck is uncut and the pistons should be around .050" in the hole with 75 cc heads the compression should be around 10.3 to 1 with a .040 head gasket and you can go to a thicker and bigger bore gasket to easily get the compression around 10 to 1 or slightly lower, although you would have no quench.

I agree with Kramer's math, with aluminum heads & no quench @ 10.3-1 I think you'll be fine......

Add to this I think you need to measure how far in the hole the pistons are... I'm thinking closer to .070-.080 Which would put the C/R in the 9.6-9.8 range..

If the heads are off your engine bring a piston to TDC & use a feeler gauge & a straight edge to get a pretty accurate (easily within .010) depth...

 

[Spanks]

So the trick you're looking for is, fatter headgaskets?

I figured that was one option. I keep reading about dished pistons also. The heads haven't even been opened yet, so returning them for different heads is an option. I was really hoping to keep the stamped rockers. From what I've read about the eddy rpm 84cc heads is I will have to use adjustable rockers

 

[Spanks]

I agree with Kramer's math, with aluminum heads & no quench @ 10.3-1 I think you'll be fine......

By my rough calculations is actually from a email I received from 440 source asking about what cam I should get. They came back with the 11.6 cr

 

[Spanks]

Assuming the deck is uncut and the pistons should be around .050" in the hole with 75 cc heads the compression should be around 10.3 to 1 with a .040 head gasket and you can go to a thicker and bigger bore gasket to easily get the compression around 10 to 1 or slightly lower, although you would have no quench.

And yes the deck is uncut

 

[1 Wild R/T]

And yes the deck is uncut

Cut/uncut doesn't matter as long as you know how far thew pistons are in the hole..

The rest of this I posted above in an edit but since you already read & commented on the post while I was editing I'm reposting here so you'll see it..

If the heads are off your engine bring a piston to TDC & use a feeler gauge & a straight edge to get a pretty accurate (easily within .010) depth...


Add to this I think you need to measure how far in the hole the pistons are... I'm thinking closer to .070-.080 Which would put the C/R in the 9.6-9.8 range..

 

[WileERobby]

you're all thinking static compression ratio. your dynamic compression ratio depends on the cam you're using & how it's installed, advanced/straight up/retarded.

 

[Spanks]

Cut/uncut doesn't matter as long as you know how far thew pistons are in the hole..

The rest of this I posted above in an edit but since you already read & commented on the post while I was editing I'm reposting here so you'll see it..

If the heads are off your engine bring a piston to TDC & use a feeler gauge & a straight edge to get a pretty accurate (easily within .010) depth...


Add to this I think you need to measure how far in the hole the pistons are... I'm thinking closer to .070-.080 Which would put the C/R in the 9.6-9.8 range..

I will do that and report back

 

[Spanks]

you're all thinking static compression ratio. your dynamic compression ratio depends on the cam you're using & how it's installed, advanced/straight up/retarded.

I have not decided on cam yet. I'm waiting to hear back from Hughes and Mancini

 

[Spanks]

I've had the car since I was 18 but this is the first engine I'm trying to build for it.

 

[1968rt]

This calc was with a 440 source head gasket at .051.

 

[Curiousyellow71]

Its great that you've kept your Road Runner so long.
There seem to be two different pistons that got into the 350hp 440 in 68. All three of the 350hp engine I have taken apart were .080-.090 down. But guys have also taken them apart to with the hp piston which are .045-.055 down. One economical option to lower the compression is 440 source .051 gasket. Since your piston are down the deck a ways...w no quench there is NO worries using a thicker gasket. Choosing the right cam can also help tremendously.
http://store.440source.com/440-Source-Head-
Gaskets/products/180/
Easy to use Compression calculator link..
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

 

[1 Wild R/T]

You need to measure your deck height before choosing a head gasket. Better to have a higher compression with some quench than a bit lower with no quench. What pistons are you running?

The pistons are to far in the hole to make quench a viable option..

 

[33 IMP]

Until you know, KNOW, how far down in the hole the pistons are, any compression ratio calculations is, at best, an educated guess. Without accurate info on your heads, deckheight, and head gaskets, its still an educated guess.

 

[Spanks]

Its great that you've kept your Road Runner so long.
There seem to be two different pistons that got into the 350hp 440 in 68. All three of the 350hp engine I have taken apart were .080-.090 down. But guys have also taken them apart to with the hp piston which are .045-.055 down. One economical option to lower the compression is 440 source .051 gasket. Since your piston are down the deck a ways...w no quench there is NO worries using a thicker gasket. Choosing the right cam can also help tremendously.
http://store.440source.com/440-Source-Head-
Gaskets/products/180/
Easy to use Compression calculator link..
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

I've read that the heat disbursement of aluminum heads will lower compression by 1. Any truth to that?