Lowering Compression Ratio

[kanuck]

I have a nice solid built 427 with 12.5 comp, it runs great but the diet of 110 octane is getting old and expensive while also limiting my range. I don't want to tear it appart to change the pistons are there any aftermarket cyl heads that have a larger chamber, the ones I have on it now are steel low riser's.

[1985 CCX]

I am not a "know all" but alloy heads are more forgiving when it comes to compression so you be looking at upper end change then all set.

[kanuck]

I know knocking it down 2 points comp is inrealistic, My thinking is between going to alum heads and with a bigger chamber lowering comp and maybe a thicker head gasket I could run it on half race half pump.

[kanuck]

double post

[kanuck]

Robert Pond has some that are 10cc larger than stock that in itself should be good for close too 2 points drop in comp ratio which is exactly where I wanted to be as far as that is concerned.

[Mark O'Neal]

Originally Posted by kanuck

I have a nice solid built 427 with 12.5 comp, it runs great but the diet of 110 octane is getting old and expensive while also limiting my range. I don't want to tear it appart to change the pistons are there any aftermarket cyl heads that have a larger chamber, the ones I have on it now are steel low riser's.

What are your current head cc.s...and bore.

[Mark O'Neal]

Originally Posted by kanuck

Robert Pond has some that are 10cc larger than stock that in itself should be good for close too 2 points drop in comp ratio which is exactly where I wanted to be as far as that is concerned.

No Sir. Increasing the head ccs will decrease your C/R by about a point.

If you're currently at 10.5:1 you'd drop to 9.57 with a 10cc increase.

I'm happy to help, but that requires more information.

[eschaider]

There are any number of c/r calculators out on the web that can save you the effort of building a spreadsheet. Here is one from Summit, click here => Compression Ratio Calculator.

If you plug in your engine bore and stroke you can back into cylinder head and dome cc figures to get to a final combustion chamber volume. I think you will find a final combustion chamber volume (TDC Dome in Chamber volume ) of 10.2 cc will very closely approximate a 12.5:1 compression ratio for 427 cubic inches.

If you increase the chamber volume by 10.2 cc's the compression will drop to 11.13:1. I think this might be a stretch. Using twice as thick a head gasket would get you to about 10.7:1 which is still pretty zippy for pump gas with the chamber designs Ford used back then.

The BBM heads have a modern heart shaped chamber shape and I believe they were 75cc in size, if I remember correctly. These would likely not fit your pistons which means you would still need to pull the engine down to buy new pistons. The other challenge is, I believe BBM is out of stock on the as cast heads but might still have some ported heads left — supply chain woes.

In the end you might just be further ahead buy biting the bullet and replacing your current pistons and leaving the heads alone — although the BBM head is a very nice head.

[kanuck]

It's a 427 that has been bored .17 thou over the pistons were 12.5 to 1 with low riser heads which are 74cc give or take. That is what I said in my post I want to lower the comp ratio and installing heads with 10 to 15 cc bigger chambers will do that. "He has a few sets that are almost 90cc" combined with going from iron to alum I am sure I will be close to where I want to be if not spot on.

[undy]

Cometic makes numerous thickness FE 427 gaskets, some as much as .120 compressed thickness. That one's BIG $$. That should get you well into the lower 10 to 1 range.

https://www.cometic.com/applications...7--427ci70l-v8

[kanuck]

Thanks fo the link.

[Dominik]

Speak to your engine builder about maximum piston to deck height (quench area).

If it becomes too big, i.e. by using a thicker gasket, you may experience exactly what you try to avoid: uncontrolled pre-ignition/detonation.

Also, your cam may be perfecly matched to your CR and you lose quite some torque and power by dropping CR. This is a lesser problem.

I feel that recurving the timing is in order once done.

[bobcowan]

Never use the head gasket to increase chamber volume. That also increases quench, which tends to increase detonation and decrease efficiency. Use the head gasket to set the quench area as small as possible, regardless of your SCR and DCR.

DCR is more important than SCR. Be sure to calculate that. You might be able to improve DCR by changing the cam.

But, I think making a high compression iron motor survive on pump gas is going to be really difficult. Unless you live at 10,000'ASL.

If it were my car (and it's not) I would pull the motor down, replace cam, pistons, and heads to match what I wanted it to do. I have had a car with 12:1 compression and a huge cam; and that's what I did.

[byron w]

Just get in touch with Brent Lykins on this forum

[HighPlainsDrifter]

Hi,
I ran a snow performance water/alcohol mix injected into motor to control detonation on my turbo mustang.
Perry