It doesn't lower the compression at all... It will allow an engine to run about one point higher compression with no other changes... Add to that the improved flow & it's a double win..
Last edited:
75cc heads lowering the compression
- Thread starter Spanks
- Start date
It doesn't lower the compression at all... It will allow an engine to run about one point higher compression with no other changes... Add to that the improved flow & it's a double win..
Spanks
Active Member
Any advice on what else to do to the engine while the heads are off? I'm still looking for which cam to run. I'll be using the holley sniper efi with the hyperspark distributor. TTI 1 7/8 long tube ceramic coated headers into 2.5 inch exhaust. The RR will be a weekend cruiser and not see the track. I'd like it to sound good at idle and be respectable for a country 1/4 mile every once in awhile.
sabre67
Well-Known Member
subscribed 
Curiousyellow71
Well-Known Member
Need more info for a cam..rockers? Springs? Hydraulic or solid? Roller or flat tappet? I'd pick a cam and intake that suits the holley sniper regardless.
Like mentioned the aluminum will dissipate more heat and be less prone to detonate and tolerate more compression. A identical head in aluminum vs cast iron is not likely allow you to run a full point of compression on the same gas. Maybe with quench and aluminum. But since you have no quench I'd guess 1/2 point. Could be higher. Pushing it usually goes badly for little gains...gas doesn't seem to get better.
Like mentioned the aluminum will dissipate more heat and be less prone to detonate and tolerate more compression. A identical head in aluminum vs cast iron is not likely allow you to run a full point of compression on the same gas. Maybe with quench and aluminum. But since you have no quench I'd guess 1/2 point. Could be higher. Pushing it usually goes badly for little gains...gas doesn't seem to get better.
Last edited:
I've seen guys claim one full point just for the cooling effect of aluminum heads & half a point for quench.... I've only built a few engines with aluminum heads & I wasn't pushing for max compression so I can't say with 100% certainty... I have built a decent number of quench engines & a half point is a safe number but I try to stay at .040 max & have built as close as .032 IMO quench @ .055 you might as well not bother...
Curiousyellow71
Well-Known Member
Could be as a max. We usually allow allow .3 for quench and .5 for aluminum on the street. I think it's a conservative...but safe.
Spanks
Active Member
The only info I could find on the springs said 150 psi. I would like to find a Hydraulic flat tappet cam and reuse the stamped rockers. Eddy performer dual plane, 1 inch tapered carb spacer. 6 quart e body hemi oil pan with hi volume oil pumpNeed more info for a cam..rockers? Springs? Hydraulic or solid? Roller or flat tappet? I'd pick a cam and intake that suits the holley sniper regardless.
Like mentioned the aluminum will dissipate more heat and be less prone to detonate and tolerate more compression. A identical head in aluminum vs cast iron is not likely allow you to run a full point of compression on the same gas. Maybe with quench and aluminum. But since you have no quench I'd guess 1/2 point. Could be higher. Pushing it usually goes badly for little gains...gas doesn't seem to get better.
Curiousyellow71
Well-Known Member
Outlawd
Well-Known Member
- Local time
- 1:55 AM
- Joined
- Dec 18, 2020
- Messages
- 765
- Reaction score
- 1,906
- Location
- United States of America
I built a 440 with eddy rpm heads and figured up my compression as if iron heads so instead of running on the edge of too much compression with the aluminum assuming to lose 1 point I'm running on edge of high compression with the aluminum as a safety net. I would dump some compression through the cam. We all know what detonation does to high performance motors, not worth the risk.
69 GeeTeeX
Well-Known Member
Make sure the camshaft is designed for Sniper EFI. EFI requires a different vacuum pulse than carburetors do. Don’t know the exact details but have done a few of these and it makes a significant difference in the way the engine runs mainly at idle. Also be aware that Sniper EFI is claimed to be self learning but my experience has been it depends on the camshaft and engine vacuum. A big lopey camshaft with low engine vacuum won’t self learn well. Not a big deal you’ll need to do more manual tuning to get it right.
Just going to toss this in spanks.
You posted about you may return the heads.
I would think twice about that.
Eddy heads are hard to get now, back ordered ect.
Oh and nice RR, & welcome from another Iowa member.
You posted about you may return the heads.
I would think twice about that.
Eddy heads are hard to get now, back ordered ect.
Oh and nice RR, & welcome from another Iowa member.
Spanks
Active Member
I was noticing that too. 440 source is out and so is trick flow
TN Mopar
Well-Known Member
Just so you know
I am running the 75 cc eddys with a 0.027 cometec gasket (.047 quench)
My pistons are .020 in the hole
My compression ratio is 10.7:1
I have the old Purple Shaft .509 cam 292 duration
I run 93 oct gas
I have no problem running the heads on the street
No issues with detonation either
When I look at your combo I agree with many of the others
your pistons are far enough down the hole that you shouldnt have any trouble running the 75cc heads
I am running the 75 cc eddys with a 0.027 cometec gasket (.047 quench)
My pistons are .020 in the hole
My compression ratio is 10.7:1
I have the old Purple Shaft .509 cam 292 duration
I run 93 oct gas
I have no problem running the heads on the street
No issues with detonation either
When I look at your combo I agree with many of the others
your pistons are far enough down the hole that you shouldnt have any trouble running the 75cc heads
Spanks
Active Member
Awesome! Thanks
Spanks
Active Member
Looks like the piston is .070 down
Spanks
Active Member
Using the summit compression calculator
Bore 4.32
Stroke 3.75
Cylinder head volume 75cc
Effective dome volume 0
Deck clearance. 07
Compression of 9.9:1
Compressed gasket .039
If I did this right, I'm happy with the results. Thanks again for the help guys
Edited to add gasket
Bore 4.32
Stroke 3.75
Cylinder head volume 75cc
Effective dome volume 0
Deck clearance. 07
Compression of 9.9:1
Compressed gasket .039
If I did this right, I'm happy with the results. Thanks again for the help guys
Edited to add gasket
Last edited:
You missed the head gasket... If theres no box for the head gasket adjust the deck clearance to compensate... Typical head gasket is .039
Spanks
Active Member
I did forget to add that here but did on the calculator. I will edit my post. Good catch Wild
Spanks
Active Member
Quick question for you. Depending on the gasket I decide to use, I'm looking at a compression ratio of 10:07.1 or 10:34.1. Which would you suggest. Also i will be using a comp cam xtreme energy 274/286 .488/.491
Similar threads
