Hemi vs Top Hat Compression Ratios

[turbo800]

Ok, I'm building some turbo specific domes for my head. Right now I'm running the MXZ 12:1 low compression head.

I cc checked my 800HO head at 33cc's & should have a CR of 13.25:1
RKT Hemi head also measured 33cc's but has a CR of 14.6:1

How can this be???

I'm hoping to cc the MXZ head tonight to compare.

 

[bikerjustin]

The RKTek domes that you cc'd may be the ones designed for his pistons which are different from an OEM piston.

 

[turbo800]

I don't think so. The head was purchased during the '01~02 time frame of the ZX days. The BB's and drop-ins weren't available back then. To add to the confusion, I cc'd two different HO heads. One was 33cc's and the other 32 cc's. The one with 32cc's could have been modified along the way.

 

[turbo800]

Measured the MXZ top hat style dome at 12:1 CR last night at 36cc's.

Nobody knows why the two different style heads can have the same cc's but different compression ratio's?

 

[BBP]

01,02 & HO are 3 different cylinders & at least 2 different heads. You're not comparing apples to apples. Mix & match & your cr's can change.

 

[turbo800]

OK, let me clarify. The MXZ head is off a '01(low comp). The HO head is off a '04 Summit REV. The RKT 14.6:1 head was purchased in the non-ho days (Hemi head). The Non-HO & HO versions have the same bore and stroke and use the same pistons...

How can a HO top hat head have the same cc's as a high compression RKT head, only difference being the dome design???

 

[LRD]

Are you measuring cc's head on the motor or just flat plating on the bench. If it's on the engine and you have the cylinder wall greased to stop the oil leaking down past the pistons and fill it just to a couple of the spark plug threads and then GENTLY rock the crank to get the highest oil level and then completely fill to the top of the head. Do all three heads exactly the same, also the longer you let the Burette tube sit the more it changes as the tranny fluid (what I always use) will keep draining down (2 or 3 cc's in 15 minutes at times. Have to be real anal about cc'ing or easy to get funny info.

Heads are a lot of work, as much as a porting job to get perfect. Porting is a lot more fun than cutting combustion chambers on my Bridgeport.

When I cut them, measure first on motor and get squish thickness and width with two pieces of solder. Then flat plate them, Then start cutting them. And when I think I've got it check them again on the motor. For designing them I have Tom turners software for figuring squish velocity, compression and safe octane requirement etc.

You can't trust all these parts to be perfect or to be what they are claimed to be. Mistakes happen.
If your measuring them all the same way, say flat plating them on the bench one or more is wrong and I'm not surprised. If all measured the same, its the cc's that count, not whether its Top Hat, Hemi, Toroidal, wide squish band, narrow squish band etc.

About four years ago with my new 08 XP TNT I ordered a major aftermarket head and two sets of domes to have low and higher compression for Mn flatland occasionally and for the mtns of WY and CO etc. Its also nearly impossible to turn rotax stock top hat chambers into the real deal, hemi heads. Asked for about 12 and 12.75. I don't like real high compression as it hurts pipe action and you lose rpm.

I get them and they are like 13 and 14 or something. Two weeks before the first trip and this is one of those guys your lucky to get on the phone sometimes. I never even called, just wrote him off and have NEVER EVER referred anymore business to him. So I put in many hours on my Bridgeport re-cutting these superior CNC'd domes. And the water jacket portion of the head that holds the domes and seats on top of the cylinders had a low spot of about 12 thou so I spent a few hours on the surface table sanding that down so it was only about 5 thou. Had some deep machining grooves too in it. The quality of the work made my stuff off my 40 year old Bridgeport look beautiful. I'd have been embarrassed to send anything out looking like that.

Just saying its easy to pay big bucks and end up with junk. Getting stuff right is what makes and brakes a engine.

Good Luck

 

[turbo800]

Thanks LRD. All cc tests were done on a flat set of parallels off the engine. As a machinist by trade, I wouldn't even attempt buildings domes on our Bridgeport...lol. Its tedious work sanding on the surface plate, but sometimes that's what it takes.

I don't have access to Tom Turners software, but I do have access to Solidworks and CNC programming software.

My domes will be cut on a very tight full 3 axis Mori Seiki mill. I don't see any reason I can't hold .001" or less tolerance on them

I'm hoping to get 12:1 CR for my turbo domes, the top side is already cut...


It appears the TSR software isn't even available anymore...

 

[PTE800]

OK, let me clarify. The MXZ head is off a '01(low comp). The HO head is off a '04 Summit REV. The RKT 14.6:1 head was purchased in the non-ho days (Hemi head). The Non-HO & HO versions have the same bore and stroke and use the same pistons...

How can a HO top hat head have the same cc's as a high compression RKT head, only difference being the dome design???

The reason that two heads with the same cc volume can have different compression ratios at TDC is because of a difference in "area" in the combustion chamber design to dome area. Some heads have a tighter radius towards the top of the chamber while others have a wider top. If you were to look at a cross sectional view of the chamber in relation to the piston dome you will see it. It is possible for two cylinder heads with identical cc volumes to create different cylinder pressures in relation to crank angle because of the area differences. Also:The only true way to check either corrected or uncorrected compression is with the head installed at TDC.

 

[LRD]

Thanks LRD. All cc tests were done on a flat set of parallels off the engine. As a machinist by trade, I wouldn't even attempt buildings domes on our Bridgeport...lol. Its tedious work sanding on the surface plate, but sometimes that's what it takes.

I don't have access to Tom Turners software, but I do have access to Solidworks and CNC programming software.

My domes will be cut on a very tight full 3 axis Mori Seiki mill. I don't see any reason I can't hold .001" or less tolerance on them

I'm hoping to get 12:1 CR for my turbo domes, the top side is already cut...


It appears the TSR software isn't even available anymore...

Don't know what happened to Tom? Sure hope he hasn't assumed room temperature? Discovered about a year ago his website was gone. I started out in the late 80's with Gordon Blairs software through SAE and always suspected that Tom cleaned it up and made it more user friendly and tweaked it to get it more accurate from racer and engine porter feedback.

On the cc's and different shape combustion chamber shapes I'd have to disagree. CC's are, figure total cc's at BDC and at TDC and maybe at the Top of the exhaust port and simple division and you have a compression ratio. Some combustion chamber shapes, squish velocity, wide squish bands etc might effect what will detonate more than another configuration related to a specific octane # but it won't change a compression ratio which is pretty simple math.

The whole thing goes out the window anyhow when your adding the pipes to the mix like a single pipe on a twin compared to twins as the twins will be able to fill the cylinder more than the single raising the actual running compression. Same idea as a turbo overfilling the combustion chamber. SLP used to have a great graffic diagram on this with recommended octane needed for the state of engine tune related to porting, single pipe, twin pipe, compression in the heads etc. The more air/fuel mix stuffed and compressed in the combustion chamber the higher octane needed.

Good Luck

Haven't worked with turbos but would suspect they have an advantage over high compression as they likely make their biggest boost at high rpm where things are happening so fast it may not leave time for detonation to happen.

 

[turbo800]

LRD, From what I read last night it appears that Tom retired and no longer supports or sells TSR.

My brother did an installed cc test on the sled. His calculation came up with -3.5cc installed, correcting for the dome of the piston. I agree with you cc's should be consistent regardless of dome profile.

If you calculate for the 14.6 CR head at 399.8+29.5(cc-3.5cc correction)/29.5= 14.55 CR it is correct, but if you do the 12:1 MXZ head at 399.8+32.5/32.5=13.3 CR which is incorrect. The HO head also comes out incorrect.

Any insight?

From dealing with turbos over the years they seem to like increased squish clearance, narrower bands, and a happy balance with grades of fuel and CR's.


Thanks.

 

[PTE800]

LRD: You are correct that it is simple math to calculate compression but the point I'm saying is that two different heads with the same (flat plate cc) can have different compression ratios at TDC. If you were to compare a hemi design verses a toroidal the TDC compression would be different and thus have a different cylinder pressure.

 

[bikerjustin]

The difference is the deck height.....?
The same reason I ended up with 14.3 to 1 on my engine with 46.25cc domes and my buddy's compression is lower-not sure the exact compression, but significantly lower.
It has to be the difference in deck height.
These are custom cut domes for a series 3 860. O Ring groove in top of cylinder welded and then decked on both engines.