Maximum sesible compression ratio

[Lids]

I am in the process of getting my engine bolted up, but have a few concerns...

1)  What is the best compromise on CR for a street car etc.

84 x 94 with 49cc heads.  It is running a raptor cam.  This gives us a CR of 10.5:1 is this too high for 98 octane fuel?

What are your experiences?

[Erlend / bug66]

I ran 10,4 last year on 98/99 octane. No ping what so ever

[Fredrik Grönlund]

Best way to determite the right compression ratio is by the Dynamic compression ratio.
There are several Calculators on the web to get this calculated.
I have understud that dynamic compression ratio should be in the range of 8-8.7:1 with pump gas.

Also if you have bought the camshaft from Johannes i think you can ask him what compression ratio he thinks could be the best one for you.

[JS]

11,0:1 static, as recommended by JPM. No problem of any sort. 98/99 octane, Raptor cam.

[richie]

I am in the process of getting my engine bolted up, but have a few concerns...

1)  What is the best compromise on CR for a street car etc.

84 x 94 with 49cc heads.  It is running a raptor cam.  This gives us a CR of 10.5:1 is this too high for 98 octane fuel?

What are your experiences?

It will be a turd    I am serious,its not your compression thats a concern.

84x94 with 45cc and 10.5/1 compression gives a deckheight required of 0.093 or nearly 2mm,for me that is way to much for a well matched combination.Your other carefully chosen parts will be wasted

And some specs on the cam would help,no I dont need to know all johannes secrets But some idea of lift and duration would help

cheers Richie

[Lids]

I am in the process of getting my engine bolted up, but have a few concerns...

1)  What is the best compromise on CR for a street car etc.

84 x 94 with 49cc heads.  It is running a raptor cam.  This gives us a CR of 10.5:1 is this too high for 98 octane fuel?

What are your experiences?

It will be a turd    I am serious,its not your compression thats a concern.

84x94 with 45cc and 10.5/1 compression gives a deckheight required of 0.093 or nearly 2mm,for me that is way to much for a well matched combination.Your other carefully chosen parts will be wasted

And some specs on the cam would help,no I dont need to know all johannes secrets But some idea of lift and duration would help

cheers Richie

Richie the cc is 49 not 45, I will find out the cam specs as I have no idea.  You are right to be concerned about the deckheight, this is our worry too.  I am trying to use my old heads, and don't really wanna stump up for a new set.

thinking of a deck height of 7/100"

[richie]

Micheal,

Thats what happens after to many hours on a plane and a time zone change,whoops my mistake
anyway 49cc is 0.070 deck so still a little to much I think,
0.055 gives you 10.9 and personally i would rather have an efficient engine with more compression than a turd with less   With all the good parts you have going into this it seems a shame to let the heads restrict it,even some minor chamber unshrouding should get you 2/3cc and 5cc with 0.052 deck is 10.5 or buy the heads you really need
I ran 10.8/1 compression in the cabrio N/A without problems on super unleaded 10 years ago with Comp elim heads

cheers Richie

[TexasTom]

Which Raptor do you have, JPM004?
What are the specs on the cam-sheet he sent with it?

For dynamic compression calculators, you will need to know the spec for the intake valve closing event. I believe that is all ... ?

TxT

[bugnut68]

I can't resist... 

6.6:1 is the maximum on 87 octane gas today, with semi hemi chambers.

  

Okie dokie, I'll bow out now, I was in a good humor.

[TexasTom]

That was another time and another place ... 

TxT

[bugnut68]

That was another time and another place ... 

TxT

Lol, I know... just figured it would generate a giggle or two.  Heh.

[neil68]

I've been running 10.6:1 CR in my 2332 cc street/strip Beetle engine:

JPM heads flycut to 50 cc
1.5 mm (0.060") DH with pistons to cylinder top + 1.5 mm copper head gaskets
91 octane (AKI/CLC) at 700-1000 m elevation which is 95 RON
94 octane at sea level which is your 98 RON

I have never had any detonation...possibly the JPM heads with 10 cooling fins are helping, but so is the OEM German FI doghouse cooling system.

I've been mostly running a Web 86C, but will be switching to a Raptor cam in a few weeks.

[DWL_Puavo]

Sorry a bit offtopic: I've always wondered that it's very usual to mix up imperial and metric measurements. Chamber size is usually quoted in metric-cc but deck height in some curious fractions of inches. Same thing with for example piston ring gaps - always quoted on inches even if barrel/piston size is quoted in millimeters. Then also rod lenght is almost exclusively in inches

And to the topic back again: with 1,5mm deck height, chamfering the heads for a few cc's would do the trick indeed.

Here's a link to easy-to-compare simple CR excel-sheet we made a few years ago: http://www.dwl.fi/engine.xls . There's comparision chart on the bottom tabs of the excel sheet so it's very easy to estimate the needed deck height or the head cc's for your application. We've been "in process" to add dynamic CR calculations there for few years - still we think that it's a very good idea

Puavo

[Shane Noone]

Hey Lids

Similar to what Richie has said and Johannes recommendations. In my experience / similar motors in the past and present have run between 10.5 and 11.5 :1 static CR using either Street Elims and Wedgeport 044' and now Comp Elims typicall 44 x 37.5 and FK87 - FK89 couldn't tell you the cc's for sure as JMR has built them all but they are around the 50 / 51 mark I think.  Anyway always running a form of "super unleaded" pump fuel and no detonation on the street. My current motor runs 11:1 on Comp Elims and at low engine speeds ( up to 3500 / 4000 rpm ) is perfectly happy even on 95 octane. For racing I will switch to 98 Shell Vmax or possibly 99 Tesco super but after reading lots of forum reviews it seems due to the "quality" of blending the Shell fuel has the edge for protection and performance for street super fuels.
So to summarise I buzz mine to 7000 rpm on 98 octane with 11:1 static - no detonation and no evident signs of overheating on the road even with the reduced fin . cooling area of Comp Elims which I am pretty sure Paul Bate verify as he also has lots of street driving and racing experience with this sort of CR and head type.

Just my two cents worth and hope you to see you at a racetrack soon

Shane.

[Lids]

Thanks for all the replies, I don't have the heads in front of me, so can't comment on the option of unshrouding etc.  They are massively welded and reworked, so much so that big beef manifolds don't cover the inlet ports!

I hope to get some answers later and then pass on this info.

Richie is right i have lots of 'nice' parts and have chosen to go a different route with the oil system, but i do want it to be reliable and streetable.  I can't afford the time or money for annual rebuilds! 

[Catbox]

If you need to make the chambers bigger don't forget you can machine the tops of the pistons to achieve more volume.

I just had this done to mine at Jaycee to make the chamber big enough that I didn't have to run a 0.080" deck height.

 
I left the shape up to Jack as he is the machinist and has probably done this a time or two....