How much C/R can one run on 91 octane pump gas?, Type 4 specific. Options

[Aaron Cox]

Strong possibility that the heads are coming off my 2L beast. while they were off, i figure i will bump up the CR to sneak some HP out.

MY motor has headgaskets and euro 94mm p/c (flat top mahles)
i think it is a tad over 8:1 currently.

been 3 years since i built it - runs great.

so my thinking is, remove the base shims and headgaskets -

I will CC the heads while they are out (cant recall the data when we built it 3 years ago)

How little deckheight can i run? what is max CR i can run on 91 premium unleaded?

If it matters, have mallory dizzy (can back off total advance if it detonates etc..) mallory CDI, and cool b7es NGK plugs.

AA

[sean_v8_914]

here is a link to a compression calculator.

http://www.btinternet.com/~mezporting/header.html

ditch teh head gasket and lap teh cylinder into the head. teh factory posted a bulliten to eliminate teh gasket. are you running a stock cam? valve lift and how deep your heads and or chambers are cut will dictate how low you can set teh deck. 0.040 is a good starting point of reference for deck.

a compression quote might launch a huge debate...
my 8.5 CR did not ping with 89 octane

how thick are your cylinder base shims?

[Aaron Cox]

here is a link to a compression calculator.

http://www.btinternet.com/~mezporting/header.html

ditch teh head gasket and lap teh cylinder into the head. teh factory posted a bulliten to eliminate teh gasket. are you running a stock cam? valve lift and how deep your heads and or chambers are cut will dictate how low you can set teh deck. 0.040 is a good starting point of reference for deck.

a compression quote might launch a huge debate...
my 8.5 CR did not ping with 89 octane

how thick are your cylinder base shims?

lapping cyl's to heads goes without saying.

i have a big cam, i believe a web 494 or an 86 grind. 3000 is about the beginning of the powerband. do nott recall actual lift numbers (had a folder with the build notes, and cannot find it)

base shims are anybody's guess without the build notes....

stock rocker ratio......

AA

[914-8]

yeah, you'll probably get a big debate.

but remember, physics dictate that more CR = more power = more heat.

Also remember that even with stock compression, 2L heads crack up pretty easily.

That being said, I was comfortable with 8.5 or so. (I think that's what it was, it was .5 over the Euro compression, whatever that is).

Drove the car a lot of miles with that using 91 and sometimes 89, never had any pinging problems or any other problems. But I didn't disassemble the engine to check how the heads were doing. (

I know that when it was disassembled during the first rebuild from the original engine, there were cracks that needed to be welded up)

[Aaron Cox]

car has a 7qt oil system.... (front cooler) so oil temps wont be an issue....

wonder what cyl head temps would do?

[914-8]

car has a 7qt oil system.... (front cooler) so oil temps wont be an issue....

wonder what cyl head temps would do?

yeah, oil temps are easy to control and keep in the optimal operating range, but all things being equal, higher compression necessarily means higher cyl head temp and combustion chamber pressures.

[Type 4 Unleashed]

AS, much as you want, 11 to 1 is a good number, and if it starts to ping, back off on the gas a little, than floor it. (IMG:style_emoticons/default/happy11.gif)

[Aaron Cox]

car has a 7qt oil system.... (front cooler) so oil temps wont be an issue....

wonder what cyl head temps would do?

yeah, oil temps are easy to control and keep in the optimal operating range, but all things being equal, higher compression necessarily means higher cyl head temp and combustion chamber pressures.

agreed

[Aaron Cox]

AS, much as you want, 11 to 1 is a good number, and if it starts to ping, back off on the gas a little, than floor it. (IMG:style_emoticons/default/happy11.gif)

ROFL

i dont remember the head CC's..... so ill have to do that.
i hear 56cc is common (gives me a baseline to work with) ((but mine have been decked at least once ugh....))

sounds like fun (IMG:style_emoticons/default/blink.gif)

[Type 4 Unleashed]

Hey, I wasn't kidding. (IMG:style_emoticons/default/happy11.gif)

Here's a tip:

Ceramic coat the faces of the valves, because the intake valve is fairly large, it's large area holds more heat, as the cold intake charge pases the hot valve, the hot valve heats up the intake charge, which promotes detonation, also ceramic coat the exhaust port, the exhaust port, is right next to the intake port, so as hot exhaust exits the port, it heats up the port, which inturns heats up the intake port, which inturns, heats up the intake charge.

By, Ceramic coating the valves, and exhaust ports, you reduce the amout of heat that is trans fered to the intake charge, which deters detonation.

Did , you know Porsche now Ceramic coats the exhaust ports on their production cars.

Do, you know why Nitrious makes so much HP, cause it's so cold.

[Aaron Cox]

Hey, I wasn't kidding. (IMG:style_emoticons/default/happy11.gif)

Here's a tip:

Ceramic coat the faces of the valves, because the intake valve is fairly large, it's large area holds more heat, as the cold intake charge pases the hot valve, the hot valve heats up the intake charge, which promotes detonation, also ceramic coat the exhaust port, the exhaust port, is right next to the intake port, so as hot exhaust exits the port, it heats up the port, which inturns heats up the intake port, which inturns, heats up the intake charge.

By, Ceramic coating the valves, and exhaust ports, you reduce the amout of heat that is trans fered to the intake charge, which deters detonation.

Did , you know Porsche now Ceramic coats the exhaust ports on their production cars.

Do, you know why Nitrious makes so much HP, cause it's so cold.

im soo broke i can barely pay attention.... (rimshot)

not ceramic coating anything....

guess i should aim for 9:1 or so?

AA

[Brando]

Go for 10.2:1 or 10.9:1 !!!

[Type 4 Unleashed]

(IMG:style_emoticons/default/agree.gif)

[Type 4 Unleashed]

Hey, I wasn't kidding. (IMG:style_emoticons/default/happy11.gif)

Here's a tip:

Ceramic coat the faces of the valves, because the intake valve is fairly large, it's large area holds more heat, as the cold intake charge pases the hot valve, the hot valve heats up the intake charge, which promotes detonation, also ceramic coat the exhaust port, the exhaust port, is right next to the intake port, so as hot exhaust exits the port, it heats up the port, which inturns heats up the intake port, which inturns, heats up the intake charge.

By, Ceramic coating the valves, and exhaust ports, you reduce the amout of heat that is trans fered to the intake charge, which deters detonation.

Did , you know Porsche now Ceramic coats the exhaust ports on their production cars.

Do, you know why Nitrious makes so much HP, cause it's so cold.

im soo broke i can barely pay attention.... (rimshot)

not ceramic coating anything....

guess i should aim for 9:1 or so?

AA

What a PUSSY.................... cat. (IMG:style_emoticons/default/happy11.gif)

[Brando]

I think the answer to your initial question, "How much CR can you run on 91 pump gas" is ... Hellifiknow... My 944 runs fine at 10.9:1 on 89 pump gas. Dad's 964 is somewhere around 11:1, newer 996s are 12:1...

I guess your only worry at that point is valve interference (IMG:style_emoticons/default/smile.gif)

And, of course, having the timing set appropriately.

[Twystd1]

Can't answer your question or hypothesize without knowing the EXACTLY which cam and the REAL cam specs for starters...

I B thinking with the right cam, exhaust, intake, heads etc...

10 to 1 is very doable for premium.

I wonder if we can check your cam specs with a dial indicator and a timing wheel??? (At Johns shop?)

Best to pull it and place it on a cam profiler. or find the markings on the cam.. (this assumes they are correct.) (cam cards can be wrong)

Like Jake says... it all in the .......................................

"Your uglier brother"

[MecGen]

Hi Aaron

I can second the 8.5:1. I have Kobelsmitch (sp) euro specs pistons, and bumped down the shims, I think the number was 8.489 :1 . Never had a problem with pump gas. I think this is a very safe ratio for the type 4, more then that, I would suspect some sealing issues might arise.
Good luck
+Karma
Later

(IMG:style_emoticons/default/beerchug.gif)

[Tom Perso]

Aaaaron...

Grant has a 2056 with a 494 cam. He was running 9.5:1 with stock pancake cooling and a stock oil cooler. This was with 93 octane gas... 28deg of total advance.

He just added an external oil cooler and bumped it up to 10:1.

I'm gonna venture out on a limp and do some "shooting from the hip" engine building. Removing both the base shim and the top shim should be OK. If you're at 8:1 now, I don't see you getting over 9:1 with both of those gone. Even so, with the cam you have (either the 86 or the 494) you'll be OK. BTW - you didn't check which one it was when you replaced the oil pump???? (IMG:style_emoticons/default/blink.gif)

Check your total timing and see where it's at. You may need to adjust it a little and bring the total advance down a little Do you have a mallory? If not, save your $$$ for one. That way, you can keep the base timing at 10deg of advance at idle and adjust your total advance to your liking.

"Bump it up to hear it thump"

Tom

[Allan]

I'm running 10:1 on 91 pump gas with no pinging issues but it's a /6 so probably doesn't help you much....

[DNHunt]

Aaron

I'm running 9.8 :1 on 91 octane with a stock cooler and stock cooling. It hasn't been a problem. Head temps are not a problem but oil temps have me a little worried. I don't have any special coatings.

Remember that's a static number and it's influenced by other things. Cam for sure, driving style and exhaust too. Clock the cam so you have a pretty good idea of the cam specs then start talking about CR.

Here's a downloadable Degree Wheel I used to check my cam.

Dave