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Mon Jul 28, 2025 3:11 pm

Figured the compression ratio on my 1915cc is at 7.8:1 and I don't think it's enough for the cam that I is installed. It has a eagle 2241 cam. Im going to need about 9:1 . My worry is the valve to piston clearances. It has flat top pistons. The heads are outlaws.

Mon Jul 28, 2025 3:19 pm

if you have .040 quench , 52cc cylinder head , that will put you at 9.1:1 what is your head cc currently?

Mon Jul 28, 2025 7:44 pm

^^^^^^

This.

Makes me wonder, what’s your deck height looking like?

I’ve got a CB 2242 with 1.25:1 rockers coming together, right around.490 lift is expected, and the AA 502s with peanut shaped chambers (expected to come in at 53-54ccs after they come out of the machine shop) roughly 9.4, maybe 9.5:1 aren’t anywhere close to valve/piston interference at .042” deck…

Mon Jul 28, 2025 10:40 pm

VWporscheGT3 wrote: if you have .040 quench , 52cc cylinder head , that will put you at 9.1:1 what is your head cc currently?
50cc

Mon Jul 28, 2025 10:49 pm

Rob Combs wrote: ^^^^^^

This.

Makes me wonder, what’s your deck height looking like?

I’ve got a CB 2242 with 1.25:1 rockers coming together, right around.490 lift is expected, and the AA 502s with peanut shaped chambers (expected to come in at 53-54ccs after they come out of the machine shop) roughly 9.4, maybe 9.5:1 aren’t anywhere close to valve/piston interference at .042” deck…
1.1:1 rockers and the deck hight I believe .1200, I don't have the measurements on me right at the moment. Crankshaft is 69.5 stroke, AA pistons. 40mm and 35mm valves.

Tue Jul 29, 2025 2:13 am

You’ve found yourself in an odd position here.

How’d you end up with.120” deck? 3.5 raised deck case? With a 69.5 crank, stock length rods, “A” pistons, and stock length cylinders the math is about right for a 3.5mm raised deck case.

Otherwise you have some strange combination of case deck, piston pin height, rod length, stroke, and cylinder length. Too early in the morning for me to work out those permutations. You’re going to have to do some measurements on your parts to know for sure.

Then there’s the heads. At 50cc, getting 9.0:1 will require.058” deck. Some here will say that’ll be fine, most will say you need to be as close to.040” as possible, best to be under .045”. (I’m in the .040-.045 school of thought.)

Sorry to say, but this is why many suggest mocking up through the cylinders and doing whatever is necessary to get your deck nailed down. Only then, when you know how many ccs you need, do you order your heads.

Wish you luck on this.

Tue Jul 29, 2025 7:59 am

Rob Combs wrote: You’ve found yourself in an odd position here.

How’d you end up with.120” deck? 3.5 raised deck case? With a 69.5 crank, stock length rods, “A” pistons, and stock length cylinders the math is about right for a 3.5mm raised deck case.

Otherwise you have some strange combination of case deck, piston pin height, rod length, stroke, and cylinder length. Too early in the morning for me to work out those permutations. You’re going to have to do some measurements on your parts to know for sure.

Then there’s the heads. At 50cc, getting 9.0:1 will require.058” deck. Some here will say that’ll be fine, most will say you need to be as close to.040” as possible, best to be under .045”. (I’m in the .040-.045 school of thought.)

Sorry to say, but this is why many suggest mocking up through the cylinders and doing whatever is necessary to get your deck nailed down. Only then, when you know how many ccs you need, do you order your heads.

Wish you luck on this.

Nothing Wrong with what Rob said here. it will be a little outside "ideal" quench , but i gaurantee you .120 deck would make a lazy burning hot running engine. so yeah, tighten up that deck , whether you cut your cylinders down or run longer rods you can get there. I would recommend cutting the cylinders down.

just for reference "Ideal" quench is between .035" and .055" just like rob says here i prefer around .040 but thats my personal preference.

Tue Jul 29, 2025 9:57 am

Assuming stock rod length here…

How’s ‘bout some 5.5 inch rods and .030” base shims? Given that the shims run a touch on the thin side, that should get him real close. (+ .106” from rod corrects to ~ .014” in the hole. Add just under .030” in shim, and as Emeril says - BAM!

Do I have this right?

Unitechs are what, ~ $120?

Or would he suffer from rod ratio issues?

Yea the engine will be a touch on the wide side but with .120 deck I can’t see how it’s not already gonna be a bit wide.

EDIT: just reran the compression ratio with the scenario above. Probably gonna put you on the high side for a 2241. I get 9.4:1 if deck comes down to.041 through.043

Sun Aug 03, 2025 1:54 pm

Couldn't be happier with my new 9.1CR top end, I will never build an N/A VW engine with anything less then a .050-.040 deck and 8.8 to 9.1 CR. Heads need to be the right CC.

Sun Aug 03, 2025 2:26 pm

DesertSasquatchXploration wrote: Couldn't be happier with my new 9.1CR top end, I will never build an N/A VW engine with anything less then a .050-.040 deck and 8.8 to 9.1 CR. Heads need to be the right CC.

Agree, my 1776. .040" deck, 9 to 1 CR with a CB 2239 cam runs alot cooler than expected. Max 205f on the fwy at 85 outside temp. 3500 rpm at 62mph. Love it :D

Sun Aug 03, 2025 2:46 pm

DesertSasquatchXploration wrote: Couldn't be happier with my new 9.1CR top end, I will never build an N/A VW engine with anything less then a .050-.040 deck and 8.8 to 9.1 CR. Heads need to be the right CC. Depends on the cam though doesn't it. That would be too much for one cam and not enough for another. It's nonsense to just say "this CR is great" without the cam details.

Sun Aug 03, 2025 3:27 pm

chrisflstf wrote: DesertSasquatchXploration wrote: Couldn't be happier with my new 9.1CR top end, I will never build an N/A VW engine with anything less then a .050-.040 deck and 8.8 to 9.1 CR. Heads need to be the right CC.

Agree, my 1776. .040" deck, 9 to 1 CR with a CB 2239 cam runs alot cooler than expected. Max 205f on the fwy at 85 outside temp. 3500 rpm at 62mph. Love it :D

I'm using the W100 cam so similar. The CR increase even solved my random idle misfire issue the idle screws are even an 1/8 turn IN from before! Its just a more efficient and overall happy engine. Went from 8.3 to 9.1 I thought it would run hotter nope its cooler sounds pretty mean out the tail pipe too you can actually hear the cam lope now.

Mon Aug 04, 2025 6:53 am

for some reason in the vw community its been thought that running more compression equals more heat. probably another one of the " so and so said it so we believe it" scenarios. good to see ppl are finally breaking away from it more and more.

far as the cam "needing" compression, the cam doesnt "need" it and you could run a stock cam with 10:1 or a "race only" cam with 8:1. not saying itd run optimally but the latter has been done all the time, ppl put a "race" cam in, add ratio rockers too then run stock-ish compression of fear of overheating all because thats the parts and theory thats pushed on them and they buy into it.
the compression the cam "needs" has to do with its rpm range, overlap, compression bleed off, idle,.. ie dynamic compression. google it if you actually want to understand how things are working

Mon Aug 04, 2025 11:36 am

Well that's the thing. There's an ideal dynamic CR for the fuel whatever the combo that never changes. later valve closing needs higher static CR to achieve that ideal dynamic CR. That's the very basic idea before you waffle about volumetric efficiency etc which is more difficult to calculate and why cam recommended static CR is always a range - got to start somewhere.

Mon Aug 04, 2025 1:09 pm

I didn't build this engine. I'm just going off the information I was given. I did find the cam card. I have a paper that has all my engine info somewhere in my office. I'll find it and post it later today. I just think the added CR would help the output a little. I know that more overlap can utilize more CR or boosted airflow. I don't want the added complexity of a turbo with dual weber idfs.

Mon Aug 04, 2025 2:37 pm

Zed999 wrote: Well that's the thing. There's an ideal dynamic CR for the fuel whatever the combo that never changes. later valve closing needs higher static CR to achieve that ideal dynamic CR. That's the very basic idea before you waffle about volumetric efficiency etc which is more difficult to calculate and why cam recommended static CR is always a range - got to start somewhere.

and I'd guess you'd say the ideal AFR is stoich

Tue Aug 05, 2025 3:22 am

BFB wrote: Zed999 wrote: Well that's the thing. There's an ideal dynamic CR for the fuel whatever the combo that never changes. later valve closing needs higher static CR to achieve that ideal dynamic CR. That's the very basic idea before you waffle about volumetric efficiency etc which is more difficult to calculate and why cam recommended static CR is always a range - got to start somewhere.

and I'd guess you'd say the ideal AFR is stoich
It is also the hottest mix possible and the one that eats valves and pistons for breakfast. Either side of stoich runs cooler. Running richer does because the evaporation of the unburnt fuel cools the engine. This is conventional wisdom taught for the last 70 or 80 years. Running leaner than stoich runs cooler because of the added cool air in the mix and the reduced overall power output.

Mon Aug 11, 2025 1:23 am

Better late then never. I guess.

Mon Aug 11, 2025 5:35 am

The math isn’t mathing.
According to my calculations, with a 1929cc, 50cc chambers, and a .01195 deck, it should be 7.8:1.

Still way too low.

Not trying to be an asshole here, but your heads were just about ruined when they were flycut so far. I never understand why a builder would Flycut the crap out of the heads, just to add it back in with deck height. #-o

The .01195 average deck is terrible, but I think if you re-arrange a few things, it might be salvageable into a decent little combo.

If you took the top end apart, and got the deck height down to a proper .045”, that wil get your compression up to 9.3, which will be just about perfect for the cam you have.

I guarantee you it will be a totally different engine. It will run cooler, have WAY better throttle response, and make quite a bit more power.

Should be an easy, inexpensive fix also. You will need to re-check your rocker geometry because the heads have moved a decent amount. This is good, because judging by the deck height, the chances of the rocker geometry being set up correctly right now are pretty slim. I am gonna guess it probably has roller ball Mazda adjuster screws on it also???

Last, post some pics of the heads. Those outlaw heads are just stock castings with big valves tossed in. If you pulled them apart, blended the bowls, did a little unshrouding, and recut the seats, they would also transform the engine.

If you spend the time to dial in the deck height, and fix the heads, your engine will all fall into place, and it will actually be a pretty correct combo, and it will work quite well.

Brian

Mon Aug 11, 2025 10:50 am

If you have 5.4 rods, switching to 5.325 would reduce your deck from .120" to .045"

There are other methods. Shorten cylinders, deck case or change shims, if used