Will be putting on fresh 1.8L AMC heads & want to know what compression (psi) is optimal. The good cylindars are currently running ~ 135psi. What is the max compression for a lasting street car?

Also, should there be cylinder head gaskets or should the "lapping" method (no gaskets) be used.

Thx in advance.

Well, Ill throw in cause no one else is answering...

The stock compression is what you should go for..if you are trying to build a street car that will run off pump gas...

Now, given that...I know there are several motors out there (I think Geoff, and McMark) that are at either 9:1 or 9.5:1...and that is with a slightly modified MPS to make up for the 2056 engine and higher compression...

Remember to watch temps...compression makes more heat...

Good luck.

Max compression? Like, what your compression ratio should be? Or are you talking what is still considered "good" on a compression test? If the former, 9.0:1 would be good.

I would suggest lapping the cylinders into the heads. I just tore apart a 2.0 that suffered from leaks because of failed head gaskets.

I was gonna add about the head gaskets...

There are two schools here..
Jake swears by never using a head gasket and lapping the cylinders to the heads...

Capn Krusty swears to always use the head gaskets, cause you will never get the lap joint to seal correclty...

You get to choose...

Rich

Ok, 1.8L AMC heads with 41x34 valves??? Too small of a exhaust valve to get all that heat out so stay with 8 to 1. or Jakes 42x36 1.8L AMC heads??


I am running 9 to 1 on pump gas no problem and 32 degrees of advance and no pinging...MPS is setup to run at 13.5 AF cruise and 12.2 at WOT.

Oil temps are in the 210-230 range on 90F days

CR and PSI don't have much in common. We were running a WEBCAM # 00-362 on an E production engine. 12 to 1 calculated CR but because of the overlap of the cam I think the PSI was below 100. The engine would hardley start , but ran to 7800 PRM.

Hi

I am going to asume you are going thru the complete routine for calculating your ratio, cc heads, barrel hims, deck hight, etc...a compression test has very little baring on ratio, only how well it seals.
I run Koblelsmitch (sp) euro piston on a motor cammed and carbed, 2l, the ratio calculated to 8.3, and I am VERY happy with the results. IMHO when you start knocking at the 9:1 door (air cooled), the motor has to be biult accordingly, or reliability will take a nose dive. That means big $$$. 8 to 8.5 is a safe number.

If I was doing a tweeked 1.8 today, this is what I'de do...
First, start with the heads, get them fly cut and cc'ed. this is the start of your base calculations. Now you know what you got to play with.
get a desent set of slugs and barrels, or have yours puched/sleeved buy some people in the vendors section. I am sure they can deck you cylinders to your liking.
If things work out to the high side, go for head gaskets, tryed and proven. If it on the low side go for the lap...Have some shimms handy.

There is a lot of material (manuals & videos) to look at and think about on how you are going to get it right the first time. I used the "How to Hot Rod your VW" book, it has a helpful T4 section. Don't forget about rocker geometrics.
And theres always the forum...

Good luck with it
Later

Thank for the replies guys. I'm still up in the air on the lapping vs gasket issue. I believe first time around they got no gaskets but my memory is a bit fuzzy & I'll find out for sure when I get the engine out and heads off.

I must have been unclear in my description about the compression b/c I'm asking about the chamber compression in psi, not about the compression ratio. I burned a valve b/c I was running too lean (operator error, I gambled and lost - now I know). That chamber registers a compression of about 50psi (poo poo). The #4 cylinder sitting next to it registers a compression of about 135psi. I havn't checked the other side for what those two register but would guess they're similar to the 135psi.

Should these be registering something more like 150 or 160 or higher? Are all my valves leaking more than they should?

too many variables... 135 seems a little low, but it's tough to say. you're in for a leakdown test

Geoff - what are the details of your heads? 2.0L?

I may need to use gaskets/shims in order to keep the CR down to acceptable range...

the psi of a 1.7L with 8.2to1 comp was about 150psi.

2056 hmm, depends on alot of variables....I should check my compression!

With a straight pattern Web 73 stay below 8.5:1, the cam simply lacks the duration to bleed any CR away.

During my testing I found that a web 73 with 8.5:1 made MORE power than it did at 9 or even 9.5:1 with the same deck height and chamber volumes.

Optimum is 8-8.5:1 with a .050 deck, I have the data.. At above 8.5:1 I could measure inaudible detonation at around peak torque with my individual cylinder knock retard system with the straight pattern web 73, when I tweeked the exhaust profile this went away and only showed up above 9.3:1.

I learned a lot developing my 9550 cam and if I were you I wouldn't run more than 8.5:1 unless you are operating at above 3,000' altitude-

Everything matters and MUSt be considered..

Jake - I was sorta hoping you would chime in on this topic. I know you've had your share of frustrations w/this community in the past and I'm happy as a pig in $hit to see posting in these parts again. Your expertise is very valuable. If your 9550 was around when I built this 4yrs ago I'd have bought it. I think it was still in development & you pointed me to the webcam 73.

I'll be sure to make carefull measurements & calculations. I'll put in shims (if necessary) to get the CR down below 8.5 per your data.

Remember, even VW themselves issued a notice that stated the head gaskets should not be used. This happened around 1990, I believe.

I was not aware of that recall. Any chance you have a link to it?

Actually the bulletin was released in 1981, but not adhered to.. It then was rekindled in 1989 and some light was shed on it..

I eliminate the gaskets in all my engines, buy my rebuild video and you'll see first hand what they do...

Relied on your rebuild video the first time around - actually have it on VHS... I'll pop it in the VCR for a refresher.

I know you know my feelings on the head gaskets from the other thread, but to add to that I just pulled apart a 914 2.0 that was smoked specifically because of those damn gaskets.

-Chris

Maybe I don't understand things correctly, but don't the gaskets act as shims which can decrease the compression ratio? Or can lapping have the same effect. Or would the heads need cc'd?

ALWAYS cc the heads, no matter what AND do all the math to come up with the actual CR, otherwise you are totally lost from day one! NEVER rely on generic numbers and always ensure that all chamber volumes are within 1cc of each other- MAX..

If you buy parts from me I loan out the syringes, deck plates and indicators to do these calculations and I'll even help you with the math..

The gaskets do act as shims to decrease the CR, so when you remove them on a COMPLETELY stock engine, you can add shims at the base of the barrels of the same thickness as the stock gaskets to maintain the stock CR . . . the problem is, especially with a rebuilt engine, is that usually things are not kept stock. Therefore it is always a good idea, as Jake points out, to CC the heads and check the deck height. I have the equipment to CC the heads (crappy lexan plate and syringe good to half a cc), I need to make something new for measuring the deck height. The piece of angle I used previously deforms too much when the barrel is tightened down. I can certainly get something together before the next NE event though. The math is easy, and there are a number of calculators on the web too. Or, just buy from Jake and get everything at once!

-Chris

Oh yeah, lapping does not have the same effect as the gasket in terms of setting your CR . . . all lapping does is (arguably) provide a better surface for sealing the cylinder to the head (you basically "grind" the cylinder in the head with a paste to clean up the contact area between the two). Therefore, in order to change or set your CR, and in order to maintain that lapped joint, you can only add shims at the base of the cylinder. Adding them between the cyl and the head will break the lapped connection. Hopefully this makes sense.

-Chris

Chris - thanks for the explanation. My fog is clearing on this topic but I could surely use your experience once the engine comes out and old heads come off. The heads that are going on are a separate set that just got valves checked (installed new stronger springs) and got the surfaces cleaned up. Time is on my side with this one but it sure would be nice to be able to get it all sorted out in a w'end (maybe w'end of 26-Aug...)

Still coming up w/a gameplan on garage & timing of dissasembly/reassembly.

-pt