TheSamba.com :: View topic - Flat top pistons vs dish top pistons? Advice needed!

Sat May 19, 2012 10:11 am

ok, advice needed. I am in the process of rebuilding Ruby's engine (75 Rivi, with a 79 2.0.) I followed the advice of a friend and bought a "VW Porsche 2.0 94mm x 71mm Flat Top Piston & Cylinder Kit 914 for Type 4." Now I am having a bit of buyers remorse as I have heard from multiple people now that I should have got the standard dish top pistons because the flats run too hot. Does anybody have any experience with this?

Sat May 19, 2012 10:46 am

I would put it together and be prepared to run high test. You should be able to offset the cost of the better fuel with the better gas mileage you will be getting.

Sat May 19, 2012 11:39 am

I am running the 94mm flat tops and really its all about setting your compression ratio right. This of course depends on the combustion chamber size and your deck ht.
Measure your combustion chamber and mock up the deck ht so you can calculate the CR. Mine is set at 8 to 1 and runs great.

Sat May 19, 2012 11:56 am

Agreed, you can run nearly any piston you like if you set the CR correctly, VW went dished to save on shims and measuring at the factory and keep the production line moving along with standarized parts.

Sat May 19, 2012 12:02 pm

Yes, you need to get it to someone familiar with vw engines. Those will work fine with proper cylinder shimming. I think running those straight up net cr of 8.8. Anything over eight is short lived in a bus.

Sat May 19, 2012 2:35 pm

chazz79 wrote: Anything over eight is short lived in a bus.

Maybe Colin will chime in with how long his lasted. Don't think it was exactly short lived.

Sat May 19, 2012 5:31 pm

much appreciated, guys! I really didnt want to have to buy another set! Poor Ruby has been sitting WAY too long, time to get the ball rolling!

Bleyseng - you are in my neck of the woods...do you do all your own work or do you have a shop you would recommend for make sure everything is a-ok?

Sat May 19, 2012 10:19 pm

I do my own work.
Setting up and measuring is pretty easy and kinda fun...readup on it to see if you can do it.

Sun May 20, 2012 12:04 am

Wildthings wrote: chazz79 wrote: Anything over eight is short lived in a bus.

Maybe Colin will chime in with how long his lasted. Don't think it was exactly short lived.

Colin wasn't 8:1 or more CR either. More like stock 7.3:1

As for the CR in a bus...IMO don't go over 8:1

Sun May 20, 2012 2:50 am

udidwht wrote: Wildthings wrote: chazz79 wrote: Anything over eight is short lived in a bus.

Maybe Colin will chime in with how long his lasted. Don't think it was exactly short lived.

Colin wasn't 8:1 or more CR either. More like stock 7.3:1

As for the CR in a bus...IMO don't go over 8:1

Colin at one time had a high compression engine 1700 in the Road Warrior, probably an 8.2:1. At least that is what I got from his posts on this subject.

Sat Mar 23, 2019 7:08 am

I have the same problem. Do you use the shims between cylinder and bloc. I read that one should not use gaskets or shims between head and cylinder

Sat Mar 23, 2019 7:42 am

Check the volume of your heads combustion chamber first and then deck ht so you can figure out the compression ratio.

Sat Mar 23, 2019 7:48 am

So, shims under cylinder and no head gaskets?

Sat Mar 23, 2019 8:21 am

cdesaul wrote: So, shims under cylinder and no head gaskets?

No cylinder head gaskets. VW put out a tech bulletin back ~1990 calling for them to be removed.

https://www.ratwell.com/mirror/www.dolphinsci.com/techbull.html

Don't forget to notch the rods.

Sat Mar 23, 2019 10:12 am

If your heads have stock 2.0 bus chambers they will be in the ballpark of 51cc's.

If your deck hight is in the neighborhood of 1.25mm (common value) it will = 8.7cc's

With flat top pistons with no valve reliefs your total unswept volume will = 59.7cc's

59.7 + 492.7 (swept volume) =552.44
552.44/59.7= 9.25:1 CR

Factory bus pistons had a 15cc dish.
If your goal is 8.0:1 you will need a total unswept volume of 70cc's.

If your chambers are indeed stock 2.0 bus and your pistons have no reliefs or dish then you will need 19cc's of deck ht volume. That will require 2.74mm of deck ht.

That's a recipe for detonation, decreased power and increased heat.

Sat Mar 23, 2019 2:31 pm

Then you will just run 100 Octane race gas at that CR... :shock:

Sat Mar 23, 2019 7:33 pm

Run the head gaskets.
Machine out dish in the flat tops. (8-10cc)
Now you can run 93 octane. Maybe.
I believe the camper top drag makes it more difficult to run higher CR anyhoo.

Sat Mar 23, 2019 8:50 pm

My type 1 1776 has 9.25 to 1 CR, I run 93octane and just have to retard max timing to 29 deg or I get ping. Might be a le to increase now with my new 40idfs but haven't tasted yet.

But, if I were you I would just set my engine up for lower compression while you have it out. Even if you hit 8.5 to 1 you'll likely be fine.

Sun Mar 24, 2019 6:19 am

Keep in mind that the volume created by using the headgaskets (4.9cc's) factors as deck ht.

More deck ht = less squish

At 2.7mm (the amount required to get the CR down to 8.0:1 with stock 2.0 Bus heads ) we are well into the excessive deck ht zone.

Less squish = less efficient combustion.

The more inefficient the combustion the more heat and detonation.

The more heat and detonation the more likely the crappy laminated headgaskets are to fail.

At 2.7mm's of deck, squish is reduced to dangerously low levels on a low speed engine. Detonation is a certainty. The knee jerk tuning response
of fuel enrichment and retarded timing to prevent detonation will lead to off the charts EGT's. Engine life and performance will suffer greatly.

My advise would be to suck it up and get the proper bus pistons with the 15cc dish. The dish in the pistons is part of the overall combustion chamber design. Bus heads rely on it for the proper CR. Other means of achieving that CR will prove unsatisfactory.

As for octane and T4's in bays, here's my two cents. Even with a CR of 7.5:1 all bus owners should be running the highest octane grade available. This statement may illicit howls of protest and plenty of folks who say they run low-test with no issues. That may be fine for around town with no heavy load and no long full throttle running.

But load that beast of burden to the penultimate straw and start climbing mountains or tacking into headwinds with long sustained full throttle running and even at a low CR you'll need every bit of detonation resistance you can get.

Like a cardiologist who's seen the effects of too many bacon cheeseburgers on arteries, so have I seen the effects of poor gas and headgasket shuffling and blown T4 head gaskets on the inherently weak T4 head. These heads consume themselves when conditions aren't right.

Thu May 30, 2024 5:11 am

So I need to get the chamber to 45 cc to get to 8.25:1???
Using 97 to 99RON.
With 2.0 dished pistons (15cc).
Will the 2.0 AMC heads take that much machining (I am measuring about 55.5>57cc stock).
Is 2mm off safe please Len, or more??? Can you use a groove/recess for the barrel rim or just flycut flat?
Ps is the 492.7 swept volume for flat tops only or all same?