Flat top pistons vs dish top pistons? Advice needed!

[karladaune]

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?

[Wildthings]

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.

[Bleyseng]

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.
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76 Porsche 914 2.1L L20c, 120hp Djet (sold)
87 Syncro Westy Titan Red 2.1L 2 knob 100k miles

[busdaddy]

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.
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[chazz79]

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.
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[Wildthings]

[karladaune]

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?

[Bleyseng]

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.
_________________
70 Ghia Black convert-9/69 build date-stock w/133k 1600 SP-barn find now with a rebuilt tranny and engine
77 Westy 2.0L w/Ljet, Camper Special engine-95hp and with LSD!(sold)
76 Porsche 914 2.1L L20c, 120hp Djet (sold)
87 Syncro Westy Titan Red 2.1L 2 knob 100k miles

[udidwht]

[Wildthings]

[cdesaul]

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

[Bleyseng]

Check the volume of your heads combustion chamber first and then deck ht so you can figure out the compression ratio.
_________________
70 Ghia Black convert-9/69 build date-stock w/133k 1600 SP-barn find now with a rebuilt tranny and engine
77 Westy 2.0L w/Ljet, Camper Special engine-95hp and with LSD!(sold)
76 Porsche 914 2.1L L20c, 120hp Djet (sold)
87 Syncro Westy Titan Red 2.1L 2 knob 100k miles

[cdesaul]

So, shims under cylinder and no head gaskets?

[udidwht]

[HAM Inc]

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.
_________________
Len Hoffman
Hoffman Automotive Machine Inc.
www.hamheads.com

[Bleyseng]

Then you will just run 100 Octane race gas at that CR...
_________________
70 Ghia Black convert-9/69 build date-stock w/133k 1600 SP-barn find now with a rebuilt tranny and engine
77 Westy 2.0L w/Ljet, Camper Special engine-95hp and with LSD!(sold)
76 Porsche 914 2.1L L20c, 120hp Djet (sold)
87 Syncro Westy Titan Red 2.1L 2 knob 100k miles

[timvw7476]

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.

[Spike0180]

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.
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[HAM Inc]

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.
_________________
Len Hoffman
Hoffman Automotive Machine Inc.
www.hamheads.com