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Early (85-87) vs 88 V6. Pistons have different compression height?! by Raydar
Started on: 11-22-2020 11:08 AM
Replies: 16 (287 views)
Last post by: Will on 11-30-2020 09:37 AM
Raydar
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Report this Post11-22-2020 11:08 AM Click Here to See the Profile for RaydarClick Here to Email RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
This question was precipitated by another post where someone was looking for a replacement crank for an 86.
I was thinking that while he was in there, he should just replace the early crank with the 88 crank, since counterweighted flywheels are becoming "uncommon".

But then I remembered hearing that the 88 pistons were somehow "different". So I did a little poking around.

I was thinking that the difference was due to different weights, but that doesn't seem to be the whole story (if that even is a factor.)

Strangely enough (per Rockauto) the '86 compression height is 1.578". The '88 compression height is 1.599". (21 thousandths?)
Since both engines are supposed to have the same stroke (and bore, of course), and the same rods, I'm wondering how this is possible. Unless the 88 just has incrementally higher compression. (Both are "spec'd" at 8.5:1)
How is this supposed to work? Was this change made due to a head gasket or block machining difference?

Thanks!
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Gall757
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Report this Post11-22-2020 12:02 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
What's the 'crush' distance on a head gasket? Seems like it would be more than 21 thousandths.....so you could make up the difference with just a torque change on the head bolts (or a different head gasket).

[This message has been edited by Gall757 (edited 11-22-2020).]

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Raydar
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Report this Post11-22-2020 12:49 PM Click Here to See the Profile for RaydarClick Here to Email RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Gall757:

What's the 'crush' distance on a head gasket? Seems like it would be more than 21 thousandths.....so you could make up the difference with just a torque change on the head bolts (or a different head gasket).



Which makes sense. I just wondered if I was missing something.

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fierogt28
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Report this Post11-22-2020 11:59 PM Click Here to See the Profile for fierogt28Click Here to Email fierogt28Send a Private Message to fierogt28Edit/Delete MessageReply w/QuoteDirect Link to This Post
The 88V6 pistons were over 14mm shorter and 3 ounces lighter.

My opinion, I always found the 88V6 engine was smoother running than the externally balanced 85-87V6.

I know, I have 87V6 and 88V6 engines in both 88 Fieros.

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Report this Post11-23-2020 02:23 PM Click Here to See the Profile for BillSClick Here to Email BillSSend a Private Message to BillSEdit/Delete MessageReply w/QuoteDirect Link to This Post
OK, if the pistons were shorter and the rods didn't change, the compression height is lower, which means that if the heads didn't change (they didn't AFAIK), the compression ratio was lower. But I had understood that all the HO engine versions were 8.9 compression. So something isn't adding up.
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Will
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Report this Post11-23-2020 02:39 PM Click Here to See the Profile for WillClick Here to Email WillSend a Private Message to WillEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Raydar:
Strangely enough (per Rockauto) the '86 compression height is 1.578". The '88 compression height is 1.599". (21 thousandths?)
[...] (Both are "spec'd" at 8.5:1)

Thanks!


 
quote
Originally posted by fierogt28:

The 88V6 pistons were over 14mm shorter and 3 ounces lighter.


 
quote
Originally posted by BillS:

OK, if the pistons were shorter and the rods didn't change, the compression height is lower, which means that if the heads didn't change (they didn't AFAIK), the compression ratio was lower. But I had understood that all the HO engine versions were 8.9 compression. So something isn't adding up.


The numbers Raydar found show the '88 pistons *taller* which jives with the idea that the later engines had slightly higher compression. I'd always read that the Fiero engines were 8.9:1... maybe the prior low output 2.8's were 8.5:1.

fierogt28's post has a... let's call it a typo
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fierogt28
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Report this Post11-24-2020 11:47 PM Click Here to See the Profile for fierogt28Click Here to Email fierogt28Send a Private Message to fierogt28Edit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks Will...

What I meant was the stock 88V6 pistons where 14.8mm shorter than the 85-87V6 pistons. From the top of the
piston to the bottom skirt. And 3.3 ounces lighter than the 85-87V6 stock pistons.

I have a spare 88V6 stock block, and when it comes apart, I will confirm measurements with what info was supplied.

This info comes from the NIFE fiero tech book.

I guess all rebuilt kits available today have the same pistons listed / made for 85-88V6s...even if the 88V6 pistons were shorter.

The specs have been listed at 8.5:1 and 8.9:1. There might be something here...


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Report this Post11-24-2020 11:58 PM Click Here to See the Profile for fierogt28Click Here to Email fierogt28Send a Private Message to fierogt28Edit/Delete MessageReply w/QuoteDirect Link to This Post
IIRC, the top of the 88V6 stock pistons are flat...

Are the top of the 85-87V6s stock pistons rounded slightly?
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pmbrunelle
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Report this Post11-25-2020 12:12 AM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
I've taken apart two 85 V6 engines, and both had flat-top pistons.
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Report this Post11-25-2020 10:57 AM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierogt28:

The specs have been listed at 8.5:1 and 8.9:1. There might be something here...



Assuming connecting rods are all the same, what you need is a dimension from the center of the wrist pin to the top of the piston (not easy to measure accurately.... )

[This message has been edited by Gall757 (edited 11-25-2020).]

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Report this Post11-25-2020 12:57 PM Click Here to See the Profile for pmbrunelleClick Here to Email pmbrunelleSend a Private Message to pmbrunelleEdit/Delete MessageReply w/QuoteDirect Link to This Post
If there really is a difference, it would take a difference of 0.016" of compression height (or something else such as gasket thickness) to explain the 0.4 CR difference.

On my Fiero engine rebuild, there was maybe about 0.005" of head gasket crush IIRC, but that was with MLS gaskets.

With a vernier caliper, you get an measurement from the uppermost edge of the hole to the piston top. Then, measure the diameter of the hole, and subtract its radius from the first measurement to obtain compression height.

It's also bizarre that compression ratio would increase, but that power ratings would decrease over V6 Fiero production; something doesn't add up.
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Report this Post11-25-2020 02:37 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by pmbrunelle:

It's also bizarre that compression ratio would increase, but that power ratings would decrease over V6 Fiero production; something doesn't add up.


The US federal government's test calculation for emissions changed in model year 87, lowering the hp rating to 135. Earlier it was 140. Apparently they used a lower RPM to get better results.

[This message has been edited by Gall757 (edited 11-25-2020).]

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Report this Post11-25-2020 05:04 PM Click Here to See the Profile for fierofoolClick Here to visit fierofool's HomePageClick Here to Email fierofoolSend a Private Message to fierofoolEdit/Delete MessageReply w/QuoteDirect Link to This Post
Weren't those HP ratings at different rpm? That could account for it.
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Will
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Report this Post11-28-2020 09:30 AM Click Here to See the Profile for WillClick Here to Email WillSend a Private Message to WillEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by fierogt28:

Thanks Will...

What I meant was the stock 88V6 pistons where 14.8mm shorter than the 85-87V6 pistons.


Umm... try again? Even 1.48mm would be quite a large amount of compression height difference.
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Report this Post11-28-2020 09:41 AM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Will:


Umm... try again? Even 1.48mm would be quite a large amount of compression height difference.


Overall piston height does not change the compression ratio. Wrist pin center to top of piston does.
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fierogt28
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Report this Post11-28-2020 07:29 PM Click Here to See the Profile for fierogt28Click Here to Email fierogt28Send a Private Message to fierogt28Edit/Delete MessageReply w/QuoteDirect Link to This Post
I'm talking about the piston from top to bottom. (piston top, to lower skirt)

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Will
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Report this Post11-30-2020 09:37 AM Click Here to See the Profile for WillClick Here to Email WillSend a Private Message to WillEdit/Delete MessageReply w/QuoteDirect Link to This Post
Ahh, ok. I thought you were referring to difference in compression height.
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