Convert S4 engine to GT specs...
05-03-2009, 01:34 PM
#1
Racer
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Convert S4 engine to GT specs...
Hi,
I have a 89 GT with a S4 engine in it...
I really like the power band and feel of a GT engine.
So I am asking for your help.
I have the original GT ECU unit.
If I install that ECU and GT cams (or modify the S4 cams to GT profile),
will I have a GT engine then..?
I mean something that has the same specs and power band..?
Or have Porsche done more to the GT engine than different ECU and cams..?
I have heard somewhere that Porsche polished the tops and had extra quality control on their GT engines.
And does the GT engines have different pistons as well?
I have the original exhaust etc too still on the car.
Please advice.
Cheers. Michael
I have a 89 GT with a S4 engine in it...
I really like the power band and feel of a GT engine.
So I am asking for your help.
I have the original GT ECU unit.
If I install that ECU and GT cams (or modify the S4 cams to GT profile),
will I have a GT engine then..?
I mean something that has the same specs and power band..?
Or have Porsche done more to the GT engine than different ECU and cams..?
I have heard somewhere that Porsche polished the tops and had extra quality control on their GT engines.
And does the GT engines have different pistons as well?
I have the original exhaust etc too still on the car.
Please advice.
Cheers. Michael
05-03-2009, 01:46 PM
#2
Three Wheelin'
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A cam grind can get you much more than GT specs. There's a thread on here where an S4 jumped up a ton with just a small weld, grind and base circle reduction.
05-03-2009, 02:21 PM
#3
Racer
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Ok excellent!
A cam grind seems to be the way to go.
I have the original gearbox luckily..!
It didn't run well with the GT ECU and S4 engine...
Felt like it had 250 ponies at the most.
Got pulled by a gang of Swedish youngsters in an Impreza...
Not cool..!!)))
So I put in a S4 unit and it runs better.
A cam grind seems to be the way to go.
I have the original gearbox luckily..!
It didn't run well with the GT ECU and S4 engine...
Felt like it had 250 ponies at the most.
Got pulled by a gang of Swedish youngsters in an Impreza...
Not cool..!!)))
So I put in a S4 unit and it runs better.
05-03-2009, 02:41 PM
#4
Nordschleife Master
forget modifying your engine to GT spec, just buy a GT engine and sell the one in the car currently. You cant just modify S4 cams to GT spec, LSA is different and not something you can just change. If you could do this most people would instead of going to the 85-86 cams which is what they do
There is a GT engine here on the board available for $5500. Buy that, sell the S4 engine for $3,000 plus and youve now got the correct engine for the chassis you have. I believe the guy selling the GT engine is including all the wiring and such too.
ALTHOUGH, your in Sweden and shipping an engine there cant be cheap...... In that case look for the 85-86 US 32V cams and start from there.
There is a GT engine here on the board available for $5500. Buy that, sell the S4 engine for $3,000 plus and youve now got the correct engine for the chassis you have. I believe the guy selling the GT engine is including all the wiring and such too.
ALTHOUGH, your in Sweden and shipping an engine there cant be cheap...... In that case look for the 85-86 US 32V cams and start from there.
05-03-2009, 02:47 PM
#5
Three Wheelin'
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Carl has nice cams on his site.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
05-03-2009, 02:52 PM
#6
Nordschleife Master
Carl has nice cams on his site.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
05-03-2009, 02:56 PM
#7
Nordschleife Master
The GT short block has a compression ratio of about 9.9-10.0, whereas the S4 short block has a compression ratio of about 9.4-9.5. The source of the difference is immediately visible when you look at the piston dish sizes and shapes.
The lobe separation angles vary greatly between different 32v cams. I was just educated on the topic by Jim Morton. S3 cams have the second wides LCA (after GTS) and I believe that the S4 cams have the narrowest.
Instead of my flawed understanding, here's a chart that may be informative:
http://www.compcams.com/Technical/FAQ/LSAproperties.asp
The lobe separation angles vary greatly between different 32v cams. I was just educated on the topic by Jim Morton. S3 cams have the second wides LCA (after GTS) and I believe that the S4 cams have the narrowest.
Instead of my flawed understanding, here's a chart that may be informative:
http://www.compcams.com/Technical/FAQ/LSAproperties.asp
Last edited by ptuomov; 05-03-2009 at 06:43 PM.
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05-03-2009, 03:00 PM
#8
Nordschleife Master
Carl has nice cams on his site.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
http://www.928motorsports.com/parts/camshafts.php
Might take some guess work out.
05-03-2009, 03:26 PM
#9
Rennlist Member
Just get some 85 cams and have them modified for the S4/GT head. You will then have a GT like engine. (take off a little base circle as well and watch the hp go up too!)
The 85 cam is actually better than the GT cam as it has less intake advance, by 8 degrees or so, which helps high end hp. exhaust cams are the same.
The 85 cam is actually better than the GT cam as it has less intake advance, by 8 degrees or so, which helps high end hp. exhaust cams are the same.
05-03-2009, 03:44 PM
#10
Racer
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Hauling a GT engine across the pond is not an option right now...
Especially for 5500!
I am not clear about it though...
I cant have the S4 cams ground and modified to GT spec or mild race..?
I can only use those rare 86 5l cams..?
How much for four of those..?
Man I wish the car had its original engine..!!)))
Especially for 5500!
I am not clear about it though...
I cant have the S4 cams ground and modified to GT spec or mild race..?
I can only use those rare 86 5l cams..?
How much for four of those..?
Man I wish the car had its original engine..!!)))
05-03-2009, 04:15 PM
#11
Race Director
In theory the GT's had a few differences...
GT cams (much better)
GT EZK (revised with 775rpm idle vs 675 S4)
GT Forged connecting rods (this wasn't advertised)
supposedly port matched heads-intake
the lower 2.72 rear end gear box (but you should have a 2.64 already so its not that different)
GT cams (much better)
GT EZK (revised with 775rpm idle vs 675 S4)
GT Forged connecting rods (this wasn't advertised)
supposedly port matched heads-intake
the lower 2.72 rear end gear box (but you should have a 2.64 already so its not that different)
05-03-2009, 04:20 PM
#12
Racer
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Just to make it clear.
Mine is a real GT with the 639 code in the back.
Its just that the engine has been swapped for a S4 unit...
And thx for all the info guys!
I really appreciate it!
Mine is a real GT with the 639 code in the back.
Its just that the engine has been swapped for a S4 unit...
And thx for all the info guys!
I really appreciate it!
05-03-2009, 04:21 PM
#13
Racer
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Just to make it clear.
Mine is a real GT with the 639 code in the back.
Its just that the engine has been swapped for a S4 unit...
And thx for all the info guys!
I really appreciate it!
Mine is a real GT with the 639 code in the back.
Its just that the engine has been swapped for a S4 unit...
And thx for all the info guys!
I really appreciate it!
05-03-2009, 04:48 PM
#14
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Rennlist Member
Rennlist Member
Boomerang -
For S4 cams to be ground to GT specs or beyond, you will have to run lash caps & modify the lifters. If the cams are welded, you can avoid running those, but people have had varying degrees of success with welded cams in the past.
On my car, I have S3 cams that have been modified to fit an S4 head. Also the cams have been reground to reduce the base circle and in turn increase the lift and duration. The reduction in base circle is slight enough that no lash caps are needed and the lifters aren't modified. All the work was done by Elgin Cams through Jim Morton. We've used a Sharktuner to bring the engine into tune and the results have been good. You can see them here: https://rennlist.com/forums/928-forum/466038-5-0l-screamer-motor-goes-to-the-dyno-w-graphs-and-video.html
The high rpm power is intoxicating. The car accelerates harder at 5000 rpm than at 3500 rpm, unlike the stock S4 cams.
For S4 cams to be ground to GT specs or beyond, you will have to run lash caps & modify the lifters. If the cams are welded, you can avoid running those, but people have had varying degrees of success with welded cams in the past.
On my car, I have S3 cams that have been modified to fit an S4 head. Also the cams have been reground to reduce the base circle and in turn increase the lift and duration. The reduction in base circle is slight enough that no lash caps are needed and the lifters aren't modified. All the work was done by Elgin Cams through Jim Morton. We've used a Sharktuner to bring the engine into tune and the results have been good. You can see them here: https://rennlist.com/forums/928-forum/466038-5-0l-screamer-motor-goes-to-the-dyno-w-graphs-and-video.html
The high rpm power is intoxicating. The car accelerates harder at 5000 rpm than at 3500 rpm, unlike the stock S4 cams.