benchracing a L98 - de-stroke it?
Thread Starter
Heel & Toe
Joined: Aug 2007
Posts: 15
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benchracing a L98 - de-stroke it?
I had a thought about changing the character of a L98 to a more high revving (fun?) character without changing the TPI, heads, etc.
I know the common wisdom (widely practiced here) that there's no substitute for cubic inches.....but what if one were to de-stroke an L98 to 302 CID (3 inch stroke) in order to make all of the breathing characteristics of the stock intake, cam, heads, and exhaust seem bigger for the displacement? Would that move the power band up the rev range, potentially adding horsepower while giving up low end torque?
A cam change might be in order, but would the TPI still be such a big limiter? I know the heads and 1.94 inch valves are good for 6,000 rpm on a 350 from an old ZZ3 that I had.
This comes from reading this forum too much... ;>]]
I know the common wisdom (widely practiced here) that there's no substitute for cubic inches.....but what if one were to de-stroke an L98 to 302 CID (3 inch stroke) in order to make all of the breathing characteristics of the stock intake, cam, heads, and exhaust seem bigger for the displacement? Would that move the power band up the rev range, potentially adding horsepower while giving up low end torque?
A cam change might be in order, but would the TPI still be such a big limiter? I know the heads and 1.94 inch valves are good for 6,000 rpm on a 350 from an old ZZ3 that I had.
This comes from reading this forum too much... ;>]]
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 15
From: backwoods upstate ny
the wall
305's w/ tpi run up against "the wall" at 5k...a 302 will probly do the same with tpi...but a 302, with it's "over-square" bore/stroke, fed by an lt1 intake, could be "interesting"...but gonna need to address the knock sensor vs solid lifters pita (again) to get to the 7k rpm that the 302 shines at...a scattershield is mandatory, even on the street.
Safety Car
Joined: Sep 2001
Posts: 4,157
Likes: 554
From: Glenbeulah Wi
I had a thought about changing the character of a L98 to a more high revving (fun?) character without changing the TPI, heads, etc.
I know the common wisdom (widely practiced here) that there's no substitute for cubic inches.....but what if one were to de-stroke an L98 to 302 CID (3 inch stroke) in order to make all of the breathing characteristics of the stock intake, cam, heads, and exhaust seem bigger for the displacement? Would that move the power band up the rev range, potentially adding horsepower while giving up low end torque?
A cam change might be in order, but would the TPI still be such a big limiter? I know the heads and 1.94 inch valves are good for 6,000 rpm on a 350 from an old ZZ3 that I had.
This comes from reading this forum too much... ;>]]
I know the common wisdom (widely practiced here) that there's no substitute for cubic inches.....but what if one were to de-stroke an L98 to 302 CID (3 inch stroke) in order to make all of the breathing characteristics of the stock intake, cam, heads, and exhaust seem bigger for the displacement? Would that move the power band up the rev range, potentially adding horsepower while giving up low end torque?
A cam change might be in order, but would the TPI still be such a big limiter? I know the heads and 1.94 inch valves are good for 6,000 rpm on a 350 from an old ZZ3 that I had.
This comes from reading this forum too much... ;>]]
You might be able to make power a little higher then the 350 but the real problem with the TPI is the length of the runners. The runners are designed (wave tuning) to provide a power band that peaks at about 4500 RPM. If you change the CI of the engine the runners length did not change so the power band remains the same. The 302 may rev slightly higher but it will still peak near 4500 RPM and you will loose torque and HP when compared to the 350.
Le Mans Master
Joined: Feb 2003
Posts: 9,087
Likes: 0
From: Chicago IL
You might be able to make power a little higher then the 350 but the real problem with the TPI is the length of the runners. The runners are designed (wave tuning) to provide a power band that peaks at about 4500 RPM. If you change the CI of the engine the runners length did not change so the power band remains the same. The 302 may rev slightly higher but it will still peak near 4500 RPM and you will loose torque and HP when compared to the 350.
The long tube runner design is incredibly long from TB to cylinder. This design will hit a wall at 4,800 if stock, maybe 5,500 if replaced with larger parts. My old '89 Formula 350 made 239 RWHP and 344 RWTQ. The dyno sheets showed a cliff for HP after 4,900 with the set up I had. The HP curve was almost flat while the TQ curve never stopped climbing.
At the track, it would be a couple cars ahead of LS1 cars at 1/2 - 3/4 track. After that, they would blow past like I was standing still and trapped way higher.
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
If your willing to change the crank to a 302 you will also need pistons. Any aftermarket pistons for a 302 are all high compression & even higher compression when used with 89 Vette heads as the combustion chambers are only 55 cc.
I am in the process of building a 302 with 89 Vette heads & a tpi for my 57 Chevy project.
here's how I made it work without having pistons custom made.
69 302 large main journal crank to fit a 350 block with a stock deck height of 9.030
Keith Black low compression pistons for a 350 with a 13cc dish.
Eagle 6" H beam rods.
Machine .030 off the piston top. This assembly = 0 (zero) deck height.
Compression ratio figures out to just under 10 to 1
Am not building a screamer, just want a crispy little motor to putt around with.
I am in the process of building a 302 with 89 Vette heads & a tpi for my 57 Chevy project.
here's how I made it work without having pistons custom made.
69 302 large main journal crank to fit a 350 block with a stock deck height of 9.030
Keith Black low compression pistons for a 350 with a 13cc dish.
Eagle 6" H beam rods.
Machine .030 off the piston top. This assembly = 0 (zero) deck height.
Compression ratio figures out to just under 10 to 1
Am not building a screamer, just want a crispy little motor to putt around with.
Melting Slicks
Joined: Mar 2004
Posts: 2,141
Likes: 2
From: Pittsburgh Pa
305's w/ tpi run up against "the wall" at 5k...a 302 will probly do the same with tpi...but a 302, with it's "over-square" bore/stroke, fed by an lt1 intake, could be "interesting"...but gonna need to address the knock sensor vs solid lifters pita (again) to get to the 7k rpm that the 302 shines at...a scattershield is mandatory, even on the street.
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Race Director
Joined: Nov 2003
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From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
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This idea makes some sense. When you put larger-bored TPIs on motors, the power bands do move up -- w/o much low-end loss. When they put a FIRST intake on a stock 350, there really wasn't any low-end loss. (But, the gains weren't that great either. I think that was due to the need for better-flowing heads.**)
If you do the math, I think a 302 would still benefit from a head that would flow at least 210cfm (maybe as much as 240cfm). Stock 113's can't do that. If you de-stroke, port 113 heads and install a bigger cam, you're idea should produce great power for a 302. Power would still roll-off between 5000 and 5300 rpms but under that WOOHOO!
If you want higher revs, don't say TPI and hi-revs in the same sentence.
gp
** TPI flows 195 cfm. 113's flow less than that w/stock cam.
If you do the math, I think a 302 would still benefit from a head that would flow at least 210cfm (maybe as much as 240cfm). Stock 113's can't do that. If you de-stroke, port 113 heads and install a bigger cam, you're idea should produce great power for a 302. Power would still roll-off between 5000 and 5300 rpms but under that WOOHOO!
If you want higher revs, don't say TPI and hi-revs in the same sentence.
gp
** TPI flows 195 cfm. 113's flow less than that w/stock cam.
Le Mans Master
Joined: Aug 2000
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From: Australia
Le Mans Master
Joined: Feb 2003
Posts: 9,087
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From: Chicago IL
It is sized right for a 305 but a bit too small for the L98.
Melting Slicks
Joined: Feb 2006
Posts: 2,753
Likes: 15
From: backwoods upstate ny
the short-arm engine must be wound higher to benefit from its reduced stroke vs the 3.48" L98 stroke engine... hydraulic lifters won't suffice due to lifter pump-up at abt 6k rpm and solid tappets must be substituted...efi needs k/s to realize its potential (excessively "rich" fuel/air mix reduces engine output), but k/s has great difficulty in "hearing" normal solid lifter lash take-up "noise" as detonation, retarding spark (per ecm program, crucial to prevent piston meltdown) with attending large power reduction.
Racer
Joined: Oct 2007
Posts: 478
Likes: 1
I think it is a very interesting idea. If I was to do it I would use the AS&M runners and the Edelbrock TPI intake manifold. Have the L98 heads ported and 2.02 intake valves installed. Maybe a cam on the order of 218/224.
Regarding the headers I don't think you want to go big maybe something on the order of 1.5" to 1.625". You would want a light weight valve train in order to spin it into the 7000rpm range.
Myself I would look into maybe the Dart Iron Eagle heads if you want to stay with iron heads. With aluminum it opens up some more possibilities.
Regarding the headers I don't think you want to go big maybe something on the order of 1.5" to 1.625". You would want a light weight valve train in order to spin it into the 7000rpm range.
Myself I would look into maybe the Dart Iron Eagle heads if you want to stay with iron heads. With aluminum it opens up some more possibilities.
Thread Starter
Heel & Toe
Joined: Aug 2007
Posts: 15
Likes: 0
If your willing to change the crank to a 302 you will also need pistons. Any aftermarket pistons for a 302 are all high compression & even higher compression when used with 89 Vette heads as the combustion chambers are only 55 cc.
I am in the process of building a 302 with 89 Vette heads & a tpi for my 57 Chevy project.
here's how I made it work without having pistons custom made.
69 302 large main journal crank to fit a 350 block with a stock deck height of 9.030
Keith Black low compression pistons for a 350 with a 13cc dish.
Eagle 6" H beam rods.
Machine .030 off the piston top. This assembly = 0 (zero) deck height.
Compression ratio figures out to just under 10 to 1
Am not building a screamer, just want a crispy little motor to putt around with.
I am in the process of building a 302 with 89 Vette heads & a tpi for my 57 Chevy project.
here's how I made it work without having pistons custom made.
69 302 large main journal crank to fit a 350 block with a stock deck height of 9.030
Keith Black low compression pistons for a 350 with a 13cc dish.
Eagle 6" H beam rods.
Machine .030 off the piston top. This assembly = 0 (zero) deck height.
Compression ratio figures out to just under 10 to 1
Am not building a screamer, just want a crispy little motor to putt around with.
Please keep us posted on how the mighty little mouse runs...
Last edited by John-in-Dallas; Sep 17, 2008 at 01:23 PM. Reason: spelling typo
Team Owner
Joined: Jul 2002
Posts: 22,267
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From: The light that never warms.
St. Jude Donor '12
So you want to make a high revving motor out of an engine that's only redeeming quality is it's low end torque? Yea, sounds great! 
I've driven a '68 Z28 with the original 302. It was a hoot, and I like high revving motors. The Ford 289 has an even shorter stroke, with a 4" bore, and it was fun in the '67 Shelby a friend had years ago. Hell, the ZR-1 I have now revs to 7000 easy, and it's probably the most fun you can have with a 350. But the TPI intake will kill you. If you really want a 302, do a Miniram and a blower like Car Craft did some years back. As I remember, it was a great little motor.
I've driven a '68 Z28 with the original 302. It was a hoot, and I like high revving motors. The Ford 289 has an even shorter stroke, with a 4" bore, and it was fun in the '67 Shelby a friend had years ago. Hell, the ZR-1 I have now revs to 7000 easy, and it's probably the most fun you can have with a 350. But the TPI intake will kill you. If you really want a 302, do a Miniram and a blower like Car Craft did some years back. As I remember, it was a great little motor.
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
Cam will be a roller with a 112 or 114 lobe seperation & maybe advanced a few degrees for better low rpm torque. I have to careful with the cam as the heads are already done. I set them up for a torquer flat tappet I have but have changed my mind & currently want a roller.
All I need to complete the assembly is the cam & lifters if I go with a roller, a set of 19# injectors for the piece meal TPI & an engine management system from Howell + a set of long tube small diameter headers which I may build to suit the application.
Trans will either be a 5 speed from an S10 or a 6 speed from a Camero. Both are here, just need to figure the gear ratios & make a choice based on that data, what rear end gearing I want & how I will use the vehicle.
Will post up when work resumes on the project.
Melting Slicks
Joined: Mar 2004
Posts: 2,141
Likes: 2
From: Pittsburgh Pa
the short-arm engine must be wound higher to benefit from its reduced stroke vs the 3.48" L98 stroke engine... hydraulic lifters won't suffice due to lifter pump-up at abt 6k rpm and solid tappets must be substituted...efi needs k/s to realize its potential (excessively "rich" fuel/air mix reduces engine output), but k/s has great difficulty in "hearing" normal solid lifter lash take-up "noise" as detonation, retarding spark (per ecm program, crucial to prevent piston meltdown) with attending large power reduction.
