408 stroker excessive blowby
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
408 stroker excessive blowby I have a built 408 gen 4 stroker motor that has excessive blowby to the point it’s blowing out of the dipstick. We took compression and leak down today. compression is 165-185 on all cylinders and leak down is10-13% across all cylinders. car made 485hp/500tq on dump the other day. I’m looking to see if anyone has a similar issue and what the fix is before we decide to tear the motor out. It seems like we have good sealing based on our testing.
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
i’m running the stock Ls1 valve covers. I have a mighty mouse catch a setup it was installed per instructions. they replaced it so we are using both rear ports to catch can now and no change in blowby. I’m running diamond pistons with the rings they supplied The compression and leak down tell us we have good sealing in cylinder.
One tech made a comment that maybe the exhaust is leaking into cylinder head and causing excessive pressure.
One tech made a comment that maybe the exhaust is leaking into cylinder head and causing excessive pressure.
TECH Fanatic
Joined: Jan 2009
Posts: 1,067
Likes: 477
From: Bostwick, GA.
"compression is 165-185 on all cylinders and leak down is10-13% across all cylinders."
Might be ok at "test speeds".
At real combustion pressure and rpm, the pistons are rattling around.
How far is the skirt out of the bore?
What's piston to wall clearance?
And what Scott asked.....
Might be ok at "test speeds".
At real combustion pressure and rpm, the pistons are rattling around.
How far is the skirt out of the bore?
What's piston to wall clearance?
And what Scott asked.....
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Gas Monkey Built a 6-Wheel Ferrari Testarossa With a Corvette LT4 Engine
Verdad Gallardo
7 Most Reliable High-Performance Engines GM Has Ever Built
Verdad Gallardo
Amazing '71 Camaro Restomod Is Modern Muscle Car Under the Skin
Verdad Gallardo
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo Anybody know the sleeve length on an L96 block? If answer is 5.45 inches part of your problem is the cylinder sleeve is too short for that stroke (5.45-4.0 = piston is in 1.45 inches - most LS pistons of this nature are 2.1-2.2 inches tall so that is like 3/4 of an inch outside the sleeve). Not to say you can not make work but over time it will develop excessive piston rock and have symptoms like you describe. The 10% - 13% is high. Also I bet piston at bottom of the bore the leak down will be higher. How many miles on the motor? The motor will still make good power like you have there but will have issues. They do have pistons to help issue not sure what you have here. How is your oil consumption? As a reference point the LS7 which has a 4 inch stroke has a sleeve length of 5.80 inches.
Last edited by Double06; Jan 18, 2025 at 02:29 PM.
I don't know the measurements off hand but this is the reason I decided on a 3.9" stroke for my ls2. I was set on 4" stroke until I talked to several people that convinced me otherwise due to insufficient sleeve length in a aluminum block. My understanding is the sleeves are longer in a iron block.
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
this is a new build used block it only has 2 runs on the dyno they pulled it off due to the pool of oil on floor from bottom of dipstick. apparently had a bunch of blowby oil was leaking
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
what should the leak down de on a motor that has just been broken in ona dyno, i thought 10-15% was normal. also i don’t think we have piston slap and id assume a gen 4 motor has same sleeve size as gen 3 6 liter. bunch of people have been doing strokers on those and never seen any issues with 4” stroke.
Mine is 1%-2% (based on 100 psi) after 2,000 miles. What are you using for crankcase breathing. The MM can is irrelevant if hooked up properly (the PCV only provides 5% to 10% of the venting) you need a vent to the outside air or to your air bridge if you are recycling. Is catch can filling with oil too? That thing needs to ventilate. The LS stroker with the short sleeve requires a specific piston for the short sleeve. Some of leak down could be going out the valves. When you do the leak down seal everything up (like valve covers) but dipstick and see how much air come out the dipstick. You might not have piston slap but a ring issue. Or it could be other things. I just did this on a BBC and I was 10%-18% (I needed a .010 bore clean up for that) and I could put my finger on dipstick and feel the air come out. How big is the cam? I presume a stick car. What is the compression ratio. IF you are using the MM catch can vent to atmosphere option (in addition to PCV feature) just make sure all the one-way valves/flaps are working correctly.
Last edited by Double06; Jan 19, 2025 at 08:34 AM.
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
i do see diamond pistons has two pistons for that bore size. one has a stock stroke and one has a 4.00 stroke. Maybe were on to something as thst may be the issue here. Can’t find the receipt to verify which ones i got. Can someone explain how the two pistons are different? is one piston skirt shorter?
The only difference between the stock stroke and the 4.0 stroke is the hole for the wrist pin will be further up and thus the piston will also be shorter. Of course this piston is for a 4 inch stroke because of this but that does not answer the block sleeve length question. I can put that piston in a longer sleeve block and it would fit fine also. In general piston height is usually about 1 inch more than compression height (compression height is the center of the wrist pin to top of the piston). So if you have a 1.12 inch compression height your piston will be about 2.12 inches tall. You may call Diamond on Monday and ask them. If your motor is fresh this could a ring or other issue as the piston rock is usually not a day one issue. You may want to ask guys at dyno shop what they think and have run into this dipstick issue before. Maybe post of photo of set up you have here maybe there is some issue we can see.
Thread Starter
Staging Lane
Joined: Dec 2019
Posts: 75
Likes: 0
From: Arizona
i think at this point i don’t really have a choice but to tear down motor and have a good LS machine shop go through all measurements and see what the hell is going on. I’m just pissed at the whole ordeal. Guy at machine shop hid in his office when i picked it up and wa no build sheet. Then later we find out we got a bunch of blowby. Something’s not right so time to tear it down i guess