Anyone tried a Destroked LS? - RX7Club.com - Mazda RX7 Forum

Anyone tried a Destroked LS?

Old 03-31-18, 04:01 AM
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Anyone tried a Destroked LS?

Hey all, new to the forum and in the process of shopping for an FD. I plan to do an LS swap on it. I've built several 90's JDM cars and done a few swaps so I'm looking forward to the project.

My question is if anyone here has done a Destroked LS in their FD (something like this: 4.8L Crank + LS3 Block: How to Build an 8,000 rpm LS Destroker - Hot Rod Network). I love higher revving V8's and I feel it would ad some extra fun/work to the project. I searched for it but didn't find much other than short discussions on doing a destroker kit. Thanks!
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Old 05-19-18, 02:16 PM
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I considered a destroked 6.0 with the 4.8 rotating assembly as i wanted more usable hp under the curve. I am build a time trial racecar and I thought it would made sense, however after really digging into it, I do not believe a destroker is worth it unless you are limited on displacement. LS engines are already over squared and can easily rev 7k+ with good heads and valves. Going through the trouble of replacing the bottom end would not be worth the extra 500-1000 rpm, unless you are after a great sound!
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Old 05-19-18, 03:36 PM
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Its the top-end that is the issue with LS.

You can spend a ton of money and build up your pushrod valve train like a Nascar LS.

You can spend silly amounts of money and put DOHC Mercury Marine heads on your LS and make more power with more revs.

Or you can let the LS do what the LS does best and made great power with high displacement from 1,000rpm to 7,000rpm for not that much money.
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Old 05-20-18, 11:15 AM
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the computer my engine sim software is on just took a dive, or i could post dyno sheets.

i have done simulations like this, before and with an american V8 the limitation is always the cylinder heads. the LS engines are better than the older stuff, but they are still the limitation
(the older stuff was designed for smaller engines ~4.5 liters, so they are just too small, the LS heads are much improved for a bigger engine, but still are limited.)

so when you de-stroke the engine, on an american v8, you tend to loose a lot (40+ lbs feet) in the 2000-4000 rpm area. peak power will be about the same, the cylinder heads are still the limiting factor. power past the peak will improve, but usually its very small (5ish hp)

the other thing the shorter stroke does though, is to lower piston speeds. if you were doing some racing, or something, where the engine would be running in the 4000-7000rpm range most of the time, then the short stroke might make sense. lower piston speed, friction will be lower, in theory a longer rod could be used.

in a street car, where most of the time its below 3,000rpm, the long stroke is better
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Old 05-20-18, 11:55 PM
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A 3.622" stroke (stock ls1/2/3) can easily rev to 7500rpm with lightweight valves. Anything beyond that and you need to start looking at a solid roller setup. Unless you have heads that flow a ton of air with a matching intake manifold (the plastic/polymer intakes typically hit a wall just over 7000rpm anyway and the power just flatlines (still makes power... just not gaining)).

A 4.0" stroke has no issues spinning to over 7000rpm either.

While it sounds great on paper to have a high revving engine ..... as others have said by destroking you are pretty much giving up a ton of torque down low. A 416 or 427 stroker motor will have gobs of power everywhere with the correct cam. Also the higher you spin the engine.... you truly will get a lot less life out of it.

My heads/cam LS1 made 430rwhp/380rwtq and went low 11'[email protected] My mild LS7 with cam only made 520rwhp/485rwtq and was sooooo much more fun, still would spin to over 7000rpm and went low 10'[email protected] It had power everywhere..... the LS1 had a fairly large cam and below 4000rpm was rather soft.... at 4000-7100rpm it was a maniac but still not like the LS7 with the additional torque.

Last edited by gnx7; 05-21-18 at 01:45 PM.
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Old 05-24-18, 09:00 AM
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What he said ^^^
Destroking only makes sense if you are displacement-limited by class rules.
The fundamental issue with destroking is that you lose more in displacement than you gain in rev potential, because peak piston acceleration varies roughly with the square of rpm. A forged 3.622" stroke 6.0 at 7500rpm gives roughly the same peak piston acceleration as a 3.267" stroke 5.4 at 7900 rpm. You've given up 10% displacement and torque to gain 5% more rev potential. Actually you gain a bit more revs than scaling by square-root of stroke because with shorter stroke you can have longer rods which will reduce peak piston accelerations, but still not enough to make up for the 10% displacement loss.

This is why INcreasing stroke works so well, particularly for a street or street/track car. More torque *everywhere*, and not much loss in rev potential.
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Old 05-27-18, 07:33 PM
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simulated results are in. test "engine" is an ls1 intake, LQ9 heads. Headers were 2" diameter 39" inches long. cam is a 216/220 @.050, .309/.319 lift. LSA is 114. i wanted something decent, but not huge. anything bigger than this and the valves wanted to float.

bottom end was a 7.0 vs a 7.0 with the 4.8 crank and rods, everything else stays the same. compression is 10:1, enough octane so that neither engine has detonation, 105 octane

the short stroke engine is 5.7 liters, so it actually night work in a class nicely.

anyways, some numbers.

the 7.0 has a peak hp of [email protected] (the 5.7 makes 423 at this Rpm), peak torque is 508lbs/ft at 4000rpm ([email protected]), piston speed at 6000rpm is 4000 ft/min

the 5.7 has peak hp of 440 at 6500rpm (the 7 liter is making 364hp). peak torque is 420lbsft at 5000, but its making 418 at 4500, and [email protected] piston speed at 6000rpm is 3268 ft/min (the 7.0 hits this at 5000rpm)

so basically to de-stroke, you loose BIG in the mid-range, a little up top. on the plus side, piston speed does come down to a sane value. if you have to limit displacement for a racing class, and or spin the engine past 5000rpm most of the time, it might make sense still, its pretty close to the same dimensions that they run in NASCAR
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Old 06-03-18, 02:37 AM
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Originally Posted by calichase View Post
I considered a destroked 6.0 with the 4.8 rotating assembly as i wanted more usable hp under the curve. I am build a time trial racecar and I thought it would made sense, however after really digging into it, I do not believe a destroker is worth it unless you are limited on displacement. LS engines are already over squared and can easily rev 7k+ with good heads and valves. Going through the trouble of replacing the bottom end would not be worth the extra 500-1000 rpm, unless you are after a great sound!
more usable hp under the curve?
confused lol (would that be over the curve referencing the TQ line?)

I've thought about this a lot as well though. Something simple like I want to use the same red line as my stock tach, or simply because a v8 at 8k sounds amazing.
But the more I think on this (read the older I get), the more I feel like I'm working backwards.
Take high rpm torqueless unreliable motor out (please rx7 club no bashing my lack of rotary love), put reliable v8 in, change out cam for a high duration lots of overlap popcorn idle high rpm screamer, trade idle quality for high rpm noises, trade gas mileage for high rpm noises, trade displacement for even more high rpm goodness... Dammit I went backwards.

I'm with all the other posts.
Now let me finish my ls400 heads and standalone. Maybe I will make it to 8k with a v8 after all.
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Old 06-04-18, 12:02 PM
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You can rev out smaller displacements LS's pretty well with not a ton invested. Destroked Ls's are cool and all but at the costs associated you may be better off with a high ci high power ls or small ci decent power and a few more revs.
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