Keeping 1600 with a 7afe block

[Cortina69]

Just a quick question to ask if any one has increased the rod size in a 7afe block to keep the engine in the 1600 class? Was just talking about the possibility this week, opening out the question.

[yoshimitsuspeed]

Just a quick question to ask if any one has increased the rod size in a 7afe block to keep the engine in the 1600 class? Was just talking about the possibility this week, opening out the question.

If you are required to stay 1600 because of racing class or any other reason then it's a great way to get the motor dialed in better for high RPM power. If you have the option of running 7A then you will more easily gain more from the added displacement. It will also allow you to make more power at a lower RPM which will help longevity.

[oldeskewltoy]

Just a quick question to ask if any one has increased the rod size in a 7afe block to keep the engine in the 1600 class? Was just talking about the possibility this week, opening out the question.

http://www.toymods.org.au/forums/thread ... troke-a-7A - a VERY good read!

[ae86cesar]

I've read around about this but never seen a finished product. I think increasing the rod ratio will help the 4ag reach higher rpm safer and with the right cams more power. Just my theory

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[sirdeuce]

Building this engine would have better power with the stock cams. Upper mid to high RPM will see the best benefit. I would like to see someone complete this build as well. Closest I've seen to such a build is a destroked type 1 VW. Class rules capped displacement at 1600cc. The builder had a 60mm crank fabbed up and ran 92mm pistons and jugs. 1595cc. With class restrictions the engine didn't really do much better than stock bore and stroke. BUT! when he changed the heads and carbs the thing was a rocket! Held it's own against most of the 2 liter cars!

[sirdeuce]

I think the best gains with a 7A block are with the 7A crank. The extra 200ccs aren't where the boost comes from, the longer stroke is. The bore is the same as the 4A, so the force on the crank is the same for both. The longer arm provides the increase at the crank's center. Without spinning the thing to higher RPM. Not sure of the 7A's block capabilities as far as RPM, but some are claiming more than 300hp turboed. If you want to increase the R/S ratio move the pin higher in the piston, maybe reduce the rod journal diameter, should be able to get an another 10mm in rod length while keeping the stronger, lighter block. If you are willing to go that far, consider steel sleeves in the block. Might be able to get up to an 85mm bore. Lotsa $$$$. Either way, I'd like to see the results of a 4A crank in a 7A block. I just don't think it would be worth the time and money.

[yoshimitsuspeed]

I've read around about this but never seen a finished product. I think increasing the rod ratio will help the 4ag reach higher rpm safer and with the right cams more power. Just my theory

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The stock 4A rotating assembly can already happily spin faster than most people will ever take it. Yes the long rod setup would have some high RPM benefit over the stock 4A but the 7A stroke would still be better than that. 7A stroke with the longest possible rod would still be better yet. At least from a performance perspective although there are other cons to moving the wristpin up into the oil control rings and such.
It all comes down to bang for the buck and the only way the long rod 4A is the best bang for the buck is when you can't increase stroke or displacement.

[yoshimitsuspeed]

Building this engine would have better power with the stock cams. Upper mid to high RPM will see the best benefit. I would like to see someone complete this build as well.

This is pretty common with real 4A race motors. I could get someone all the components they need to do this build but it wouldn't be cheap and like you said above if you can run a 7A stroke that is the better option. The only time they use this setup in race motors is when more displacement is not an option. Unfortunately I couldn't provide much info on specifics like gains, or comparison dynos as all these builds will be done on the DL.
There was a build on the MR2OC where a guy had a 4AGTE in his Miata that he destroked with a 5A crank to race in a 1.5 liter class. It was a pretty cool build and I think he was making something like 600 hp. There is a longer thread on the build on some Miata forum somewhere.

[ae86cesar]

I've read around about this but never seen a finished product. I think increasing the rod ratio will help the 4ag reach higher rpm safer and with the right cams more power. Just my theory

Sent from my SM-N910T3 using Tapatalk

The stock 4A rotating assembly can already happily spin faster than most people will ever take it. Yes the long rod setup would have some high RPM benefit over the stock 4A but the 7A stroke would still be better than that. 7A stroke with the longest possible rod would still be better yet. At least from a performance perspective although there are other cons to moving the wristpin up into the oil control rings and such.
It all comes down to bang for the buck and the only way the long rod 4A is the best bang for the buck is when you can't increase stroke or displacement.

So you're saying a 7age would be better with custom pistons with the pin moved up as much as possible and custom rods? What problems would moving the pin higher cause?

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[yoshimitsuspeed]

I've read around about this but never seen a finished product. I think increasing the rod ratio will help the 4ag reach higher rpm safer and with the right cams more power. Just my theory

Sent from my SM-N910T3 using Tapatalk

The stock 4A rotating assembly can already happily spin faster than most people will ever take it. Yes the long rod setup would have some high RPM benefit over the stock 4A but the 7A stroke would still be better than that. 7A stroke with the longest possible rod would still be better yet. At least from a performance perspective although there are other cons to moving the wristpin up into the oil control rings and such.
It all comes down to bang for the buck and the only way the long rod 4A is the best bang for the buck is when you can't increase stroke or displacement.

So you're saying a 7age would be better with custom pistons with the pin moved up as much as possible and custom rods? What problems would moving the pin higher cause?

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Exactly, on a true race build you would move the pin up to just below the lower compression ring and run a longer rod. The downside is that the oil control ring will not be supported there which can lead to expedited wear and increased oil consumption.
A worthwhile mod for a full blown racing motor where every little bit counts. For your average street car it's just not worth it.

[oldeskewltoy]

longer stroke = piston slower, combustion pressures on top of piston for longer time... more power, typically @ lower rpm. Example early Atlantics used std rod they made 240hp @ 10,000-10,500, longer rod engines make similar power but about 1000 rpm lower.

shorter rod = higher piston speeds, good for induction in a small displacement engine.


Interestingly.... the 4A and 7A have nearly the same rod to stroke ratio

[ae86cesar]

longer stroke = piston slower, combustion pressures on top of piston for longer time... more power, typically @ lower rpm. Example early Atlantics used std rod they made 240hp @ 10,000-10,500, longer rod engines make similar power but about 1000 rpm lower.

shorter rod = higher piston speeds, good for induction in a small displacement engine.


Interestingly.... the 4A and 7A have nearly the same rod to stroke ratio

So all else the same, a longer rod will make the same power but at a lower rev? I know custom rods are around the $1000 range, but how about custom pistons with the pin raised?

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[oldeskewltoy]

longer stroke = piston slower, combustion pressures on top of piston for longer time... more power, typically @ lower rpm. Example early Atlantics used std rod they made 240hp @ 10,000-10,500, longer rod engines make similar power but about 1000 rpm lower.

shorter rod = higher piston speeds, good for induction in a small displacement engine.


Interestingly.... the 4A and 7A have nearly the same rod to stroke ratio

So all else the same, a longer rod will make the same power but at a lower rev? I know custom rods are around the $1000 range, but how about custom pistons with the pin raised?

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As to long rod... in a 4AG... yes... In other engines... I can't speak to it... although I've read a long rod in a 2TG does help with power in that engine.

As to custom pistons - talk to a piston company and ask them... they know far more then we...

What you are after here are minutia... small differences. Much more significant is the right headwork, the right cams, the the best compression to make use of the cams and head.